Is the notion that species interactions are stronger and more specialized in the tropics a zombie idea?

Effects of climate on leaf phenolics, insect herbivory, and their relationship in pedunculate oak (Quercus robur) across its geographic range in Europe

An increase in biotic interactions towards lower latitudes is one of the most consistent patterns in ecology. Higher temperatures and more stable climatic conditions at low latitudes are thought to enhance biotic interactions, accelerating biological evolution and leading to stronger anti-herbivore defences in plants. However, some studies report contradictory findings, highlighting the need for further investigation into the underlying mechanisms. We used a combination of field observations and feeding trials in controlled environments to investigate the effect of climate on chemical defences and insect herbivory in pedunculate oak (Quercus robur L.) throughout most of its geographic range in Europe, while controlling for physical defences. The concentration of lignin, flavonoids, and total phenolics increased significantly with temperature, whereas both field herbivory and weight of spongy moth (Lymantria dispar L.) larvae were negatively influenced by temperature. Lignin concentration positively influenced the weight of spongy moth larvae whereas it had no effect on field herbivory. We found no evidence of strong positive relationships between insect herbivory and larvae growth with leaf defences. Our study underscores the complexity of plant-herbivore interactions along climatic gradients and highlights the need for further research to disentangle these intricate relationships.

Global Avian Frugivore-Fruit Trait Matching Decreases Toward the Tropics

Trait matching, the phenomenon where ecological interactions are mediated by compatibility, constitutes a cornerstone of frugivore-fruit interaction network dynamics. Given that biotic interactions have long been hypothesized to be more intense or specialized in the tropics, the intensity of trait matching patterns might likewise exhibit a latitudinal gradient in frugivory networks, yet this remains unverified. Here, we established a dataset encompassing 200 avian frugivorous networks to explore the relationships between the body mass and gape size of frugivore birds and fruit traits (size and color) on a global scale. Our results indicated that frugivore traits were closely associated with fruit traits regardless of the climate, demonstrating a biotic match between the two counterparts. We detected a significant decrease in frugivore-fruit trait matching toward the tropics, which challenges prevailing concepts considering the high biodiversity therein. Our structural equation modeling clarified that latitude and temperature exert an indirect influence on trait matching by affecting gape size and fruit traits. These discoveries emphasize the impact of the latitudinal gradient of temperature in driving the observed patterns of trait matching. The weaker trait matching in tropical regions may suggest more complex interactions therein and also highlights the potential for altered network structures amid global climate change.

Leaf traits and insect herbivory levels in two Mediterranean oaks and their hybrids through contrasting environmental gradients

Insect herbivory has attracted enormous attention from researchers due to its effects on plant fitness. However, there remain questions such as what are the most important leaf traits that determine consumption levels, whether there are latitudinal gradients in herbivore pressure, or whether there are differences in susceptibility between hybrids and their parental species. In this work, we address all these issues in two species of Mediterranean Quercus (Quercus faginea subsp. faginea Lam. and Quercus pyrenaica Wild.) and their hybrids. Over 2 years, we analyzed leaf emergence and 11 leaf traits (biomechanical, chemical and morphological), as well as the levels of herbivory by insects in leaves of the three genetic groups in different locations distributed along a climatic gradient. The hybrids showed intermediate values between both species in leaf emergence, chemical traits and structural reinforcement. By contrast, they were more similar to Q. faginea in leaf size and shape. Despite their intermediate leaf traits, hybrids always showed lower losses by consumption than both parental species, which suggests that they possess inherent higher resistance to herbivores, which cannot be explained by their dissimilarities in leaf traits. Within each genetic group, differences in leaf size were the most important determinant of differences in herbivory losses, which increased with leaf size. In turn, leaf size increased significantly with the increase in mean annual temperatures across the climatic gradient, in parallel with herbivory losses. In conclusion, contrary to our expectations, hybrids maintained lower levels of herbivory than their parent species. Given the potential negative effect of leaf herbivory on carbon fixation, this advantage of the hybrids would imply a threat to the persistence of both pure species. More research is needed to elucidate possible alternative mechanisms that allow for maintaining species integrity in the absence of reproductive barriers.

Weaker Plant-Frugivore Trait Matching Towards the Tropics and on Islands

Biotic interactions play an important role in species diversification and maintenance and, thus, are regarded as the architecture of biodiversity. Since Darwin and Wallace, biologists have debated whether biotic interactions are stronger towards the tropics and on continents, when compared to temperate regions and islands. Here, based on 354 avian frugivory networks accounting for 22,199 interactions between 1247 bird species and 2126 plant species, we quantified trait matching strength, which reflects interaction strength and specificity, across gradients of latitude and insularity globally. We found that matching between beak size and fruit size was significantly stronger towards the poles and on continents, when compared with the tropics and on islands. As underlining ecological factors, trait matching was stronger with a larger proportion of frugivory (measured as the mean proportion of fruits in bird diets) and network-level mean beak size, and with a smaller proportion of fleshy-fruited species (measured as the proportion of fleshy-fruited plant species in the botanical country where the network was located). These findings suggest that the latitudinal and insular patterns in trait matching are driven by biotic factors that may relate to trait co-evolution between interacting species and optimal foraging for bird species.

Tree phytochemical diversity and herbivory are higher in the tropics

A long-standing but poorly tested hypothesis in plant ecology and evolution is that biotic interactions play a more important role in producing and maintaining species diversity in the tropics than in the temperate zone. A core prediction of this hypothesis is that tropical plants deploy a higher diversity of phytochemicals within and across communities because they experience more herbivore pressure than temperate plants. However, simultaneous comparisons of phytochemical diversity and herbivore pressure in plant communities from the tropical to the temperate zone are lacking. Here we provide clear support for this prediction by examining phytochemical diversity and herbivory in 60 tree communities ranging from species-rich tropical rainforests to species-poor subalpine forests. Using a community metabolomics approach, we show that phytochemical diversity is higher within and among tropical tree communities than within and among subtropical and subalpine communities, and that herbivore pressure and specialization are highest in the tropics. Furthermore, we show that the phytochemical similarity of trees has little phylogenetic signal, indicating rapid divergence between closely related species. In sum, we provide several lines of evidence from entire tree communities showing that biotic interactions probably play an increasingly important role in generating and maintaining tree diversity in the lower latitudes.

Latitudinal specificity of plant-avian frugivore interactions

Broad-scale assessments of plant-frugivore interactions indicate the existence of a latitudinal gradient in interaction specialization. The specificity (i.e. the similarity of the interacting partners) of plant-frugivore interactions could also change latitudinally given that differences in resource availability could favour species to become more or less specific in their interactions across latitudes. Species occurring in the tropics could be more taxonomically, phylogenetically and functionally specific in their interactions because of a wide range of resources that are constantly available in these regions that would allow these species to become more specialized in their resource usage. We used a data set on plant-avian frugivore interactions spanning a wide latitudinal range to examine these predictions, and we evaluated the relationship between latitude and taxonomic, phylogenetic and functional specificity of plant and frugivore interactions. These relationships were assessed using data on population interactions (population level), species means (species level) and community means (community level). We found that the specificity of plant-frugivore interactions is generally not different from null models. Although statistically significant relationships were often observed between latitude and the specificity of plant-frugivore interactions, the direction of these relationships was variable and they also were generally weak and had low explanatory power. These results were consistent across the three specificity measures and levels of organization, suggesting that there might be an interplay between different mechanisms driving the interactions between plants and frugivores across latitudes.

Global insect herbivory and its response to climate change

Herbivorous insects consume a large proportion of the energy flow in terrestrial ecosystems and play a major role in the dynamics of plant populations and communities. However, high-resolution, quantitative predictions of the global patterns of insect herbivory and their potential underlying drivers remain elusive. Here, we compiled and analyzed a dataset consisting of 9,682 records of the severity of insect herbivory from across natural communities worldwide to quantify its global patterns and environmental determinants. Global mapping revealed strong spatial variation in insect herbivory at the global scale, showing that insect herbivory did not significantly vary with latitude for herbaceous plants but increased with latitude for woody plants. We found that the cation-exchange capacity in soil was a main predictor of levels of herbivory on herbaceous plants, while climate largely determined herbivory on woody plants. We next used well-established scenarios for future climate change to forecast how spatial patterns of insect herbivory may be expected to change with climate change across the world. We project that herbivore pressure will intensify on herbaceous plants worldwide but would likely only increase in certain biomes (e.g., northern coniferous forests) for woody plants. Our assessment provides quantitative evidence of how environmental conditions shape the spatial pattern of insect herbivory, which enables a more accurate prediction of the vulnerabilities of plant communities and ecosystem functions in the Anthropocene.

Weak latitudinal trends in reproductive traits of Afromontane forest trees

BACKGROUND AND AIMS

Is the increase in species diversity patterns towards lower latitudes linked to reproductive traits? Plant reproductive organs influence reproductive isolation and hence species divergence. Abiotic differences between temperate and tropical regions can also directly impact on plant reproductive traits. Here we provide a novel overview of southern hemisphere, Afromontane forest tree taxonomical patterns and ask whether reproductive traits relate to latitude, while accounting for environmental (tree height) and evolutionary (biogeographical affinity) selective forces.

METHODS

We compiled a novel dataset with (1) flower colour, size and pollination syndrome and (2) fruit colour, size and dispersal syndrome for 331 tree species found in six Afromontane forest regions. We categorized each species into latitudinal distribution using these six regions, spanning the southern Cape (34º S) to Mount Kenya (0º S). Additionally, we gathered maximum tree height (m) for each species and determined the global distribution of all 196 tree genera (Afrotropical, Palaeotropical or Pantropical).

KEY RESULTS

Species, genera and families showed a general decrease in richness away from tropical and subtropical forests towards warm temperate forests. Southern Afrotemperate forests (the furthest south) had the highest tree endemism. There was no relationship between latitude and the reproductive traits tested here. Biogeographical affinity related to fruit colour and dispersal syndrome, with palaeotropical genera showing relative increases in black-purple fruit colour compared with pantropical genera, and palaeotropical genera showing relative increases in biotic seed dispersal compared with Afrotropical genera, which showed higher relative abiotic seed dispersal. Taller trees had a higher chance to be wind or insect pollinated (compared with bird pollinated) and had larger fruits.

CONCLUSIONS

Latitude explained patterns in Afromontane tree taxonomic diversity; however, tree reproductive traits did not relate to latitude. We suggest that phylogenetic conservatism or convergence, or both, explain the reported patterns.

Following the feeder: A global synthesis of disturbance-based foraging associations of birds

Species interactions link animal behaviour to community structure and macroecological patterns of biodiversity. One common type of trophic species interaction is disturbance foraging-the act of obtaining food at a disturbance created by another organism. Disturbance foraging is widespread across the animal kingdom, especially among birds, yet previous research has been largely anecdotal and we still lack a synthetic understanding of how this behaviour varies geographically, phylogenetically and ecologically. To address these gaps, we conducted a comprehensive literature review to test focal hypotheses about disturbance foraging behaviour in birds. We found that avian disturbance foraging was geographically ubiquitous, occurring in both aquatic and terrestrial habitats across six continents and four oceans. Consistent with predictions based on established species diversity gradients in different habitat types, the majority of terrestrial observations occurred at tropical latitudes, whereas aquatic observations took place most frequently in temperate marine waters. Although disturbance foraging was widespread across the avian phylogeny, contrary to our prediction, the behaviour was also conserved phylogenetically (Pagel's λ = 0.7) and clustered within suboscine landbirds in terrestrial environments and seabirds in aquatic environments. Similarly, although disturbers were taxonomically diverse as we predicted, interactions were unexpectedly dominated by swarm-raiding ants in terrestrial environments and cetaceans in aquatic environments. Diet and body mass were also important predictors of disturbance foraging associations: Responders followed disturbers with similar diets and larger body sizes. Overall, our hypothesis-testing framework provides insight into the importance of geography, phylogeny and ecology as predictors of disturbance foraging behaviour. We anticipate that this comprehensive assessment of disturbance foraging will serve to generate additional hypotheses and spark future research and management considerations about this fascinating but poorly studied suite of species interactions, especially as biotic interactions face unprecedented risks in our rapidly changing world.

Plant size, latitude, and phylogeny explain within-population variability in herbivory

Interactions between plants and herbivores are central in most ecosystems, but their strength is highly variable. The amount of variability within a system is thought to influence most aspects of plant-herbivore biology, from ecological stability to plant defense evolution. Our understanding of what influences variability, however, is limited by sparse data. We collected standardized surveys of herbivory for 503 plant species at 790 sites across 116° of latitude. With these data, we show that within-population variability in herbivory increases with latitude, decreases with plant size, and is phylogenetically structured. Differences in the magnitude of variability are thus central to how plant-herbivore biology varies across macroscale gradients. We argue that increased focus on interaction variability will advance understanding of patterns of life on Earth.

Linking temperature dependence of fitness effects of mutations to thermal niche adaptation

Fitness effects of mutations may generally depend on temperature that influences all rate-limiting biophysical and biochemical processes. Earlier studies suggested that high temperatures may increase the availability of beneficial mutations ('more beneficial mutations'), or allow beneficial mutations to show stronger fitness effects ('stronger beneficial mutation effects'). The 'more beneficial mutations' scenario would inevitably be associated with increased proportion of conditionally beneficial mutations at higher temperatures. This in turn predicts that populations in warm environments show faster evolutionary adaptation but suffer fitness loss when faced with cold conditions, and those evolving in cold environments become thermal-niche generalists ('hotter is narrower'). Under the 'stronger beneficial mutation effects' scenario, populations evolving in warm environments would show faster adaptation without fitness costs in cold environments, leading to a 'hotter is (universally) better' pattern in thermal niche adaptation. We tested predictions of the two competing hypotheses using an experimental evolution study in which populations of two model bacterial species, Escherichia coli and Pseudomonas fluorescens, evolved for 2400 generations at three experimental temperatures. Results of reciprocal transplant experiments with our P. fluorescens populations were largely consistent with the 'hotter is narrower' prediction. Results from the E. coli populations clearly suggested stronger beneficial mutation effects at higher assay temperatures, but failed to detect faster adaptation in populations evolving in warmer experimental environments (presumably because of limitation in the supply of genetic variation). Our results suggest that the influence of temperature on mutational effects may provide insight into the patterns of thermal niche adaptation and population diversification across thermal conditions.

Network and parasitological analyses reveal latitudinal gradient in bats-ectoparasitic fly interactions across the Neotropic

Ecological interactions between parasites and their hosts play a fundamental role in evolutionary processes. Selection pressures are exerted on parasites and their hosts, usually resulting in high levels of specificity. Such is the case of ectoparasitic bat-flies, but how large-scale spatial gradients affect the dynamics of their interactions with their bat hosts is still unknown. In the present study, we investigated interaction patterns between bats and their ectoparasitic flies (Streblidae and Nycteribiidae), both presenting their peak of diversity in the Neotropical region, along a latitudinal gradient. Using network analyses and parasitic indices, grounded on the latitudinal diversity gradient pattern, we evaluated how spatial gradients affect species interactions and parasitic indices at the biogeopraphic scale, with increasing species richness in interaction networks closer to the tropics, leading to increases in network modularity, size, and specialization, and to a decrease in nesting and connectivity. We conducted a literature review, focusing on studies done in the Neotropical region, and own data. We obtained a bat richness of 97 species parasitized by 128 species of ectoparasitic flies, distributed into 57 interaction networks between latitudes 29° S and 19° N in the Neotropic. Network metrics and parasitic indices varied along the latitudinal gradient, with changes in the richness of bats and their ectoparasitic flies and in the structure of their interactions; network specialization, modularity, and connectance increase with latitude, while network size decreases with latitude. Regions closer to the equator had higher parasite loads. Our results show that interaction network metrics present a latitudinal gradient and that such interactions, when observed at a local scale, hide variations that only become perceptible at larger scales. In this way, ectoparasites such as bat flies are not only influenced by the ecology and biology of their hosts, but also by other environmental factors acting directly on their distribution and survival.

The future of tropical insect diversity: strategies to fill data and knowledge gaps

The decline of insect diversity is a much-discussed, yet understudied phenomenon, particularly in the tropics, where the majority of insect abundance, diversity and biomass is found. Integrated approaches involving traditional taxonomic methods, new molecular approaches, and novel monitoring and identification tools and applications are needed to address related and challenging questions regarding how many species of tropical insects exist, their distributions and natural history, the relative impacts of global change drivers on insect diversity across complex tropical landscapes, and the effects of insect declines on ecosystem functions and services. The main barriers to addressing these challenges are a lack of capacity and funding for research on insects in tropical countries and a lack of recognition of their importance for ecosystem functioning and human wellbeing. Insects must be brought into policy agendas, local capacity and funding through cross-boundary collaborations and equitable scientific practices increased, and their importance emphasized.

Exceptional endemicity of Aotearoa New Zealand biota shows how taxa dispersal traits, but not phylogeny, correlate with global species richness

Species' with more limited dispersal and consequently less gene flow are more likely to form new spatially segregated species and thus contribute disproportionally to endemic biota and global species richness. Aotearoa New Zealand has exceptional endemicity, with 52% of its 54,000 named species endemic, including 32%, 39% and 68% for freshwater, marine and terrestrial environments respectively. The lower endemicity of freshwater biota (excluding insects) is attributed to their need to disperse between habitats that are temporary on evolutionary timescales. The percent endemicity of higher taxa (Order to Kingdom), a measure of phylogenetic relationships, was not correlated with regional and global species richness. However, there was a positive correlation between endemicity and species richness across dispersal trait groups based on their environment, typical body size, mobility (including flight), and if marine, whether pelagic or benthic. Typically flighted taxa had high endemicity contrary to the dispersal-endemicity hypothesis, but reflecting exceptional isolation by distance and time, and reduced flight ability as occurs on islands. It is proposed that the high richness and endemicity of mobile macrofauna is caused by a combination of niche specialisation opportunities and predation limiting dispersal respectively. Thus, dispersal traits better predicted endemicity and global species richness than phylogeny.

Comparative phylogeography reveals widespread cryptic diversity driven by ecology in Panamanian birds

Widespread species often harbor unrecognized genetic diversity, and investigating the factors associated with such cryptic variation can help us better understand the forces driving diversification. Here, we identify potential cryptic species based on a comprehensive dataset of COI mitochondrial DNA barcodes from 2,333 individual Panamanian birds across 429 species, representing 391 (59%) of the 659 resident landbird species of the country, as well as opportunistically sampled waterbirds. We complement this dataset with additional publicly available mitochondrial loci, such as ND2 and cytochrome b, obtained from whole mitochondrial genomes from 20 taxa. Using barcode identification numbers (BINs), we find putative cryptic species in 19% of landbird species, highlighting hidden diversity in the relatively well-described avifauna of Panama. Whereas some of these mitochondrial divergence events corresponded with recognized geographic features that likely isolated populations, such as the Cordillera Central highlands, the majority (74%) of lowland splits were between eastern and western populations. The timing of these splits are not temporally coincident across taxa, suggesting that historical events, such as the formation of the Isthmus of Panama and Pleistocene climatic cycles, were not the primary drivers of cryptic diversification. Rather, we observed that forest species, understory species, insectivores, and strongly territorial species-all traits associated with lower dispersal ability-were all more likely to have multiple BINs in Panama, suggesting strong ecological associations with cryptic divergence. Additionally, hand-wing index, a proxy for dispersal capability, was significantly lower in species with multiple BINs, indicating that dispersal ability plays an important role in generating diversity in Neotropical birds. Together, these results underscore the need for evolutionary studies of tropical bird communities to consider ecological factors along with geographic explanations, and that even in areas with well-known avifauna, avian diversity may be substantially underestimated.

Latitudinal gradients in biotic interactions: Are cacti pollination systems more specialized in the tropics?

Biotic interactions are said to be more specialized in the tropics, and this was also proposed for the pollination systems of columnar cacti from North America. However, this has not yet been tested for a wider set of cactus species. Here, we use the available information about pollination in the Cactaceae to explore the geographic patterns of this mutualistic interaction, and test if there is a latitudinal gradient in its degree of specialization. We performed a bibliographic search of all publications on the pollination of cacti species and summarized the information to build a database. We used generalized linear models to evaluate if the degree of specialization in cacti pollination systems is affected by latitude, using two different measures: the number of pollinator guilds (functional specialization) and the number of pollinator species (ecological specialization). Our database contained information about the pollination of 148 species. The most frequent pollinator guilds were bees, birds, moths and bats. There was no apparent effect of latitude on the number of guilds that pollinate a cactus species. However, latitude had a small but significant effect on the number of pollinator species that service a given cactus species. Bees are found as pollinators of most cactus species, along a wide latitudinal gradient. Bat and bird pollination is more common in the tropics than in the extra-tropics. The available information suggests that cacti pollination systems are slightly more ecologically specialized in the tropics, but it does not support any trend with regard to functional specialization.

Female dewlap ornaments are evolutionarily labile and associated with increased diversification rates in Anolis lizards

The evolution of costly signalling traits has largely focused on male ornaments. However, our understanding of ornament evolution is necessarily incomplete without investigating the causes and consequences of variation in female ornamentation. Here, we study the Anolis lizard dewlap, a trait extensively studied as a male secondary sexual characteristic but present in females of several species. We characterized female dewlaps for 339 species to test hypotheses about their evolution. Our results did not support the hypothesis that female dewlaps are selected against throughout the anole phylogeny. Rather, we found that female dewlaps were evolutionary labile. We also did not find support for the adaptive hypothesis that interspecific competition drove the evolution of female dewlaps. However, we did find support for the pleiotropy hypothesis as species with larger females and reduced sexual size dimorphism were more likely to possess female dewlaps. Lastly, we found that female dewlap presence influenced diversification rates in anoles, but only secondarily to a hidden state. Our results demonstrate that female ornamentation is widespread in anoles and the traditional hypothesis of divergent selection between the sexes does not fully explain their evolution. Instead, female ornamentation is likely to be subject to complex adaptive and non-adaptive evolutionary forces.

Drosophila melanogaster microbiome is shaped by strict filtering and neutrality along a latitudinal cline

Microbiomes affect many aspects of host biology, but the eco-evolutionary forces that shape their diversity in natural populations remain poorly understood. Geographical gradients, such as latitudinal clines, generate predictable patterns in biodiversity at macroecological scales, but whether these macroscale processes apply to host-microbiome interactions is an open question. To address this question, we sampled the microbiomes of 13 natural populations of Drosophila melanogaster along a latitudinal cline in the eastern United States. The microbiomes were surprisingly consistent across the cline, as latitude did not predict either alpha or beta diversity. Only a narrow taxonomic range of bacteria were present in all microbiomes, indicating that strict taxonomic filtering by the host and neutral ecological dynamics are the primary factors shaping the fly microbiome. Our findings reveal the complexity of eco-evolutionary interactions shaping microbial variation in D. melanogaster and highlight the need for additional sampling of the microbiomes in natural populations along environmental gradients.

Tropical-temperate comparisons in insect seed predation vary between study levels and years

The biotic interaction hypothesis, which states the species interaction becomes stronger in the tropics, is deeply rooted in classic ecological literature and widely accepted to contribute to the latitudinal gradients of biodiversity. Tests in latitudinal insect-plant interaction have emphasized leaf-eating insects on a single or a few plant species rather than within an entire community and mixed accumulating evidence, leaving the biotic interaction hypothesis disputed. We aimed to test the hypothesis by quantifying insect seed predation in a pair of tropical and temperate forest communities with similar elevations. We applied a consistent study design to sample predispersal seeds with systematically set seed traps in 2019-2020 and examined internally feeding insects. The intensity of seed predation was measured and further applied to tropical versus temperate comparison at two levels (cross-species and community-wide). Our results showed every latitudinal pattern associated with different study levels and years, that is, negative (greater granivory in the tropics in community-wide comparison in 2020), positive (less granivory in the tropics in community-wide and cross-species comparison in 2019), and missing (similar level of granivory in the tropics in cross-species comparisons in 2020). The cross-species level analyses ignore differences among species in seed production and weaken or even lose the latitudinal trend detected by community-wide comparisons. The between-year discrepancy in tropical-temperate comparisons relates to the highly variable annual seed composition in the temperate forest due to mast seeding of dominant species. Our study highlights that long-term community-level researches across biomes are essential to assess the latitudinal biotic interaction hypothesis.

Predator control of marine communities increases with temperature across 115 degrees of latitude

Early naturalists suggested that predation intensity increases toward the tropics, affecting fundamental ecological and evolutionary processes by latitude, but empirical support is still limited. Several studies have measured consumption rates across latitude at large scales, with variable results. Moreover, how predation affects prey community composition at such geographic scales remains unknown. Using standardized experiments that spanned 115° of latitude, at 36 nearshore sites along both coasts of the Americas, we found that marine predators have both higher consumption rates and consistently stronger impacts on biomass and species composition of marine invertebrate communities in warmer tropical waters, likely owing to fish predators. Our results provide robust support for a temperature-dependent gradient in interaction strength and have potential implications for how marine ecosystems will respond to ocean warming.

How network size strongly determines trophic specialisation: A technical comment on Luna et al. (2022)

Luna et al. (2022) concluded that the environment contributes to explaining specialisation in open plant-pollinator networks. When reproducing their study, we instead found that network size alone largely explained the variation in their specialisation metrics. Thus, we question whether empirical network specialisation is driven by the environment.

The latitudinal gradient in rates of evolution for bird beaks, a species interaction trait

Where is evolution fastest? The biotic interactions hypothesis proposes that greater species richness creates more ecological opportunity, driving faster evolution at low latitudes, whereas the 'empty niches' hypothesis proposes that ecological opportunity is greater where diversity is low, spurring faster evolution at high latitudes. We tested these contrasting predictions by analysing rates of beak evolution for a global dataset of 1141 avian sister species. Rates of beak size evolution are similar across latitudes, with some evidence that beak shape evolves faster in the temperate zone, consistent with the empty niches hypothesis. The empty niches hypothesis is further supported by a meta-analysis showing that rates of trait evolution and recent speciation are generally faster in the temperate zone, whereas rates of molecular evolution are slightly faster in the tropics. Our results suggest that drivers of evolutionary diversification are either similar across latitudes or more potent in the temperate zone, thus calling into question multiple hypotheses that invoke faster tropical evolution to explain the latitudinal diversity gradient.

Dietary abundance distributions: Dominance and diversity in vertebrate diets

Diet composition is among the most important yet least understood dimensions of animal ecology. Inspired by the study of species abundance distributions (SADs), we tested for generalities in the structure of vertebrate diets by characterising them as dietary abundance distributions (DADs). We compiled data on 1167 population-level diets, representing >500 species from six vertebrate classes, spanning all continents and oceans. DADs near-universally (92.5%) followed a hollow-curve shape, with scant support for other plausible rank-abundance-distribution shapes. This strong generality is inherently related to, yet incompletely explained by, the SADs of available food taxa. By quantifying dietary generalisation as the half-saturation point of the cumulative distribution of dietary abundance (sp50, minimum number of foods required to account for 50% of diet), we found that vertebrate populations are surprisingly specialised: in most populations, fewer than three foods accounted for at least half the diet. Variation in sp50 was strongly associated with consumer type, with carnivores being more specialised than herbivores or omnivores. Other methodological (sampling method and effort, taxonomic resolution), biological (body mass, frugivory) and biogeographic (latitude) factors influenced sp50 to varying degrees. Future challenges include identifying the mechanisms underpinning the hollow-curve DAD, its generality beyond vertebrates, and the biological determinants of dietary generalisation.

Functional biogeography of parasite traits: hypotheses and evidence

Functional biogeography, or the study of trait-based distributional patterns, not only complements our understanding of spatial patterns in biodiversity, but also sheds light on the underlying processes generating them. In parallel with the well-studied latitudinal diversity gradient, decades-old ecogeographical rules also postulate latitudinal variation in species traits. Notably, species in the tropics are predicted to have smaller body sizes (Bergmann's rule), narrower niches (MacArthur's rule) and smaller geographical ranges (Rapoport's rule) than their counterparts at higher latitudes. Although originally proposed for free-living organisms, these rules have been extended to parasitic organisms as well. In this review, I discuss the mechanistic hypotheses most likely to explain latitudinal gradients in parasite traits, and assess the empirical evidence obtained from comparative studies testing the above three rules as well as latitudinal gradients in other parasite traits. Overall, there is only weak empirical support for latitudinal gradients in any parasite trait, with little consistency among comparative analyses. The most parsimonious explanation for the existence of geographical patterns in parasite traits is that they are primarily host-driven, i.e. ecological traits of parasites track those of their hosts, with a direct influence of bioclimatic factors playing a secondary role. Thus, geographical patterns in parasite traits probably emerge as epiphenomena of parallel patterns in their hosts.

This article is part of the theme issue ‘Infectious disease macroecology: parasite diversity and dynamics across the globe’.

Tempo and mode of morphological evolution are decoupled from latitude in birds

The latitudinal diversity gradient is one of the most striking patterns in nature, yet its implications for morphological evolution are poorly understood. In particular, it has been proposed that an increased intensity of species interactions in tropical biota may either promote or constrain trait evolution, but which of these outcomes predominates remains uncertain. Here, we develop tools for fitting phylogenetic models of phenotypic evolution in which the impact of species interactions-namely, competition-can vary across lineages. Deploying these models on a global avian trait dataset to explore differences in trait divergence between tropical and temperate lineages, we find that the effect of latitude on the mode and tempo of morphological evolution is weak and clade- or trait dependent. Our results indicate that species interactions do not disproportionately impact morphological evolution in tropical bird families and question the validity of previously reported patterns of slower trait evolution in the tropics.

Latitudinal gradient in the intensity of biotic interactions in terrestrial ecosystems: Sources of variation and differences from the diversity gradient revealed by meta-analysis

The Latitudinal Biotic Interaction Hypothesis (LBIH) states that the intensity of biotic interactions increases from high to low latitudes. This hypothesis, which may partly explain latitudinal gradients in biodiversity, remains hotly debated, largely due to variable outcomes of published studies. We used meta-analysis to identify the scope of the LBIH in terrestrial ecosystems. For this purpose, we explored the sources of variation in the strength of latitudinal changes in herbivory, carnivory and parasitism (119 publications) and compared these gradients with gradients in the diversity of the respective groups of animals (102 publications). Overall, both herbivory and carnivory decreased towards the poles, while parasitism increased. The latitudinal gradient in herbivory and carnivory was threefold stronger above 50-60° than at lower latitudes and was significant due to interactions involving ectothermic consumers, studies using standardised prey (i.e. prey lacking local anti-predator adaptations) and studies aimed at testing LBIH. The poleward decrease in biodiversity did not differ between ectothermic and endothermic animals or among climate zones and was fourfold stronger than decrease in herbivory and carnivory. The discovered differences between the gradients in biotic interactions and biodiversity suggest that these two global macroecological patterns are likely shaped by different factors.

Plantago spp. as Models for Studying the Ecology and Evolution of Species Interactions across Environmental Gradients

AbstractA central challenge in ecology and evolutionary biology is to understand how variation in abiotic and biotic factors combine to shape the distribution, abundance, and diversity of focal species. Environmental gradients, whether natural (e.g., latitude, elevation, ocean proximity) or anthropogenic (e.g., land-use intensity, urbanization), provide compelling settings for addressing this challenge. However, not all organisms are amenable to the observational and experimental approaches required for untangling the factors that structure species along gradients. Here we highlight herbaceous plants in the genus Plantago as models for studying the ecology and evolution of species interactions along abiotic gradients. Plantago lanceolata and P. major are native to Europe and Asia but distributed globally, and they are established models for studying population ecology and interactions with herbivores, pathogens, and soil microbes. Studying restricted range congeners in comparison with those cosmopolitan species can provide insight into abiotic and biotic determinants of range size and population structure. We highlight one such species, P. rugelii, which is endemic to eastern North America. We give an overview of the literature on these focal Plantago species and explain why they are logical candidates for studies of species interactions across environmental gradients. Finally, we emphasize collaborative and community science approaches that can facilitate such research and note the amenability of Plantago for authentic research projects in science education.

Range-wide variations in common milkweed traits and their effect on monarch larvae

PREMISE

Leaf economic spectrum (LES) theory has historically been employed to inform vegetation models of ecosystem processes, but largely neglects intraspecific variation and biotic interactions. We attempt to integrate across environment-plant trait-herbivore interactions within a species at a range-wide scale.

METHODS

We measured traits in 53 populations spanning the range of common milkweed (Asclepias syriaca) and used a common garden to determine the role of environment in driving patterns of intraspecific variation. We used a feeding trial to determine the role of plant traits in monarch (Danaus plexippus) larval development.

RESULTS

Trait-trait relationships largely followed interspecific patterns in LES theory and persisted in a common garden when individual traits change. Common milkweed showed intraspecific variation and biogeographic clines in traits. Clines did not persist in a common garden. Larvae ate more and grew larger when fed plants with more nitrogen. A longitudinal environmental gradient in precipitation corresponded to a resource gradient in plant nitrogen, which produces a gradient in larval performance.

CONCLUSIONS

Biogeographic patterns in common milkweed traits can sometimes be predicted from LES, are largely driven by environmental conditions, and have consequences for monarch larval performance. Changes to nutrient dynamics of landscapes with common milkweed could potentially influence monarch population dynamics. We show how biogeographic trends in intraspecific variation can influence key ecological interactions, especially in common species with large distributions.

Enemies mediate distance- and density-dependent mortality of tree seeds and seedlings: a meta-analysis of fungicide, insecticide and exclosure studies

Conspecific negative distance- and density-dependence is often assumed to be one of the most important mechanisms controlling forest community assembly and species diversity globally. Plant pathogens, and insect and mammalian herbivores, are the most common natural enemy types that have been implicated in this phenomenon, but their general effects at different plant life stages are still unclear. Here, we conduct a meta-analysis of studies that involved robust manipulative experiments, using fungicides, insecticides and exclosures, to assess the contributions of different natural enemy types to distance- and density-dependent effects at seed and seedling stages. We found that distance- and density-dependent mortality caused by natural enemies was most likely at the seedling stage and was greater at higher mean annual temperatures. Conspecific negative distance- and density-dependence at the seedling stage is significantly weakened when fungicides were applied. By contrast, negative conspecific distance- and density-dependence is not a general pattern at the seed stage. High seed mass reduced distance- and density-dependent mortality at the seed stage. Seed studies excluding only large mammals found significant negative conspecific distance-dependent mortality, but exclusion of all mammals resulted in a non-significant effect of conspecifics. Our study suggests that plant pathogens are a major cause of distance- and density-dependent mortality at the seedling stage, while the impacts of herbivores on seedlings have been understudied. At the seed stage, large and small mammals, respectively, weaken and enhance negative conspecific distance-dependent mortality. Future research should identify specific agents of mortality, investigate the interactions among different enemy types and assess how global change may affect natural enemies and thus influence the strength of conspecific distance- and density-dependence.

Ecological Interactions and Macroevolution: A New Field with Old Roots

Linking interspecific interactions (e.g., mutualism, competition, predation, parasitism) to macroevolution (evolutionary change on deep timescales) is a key goal in biology. The role of species interactions in shaping macroevolutionary trajectories has been studied for centuries and remains a cutting-edge topic of current research. However, despite its deep historical roots, classic and current approaches to this topic are highly diverse. Here, we combine historical and contemporary perspectives on the study of ecological interactions in macroevolution, synthesizing ideas across eras to build a zoomed-out picture of the big questions at the nexus of ecology and macroevolution. We discuss the trajectory of this important and challenging field, dividing research into work done before the 1970s, research between 1970 and 2005, and work done since 2005. We argue that in response to long-standing questions in paleobiology, evidence accumulated to date has demonstrated that biotic interactions (including mutualism) can influence lineage diversification and trait evolution over macroevolutionary timescales, and we outline major open questions for future research in the field.

Chemical responses of an invasive plant to herbivory and abiotic environments reveal a novel invasion mechanism

Invasive plant environments differ along latitudes and between native and introduced ranges. In response to herbivory and abiotic stresses that vary with latitudes and between ranges, invasive plants may shift their secondary chemicals to facilitate invasion success. However, it remains unclear whether and how invasive plant chemical responses to herbivory and chemical responses to abiotic environments are associated. We conducted large scale field surveys of herbivory on the invasive tallow tree (Triadica sebifera) along latitudes in both its native (China) and introduced ranges (United States) and collected leaf samples for analyses of tannins and flavonoids. We used data on climate and solar radiation to examine these chemical responses to abiotic environments and their variations along these latitudes and between ranges. We also re-analyzed previously published data from multiple common garden experiments on tallow tree to investigate genetic divergence of secondary chemical concentrations between introduced and native populations. We found foliar tannins and herbivory (chewing, sucking) were higher in the native range compared to the invasive range. Allocation to tannins versus flavonoids decreased with latitude in the native range but did not vary in the invasive range. Analyses of previously published common garden experimental data indicated genetic divergence contributes to chemical concentration differences between ranges. Our field data further indicated that the latitudinal patterns were primarily phenotypic responses to herbivory in China while in US they were primarily phenotypic responses to abiotic environments. The variation of tannins may be linked to flavonoids, given tannins and flavonoids share a biosynthesis pathway. Together, our results suggest that invasive plants adjust their secondary metabolism to decrease chemicals that primarily defend against herbivory and increase those that help them to respond to their abiotic environment. These findings deepen our understanding of how invasive plants adapt to biogeographically heterogeneous environments through trade-offs between secondary chemical responses.

Adaptation and Latitudinal Gradients in Species Interactions: Nest Predation in Birds

AbstractAre biotic interactions stronger in the tropics? Here, we investigate nest predation in birds, a canonical example of a strong tropical biotic interaction. Counter to expectations, daily rates of nest predation vary minimally with latitude. However, life-history traits that influence nest predation have diverged between latitudes. For example, tropical species have evolved a longer average nesting period, which is associated with reduced rates of nest attendance by parents. Daily nest mortality declines with nesting period length within regions, but tropical species have a higher intercept. Consequently, for the same nesting period length, tropical species experience higher daily nest predation rates than temperate species. The implication of this analysis is that the evolved difference in nesting period length between latitudes produces a flatter latitudinal gradient in daily nest predation than would otherwise be predicted. We propose that adaptation may frequently dampen geographic patterns in interaction rates.

Geographical variation of multiplex ecological networks in marine intertidal communities

Understanding the drivers of geographical variation in species distributions, and the resulting community structure, constitutes one of the grandest challenges in ecology. Geographical patterns of species richness and composition have been relatively well studied. Less is known about how the entire set of trophic and non-trophic ecological interactions, and the complex networks that they create by gluing species together in complex communities, change across geographical extents. Here, we compiled data of species composition and three types of ecological interactions occurring between species in rocky intertidal communities across a large spatial extent (~970 km of shoreline) of central Chile, and analyzed the geographical variability in these multiplex networks (i.e., comprising several interaction types) of ecological interactions. We calculated nine network summary statistics common across interaction types, and additional network attributes specific to each of the different types of interactions. We then investigated potential environmental drivers of this multivariate network organization. These included variation in sea surface temperature and coastal upwelling, the main drivers of productivity in nearshore waters. Our results suggest that structural properties of multiplex ecological networks are affected by local species richness and modulated by factors influencing productivity and environmental predictability. Our results show that non-trophic negative interactions are more sensitive to spatially structured temporal environmental variation than feeding relationships, with non-trophic positive interactions being the least labile to it. We also show that environmental effects are partly mediated through changes in species richness and partly through direct influences on species interactions, probably associated to changes in environmental predictability and to bottom-up nutrient availability. Our findings highlight the need for a comprehensive picture of ecological interactions and their geographical variability if we are to predict potential effects of environmental changes on ecological communities.

Segregation structure in Odonata assemblages follows the latitudinal gradient

Latitude is known to deeply affect life with effects generalizable into ecological rules; the increasing species diversity toward tropics is the most paradigmatic. Several hypotheses tested patterns of biotic interactions' intensity along latitude. Negative interactions (i.e. competition and predation) are expected to be among the processes that produce checkerboard distribution of species. However, no relationship between checkerboardness and latitude has been uncovered. We tested Odonata assemblages worldwide for segregation patterns using a faunistic dataset (395 species arranged in 386 natural communities) spanning a wide latitudinal range (87°). We used co-occurrence analyses (C-score index and Standardized Effect Size) as an estimate of checkerboardness then correlated the occurrence of segregation to latitude. Odonata followed the Latitudinal Diversity Gradient at the regional scale (i.e. country scale) within our analyzed assemblages spanning, whereas local richness (i.e. community scale) did not follow the same pattern. Odonata assemblages structured with segregation are more common going from high to low latitudes, and local species richness have no effect on the pattern. We summarized hypotheses on how biotic interactions or ecological and historical processes can influence the spatial patterns in the checkerboards of assemblages and presented promising ways to help to gain a better mechanistic understanding of the drivers of the Latitudinal Diversity Gradient.

Latitudinal variation in seed predation correlates with latitudinal variation in seed defensive and nutritional traits in a widespread oak species

BACKGROUND AND AIMS

Classic theory on geographical gradients in plant-herbivore interactions assumes that herbivore pressure and plant defences increase towards warmer and more stable climates found at lower latitudes. However, the generality of these expectations has been recently called into question by conflicting empirical evidence. One possible explanation for this ambiguity is that most studies have reported on patterns of either herbivory or plant defences whereas few have measured both, thus preventing a full understanding of the implications of observed patterns for plant-herbivore interactions. In addition, studies have typically not measured climatic factors affecting plant-herbivore interactions, despite their expected influence on plant and herbivore traits.

METHODS

Here we tested for latitudinal variation in insect seed predation and seed traits putatively associated with insect attack across 36 Quercus robur populations distributed along a 20° latitudinal gradient. We then further investigated the associations between climatic factors, seed traits and seed predation to test for climate-based mechanisms of latitudinal variation in seed predation.

KEY RESULTS

We found strong but contrasting latitudinal clines in seed predation and seed traits, whereby seed predation increased whereas seed phenolics and phosphorus decreased towards lower latitudes. We also found a strong direct association between temperature and seed predation, with the latter increasing towards warmer climates. In addition, temperature was negatively associated with seed traits, with populations at warmer sites having lower levels of total phenolics and phosphorus. In turn, these negative associations between temperature and seed traits led to a positive indirect association between temperature and seed predation.

CONCLUSIONS

These results help unravel how plant-herbivore interactions play out along latitudinal gradients and expose the role of climate in driving these outcomes through its dual effects on plant defences and herbivores. Accordingly, this emphasizes the need to account for abiotic variation while testing concurrently for latitudinal variation in plant traits and herbivore pressure.

Pelagic fish predation is stronger at temperate latitudes than near the equator

Species interactions are widely thought to be strongest in the tropics, potentially contributing to the greater number of species at lower latitudes. Yet, empirical tests of this "biotic interactions" hypothesis remain limited and often provide mixed results. Here, we analyze 55 years of catch per unit effort data from pelagic longline fisheries to estimate the strength of predation exerted by large predatory fish in the world's oceans. We test two central tenets of the biotic interactions hypothesis: that predation is (1) strongest near the equator, and (2) positively correlated with species richness. Counter to these predictions, we find that predation is (1) strongest in or near the temperate zone and (2) negatively correlated with oceanic fish species richness. These patterns suggest that, at least for pelagic fish predation, common assumptions about the latitudinal distribution of species interactions do not apply, thereby challenging a leading explanation for the latitudinal gradient in species diversity.

Opposite latitudinal patterns for bird and arthropod predation revealed in experiments with differently colored artificial prey

The strength of biotic interactions is generally thought to increase toward the equator, but support for this hypothesis is contradictory. We explored whether predator attacks on artificial prey of eight different colors vary among climates and whether this variation affects the detection of latitudinal patterns in predation. Bird attack rates negatively correlated with model luminance in cold and temperate environments, but not in tropical environments. Bird predation on black and on white (extremes in luminance) models demonstrated different latitudinal patterns, presumably due to differences in prey conspicuousness between habitats with different light regimes. When attacks on models of all colors were combined, arthropod predation decreased, whereas bird predation increased with increasing latitude. We conclude that selection for prey coloration may vary geographically and according to predator identity, and that the importance of different predators may show contrasting patterns, thus weakening the overall latitudinal trend in top-down control of herbivorous insects.

Latitude, temperature, and habitat complexity predict predation pressure in eelgrass beds across the Northern Hemisphere

Latitudinal gradients in species interactions are widely cited as potential causes or consequences of global patterns of biodiversity. However, mechanistic studies documenting changes in interactions across broad geographic ranges are limited. We surveyed predation intensity on common prey (live amphipods and gastropods) in communities of eelgrass (Zostera marina) at 48 sites across its Northern Hemisphere range, encompassing over 37° of latitude and four continental coastlines. Predation on amphipods declined with latitude on all coasts but declined more strongly along western ocean margins where temperature gradients are steeper. Whereas in situ water temperature at the time of the experiments was uncorrelated with predation, mean annual temperature strongly positively predicted predation, suggesting a more complex mechanism than simply increased metabolic activity at the time of predation. This large-scale biogeographic pattern was modified by local habitat characteristics; predation declined with higher shoot density both among and within sites. Predation rates on gastropods, by contrast, were uniformly low and varied little among sites. The high replication and geographic extent of our study not only provides additional evidence to support biogeographic variation in predation intensity, but also insight into the mechanisms that relate temperature and biogeographic gradients in species interactions.

The geographical variation of network structure is scale dependent: understanding the biotic specialization of host-parasitoid networks

Research on the structure of ecological networks suggests that a number of universal patterns exist. Historically, biotic specialization has been thought to increase towards the Equator. Yet, recent studies have challenged this view showing non-conclusive results. Most studies analysing the geographical variation in biotic specialization focus, however, only on the local scale. Little is known about how the geographical variation of network structure depends on the spatial scale of observation (i.e., from local to regional spatial scales). This should be remedied, as network structure changes as the spatial scale of observation changes, and the magnitude and shape of these changes can elucidate the mechanisms behind the geographical variation in biotic specialization. Here we analyse four facets of biotic specialization in host-parasitoid networks along gradients of climatic constancy, classifying the networks according to their spatial extension (local or regional). Namely, we analyse network connectance, consumer diet overlap, consumer diet breadth, and resource vulnerability at both local and regional scales along the gradients of both current climatic constancy and historical climatic change. While at the regional scale none of the climatic variables are associated to biotic specialization, at the local scale, network connectance, consumer diet overlap, and resource vulnerability decrease with current climatic constancy, whereas consumer generalism increases (i.e., broader diet breadths in tropical areas). Similar patterns are observed along the gradient of historical climatic change. We provide an explanation based on different beta-diversity for consumers and resources across the geographical gradients. Our results show that the geographical gradient of biotic specialization is not universal. It depends on both the facet of biotic specialization and the spatial scale of observation.

Elevational cline in herbivore abundance driven by a monotonic increase in trophic-level sensitivity to aridity

The abiotic environment drives species abundances and distributions both directly and indirectly through effects on multi-trophic species interactions. However, few studies have documented the individual and combined consequences of these direct and indirect effects. We studied an ant-tended aphid along an elevational gradient, where lower elevations were more arid. Hypotheses of stronger species interactions at lower elevations and a greater sensitivity of higher trophic levels to climate led us to predict increased top-down control of aphids by natural enemies (third trophic level) but even stronger protection from mutualist ants (fourth trophic level) with increasing aridity. As a result, we predicted that mutualism strength and aphid abundance would increase with aridity. We documented patterns of aphid abundance and tested for both the direct and multi-trophic indirect effects of aridity on aphid performance. To do so, we used both observational and manipulative methods across two years in replicate high- and low-elevation valleys, where summer temperatures decreased by 3.7°C and precipitation increased by 27 mm/mo from low to high elevations. Aphid colonies were 75% larger in the most (vs. least) arid sites, and this was best explained by changes in interactions with predators and ants. Aphids were unaffected by the direct effects of the abiotic environment or its indirect effects via host plant quality. In contrast, natural enemy effects increased with aridity; under ant exclusion, natural enemies had no effect on aphids in the least arid sites but depressed colony growth by 252% in the most arid sites. Ant activity also increased with aridity, with ants discovering more aphid colonies and experimental baits and allocating more foragers per aphid, although there was no effect of aridity on ant abundance or community composition. Correspondingly, the mutualist services provided by ants increased with aridity; ants provided no benefits to aphids in the least arid sites but doubled colony growth in the most arid sites. In summary, an elevational cline in herbivore abundance was driven by a monotonic increase in trophic-level sensitivity to aridity. These findings illustrate that predicting species responses to climate change will require a multi-trophic perspective.

No Indication of High Host-Plant Specificity in Afrotropical Geometrid Moths

Specificity is one of the fundamental concepts in ecology. Host specificity of phytophagous insects has been of particular interest because of its crucial role in diversification and life-history evolution. However, the majority of tropical insects remain insufficiently explored with respect to their host-plant relations. A lack of respective data is also hindering the debate over whether higher levels of host-plant specificity prevail in tropical insects compared to temperate ones. We investigated host-plant specificity of forest geometrid moths (Lepidoptera: Geometridae) in equatorial Africa using host-plant acceptability trials with neonate larvae, with the addition of field observations. We compare our experimental data to the (well-known) host-specificity patterns of closely related temperate (hemiboreal) species. Similarly to the temperate region, there were broadly polyphagous tropical species in several clades of Geometridae utilizing hosts belonging to different plant families. Phylogenetic comparative analysis returned no significant differences in host specificity between the two regions. Our study contributes to the evidence that host-plant specificity of herbivores is not necessarily substantially higher in tropical than temperate regions.

Challenges and advances in the study of latitudinal gradients in multitrophic interactions, with a focus on consumer specialization

Increases in data availability and geographic ranges of studies have allowed for more thorough tests of latitudinal gradients in trophic interactions, with numerous recent studies testing hypotheses that strength of interactions, herbivory, plant chemical defense, and dietary specialization all increase with decreasing latitude. We review the issues surrounding these latitudinal gradients, discuss some methodological challenges, and provide some caveats relevant to inferences from existing approaches. To examine some potential issues with studies on latitudinal gradients in dietary specialization, we simulate a latitudinal gradient of communities that increase in diversity and specialization towards the equator then test the power of different sampling designs for detecting the gradient. Based on this simple simulation, as well as apparent incongruities in the literature, we conclude that subtle differences in sampling design can be responsible for failure to detect existing gradients. Despite calls for rejecting some latitudinal gradient hypotheses, it is clear that a great deal of careful research remains to determine important correlates of the well-established latitudinal gradient in diversity. In particular, future studies should focus on replicated gradients, greater emphasis on continuous sampling, and use of taxonomic controls that allow for meaningful analyses across latitudes.

Tolerance to seed predation mediated by seed size increases at lower latitudes in a Mediterranean oak

BACKGROUND AND AIMS

The ability of plants to allocate energy to resistance against herbivores changes with abiotic conditions and thus may vary along geographical clines, with important consequences for plant communities. Seed size is a plant trait potentially influencing plant tolerance to endoparasites, and seed size often varies across latitude. Consequently, plant tolerance to endoparasites may change across geographical clines.

METHODS

The interaction between Quercus ilex (holm oak) and seed-predating Curculio spp. (weevils) was explored along most of the latitudinal range of Q. ilex. This included quantification of variation in seed size, survival likelihood of infested seeds, multi-infestation of acorns and community composition of Curculio weevils in acorns.

KEY RESULTS

Larger seeds had a higher probability of surviving weevil attack (i.e. embryo not predated). Southern populations of oak produced on average four times larger seeds than those of northern populations. Consequently, the probability of survival of infested acorns decreased with latitude. The community composition of Curculio varied, with large weevils (C. elephas) dominating in southern populations and small weevils (C. glandium) dominating in northern populations. However, damage tolerance was robust against this turnover in predator functional traits. Furthermore, we did not detect any change in multi-infestation of acorns along the geographical gradient.

CONCLUSIONS

Quercus ilex tolerance to seed predation by Curculio weevils increases toward the southern end of its distribution. Generally, studies on geographical variation in plant defence against enemies largely ignore seed attributes or they focus on seed physical barriers. Thus, this research suggests another dimension in which geographical trends in plant defences should be considered, i.e. geographical variation in tolerance to seed predators mediated by seed size.