Generalist birds promote tropical forest regeneration and increase plant diversity via rare-biased seed dispersal

Functional connectivity of animal-dispersed plant communities depends on the interacting effects of network specialization and resource diversity

Plant functional connectivity-the dispersal of plant propagules between habitat patches-is often ensured through animal movement. Yet, there is no quantitative framework to analyse how plant-animal interactions and the movement of seed dispersers influence community-level plant functional connectivity. We propose a trait-based framework to quantify plant connectivity with a model integrating plant-frugivore networks, animal-mediated seed-dispersal distances and the selection of target patches by seed dispersers. Using this framework, we estimated how network specialization, between-patch distance and resource diversity in a target patch affect the number and diversity of seeds dispersed to that patch. Specialized networks with a high degree of niche partitioning in plant-frugivore interactions reduced functional connectivity by limiting the diversity of seeds dispersed over long distances. Resource diversity in the target patch increased both seed number and diversity, especially in specialized networks and within short and intermediate distances between patches. Notably, resource diversity was particularly important at intermediate distances, where the number and diversity of seeds reaching a patch increased more strongly with resource diversity than at longer distances. Using a trait-based framework, we show that resource diversity in the target patch is a major driver of connectivity in animal-dispersed plant communities.

Interconnecting fragmented forests: Small and mobile birds are cornerstones in the plant-frugivore meta-network

Habitat fragmentation is causing the collapse of seed dispersal interactions and ecosystem functioning. When management and conservation strategies aim to sustain ecosystem functioning of fragmented forests, species' traits and functional performance are critical in guiding decisions. However, to date, we lack a quantitative understanding of the role of frugivores' body size and dispersal ability in ecosystem sustainability among fragmented forests. Focusing on avian frugivory and seed dispersal in a multi-island setting, we address the data gap by recording more than 20,000 frugivory events in an artificial insular fragmented landscape constructed in 1959 and nearby unfragmented forests on the mainland. We show that large-bodied and dispersal-limited frugivorous birds are largely confined to large islands and the unfragmented mainland, whereas on small islands, small-bodied and highly mobile birds predominantly engage in frugivory interactions. The plant-frugivore meta-network exhibits a distinct compartmentalization, driven by island area and bird mobility. Birds with smaller size and greater mobility have higher topological importance, and the presence of small-bodied birds significantly enhances meta-network robustness. These results suggest that among insular fragmented forests where frugivory interactions are degraded, small-bodied and highly mobile birds disproportionately contribute to meta-community cohesion and ecosystem functioning because of the lack of large-bodied and dispersal-limited birds. We thus advocate for the restoration of landscapes to facilitate seed dispersal and functional connectivity, ensuring the presence of large patches along with small patches as stepping-stones. Meanwhile, we recommend prioritizing conservation on small-bodied and highly mobile birds in fragmented landscapes, a subset of underappreciated species that yet play crucial roles in ecosystem functioning.

Active restoration increases tree species richness and recruitment of large-seeded taxa after 16-18 years

Tropical forest restoration presents a potential lifeline to mitigate climate change and biodiversity crises in the Anthropocene. Yet, the extent to which human interventions, such as tree planting, accelerate the recovery of mature functioning ecosystems or redirect successional trajectories toward novel states remains uncertain due to a lack of long-term experiments. In 2004-2006, we established three 0.25-ha plots at 10 sites in southern Costa Rica to test three forest restoration approaches: natural regeneration (no planting), applied nucleation (planting in patches), and plantation (full planting). In a comprehensive survey after 16-18 years of recovery, we censused >80,000 seedlings, saplings, and trees from at least 255 species across 26 restoration plots (nine natural regeneration, nine applied nucleation, eight plantation) and six adjacent reference forests to evaluate treatment effects on recruitment patterns and community composition. Both applied nucleation and plantation treatments resulted in significantly elevated seedling and sapling establishment and more predictable community composition compared with natural regeneration. Similarity of vegetation composition to reference forest tended to scale positively with treatment planting intensity. Later-successional species with seeds ≥5 mm had significantly greater seedling and sapling abundance in the two planted treatments, and plantation showed similar recruitment densities of large-seeded (≥10 mm) species to reference forest. Plantation tended toward a lower abundance of early-successional recruits than applied nucleation. Trees (≥5 cm dbh) in all restoration treatments continued to be dominated by a few early-successional species and originally transplanted individuals. Seedling recruits of planted taxa were more abundant in applied nucleation than the other treatments though few transitioned into the sapling layer. Overall, our findings show that active tree planting accelerates the establishment of later-successional trees compared with natural regeneration after nearly two decades. While the apparent advantages of higher density tree planting on dispersal and understory establishment of larger seeded, later-successional species recruitment is notable, more time is needed to assess whether these differences will persist and transition to the more rapid development of a mature later-successional canopy. Our results underscore the need for ecological restoration planning and monitoring that targets biodiversity recovery over multiple decades.

Negative density dependence characterizes mutualistic interactions between birds and fruiting plants across latitudes

Negative density dependence (NDD) in biotic interactions of interference such as plant-plant competition, granivory and herbivory are well-documented mechanisms that promote species' coexistence in diverse plant communities worldwide. Here, we investigated the generality of a novel type of NDD mechanism that operates through the mutualistic interactions of frugivory and seed dispersal among fruit-eating birds and plants. By sampling community-wide frugivory interactions at high spatial and temporal resolution in Pennsylvania, Puerto Rico, Peru, Brazil and Argentina, we evaluated whether interaction frequencies between birds and fruit resources occurred more often (selection), as expected, or below expectations (under-utilization) set by the relative fruit abundance of the fruit resources of each plant species. Our models considered the influence of temporal scales of fruit availability and bird phylogeny and diets, revealing that NDD characterizes frugivory across communities. Irrespective of taxa or dietary guild, birds tended to select fruits of plant species that were proportionally rare in their communities, or that became rare following phenological fluctuations, while they mostly under-utilized abundant fruit resources. Our results demonstrate that negative density-dependence in frugivore-plant interactions provides a strong equalizing mechanism for the dispersal processes of fleshy-fruited plant species in temperate and tropical communities, likely contributing to building and sustaining plant diversity. This article is part of the theme issue 'Diversitydependence of dispersal: interspecific interactions determine spatial dynamics'.

Generalist and topologically central avian frugivores promote plant invasion unequally across land-bridge islands

Seed dispersal by frugivorous birds facilitates plant invasions, but it is poorly known how invasive plants integrate into native communities in fragmented landscapes. We surveyed plant-frugivore interactions, including an invasive plant (Phytolacca americana), on 22 artificial land-bridge islands (fragmented forests) in the Thousand Island Lake, China. Focusing on frugivory interactions that may lead to seed dispersal, we built ecological networks of studied islands both at the local island (community) and at landscape (metacommunity) levels. On islands with P. americana, we found that P. americana impacted local avian frugivory networks more on islands with species-poor plant communities and on isolated islands. Moreover, as P. americana interacted mainly with local core birds (generalists), this indicates reduced seed dispersal of native plants on invaded islands. At the landscape level, P. americana had established strong interactions with generalist birds that largely maintain seed-dispersal functions across islands, as revealed by their topologically central roles both in the regional plant-bird trophic network and in the spatial metanetwork. This indicates that generalist frugivorous birds may have facilitated the dispersal of P. americana across islands, making P. americana well integrated into the plant-frugivore mutualistic metacommunity. Taken together, our study demonstrates that the impact of plant invasion is context-dependent and that generalist native frugivores with high dispersal potential may accelerate plant invasion in fragmented landscapes. These findings highlight the importance of taking the functional roles of animal mutualists and habitat fragmentation into account when managing plant invasions and their impact on native communities.

Frugivore-mediated seed dispersal in fragmented landscapes: Compositional and functional turnover from forest to matrix

Seed dispersal by frugivores is a fundamental function for plant community dynamics in fragmented landscapes, where forest remnants are typically embedded in a matrix of anthropogenic habitats. Frugivores can mediate both connectivity among forest remnants and plant colonization of the matrix. However, it remains poorly understood how frugivore communities change from forest to matrix due to the loss or replacement of species with traits that are less advantageous in open habitats and whether such changes ultimately influence the composition and traits of dispersed plants via species interactions. Here, we close this gap by using a unique dataset of seed-dispersal networks that were sampled in forest patches and adjacent matrix habitats of seven fragmented landscapes across Europe. We found a similar diversity of frugivores, plants, and interactions contributing to seed dispersal in forest and matrix, but a high turnover (replacement) in all these components. The turnover of dispersed seeds was smaller than that of frugivore communities because different frugivore species provided complementary seed dispersal in forest and matrix. Importantly, the turnover involved functional changes toward larger and more mobile frugivores in the matrix, which dispersed taller, larger-seeded plants with later fruiting periods. Our study provides a trait-based understanding of frugivore-mediated seed dispersal through fragmented landscapes, uncovering nonrandom shifts that can have cascading consequences for the composition of regenerating plant communities. Our findings also highlight the importance of forest remnants and frugivore faunas for ecosystem resilience, demonstrating a high potential for passive forest restoration of unmanaged lands in the matrix.

Feedback loops between 3D vegetation structure and ecological functions of animals

Ecosystems function in a series of feedback loops that can change or maintain vegetation structure. Vegetation structure influences the ecological niche space available to animals, shaping many aspects of behaviour and reproduction. In turn, animals perform ecological functions that shape vegetation structure. However, most studies concerning three-dimensional vegetation structure and animal ecology consider only a single direction of this relationship. Here, we review these separate lines of research and integrate them into a unified concept that describes a feedback mechanism. We also show how remote sensing and animal tracking technologies are now available at the global scale to describe feedback loops and their consequences for ecosystem functioning. An improved understanding of how animals interact with vegetation structure in feedback loops is needed to conserve ecosystems that face major disruptions in response to climate and land-use change.

How effective are perches in promoting bird-mediated seed dispersal for natural forest regeneration? A systematic review protocol

BACKGROUND

Forest landscape restoration (FLR), often through tree planting, is one of the priorities in many global and national initiatives for carbon offsetting as part of climate change mitigation and biodiversity conservation. However, active efforts to meet FLR objectives entail substantial costs for the procurement of planting stocks and require an experienced workforce for planting and nurturing tree seedlings. Alternatively, restoration projects can be more cost-effective and potentially may have greater biodiversity gain through assisting and accelerating natural forest regeneration. The use of perches is one of the strategies under Assisted Natural Regeneration (ANR) and is used to attract avian seed dispersers to degraded habitats for increased tree seed supply and seedling establishment. This systematic review and potential meta-analysis aim to determine the effectiveness of artificial and natural perches in promoting natural forest regeneration. Specifically, we will evaluate their effectiveness in driving seed richness, seed density, seedling richness, and seedling density. The results will synthesize available evidence on the topic, identify knowledge gaps we need filling to upscale the strategy, and inform their use in concert with other ANR strategies.

METHODS

The search strategy was informed through a literature scan and discussions with stakeholders and experts. A total of eight databases, which include an organizational library and a web-based search engine, will be searched using the refined search string in English. The search string was formed using keywords corresponding to the PICO structure of the research question, and its comprehensiveness was evaluated using 10 benchmark articles. The search results will be screened by the review team (composed of a primary and at least two secondary reviewers) using the set eligibility criteria at the title and abstract level, followed by the full-text screening. The screened studies will then undergo critical appraisal using the assessment criteria based on risk of bias and methods. Data from the accepted studies will be extracted to the standard data sheet for meta-analysis. Effect size (Hedges' g) will be computed to determine whether perches are effective in increasing seed dispersal and seedling establishment in degraded sites. The effect of potential modifiers relating to the landscape will be explored via mixed models.

Sympatric primate seed dispersers and predators jointly contribute to plant diversity in a subtropical forest

Mutualistic and antagonistic plant-animal interactions differentially contribute to the maintenance of species diversity in ecological communities. Although both seed dispersal and predation by fruit-eating animals are recognized as important drivers of plant population dynamics, the mechanisms underlying how seed dispersers and predators jointly affect plant diversity remain largely unexplored. Based on mediating roles of seed size and species abundance, we investigated the effects of seed dispersal and predation by two sympatric primates (Nomascus concolor and Trachypithecus crepusculus) on local plant recruitment in a subtropical forest of China. Over a 26 month period, we confirmed that these primates were functionally distinct: gibbons were legitimate seed dispersers who dispersed seeds of 44 plant species, while langurs were primarily seed predators who destroyed seeds of 48 plant species. Gibbons dispersed medium-seeded species more effectively than small- and large-seeded species, and dispersed more seeds of rare species than common and dominant species. Langurs showed a similar predation rate across different sizes of seeds, but destroyed a large number of seeds from common species. Due to gut passage effects, gibbons significantly shortened the duration of seed germination for 58% of the dispersed species; however, for 54% of species, seed germination rates were reduced significantly. Our study underlined the contrasting contributions of two primate species to local plant recruitment processes. By dispersing rare species and destroying the seeds of common species, both primates might jointly maintain plant species diversity. To maintain healthy ecosystems, the conservation of mammals that play critical functional roles needs to receive further attention.

Animal seed dispersal recovery during passive restoration in a forested landscape

Seed dispersal by animals is key for restoration of tropical forests because it maintains plant diversity and accelerates community turnover. Therefore, changes in seed dispersal during forest restoration can indicate the recovery of species interactions, and yet these changes are rarely considered in forest restoration planning. In this study, we examined shifts in the importance of different seed dispersal modes during passive restoration in a tropical chronosequence spanning more than 100 years, by modelling the proportion of trees dispersed by bats, small birds, large birds, flightless mammals and abiotic means as a function of forest age. Contrary to expectations, tree species dispersed by flightless mammals dominated after 20 years of regeneration, and tree richness and abundance dispersed by each mode mostly recovered to old growth levels between 40 and 70 years post-abandonment. Seed dispersal by small birds declined over time during regeneration, while bat dispersal played a minor role throughout all stages of succession. Results suggest that proximity to old growth forests, coupled with low hunting, explained the prevalence of seed dispersal by animals, especially by flightless mammals at this site. We suggest that aspects of seed dispersal should be monitored when restoring forest ecosystems to evaluate the reestablishment of species interactions. This article is part of the theme issue 'Understanding forest landscape restoration: reinforcing scientific foundations for the UN Decade on Ecosystem Restoration'.

Earliest evidence for fruit consumption and potential seed dispersal by birds

The Early Cretaceous diversification of birds was a major event in the history of terrestrial ecosystems, occurring during the earliest phase of the Cretaceous Terrestrial Revolution, long before the origin of the bird crown-group. Frugivorous birds play an important role in seed dispersal today. However, evidence of fruit consumption in early birds from outside the crown-group has been lacking. Jeholornis is one of the earliest-diverging birds, only slightly more crownward than Archaeopteryx, but its cranial anatomy has been poorly understood, limiting trophic information which may be gleaned from the skull. Originally hypothesised to be granivorous based on seeds preserved as gut contents, this interpretation has become controversial. We conducted high-resolution synchrotron tomography on an exquisitely preserved new skull of Jeholornis, revealing remarkable cranial plesiomorphies combined with a specialised rostrum. We use this to provide a near-complete cranial reconstruction of Jeholornis, and exclude the possibility that Jeholornis was granivorous, based on morphometric analyses of the mandible (3D) and cranium (2D), and comparisons with the 3D alimentary contents of extant birds. We show that Jeholornis provides the earliest evidence for fruit consumption in birds, and indicates that birds may have been recruited for seed dispersal during the earliest stages of the avian radiation. As mobile seed dispersers, early frugivorous birds could have expanded the scope for biotic dispersal in plants, and might therefore explain, at least in part, the subsequent evolutionary expansion of fruits, indicating a potential role of bird–plant interactions in the Cretaceous Terrestrial Revolution.

Birds and plants have a close relationship that has developed over millions of years. Birds became diverse and abundant around 135 million years ago. Shortly after, plants started developing new and different kinds of fruits. Today, fruit-eating birds help plants to reproduce by spreading seeds in their droppings. This suggests that birds and plants have coevolved, changing together over time. But it is not clear exactly how their relationship started.

One species that might hold the answers is an early bird species known as Jeholornis. It lived in China in the Early Cretaceous, around 120 million years ago. Palaeontologists have discovered preserved seeds inside its fossilised remains. The question is, how did they get there? Some birds eat seeds directly, cracking them open or grinding them up in the stomach to extract the nutrients inside. Other birds swallow seeds when they are eating fruit. If Jeholornis belonged to this second group, it could represent one of the early steps in plant-bird coevolution.

Hu et al. scanned and reconstructed a preserved Jeholornis skull and compared it to the skulls, especially the mandibles, of modern birds, including species that grind seeds, species that crack seeds and species that eat fruits, leaving the seeds whole. The analyses ruled out seed cracking. But it could not distinguish between seed grinding and fruit eating. Hu et al. therefore compared the seed remains found inside Jeholornis fossils to seeds eaten by modern birds. The fossilised seeds were intact and showed no evidence of grinding. This suggests that Jeholornis ate whole fruits for at least part of the year.

At around the time Jeholornis was alive, the world was entering a phase called the Cretaceous Terrestrial Revolution, which was characterized by an explosion of new species and an expansion of both flowering plants and birds. This finding opens new avenues for scientists to explore how plant and birds might have evolved together. Similar analyses could unlock new information about how other species interacted with their environments.

Passive directed dispersal of plants by animals

Conceptual gaps and imprecise terms and definitions may obscure the breadth of plant-animal dispersal relationships involved in directed dispersal. The term 'directed' indicates predictable delivery to favourable microsites. However, directed dispersal was initially considered uncommon in diffuse mutualisms (i.e. those involving many species), partly because plants rarely influence post-removal propagule fate without specialized adaptations. This rationale implies that donor plants play an active role in directed dispersal by manipulating vector behaviour after propagule removal. However, even in most classic examples of directed dispersal, participating plants do not influence animal behaviour after propagule removal. Instead, such plants may take advantage of vector attraction to favourable plant microsites, indicating a need to expand upon current interpretations of directed dispersal. We contend that directed dispersal can emerge whenever propagules are disproportionately delivered to favourable microsites as a result of predictably skewed vector behaviour. Thus, we propose distinguishing active and passive forms of directed dispersal. In active directed dispersal, the donor plant achieves disproportionate arrival to favourable microsites by influencing vector behaviour after propagule removal. By contrast, passive directed dispersal occurs when the donor plant takes advantage of vector behaviour to arrive at favourable microsites. Whereas predictable post-removal vector behaviour is dictated by characteristics of the donor plant in active directed dispersal, characteristics of the destination dictate predictable post-removal vector behaviour in passive directed dispersal. Importantly, this passive form of directed dispersal may emerge in more plant-animal dispersal relationships because specialized adaptations in donor plants that influence post-removal vector behaviour are not required. We explore the occurrence and consequences of passive directed dispersal using the unifying generalized gravity model of dispersal. This model successfully describes vectored dispersal by incorporating the influence of the environment (i.e. attractiveness of microsites) on vector movement. When applying gravity models to dispersal, the three components of Newton's gravity equation (i.e. gravitational force, object mass, and distance between centres of mass) become analogous to propagules moving towards a location based on characteristics of the donor plant, the destination, and relocation processes. The generalized gravity model predicts passive directed dispersal in plant-animal dispersal relationships when (i) animal vectors are predictably attracted to specific destinations, (ii) animal vectors disproportionately disperse propagules to those destinations, and (iii) those destinations are also favourable microsites for the dispersed plants. Our literature search produced evidence for these three conditions broadly, and we identified 13 distinct scenarios where passive directed dispersal likely occurs because vector behaviour is predictably skewed towards favourable microsites. We discuss the wide applicability of passive directed dispersal to plant-animal mutualisms and provide new insights into the vulnerability of those mutualisms to global change.

Simplified Communities of Seed-Dispersers Limit the Composition and Flow of Seeds in Edge Habitats

Edge effects, driven by human modification of landscapes, can have critical impacts on ecological processes such as species interactions, with cascading impacts on biodiversity as a whole. Characterizing how edges affect vital biotic interactions such as seed dispersal by frugivores is important for better understanding potential mechanisms that drive species coexistence and diversity within a plant community. Here, we investigated how differences between frugivore communities at the forest edge and interior habitats of a diverse tropical rainforest relate to patterns of animal-mediated seed dispersal and early seedling recruitment. We found that the lemur communities across the forest edge-interior gradient in this system showed the highest species richness and variability in body sizes at intermediate distances; the community of birds showed the opposite pattern for species richness. Three large-bodied frugivores, known to be effective dispersers of large seeds, tended to avoid the forest edge. As result, the forest edges received a lower rate of animal-mediated seed dispersal compared to the interior habitats. In addition, we also found that the seeds that were actively dispersed by animals in forest edge habitats were smaller in size than seeds dispersed in the forest interior. This pattern was found despite a similarity in seed size of seasonally fruiting adult trees and shrubs between the two habitats. Despite these differences in dispersal patterns, we did not observe any differences in the rates of seedling recruitment or seed-size distribution of successful recruit species. Our results suggest that a small number of frugivores may act as a potential biotic filter, acting on seed size, for the arrival of certain plant species to edge habitats, but other factors may be more important for driving recruitment patterns, at least in the short term. Further research is needed to better understand the potential long-term impacts of altered dispersal regimes relative to other environmental factors on the successional dynamics of edge communities. Our findings are important for understanding potential ecological drivers of tree community changes in forest edges and have implications for conservation management and restoration of large-seeded tree species in disturbed habitats.

Uniting niche differentiation and dispersal limitation predicts tropical forest succession

Tropical secondary forests are increasingly important for carbon sequestration and biodiversity conservation worldwide; yet, we still cannot accurately predict community turnover during secondary succession. We propose that integrating niche differentiation and dispersal limitation will generate an improved theoretical explanation of tropical forest succession. The interaction between seed sources and dispersers regulates seed movement throughout succession, and recent technological advances in animal tracking and molecular analyses enable us to accurately monitor seed movement as never before. We propose a framework to bridge the gap between niche differentiation and dispersal limitation. The Source-Disperser Limitation Framework (SDLF) provides a way to better predict secondary tropical forest succession across gradients of landscape disturbance by integrating seed sources and frugivore behavior.

Dispersers and environment drive global variation in fruit colour syndromes

The colours of fleshy fruits play a critical role in plant dispersal by advertising ripe fruits to consumers. Fruit colours have long been classified into syndromes attributed to selection by animal dispersers, despite weak evidence for this hypothesis. Here, we test the relative importance of biotic (bird and mammal frugivory) and abiotic (wet season temperatures, growing season length and UV-B radiation) factors in determining fruit colour syndrome in 3163 species of fleshy-fruited plants. We find that both dispersers and environment are important, and they interact. In warm areas, contrastive, bird-associated fruit colours increase with relative bird frugivore prevalence, whereas in cold places these colours dominate even where mammalian dispersers are prevalent. We present near-global maps of predicted fruit colour syndrome based on our species-level model and our newly developed characterisations of relative importance of bird and mammal frugivores.

Winner-Loser Species Replacements in Human-Modified Landscapes

Community assembly arguably drives the provision of ecosystem services because they critically depend on which and how species coexist. We examine conspicuous cases of 'winner and loser' replacements (WLRs) in tropical forests to provide a framework integrating drivers, impacts on ecological organization, and reconfiguration of ecosystem service provisioning. Most WLRs involve native species and result from changes in resource availability rather than from altered competition among species. In this context, species dispersal is a powerful force controlling community (re)assembly. Furthermore, replacements imply a nearly complete functional reorganization of assemblages and new 'packages' of ecosystem services and disservices provided by winners. WLRs can thus elucidate the multiple transitions experienced by tropical forests, and have theoretical/applied implications, including the role that human-modified landscapes may play in global-scale sustainability.

Frugivory Specialization in Birds and Fruit Chemistry Structure Mutualistic Networks across the Neotropics

AbstractThe interaction between fruit chemistry and the physiological traits of frugivores is expected to shape the structure of mutualistic seed dispersal networks, but it has been understudied compared with the role of morphological trait matching in structuring interaction patterns. For instance, highly frugivorous birds (i.e., birds that have fruits as the main component of their diets), which characteristically have fast gut passage times, are expected to avoid feeding on lipid-rich fruits because of the long gut retention times associated with lipid digestion. Here, we compiled data from 84 studies conducted in the Neotropics that used focal plant methods to record 35,815 feeding visits made by 317 bird species (155 genera in 28 families) to 165 plant species (82 genera in 48 families). We investigated the relationship between the degree of frugivory of birds (i.e., how much of their diet is composed of fruit) at the genus level and their visits to plant genera that vary in fruit lipid content. We used a hierarchical modeling of species communities approach that accounted for the effects of differences in body size, bird and plant phylogeny, and spatial location of study sites. We found that birds with a low degree of frugivory (e.g., predominantly insectivores) tend to have the highest increase in visitation rates as fruits become more lipid rich, while birds that are more frugivorous tend to increase visits at a lower rate or even decrease visitation rates as lipids increase in fruits. This balance between degree of frugivory and visitation rates to lipid-poor and lipid-rich fruits provides a mechanism to explain specialized dispersal systems and the occurrence of certain physiological nutritional filters, ultimately helping us to understand community-wide interaction patterns between birds and plants.

Improvement of vegetation structure enhances bird functional traits and habitat resilience in an area of ongoing restoration in the Atlantic Forest

Ecological restoration is a traditional option for recovering biodiversity and ecosystem functions. Birds perform pollination, seed dispersal, and pest-control services, which catalyze increases in habitat structure. Habitat complexity changes bird composition, but there is little evidence of its effects on bird functional diversity in Neotropical restorations. We tested whether bird functional diversity and composition respond to increased habitat complexity. Point-counts were performed (January-December 2015) in an area undergoing restoration (536 ha) in the Atlantic Forest of southeastern Brazil, in restorations with less and more structured vegetation and pastures and forest-fragments. The functional bird traits considered were diet, habitat, biomass, environmental sensitivity, and foraging strata. Increased habitat complexity was evaluated using plant characteristics (exotic grass, canopy, herbaceous cover, and diameter at breast height). A total of 172 bird species (5% endemic; 12% migratory) were recorded. Increased vegetation structure in both restored sites and forest-fragments drove a reorganization and addition of functional bird traits, which positively influenced functional richness, dispersion, and evenness. Shifts in plant-characteristics rearranged bird functional traits (diet-forest-dependence and diet-strata-foraging). The rapid development of vegetation structure is a key factor for restoration because it provides additional habitat for semi-dependent forest birds and enhances resilience and sustainability in new man-made forests.

Resistance Genes Affect How Pathogens Maintain Plant Abundance and Diversity

AbstractSpecialized pathogens are thought to maintain plant community diversity; however, most ecological studies treat pathogens as a black box. Here we develop a theoretical model to test how the impact of specialized pathogens changes when plant resistance genes (R-genes) mediate susceptibility. This work synthesizes two major hypotheses: the gene-for-gene model of pathogen resistance and the Janzen-Connell hypothesis of pathogen-mediated coexistence. We examine three scenarios. First, R-genes do not affect seedling survival; in this case, pathogens promote diversity. Second, seedlings are protected from pathogens when their R-gene alleles and susceptibility differ from those of nearby conspecific adults, thereby reducing transmission. If resistance is not costly, pathogens are less able to promote diversity because populations with low R-gene diversity suffer higher mortality, putting those populations at a disadvantage and potentially causing their exclusion. R-gene diversity may also be reduced during population bottlenecks, creating a priority effect. Third, when R-genes affect survival but resistance is costly, populations can avoid extinction by losing resistance alleles, as they cease paying a cost that is unneeded. Thus, the impact pathogens can have on tree diversity depends on the mechanism of plant-pathogen interactions. Future empirical studies should examine which of these scenarios most closely reflects the real world.

Frugivore species maintain their structural role in the trophic and spatial networks of seed dispersal interactions

Trophic relationships have inherent spatial dimensions associated with the sites where species interactions, or their delayed effects, occur. Trophic networks among interacting species may thus be coupled with spatial networks linking species and habitats whereby animals connect patches across the landscape thanks to their high mobility. This trophic and spatial duality is especially inherent in processes like seed dispersal by animals, where frugivores consume fruit species and deposit seeds across habitats. We analysed the frugivore-plant interactions and seed deposition patterns of a diverse assemblage of frugivores in a heterogeneous landscape in order to determine whether the roles of frugivores in network topology are correlated across trophic and spatial networks of seed dispersal. We recorded fruit consumption and seed deposition by birds and mammals during 2 years in the Cantabrian Range (N Spain). We then constructed two networks of trophic (i.e. frugivore-plant) and spatial (i.e. frugivore-seed deposition habitat) interactions and estimated the contributions of each frugivore species to the network structure in terms of nestedness, modularity and complementary specialization. We tested whether the structural role of frugivore species was correlated across the trophic and spatial networks, and evaluated the influence of each frugivore abundance and body mass in that relationship. Both the trophic and the spatial networks were modular and specialized. Trophic modules matched medium-sized birds with fleshy-fruited trees, and small bird and mammals with small-fruit trees and shrubs. Spatial modules associated birds with woody canopies, and mammals with open habitats. Frugivore species maintained their structural role across the trophic and spatial networks of seed dispersal, even after accounting for frugivore abundance and body mass. The modularity found in our system points to complementarity between birds and mammals in the seed dispersal process, a fact that may trigger landscape-scale secondary succession. Our results open up the possibility of predicting the consumption pattern of a diverse frugivore community, and its ecological consequences, from the uneven distribution of fleshy-fruit resources in the landscape.

Modifications of the rain forest frugivore community are associated with reduced seed removal at the community level

Tropical rain forests worldwide are under increasing pressure from human activities, which are altering key ecosystem processes such as plant-animal interactions. However, while the direct impact of anthropogenic disturbance on animal communities has been well studied, the consequences of such defaunation for mutualistic interactions such as seed dispersal remains chiefly understood at the plant species level. We asked whether communities of endozoochorous tree species had altered seed removal in forests affected by hunting and logging and if this could be related to modifications of the frugivore community. At two contrasting forest sites in French Guiana, Nouragues (protected) and Montagne de Kaw (hunted and partly logged), we focused on four families of animal-dispersed trees (Sapotaceae, Myristicaceae, Burseraceae, and Fabaceae), which represent 88% of all endozoochorous trees that were fruiting at the time and location of the study. We assessed the abundance of the seed dispersers and predators of these four focal families by conducting diurnal distance sampling along line transects. Densities of several key seed dispersers such as large-bodied primates were greatly reduced at Montagne de Kaw, where the specialist frugivore Ateles paniscus is probably extinct. In parallel, we estimated seed removal rates from fruit and seed counts conducted in 1-m² quadrats placed on the ground beneath fruiting trees. Seed removal rates dropped from 77% at Nouragues to 47 % at Montagne de Kaw, confirming that the loss of frugivores associated with human disturbance impacts seed removal at the community level. In contrast to Sapotaceae, whose seeds are dispersed by mammals only, weaker declines in seed removal for Burseraceae and Myristicaceae suggest that some compensation may occur for these bird- and mammal-dispersed families, possibly because of the high abundance of Toucans at the disturbed site. The defaunation process currently occurring across many tropical forests could dramatically reduce the diversity of entire communities of animal-dispersed trees through seed removal limitation.

The underappreciated role of rodent generalists in fungal spore dispersal networks

Animals are often the primary dispersers of seeds and fungal spores. Specialist species that consume fruits or fungal fruiting bodies (sporocarps) as their main food source are thought to play a more important role in dispersal networks compared to generalist species. However, dispersal networks are often based on occurrence data, overlooking the influence of animal abundance and dispersal effectiveness on network interactions. Using rodent-mycorrhizal fungi networks, we determined how diet specialization and abundance influence the role of rodent species in dispersing fungal spores in temperate forests of northern New Hampshire, USA. We tracked the interactions of five rodent species and 34 fungal taxa over a 3-yr period across hardwood, mixed, and softwood forest stands. We accounted for fluctuations in rodent abundance and differences in the number of spores dispersed in rodent scat. Myodes gapperi, a fungal specialist, dispersed a more diverse spore community than rodent generalists and was consistently the most important disperser in forest types with high fungal availability. Nevertheless, during years when generalist species such as Tamias striatus and Peromyscus maniculatus reached high abundance, their relative importance (species strength) in networks approached or even surpassed that of M. gapperi, particularly in forest types where M. gapperi was less common and fungal availability was low. Increased numbers of generalists enhanced network interaction diversity and the number of fungal taxa dispersed, the timing of which was coincident with seedling establishment following masting, a stage when inoculation by mycorrhizal fungi is critical for growth and survival. Our findings suggest that although specialists play key roles in dispersing mycorrhizal fungal spores, generalists play a heretofore underappreciated role.

Can network metrics predict vulnerability and species roles in bird-dispersed plant communities? Not without behaviour

Network metrics are widely used to infer the roles of mutualistic animals in plant communities and to predict the effect of species' loss. However, their empirical validation is scarce. Here we parameterized a joint species model of frugivory and seed dispersal with bird movement and foraging data from tropical and temperate communities. With this model, we investigate the effect of frugivore loss on seed rain, and compare our predictions to those of standard coextinction models and network metrics. Topological coextinction models underestimated species loss after the removal of highly linked frugivores with unique foraging behaviours. Network metrics informed about changes in seed rain quantity after frugivore loss. However, changes in seed rain composition were only predicted by partner diversity. Nestedness, closeness, and d' specialisation could not anticipate the effects of rearrangements in plant-frugivore communities following species loss. Accounting for behavioural differences among mutualists is critical to improve predictions from network models.