Civet latrines in three habitats of a coffee dominated landscape of the Western Ghats biodiversity hotspot

Civets are frugivorous animals in the Order Carnivora. They are relatively less shy towards people and anthropogenic habitats. It has been reported that the civets' preference of defecating in open sites enable them to be important seed dispersers of degraded forests and urban ecosystems of Asia and Africa. We surveyed for scats of palm civet (Paradoxurus hermaphroditus) in forest fragments of sacred groves (closed), coffee plantations (partly closed) and home gardens (relatively open) during the fruit ripening period of Coffee and Caryota urens - the two preferred fruits of civet - to report the microhabitat characteristics and seed composition of civet latrines. The microhabitat of each scat position - whether on or off the ground and the shade type - was recorded. The scat analysis showed the presence of 4234 seeds belonging to coffee (90.2%), C. urens (9.7%), and an anonymous Rubiacea species (0.10%) in a total of 105 scats collected from coffee plantations (55), home gardens (5), and sacred groves (45). The number of scats sampled from the three habitats was different, but not the number of seeds per scat. Overall, the number of scats increased with the canopy cover, but the trend was different for different habitats. In home garden and coffee plantations, it decreased, but in sacred groves, it increased with the canopy cover. The number of scats sampled above the ground - on tree branches, logs and built-up structures- was more than that was on the ground. The findings contradict the general belief that the civet latrines occur more in open areas than the shaded areas. Because the civet latrines are seen more above ground than on the ground, their efficiency as seed dispersal agent may be examined critically in different contexts.

Pinanga palms revisited 20 years on: what can changes in Pinanga species populations tell us about rainforest understory palm persistence?

Borneo is the centre of diversity of the palm genus Pinanga. At least 13 understory species have been recorded in the Ulu Temburong National Park in Brunei, but little is known of their persistence. Changes in populations of Pinanga understory palms may be indicative of more widespread changes due to climate change, such as changes in rainfall, which may be important for the palm diversity in the protected area. However, we know little about the population dynamics of these palms, how persistent their populations are or if they behave similarly over long time frames. In 1998, populations of five co-occurring species of Pinanga at several locations in the Ulu Temburong National Park were documented. This project aimed to undertake a comprehensive resurvey of the original five Pinanga palm species populations in order to assess if they showed similar population changes across sites and species after two decades. Overall, most species maintained their population size in the surveyed region but not consistently among sites, and one species significantly declined in abundance. There was considerable variation in population growth rate (R) within and among species and sites that was significantly correlated with density and the percentage of multi-stemmed plants. There was evidence of pulsed recruitment in some species and or sites rather than steady or exponential patterns of population growth.

Mycorrhizal Switching and the Role of Fungal Abundance in Seed Germination in a Fully Mycoheterotrophic Orchid, Gastrodia confusoides

Mycorrhizal associations are essential for orchid germination and seedling establishment, and thus may constrain the distribution and abundance of orchids under natural conditions. Previous studies have shown that germination and seedling establishment in several orchids often decline with increasing distance from adult plants, resulting in non-random spatial patterns of seedling establishment. In contrast, individuals of the fully mycoheterotrophic orchid Gastrodia confusoides often tend to have random aboveground spatial patterns of distribution within bamboo forests. Since G. confusoides is parasitic on litter-decaying fungi, its random spatial patterns of distribution may be due to highly scattered patterns of litter-decaying fungi within bamboo forests. To test this hypothesis, we first identified the main mycorrhizal fungi associating with developing seeds and adult plants at a bamboo forest site in Taiwan using Miseq high-throughput DNA sequencing. Next, we combined seed germination experiments with quantitative PCR (qPCR) analyses to investigate to what extent the abundance of mycorrhizal fungi affected spatial patterns of seed germination. Our results show that seed germination and subsequent growth to an adult stage in G. confusoides required a distinct switch in mycorrhizal partners, in which protocorms associated with a single Mycena OTU, while adults mainly associated with an OTU from the genus Gymnopus. A strong, positive relationship was observed between germination and Mycena abundance in the litter, but not between germination and Gymnopus abundance. Fungal abundance was not significantly related to the distance from the adult plants, and consequently germination was also not significantly related to the distance from adult plants. Our results provide the first evidence that the abundance of litter-decaying fungi varies randomly within the bamboo forest and independently from G. confusoides adults.

Are we missing the forest for the trees? Conspecific negative density dependence in a temperate deciduous forest

One of the central goals of ecology is to determine the mechanisms that enable coexistence among species. Evidence is accruing that conspecific negative density dependence (CNDD), the process by which plant seedlings are unable to survive in the area surrounding adults of their same species, is a major contributor to tree species coexistence. However, for CNDD to maintain community-level diversity, three conditions must be met. First, CNDD must maintain diversity for the majority of the woody plant community (rather than merely specific groups). Second, the pattern of repelled recruitment must increase in with plant size. Third, CNDD should extend to the majority of plant life history strategies. These three conditions are rarely tested simultaneously. In this study, we simultaneously test all three conditions in a woody plant community in a North American temperate forest. We examined whether understory and canopy woody species across height categories and dispersal syndromes were overdispersed-a spatial pattern indicative of CNDD-using spatial point pattern analysis across life history stages and strategies. We found that there was a strong signal of overdispersal at the community level. Across the whole community, larger individuals were more overdispersed than smaller individuals. The overdispersion of large individuals, however, was driven by canopy trees. By contrast, understory woody species were not overdispersed as adults. This finding indicates that the focus on trees for the vast majority of CNDD studies may have biased the perception of the prevalence of CNDD as a dominant mechanism that maintains community-level diversity when, according to our data, CNDD may be restricted largely to trees.

Ectomycorrhizal fungi drive positive phylogenetic plant-soil feedbacks in a regionally dominant tropical plant family

While work in temperate forests suggests that there are consistent differences in plant-soil feedback (PSF) between plants with arbuscular and ectomycorrhizal associations, it is unclear whether these differences exist in tropical rainforests. We tested the effects of mycorrhizal type, phylogenetic relationships to overstory species, and soil fertility on the growth of tree seedlings in a tropical Bornean rainforest with a high diversity of both ectomycorrhizal and arbuscular mycorrhizal trees. We found that ectomycorrhizal tree seedlings had higher growth in soils conditioned by close relatives and that this was associated with higher mycorrhizal colonization. By contrast, arbuscular mycorrhizal tree seedlings generally grew more poorly in soils conditioned by close relatives. For ectomycorrhizal species, the phylogenetic trend was insensitive to soil fertility. For arbuscular mycorrhizal seedlings, however, the effect of growing in soils conditioned by close relatives became increasingly negative as soil fertility increased. Our results demonstrate consistent effects of mycorrhizal type on plant-soil feedbacks across forest biomes. The positive effects of ectomycorrhizal symbiosis may help explain biogeographic variation across tropical forests, such as familial dominance of the Dipterocarpaceae in southeast Asia. However, positive feedbacks also raise questions about the role of PSFs in maintaining tropical diversity.

Plant species abundance and phylogeny explain the structure of recruitment networks

Established plants can affect the recruitment of young plants, filtering out some and allowing the recruitment of others, with profound effects on plant community dynamics. Recruitment networks (RNs) depict which species recruit under which others. We investigated whether species abundance and phylogenetic distance explain the structure of RNs across communities. We estimated the frequency of canopy-recruit interactions among woody plants in 10 forest assemblages to describe their RNs. For each RN, we determined the functional form (linear, power or exponential) best describing the relationship of interaction frequency with three predictors: canopy species abundance, recruit species abundance and phylogenetic distance. We fitted models with all combinations of predictor variables, from which we simulated RNs. The best functional form of each predictor was the same in most communities (linear for canopy species abundance, power for recruit species abundance and exponential for phylogenetic distance). The model including all predictor variables was consistently the best in explaining interaction frequency and showed the best performance in predicting RN structure. Our results suggest that mechanisms related to species abundance are necessary but insufficient to explain the assembly of RNs. Evolutionary processes affecting phylogenetic divergence are critical determinants of RN structure.

The genetic dimension of pest pressure in the tropical rainforest

Wet tropical forests are among the most diverse ecosystems on Earth and can host several hundreds of tree species per hectare. To maintain such diversity, the community must contain large numbers of relatively rare species rather than be dominated by a few very common trees, as is often the case in temperate forests. Explaining the mechanisms preventing dominance by common species has been a major task of tropical forest ecology. One of the most promising mechanisms is negative density dependence (NDD) of tree abundance driven by pests, including fungal diseases ('pest pressure'). NDD entails that the chance of survival of a sapling increases with the distance from a mature tree of the same species, thus preventing species from becoming locally dominant. Curiously, the strength of NDD is negatively correlated with abundance, meaning that tree species that are more common generally show weaker NDD (Comita et al. ). Interactions between plants and soil pathogens have been shown to play an important role in NDD (Klironomos ), and rare species are apparently more strongly affected (Mangan et al. ). However, the genetic mechanisms underlying this phenomenon have remained obscure. In this issue of Molecular Ecology, Marden et al. () suggest that reduced diversity of the genes involved in pathogen recognition (Resistance genes or R genes) could explain why NDD is stronger in locally rare species.

Distance-dependent seedling mortality and long-term spacing dynamics in a neotropical forest community

Negative distance dependence (NDisD), or reduced recruitment near adult conspecifics, is thought to explain the astounding diversity of tropical forests. While many studies show greater mortality at near vs. far distances from adults, these studies do not seek to track changes in the peak seedling curve over time, thus limiting our ability to link NDisD to coexistence. Using census data collected over 12 years from central Panama in conjunction with spatial mark-connection functions, we show evidence for NDisD for many species, and find that the peak seedling curve shifts away from conspecific adults over time. We find wide variation in the strength of NDisD, which was correlated with seed size and canopy position, but other life-history traits showed no relationship with variation in NDisD mortality. Our results document shifts in peak seedling densities over time, thus providing evidence for the hypothesized spacing mechanism necessary for diversity maintenance in tropical forests.

Conspecific Leaf Litter-Mediated Effect of Conspecific Adult Neighborhood on Early-Stage Seedling Survival in A Subtropical Forest

Conspecific adults have strong negative effect on the survival of nearby early-stage seedlings and thus can promote species coexistence by providing space for the regeneration of heterospecifics. The leaf litter fall from the conspecific adults, and it could mediate this conspecific negative adult effect. However, field evidence for such effect of conspecific leaf litter remains absent. In this study, we used generalized linear mixed models to assess the effects of conspecific leaf litter on the early-stage seedling survival of four dominant species (Machilus leptophylla, Litsea elongate, Acer pubinerve and Distylium myricoides) in early-stage seedlings in a subtropical evergreen broad-leaved forest in eastern China. Our results consistently showed that the conspecific leaf litter of three species negatively affected the seedling survival. Meanwhile, the traditional conspecific adult neighborhood indices failed to detect this negative conspecific adult effect. Our study revealed that the accumulation of conspecific leaf litter around adults can largely reduce the survival rate of nearby seedlings. Ignoring it could result in underestimation of the importance of negative density dependence and negative species interactions in the natural forest communities.

Effects of density dependence in a temperate forest in northeastern China

Negative density dependence may cause reduced clustering among individuals of the same species, and evidence is accumulating that conspecific density-dependent self-thinning is an important mechanism regulating the spatial structure of plant populations. This study evaluates that specific density dependence in three very large observational studies representing three successional stages in a temperate forest in northeastern China. The methods include standard spatial point pattern analysis and a heterogeneous Poisson process as the null model to eliminate the effects of habitat heterogeneity. The results show that most of the species exhibit conspecific density-dependent self-thinning. In the early successional stage 11 of the 16 species, in the intermediate successional stage 18 of the 21 species and in the old growth stage all 21 species exhibited density dependence after removing the effects of habitat heterogeneity. The prevalence of density dependence thus varies among the three successional stages and exhibits an increase with increasing successional stage. The proportion of species showing density dependence varied depending on whether habitat heterogeneity was removed or not. Furthermore, the strength of density dependence is closely related with species abundance. Abundant species with high conspecific aggregation tend to exhibit greater density dependence than rare species.

Spatiotemporal strategies that facilitate recruitment in a habitat specialist tree species

Our understanding of processes underlying plant recruitment emerges from species and habitats that are widely distributed at regional and global scales. However, the applicability of dispersal-recruitment models and the role of dispersal limitation versus microsite limitation have not been examined for specialized habitats. In patchy, freshwater Myristica swamp forests (Western Ghats, India), we examine the roles of primary seed dispersal, secondary seed removal and microsite suitability for the establishment of a swamp specialist tree, Myristica fatua We estimated primary seed shadows, performed secondary removal experiments and enumerated recruits in swamp sites. Steady-state fruiting was observed with the extended production (>7 months) of small numbers of fruits. Frugivores dropped most of the large and heavy seeds under parent crowns, while a few seeds were transported over short distances by hornbills. Seed placement experiments indicated that removal, germination and establishment were similar within swamp microsites, while seeds failed to survive in matrix habitats surrounding the swamp. Crabs, which were major secondary removers of M. fatua, did not alter the initial seed dispersal patterns substantially, which led to the retention of seeds within the swamp. Distribution of saplings and adults from previous seasons also suggest that dispersal-recruitment dynamics in the swamp specialist M. fatua did not strictly follow predictions of Janzen-Connell model while abiotic effects were significant. Large seeds, steady-state fruiting and small crop sizes may be significant selective forces facilitating escape from density and distance-dependent effects in space and time in specialist plant species such as M. fatua.

Persistence of Neighborhood Demographic Influences over Long Phylogenetic Distances May Help Drive Post-Speciation Adaptation in Tropical Forests

Studies of forest dynamics plots (FDPs) have revealed a variety of negative density-dependent (NDD) demographic interactions, especially among conspecific trees. These interactions can affect growth rate, recruitment and mortality, and they play a central role in the maintenance of species diversity in these complex ecosystems. Here we use an equal area annulus (EAA) point-pattern method to comprehensively analyze data from two tropical FDPs, Barro Colorado Island in Panama and Sinharaja in Sri Lanka. We show that these NDD interactions also influence the continued evolutionary diversification of even distantly related tree species in these FDPs. We examine the details of a wide range of these interactions between individual trees and the trees that surround them. All these interactions, and their cumulative effects, are strongest among conspecific focal and surrounding tree species in both FDPs. They diminish in magnitude with increasing phylogenetic distance between heterospecific focal and surrounding trees, but do not disappear or change the pattern of their dependence on size, density, frequency or physical distance even among the most distantly related trees. The phylogenetic persistence of all these effects provides evidence that interactions between tree species that share an ecosystem may continue to promote adaptive divergence even after the species’ gene pools have become separated. Adaptive divergence among taxa would operate in stark contrast to an alternative possibility that has previously been suggested, that distantly related species with dispersal-limited distributions and confronted with unpredictable neighbors will tend to converge on common strategies of resource use. In addition, we have also uncovered a positive density-dependent effect: growth rates of large trees are boosted in the presence of a smaller basal area of surrounding trees. We also show that many of the NDD interactions switch sign rapidly as focal trees grow in size, and that their cumulative effect can strongly influence the distributions and species composition of the trees that surround the focal trees during the focal trees’ lifetimes.

Prevalence and strength of density-dependent tree recruitment

Density dependence could maintain diversity in forests, but studies continue to disagree on its role. Part of the disagreement results from the fact that different studies have evaluated different responses (survival, recruitment, or growth) of different stages (seeds, seedlings, or adults) to different inputs (density of seedlings, density or distance to adults). Most studies are conducted on a single site and thus are difficult to generalize. Using USDA Forest Service's Forest Inventory and Analysis data, we analyzed over a million seedling-to-sapling recruitment observations of 50 species from the eastern United States, controlling for the effects of climate. We focused on the per-seedling recruitment rate, because it is most likely to promote diversity and to be identified in observational or experimental data. To understand the prevalence of density dependence, we quantified the number of species with significant positive or negative effects. To understand the strength of density dependence, we determined the magnitude of effects among con- and heterospecifics, and how it changes with overall species abundance. We found that density dependence is pervasive among the 50 species, as the majority of them have significant effects and mostly negative. Density-dependence effects are stronger from conspecific than heterospecfic adult neighbors, consistent with the predictions of the Janzen-Connell hypothesis. Contrary to recent reports, density-dependence effects are more negative for common than rare species, suggesting disproportionately stronger population regulation in common species. We conclude that density dependence is pervasive, and it is strongest from conspecific neighbors of common species. Our analysis provides direct evidence that density dependence reaulates opulation dynamics of tree species in eastern U.S. forests.

Stochastically driven adult-recruit associations of tree species on Barro Colorado Island

The spatial placement of recruits around adult conspecifics represents the accumulated outcome of several pattern-forming processes and mechanisms such as primary and secondary seed dispersal, habitat associations or Janzen–Connell effects. Studying the adult–recruit relationship should therefore allow the derivation of specific hypotheses on the processes shaping population and community dynamics. We analysed adult–recruit associations for 65 tree species taken from six censuses of the 50 ha neotropical forest plot on Barro Colorado Island (BCI), Panama. We used point pattern analysis to test, at a range of neighbourhood scales, for spatial independence between recruits and adults, to assess the strength and type of departure from independence, and its relationship with species properties. Positive associations expected to prevail due to dispersal limitation occurred only in 16% of all cases; instead a majority of species showed spatial independence (≈73%). Independence described the placement of recruits around conspecific adults in good approximation, although we found weak and noisy signals of species properties related to seed dispersal. We hypothesize that spatial mechanisms with strong stochastic components such as animal seed dispersal overpower the pattern-forming effects of dispersal limitation, density dependence and habitat association, or that some of the pattern-forming processes cancel out each other.

Arbuscular mycorrhizal fungi counteract the Janzen-Connell effect of soil pathogens

Soilborne pathogens can contribute to diversity maintenance in tree communities through the Janzen-Connell effect, whereby the pathogenic reduction of seedling performance attenuates with distance from conspecifics. By contrast, arbuscular mycorrhizal fungi (AMF) have been reported to promote seedling performance; however, it is unknown whether this is also distance dependent. Here, we investigate the distance dependence of seedling performance in the presence of both pathogens and AMF. In a subtropical forest in south China, we conducted a four-year field census of four species with relatively large phylogenetic distances and found no distance-dependent mortality for newly germinated seedlings. By experimentally separating the effects of AMF and pathogens on seedling performance of six subtropical tree species in a shade house, we found that soil pathogens significantly inhibited seedling survival and growth while AMF largely promoted seedling growth, and these effects were host specific and declined with increasing conspecific distance. Together, our field and experimental results suggest that AMF can neutralize the negative effect of pathogens and that the Janzen-Connell effect may play a less prominent role in explaining diversity of nondominant tree species than previously thought.

Experimental evidence for an intraspecific Janzen-Connell effect mediated by soil biota

The negative effect of soil pathogens on seedling survival varies considerably among conspecific individuals, but the underlying mechanisms are largely unknown. For variation between heterospecifics, a common explanation is the Janzen-Connell effect: negative density dependence in survival due to specialized pathogens aggregating on common hosts. We test whether an intraspecific Janzen-Connell effect exists, i.e., whether the survival chances of one population's seedlings surrounded by a different conspecific population increase with genetic difference, spatial distance, and trait dissimilarity between them. In a shade-house experiment, we grew seedlings of five populations of each of two subtropical tree species (Castanopsis fissa and Canarium album) for which we measured genetic distance using intersimple sequence repeat (ISSR) analysis and eight common traits/characters, and we treated them with soil material or soil biota filtrate collected from different populations. We found that the relative survival rate increased with increasing dissimilarity measured by spatial distance, genetic distance, and trait differences between the seedling and the population around which the soil was collected. This effect disappeared after soil sterilization. Our results provide evidence that genetic variation, trait similarity, and spatial distance can explain intraspecific variation in plant-soil biotic interactions and suggest that limiting similarity also occurs at the intraspecific level.

CTFS-ForestGEO: a worldwide network monitoring forests in an era of global change

Global change is impacting forests worldwide, threatening biodiversity and ecosystem services including climate regulation. Understanding how forests respond is critical to forest conservation and climate protection. This review describes an international network of 59 long-term forest dynamics research sites (CTFS-ForestGEO) useful for characterizing forest responses to global change. Within very large plots (median size 25 ha), all stems ≥ 1 cm diameter are identified to species, mapped, and regularly recensused according to standardized protocols. CTFS-ForestGEO spans 25 °S-61 °N latitude, is generally representative of the range of bioclimatic, edaphic, and topographic conditions experienced by forests worldwide, and is the only forest monitoring network that applies a standardized protocol to each of the world's major forest biomes. Supplementary standardized measurements at subsets of the sites provide additional information on plants, animals, and ecosystem and environmental variables. CTFS-ForestGEO sites are experiencing multifaceted anthropogenic global change pressures including warming (average 0.61 °C), changes in precipitation (up to ± 30% change), atmospheric deposition of nitrogen and sulfur compounds (up to 3.8 g N m(-2) yr(-1) and 3.1 g S m(-2) yr(-1)), and forest fragmentation in the surrounding landscape (up to 88% reduced tree cover within 5 km). The broad suite of measurements made at CTFS-ForestGEO sites makes it possible to investigate the complex ways in which global change is impacting forest dynamics. Ongoing research across the CTFS-ForestGEO network is yielding insights into how and why the forests are changing, and continued monitoring will provide vital contributions to understanding worldwide forest diversity and dynamics in an era of global change.

Influences of forest structure and landscape features on spatial variation in species composition in a palm community in central Amazonia

The mechanisms that maintain palm species diversity in tropical rain forests are still debated. Spatial variation in forest structure produces small-scale environmental heterogeneity, which in turn can affect plant survival and reproductive performance. An understanding of how palms respond to variation in forest heterogeneity may help to explain the diversity and structure of their assemblages. We used multivariate ordination statistics and multiple linear models to analyse how palm assemblages are affected by forest structure and landscape features in central Amazonia. In 72 (250×4 m) forest plots distributed over an area of 64 km2, we recorded all seedling and adult palms, and measured topographic and soil variables, and components of forest structure and tree abundance. We found 16976 adults and 18935 seedlings of 46 palm species and five varieties including two morphological forms making a total of 50 botanical entities. Results show that landscape features (altitude, slope, proportions of soil sand and clay) and various components of forest structure (such as degree of forest openness, abundance of forest trees, logs and snags, and leaf litter mass), influence spatial variation in richness, abundance and species composition of palms, creating ecological gradients in palm community composition. Despite the statistically significant effects of environmental variables, most species occurred throughout the full range of the ecological gradients we studied, indicating that there is either relatively weak niche specialization in the palms, or that the competition between the species is mediated by diffuse demographic processes that cannot be evaluated only through studies of species distributions.

Negative density dependence regulates two tree species at later life stage in a temperate forest

Numerous studies have demonstrated that tree survival is influenced by negative density dependence (NDD) and differences among species in shade tolerance could enhance coexistence via resource partitioning, but it is still unclear how NDD affects tree species with different shade-tolerance guilds at later life stages. In this study, we analyzed the spatial patterns for trees with dbh (diameter at breast height) ≥2 cm using the pair-correlation g(r) function to test for NDD in a temperate forest in South Korea after removing the effects of habitat heterogeneity. The analyses were implemented for the most abundant shade-tolerant (Chamaecyparis obtusa) and shade-intolerant (Quercus serrata) species. We found NDD existed for both species at later life stages. We also found Quercus serrata experienced greater NDD compared with Chamaecyparis obtusa. This study indicates that NDD regulates the two abundant tree species at later life stages and it is important to consider variation in species' shade tolerance in NDD study.

The relative importance of Janzen-Connell effects in influencing the spatial patterns at the Gutianshan subtropical forest

The Janzen-Connell hypothesis is among the most important theories put forward to explain species coexistence in species-rich communities. However, the relative importance of Janzen-Connell effects with respect to other prominent mechanisms of community assembly, such as dispersal limitation, self-thinning due to competition, or habitat association, is largely unresolved. Here we use data from a 24-ha Gutianshan subtropical forest to address it. First we tested for significant associations of adults, juveniles, and saplings with environmental variables. Second we evaluated if aggregation decreased with life stage. In a third analysis we approximately factored out the effect of habitat association and comprehensively analyzed the spatial associations of intraspecific adults and offspring (saplings, juveniles) of 46 common species at continuous neighborhood distances. We found i) that, except for one, all species were associated with at least one environmental variable during at least one of their life stages, but the frequency of significant habitat associations declined with increasing life stage; ii) a decline in aggregation with increasing life stage that was strongest from juveniles to adults; and iii) intraspecific adult-offspring associations were dominated by positive relationships at neighborhood distances up to 10 m. Our results suggest that Janzen-Connell effects were not the dominant mechanisms in structuring the spatial patterns of established trees in the subtropical Gutianshan forest. The spatial patterns may rather reflect the joint effects of size-dependent self-thinning, dispersal limitation and habitat associations. Our findings contribute to a more comprehensive understanding of the relative importance of Janzen-Connell effects in influencing plant community structure under strong topographic heterogeneity.

Spatial distribution and interspecific associations of tree species in a tropical seasonal rain forest of China

Studying the spatial pattern and interspecific associations of plant species may provide valuable insights into processes and mechanisms that maintain species coexistence. Point pattern analysis was used to analyze the spatial distribution patterns of twenty dominant tree species, their interspecific spatial associations and changes across life stages in a 20-ha permanent plot of seasonal tropical rainforest in Xishuangbanna, China, to test mechanisms maintaining species coexistence. Torus-translation tests were used to quantify positive or negative associations of the species to topographic habitats. The results showed: (1) fourteen of the twenty tree species were negatively (or positively) associated with one or two of the topographic variables, which evidences that the niche contributes to the spatial pattern of these species. (2) Most saplings of the study species showed a significantly clumped distribution at small scales (0-10 m) which was lost at larger scales (10-30 m). (3) The degree of spatial clumping deceases from saplings, to poles, to adults indicates that density-dependent mortality of the offspring is ubiquitous in species. (4) It is notable that a high number of positive small-scale interactions were found among the twenty species. For saplings, 42.6% of all combinations of species pairs showed positive associations at neighborhood scales up to five meters, but only 38.4% were negative. For poles and adults, positive associations at these distances still made up 45.5% and 29.5%, respectively. In conclusion, there is considerable evidence for the presence of positive interactions among the tree species, which suggests that species herd protection may occur in our plot. In addition, niche assembly and limited dispersal (likely) contribute to the spatial patterns of tree species in the tropical seasonal rain forest in Xishuangbanna, China.

Rodent seed predation: effects on seed survival, recruitment, abundance, and dispersion of bird-dispersed tropical trees

Tropical tree species vary widely in their pattern of spatial dispersion. We focus on how seed predation may modify seed deposition patterns and affect the abundance and dispersion of adult trees in a tropical forest in India. Using plots across a range of seed densities, we examined whether seed predation levels by terrestrial rodents varied across six large-seeded, bird-dispersed tree species. Since inter-specific variation in density-dependent seed mortality may have downstream effects on recruitment and adult tree stages, we determined recruitment patterns close to and away from parent trees, along with adult tree abundance and dispersion patterns. Four species (Canarium resiniferum, Dysoxylum binectariferum, Horsfieldia kingii, and Prunus ceylanica) showed high predation levels (78.5-98.7%) and increased mortality with increasing seed density, while two species, Chisocheton cumingianus and Polyalthia simiarum, showed significantly lower seed predation levels and weak density-dependent mortality. The latter two species also had the highest recruitment near parent trees, with most abundant and aggregated adults. The four species that had high seed mortality had low recruitment under parent trees, were rare, and had more spaced adult tree dispersion. Biotic dispersal may be vital for species that suffer density-dependent mortality factors under parent trees. In tropical forests where large vertebrate seed dispersers but not seed predators are hunted, differences in seed vulnerability to rodent seed predation and density-dependent mortality can affect forest structure and composition.

Herbivores on a dominant understory shrub increase local plant diversity in rain forest communities

Indirect effects of trophic interactions on biodiversity can be large and common, even in complex communities. Previous experiments with dominant understory Piper shrubs in a Costa Rican rain forest revealed that increases in herbivore densities on these shrubs caused widespread seedling mortality as a result of herbivores moving from Piper to seedlings of many different plant genera. We tested components of the Janzen-Connell hypothesis by conducting focused studies on the effects of specialist and generalist Piper herbivores on local seedling diversity. Whereas specialist herbivores are predicted to increase mortality to neighboring seedlings that are closely related to the source plant, true generalists moving from source plants may cause density-dependent mortality of many species, and possibly increase richness if new species replace abundant species that have been thinned by herbivores. Therefore, we hypothesized that seedling richness would be greater in understory control plots created in patches of Piper that had normal densities of generalist herbivores compared to plots from which we removed generalist herbivores manually from all Piper shrubs. After 15 months, generalist-herbivore-removal plots had > 40% fewer seedlings, > 40% fewer species, and 40% greater seedling evenness, on average, than control plots with generalist herbivores intact. Using a complementary approach in unmanipulated plots in four forests, we used path analysis to test for a positive association between seedling diversity and herbivore damage on Piper species. In unmanipulated plots, for both generalist and specialist herbivores, our data were significant fits to the causal model that Piper herbivores decrease evenness and increase plant species richness, corroborating the experimental results. Because herbivores changed how individuals were apportioned among the species and families present (lower evenness), one interpretation of these associations between herbivores on Piper shrubs and local seedling richness is that high seedling mortality in dominant families allowed the colonization or survival of less common species. If interspecific or apparent competition allowed for a relative increase in species richness, then the Janzen-Connell hypothesis may extend its predictions to generalist seedling predators. We speculate that apparent competition may explain some of the deviations from neutral model predictions, especially at small scales.

Foliar fungal pathogens and grassland biodiversity

By attacking plants, herbivorous mammals, insects, and belowground pathogens are known to play an important role in maintaining biodiversity in grasslands. Foliar fungal pathogens are ubiquitous in grassland ecosystems, but little is known about their role as drivers of community composition and diversity. Here we excluded foliar fungal pathogens from perennial grassland by using fungicide to determine the effect of natural levels of disease on an otherwise undisturbed plant community. Importantly, we excluded foliar fungal pathogens along with rabbits, insects, and mollusks in a full factorial design, which allowed a comparison of pathogen effects along with those of better studied plant enemies. This revealed that fungal pathogens substantially reduced aboveground plant biomass and promoted plant diversity and that this especially benefited legumes. The scale of pathogen effects on productivity and biodiversity was similar to that of rabbits and insects, but different plant species responded to the exclusion of the three plant enemies. These results suggest that theories of plant coexistence and management of biodiversity in grasslands should consider foliar fungal pathogens as potentially important drivers of community composition.

Community-level consequences of density dependence and habitat association in a subtropical broad-leaved forest

How extraordinary numbers of species can coexist in hyper-diverse communities remains unresolved. While numerous hypotheses have been proposed based on observational and theoretical investigations, little is known about which mechanisms are truly active in forest communities and less is known about their relative contributions to community assembly. In this study, generalized linear mixed models with crossed random effects were used to assess the relative contributions of density dependence and habitat association to community-level diversity maintenance. Species habitat associations were classified based on soil nutrients, topography and species composition. Local neighbourhood effects were also addressed with spatially explicit models of seedling survival. The results shown here reveal that local- and community-level seedling dynamics were consistent with density-dependent predictions, although habitat association played a more important role in shaping short-term seedling survival. We conclude that density dependence could promote species coexistence on the premise of habitat partitioning.

Spatially explicit neutral models for population genetics and community ecology: Extensions of the Neyman-Scott clustering process

Spatially explicit models relating to plant populations have developed little since Felsenstein (1975) pointed out that if limited seed dispersal causes clustering of individuals, such models cannot reach an equilibrium. This paper aims to resolve this issue by modifying the Neyman-Scott cluster point process. The new point processes are dynamic models with random immigration, and the continuous increase in the clustering of individuals stops at some level. Hence, an equilibrium state is achieved, and new individual-based spatially explicit neutral coalescent models are established. By fitting the spatial structure at equilibrium to individual spatial distribution data, we can indirectly estimate seed dispersal and effective population density. These estimates are improved when genetic data are available, and become even more sophisticated if spatial distribution and genetic data pertaining to the offspring are also available.

Central-place seed foraging and vegetation patterns

We investigate how central-place seed foragers with a nest in the proximity of one or more seed sources determine the formation of different vegetation patterns. In particular, we discuss the ecological conditions that lead to the formation of hump-shaped (Janzen-Connell) patterns in a two-dimensional landscape. Our analysis shows that central-place predation can generate Janzen-Connell patterns even if predators' movement strategies are exclusively based on resource abundance, both in the single-plant/single-nest case and in a forest with several seed sources. We also show that social foraging may either promote or work against the formation of Janzen-Connell patterns, depending upon the way foragers take advantage of social interactions.