Frugivore Abundance Research Articles

Spatial and seasonal use of habitat by birds in northern Nigeria

ABSTRACTCapsule: Although woodland habitat supports higher avian species diversity and abundance, shrubland may be more important for migratory species.Aims: To investigate the use of habitat, site and season by bird species according to feeding guild, residence status and population trend in Dutse, Northern Nigeria.Methods: A point transect was used to count and identify birds between 0630 and 1100 hours from August 2015 to February 2016 across habitats, sites and seasons in Dutse.Results: Frugivore abundance was significantly higher in woodland habitat. The woodland and shrubland habitats had a higher proportion of resident and migratory species, respectively. Birds with stable population trends were more abundant in woodland, whereas the abundance of birds with decreasing trends were generally similar across most sites. Most of the birds with decreasing trends were migratory species. The dry season had a higher abundance and diversity of birds across different feeding guilds, residence status and population trend status than the wet season.Conclusion: Both woodland and shrubland in Dutse were important bird habitats, but migrant species preferentially used shrubland. The shrubland habitat and the dry season should be prioritized for conservation consideration, especially for migrant birds in Dutse.

Frugivore biodiversity and complementarity in interaction networks enhance landscape‐scale seed dispersal function

Abstract Animal biodiversity matters for the provision of ecosystem functions derived from trophic activity. However, the mechanisms underlying this pattern remain elusive since animal abundance and diversity, which are the components commonly used for representing biodiversity, provide poor information about ecological complementarity in species assemblages. An approach based on species interaction networks may overcome this constraint. Here, we relate frugivore biodiversity and frugivore–plant network structure with landscape‐scale seed dispersal function. We sampled, for two years, and at fourteen plots with variable assemblages of frugivores and plants in the Cantabrian Range (N Spain), data on the abundance and diversity of frugivorous birds, the consumption of fleshy fruits of woody plants and the landscape‐scale patterns of avian seed deposition. As a measure of interaction complementarity in seed dispersal networks, we estimated the degree to which frugivore and plant species specialize in their interacting partners. Specialization varied strongly across the seed dispersal networks of the different plots, being higher in networks harbouring smaller bird species that dispersed mostly small‐fruited plants, and also in networks with late‐ripening, dominant fruiting species dispersed mostly by wintering birds. Bird abundance markedly affected seed deposition. Plots harbouring more birds received a higher density of dispersed seeds and showed higher probabilities of seed arrival and seed deposition in open microhabitats. Bird diversity also had a positive effect on the density of dispersed seed and, to a lesser extent, seed arrival probability. Independently of frugivore abundance and diversity, the density of dispersed seeds increased in plots where seed dispersal networks showed a higher degree of specialization. This study considers the structure of interaction networks to re‐address the relationship between biodiversity and ecosystem functionality, evidencing that specialization in frugivore–plant networks drives the large‐scale process of seed dispersal. These results encourage the consideration of interaction complementarity as an underlying mechanism linking animal biodiversity and trophic‐related functions.

Invasive alien trees reduce bird species richness and abundance of mutualistic frugivores and nectarivores; a bird's eye view on a conflict of interest species in riparian habitats

AbstractInvasive alien plants have major ecological effects, in particular in riparian habitats. While effects of alien tree invasions on riparian plants are well studied, effects on animals are less well understood. Invasive alien trees can have a positive effect by adding habitat and food sources, or have a negative effect, by replacing native food plants. Here we use birds as indicators to determine the impacts of an invasive Eucalyptus tree species in riparian areas of the Cape Floristic Region (CFR) of South Africa. Birds are an ideal study group because they are mobile, respond quickly to habitat changes and feed at different trophic levels. Fixed‐point bird counts were done during winter and spring at near‐pristine and Eucalyptus camaldulensis invaded riparian habitats. A total of 1142 birds from 44 species were recorded. Bird assemblages in invaded sites are almost a complete subset (24 species) of those in near‐pristine areas (42 species). Invaded areas were missing 18 species and contained a total of 128 fewer individuals. This is due to declines in insectivores, frugivores, granivores, raptors and omnivores and the absence of nectarivores in invaded sites. From a bird's perspective, the prioritisation of E. camaldulensis removal from the CFR's river systems is justified, but whether bird species will return to cleared areas needs to be determined.

Forest cover, hunting pressure, and fruit availability influence seed dispersal in a forest‐savanna mosaic in the Congo Basin

AbstractForest fragmentation, reduced forest cover, and hunting pressure are the main threats affecting animal‐mediated seed dispersal. However, their combined effects on seed dispersal rates have been simultaneously investigated only rarely, and never in Africa. We aimed to disentangle the effects of forest cover, hunting pressure, frugivore abundance, and fruit availability at the local and landscape scales on the seed dispersal rates of Staudtia kamerunensis (Myristicaceae). To estimate the percentages of seed dispersal failure (undispersed seeds), we quantitated fruit remains below fruiting trees distributed across five contrasting sites in a semi‐natural forest‐savanna mosaic in the Democratic Republic of Congo. We used statistical analyses accounting for spatial autocorrelation and found that forest cover in the surrounding landscape, hunting level, the associated abundance of dispersers, and fruit availability all had significant effects on the percentage of seed dispersal failure. The combination of high fruit availability and reduced abundance of seed dispersers could accelerate seed disperser satiation, causing the seed dispersal system to be saturated. Our study highlights how two major factors associated with anthropogenic activities, forest cover and hunting, affect seed dispersal by animals. These findings could have far‐reaching implications for our understanding of tree‐frugivore interactions and the conservation of tropical communities.

Importance of riparian habitat for frugivorous bats in a tropical dry forest in western Mexico

Abstract:Riparian forests (RF) provide shelter, feeding resources and commuting areas and are fundamental for local fauna during long drought periods in tropical dry forests. Nevertheless, information regarding vertebrate ecology in tropical RF is scarce. The studies of bats in riparian habitats have been mostly focused on insectivorous assemblages in non-tropical regions. We contrasted the occurrence of frugivorous bats between RF and upland dry forest (UDF) in a Neotropical dry forest region and evaluated how vegetation structure modulates frugivorous bat presence and abundance. We sampled bat ensembles and vegetation structure in three UDF and three RF sites in Jalisco, Mexico. Bat ensembles and populations were analysed with generalized linear models, including as independent factors: (1) habitat and (2) vegetation attributes. In 72 nights we captured 840 bats belonging to seven species. No effect of habitat type or vegetation structure on species richness was found, but habitat type did affect bat species composition and abundance. Moreover, vegetation structural traits affected frugivorous bat abundance. Frugivore abundance was higher in RF than in UDF probably because RF provides greater resources and more roosting sites. Riparian forests also present more canopy cover and accessible flight spaces. Our results support the importance of RF for frugivorous bats in terms of abundance and composition. Additionally, the lack of differences in species richness between UDF and RF highlights the importance of both habitats for maintaining frugivorous bats, as well as the related ecosystem processes and services in tropical dry forest landscapes.

Vertical stratification of an avian community in New Guinean tropical rainforest

AbstractVertical stratification of avian communities has been studied in both temperate and tropical forests; however, the majority of studies used ground‐based methods. In this study we used ground‐to‐canopy mist nets to collect detailed data on vertical bird distribution in primary rain forest in Wanang Conservation Area in Papua New Guinea (Madang Province). In total 850 birds from 86 species were caught. Bird abundance was highest in the canopy followed by the understory and lowest in the midstory. Overall bird diversity increased towards the canopy zone. Insectivorous birds represented the most abundant and species‐rich trophic guild and their abundances decreased from the ground to canopy. The highest diversity of frugivorous and omnivorous birds was confined to higher vertical strata. Insectivorous birds did not show any pattern of diversity along the vertical gradient. Further, insectivores preferred strata with thick vegetation, while abundance and diversity of frugivores increased with decreasing foliage density. Our ground‐to‐canopy (0–27 m) mist netting, when compared to standard ground mist netting (0–3 m), greatly improved bird diversity assessment and revealed interesting patterns of avian community stratification along vertical forest strata.

Active restoration facilitates bird community recovery in an Afrotropical rainforest

Active restoration of rainforests is essential in enhancing the recovery of biodiversity in many tropical regions suffering from deforestation. However, the value of actively restored rainforests for birds is still relatively poorly understood because of the short history of active restoration programs. Here, we studied the recovery of bird communities in Kibale National Park, Uganda. We sampled birds along a restoration gradient consisting of six restoration areas (3–16years old) and used five primary forests as reference areas. We found that bird community compositions changed in an orderly fashion along the restoration gradient. The abundance of arboreal insectivore frugivores and forest specialists increased, while that of foliage-gleaning insectivores, granivores and forest visitors decreased towards older sites. The similarity of the bird communities of the restored forests to those of the nearby primary forests increased linearly with time. If this recovery rate is extrapolated to the future, the bird communities could reach a pre-disturbance state after only approximately 20years of active restoration. However, previous studies have shown that recovery is typically a non-linear process, and the time needed for bird communities to recover is more likely longer than this. Nevertheless, our study provides evidence that bird communities benefit from active rainforest restoration after human-induced deforestation.

Landscape composition is more important than landscape configuration for phyllostomid bat assemblages in a fragmented biodiversity hotspot

Human-modified landscapes are composed of different types of land covers in differing proportions (landscape composition), and each with differing spatial physiognomy (landscape configuration). Unfortunately, the information on the relative impact of these two components of landscape structure on biological assemblages is scarce, but urgently needed to improve conservation strategies. We assessed the relative influence of the composition (landscape forest cover and matrix composition) and configuration (degree of forest fragmentation and forest edge density) of 100-ha and 500-ha landscapes on the abundance, diversity, and evenness of phyllostomid bat assemblages in the Lacandona rainforest, Mexico. We assessed the complete bat assemblage and frugivorous bats separately, and we grouped frugivores into understory foragers and canopy foragers. Landscape forest cover was the main predictor of the complete bat assemblage, positively affecting species diversity, particularly the number of rare species. Thus, community evenness decreased in landscapes with higher forest cover. Although weaker, species diversity was positively related to fragmentation and negatively associated with edge density. Landscape composition also was relatively more important than configuration for frugivores. The number of common and dominant frugivorous species and the abundance of understory frugivores increased in landscapes with lower forest cover and dominated by secondary forests in the matrix. The abundance of canopy frugivores showed the opposite response. Thus, to preserve bat assemblages and their important functional roles, conservation initiatives should prioritize a reduction in deforestation and the increase of secondary forests in the matrix. Maintaining all remaining forest patches is needed to favor landscape complementation and supplementation dynamics.

Incorporating seed fate into plant–frugivore networks increases interaction diversity across plant regeneration stages

Plant–animal mutualistic interactions, such as pollination and seed dispersal, affect ecosystem functioning by driving plant population dynamics. However, little is known of how the diversity of interactions in these mutualistic networks determines plant regeneration dynamics. To fill this gap, interaction networks should not only account for the number of seeds dispersed by animals, but also for seed fate after dispersal. Here, we compare plant–animal networks at both the seed dispersal and seedling recruitment stage to evaluate how interaction diversity, represented by different network metrics, changes throughout the process of plant regeneration. We focused on a system with six species of frugivorous birds and three species of fleshy‐fruited trees in the temperate secondary forest of the Cantabrian Range (northern Iberian Peninsula). We considered two plant cohorts corresponding to two fruiting years showing strong differences in fruit and frugivore abundance. Seed dispersal interactions were estimated from a spatially‐explicit, field‐validated model predicting tree and bird species‐specific seed deposition in different microhabitats. These interactions were further transformed into interactions at the seedling recruitment stage by accounting for plant‐ and microhabitat‐specific seed fates estimated from field sampling. We found that network interaction diversity varied across plant regeneration stages and cohorts, both in terms of the evenness and the number of paired interactions. Tree–bird interactions were more evenly distributed across species pairs at the recruitment stage than at the seed deposition stage, although some interactions disappeared in the seed‐to‐seedling transition for one plant cohort. The variations in interaction diversity were explained by between‐plant differences in post‐dispersal seed fate and in inter‐annual fruit production, rather than by differences between frugivores in seed deposition patterns. These results highlight the need for integrating plant traits and disperser quality to predict the functional outcome of plant–animal mutualistic networks.

Frugivory in Canopy Plants in a Western Amazonian Forest: Dispersal Systems, Phylogenetic Ensembles and Keystone Plants.

Frugivory is a widespread mutualistic interaction in which frugivores obtain nutritional resources while favoring plant recruitment through their seed dispersal services. Nonetheless, how these complex interactions are organized in diverse communities, such as tropical forests, is not fully understood. In this study we evaluated the existence of plant-frugivore sub-assemblages and their phylogenetic organization in an undisturbed western Amazonian forest in Colombia. We also explored for potential keystone plants, based on network analyses and an estimate of the amount of fruit going from plants to frugivores. We carried out diurnal observations on 73 canopy plant species during a period of two years. During focal tree sampling, we recorded frugivore identity, the duration of each individual visit, and feeding rates. We did not find support for the existence of sub assemblages, such as specialized vs. generalized dispersal systems. Visitation rates on the vast majority of canopy species were associated with the relative abundance of frugivores, in which ateline monkeys (i.e. Lagothrix and Ateles) played the most important roles. All fruiting plants were visited by a variety of frugivores and the phylogenetic assemblage was random in more than 67% of the cases. In cases of aggregation, the plant species were consumed by only primates or only birds, and filters were associated with fruit protection and likely chemical content. Plants suggested as keystone species based on the amount of pulp going from plants to frugivores differ from those suggested based on network approaches. Our results suggest that in tropical forests most tree-frugivore interactions are generalized, and abundance should be taken into account when assessing the most important plants for frugivores.

Intensive land-use and high native fruit availability reduce fruit removal of the invasive Solanum mauritianum in South Africa

Invasive plant species threaten ecosystems worldwide. Management of invasive plants considerably benefits from an understanding of the processes that drive invasive spread, like seed dispersal. In the subtropics, many invasive species rely on vertebrate frugivores for dispersal. In turn, frugivore distribution across the landscape depends on habitat and fruit availability. Therefore, the loss of habitat owing to land-use change, and low fruit availability may reduce the local frugivore abundance and thereby impact invasive fruit removal and spread. However, the interplay among these factors remains poorly understood. In the intensively used agricultural landscape in KwaZulu-Natal, eastern South Africa, we investigated the effects of land-use and native and invasive fruit availability on frugivore abundance and analyzed how potential changes translate into fruit removal rates of the invasive fleshy-fruiting shrub Solanum mauritianum Scopoli (Solanaceae). Increasing land-use intensity reduced frugivore abundance, which in turn reduced fruit removal rates of the invasive species. Increasing native fruit availability did not influence frugivore abundance, but reduced fruit removal rates of S. mauritianum, indicating a fruit preference of frugivores for the simultaneously fruiting native species. In conclusion, fruit removal of the species is highest in locations with low land-use and low native fruit availability. Thus, these areas should be priority areas for invasive species management. Additionally, plantings of native fruiting species may provide alternative preferred food resources, reduce invasive fruit removal and ultimately improve invasive species management.

Response of frugivorous primates to changes in fruit supply in a northern Amazonian forest.

Few attempts have been made to understand how spatiotemporal changes in fruit supply influence frugivores in tropical forests. The marked spatiotemporal variation in fruit supply can affect frugivore abundance and distribution, but studies addressing the effects of this variation on primates are scarce. The present study aimed to investigate how the spatiotemporal distribution of fruits influences the local distribution of three frugivorous primates in the eastern part of the Maracá Ecological Station, a highly seasonal Amazonian rainforest. Specifically, it was hypothesised that primate distribution will track changes in fruit supply, resulting that sites with high fruit availability should be heavily used by primates. During a 1-year study, fruit supply (ground fruit surveys) and primate density (line-transects) were monitored in twelve 2 km-long transects at monthly intervals. Fruit supply varied seasonally, being low during the dry season. The density of Ateles belzebuth was positively related to fruit supply during fruit shortage, but Cebus olivaceus and Alouatta macconnelli did not follow the same pattern. The supply of Sapotaceae fruit was an important component determining local distribution of A. belzebuth during the overall fruit shortage. Highly frugivorous primates such as A. belzebuth respond to seasonal decline in fruit supply by congregating at places with high fruit supply in this forest, particularly, those with many individuals of species of Sapotaceae. This study underscores the importance of small-scale spatiotemporal changes of fruit supply as a key component of frugivorous primate ecology in highly seasonal environments.

Landscape context mediates avian habitat choice in tropical forest restoration.

Birds both promote and prosper from forest restoration. The ecosystem functions birds perform can increase the pace of forest regeneration and, correspondingly, increase the available habitat for birds and other forest-dependent species. The aim of this study was to learn how tropical forest restoration treatments interact with landscape tree cover to affect the structure and composition of a diverse bird assemblage. We sampled bird communities over two years in 13 restoration sites and two old-growth forests in southern Costa Rica. Restoration sites were established on degraded farmlands in a variety of landscape contexts, and each included a 0.25-ha plantation, island treatment (trees planted in patches), and unplanted control. We analyzed four attributes of bird communities including frugivore abundance, nectarivore abundance, migrant insectivore richness, and compositional similarity of bird communities in restoration plots to bird communities in old-growth forests. All four bird community variables were greater in plantations and/or islands than in control treatments. Frugivore and nectarivore abundance decreased with increasing tree cover in the landscape surrounding restoration plots, whereas compositional similarity to old-growth forests was greatest in plantations embedded in landscapes with high tree cover. Migrant insectivore richness was unaffected by landscape tree cover. Our results agree with previous studies showing that increasing levels of investment in active restoration are positively related to bird richness and abundance, but differences in the effects of landscape tree cover on foraging guilds and community composition suggest that trade-offs between biodiversity conservation and bird-mediated ecosystem functioning may be important for prioritizing restoration sites.

Phyllostomid bat occurrence in successional stages of neotropical dry forests.

Tropical dry forests (TDFs) are highly endangered tropical ecosystems being replaced by a complex mosaic of patches of different successional stages, agricultural fields and pasturelands. In this context, it is urgent to understand how taxa playing critical ecosystem roles respond to habitat modification. Because Phyllostomid bats provide important ecosystem services (e.g. facilitate gene flow among plant populations and promote forest regeneration), in this study we aimed to identify potential patterns on their response to TDF transformation in sites representing four different successional stages (initial, early, intermediate and late) in three Neotropical regions: México, Venezuela and Brazil. We evaluated bat occurrence at the species, ensemble (abundance) and assemblage level (species richness and composition, guild composition). We also evaluated how bat occurrence was modulated by the marked seasonality of TDFs. In general, we found high seasonal and regional specificities in phyllostomid occurrence, driven by specificities at species and guild levels. For example, highest frugivore abundance occurred in the early stage of the moistest TDF, while highest nectarivore abundance occurred in the same stage of the driest TDF. The high regional specificity of phyllostomid responses could arise from: (1) the distinctive environmental conditions of each region, (2) the specific behavior and ecological requirements of the regional bat species, (3) the composition, structure and phenological patterns of plant assemblages in the different stages, and (4) the regional landscape composition and configuration. We conclude that, in tropical seasonal environments, it is imperative to perform long-term studies considering seasonal variations in environmental conditions and plant phenology, as well as the role of landscape attributes. This approach will allow us to identify potential patterns in bat responses to habitat modification, which constitute an invaluable tool for not only bat biodiversity conservation but also for the conservation of the key ecological processes they provide.

Fruit removal rate depends on neighborhood fruit density, frugivore abundance, and spatial context

Fleshy-fruited plants depend fundamentally on interactions with frugivores for effective seed dispersal. Recent models of frugivory within spatially explicit networks make two general predictions regarding these interactions: rate of fruit removal increases (i.e., is facilitated) as densities of conspecific neighborhood fruits increase, and fruit removal rate varies positively with frugivore abundance. We conducted a field experiment that constitutes the first empirical and simultaneous test of these two primary predictions. We manipulated neighborhood abundances of arrowwood (Viburnum recognitum and Viburnum dentatum) fruits in southern New England's maritime shrub community and monitored removal rates by autumn-migrating birds. Focal arrowwood plants in neighborhoods with high conspecific fruit density sustained moderately decreased fruit removal rates (i.e., competition) relative to those in low-density neighborhoods, a result that agrees with most field research to date but contrasts with theoretical expectation. We suggest the spatial contexts that favor competition (i.e., high-abundance neighborhoods and highly aggregated landscapes) are considerably more common than the relatively uniform, low-aggregation fruiting landscapes that promote facilitation. Patterns of arrowwood removal by avian frugivores generally varied positively with, and apparently in response to, seasonal changes in migratory frugivore abundance. However, we suggest that dense stands of arrowwood concentrated frugivore activity at the neighborhood scale, thus counteracting geographic patterns of frugivore abundance. Our results underscore the importance of considering spatial context (e.g., fruit distribution and aggregation, frugivory hubs) in plant-avian frugivore interactions.

Effects of habitat alteration on the effectiveness of plant-avian seed dispersal mutualisms: Consequences for plant regeneration

Abstract Anthropogenic habitat alteration may affect the dispersal service provided by avian seed dispersers, ultimately causing regeneration collapse, through a decay in both the quantitative (seed removal) and qualitative (seed arrival to safe sites) components of seed dispersal effectiveness. However, despite its implications for management in real-world landscapes, few studies have investigated the shifts in components of seed dispersal effectiveness resulting from habitat alteration. We advocate the use of stage-specific transition probabilities, combined with data on seed shadows and bird abundance and mobility, for a mechanistic inference of the consequences for recruitment of the disruption of plant-frugivore mutualism in altered habitats. Such an approach allows the identification of regeneration bottlenecks, evaluates the differential contribution to recruitment of quantitative and qualitative components of seed dispersal, and provides the means to compare seed dispersal limitation. We exemplify our conceptual approach with studies of seed dispersal and recruitment in the wild olive tree in unaltered and severely altered adjacent sites. We show that simplification of the habitat substantially affected bird abundance, diversity and mobility, which caused a reduction in fruit removal and a concomitant simplification of the seed shadows compared to the unaltered site. Linked to these shifts, postdispersal seed survival and seedling emergence and survival were affected. The final outcome of habitat alteration was the collapse of the regeneration dynamics with very few seeds escaping the influence of maternal plants and reaching the safest sites for recruitment (dispersal limitation). As predicted, the collapse in the regeneration dynamics resulted from severe decays in the quantitative but especially in the qualitative components of seed dispersal effectiveness. Management of fleshy-fruited plant populations in altered habitats should thus pay attention to landscape elements that promote frugivore abundance, diversity and mobility and that alleviate the dispersal limitation.

Avian frugivore assemblages on Celtis gomphophylla, Baker, in Budongo Forest Reserve, Uganda

AbstractThe relationship between selective logging and avian frugivores feeding on Celtis gomphophylla was examined in Budongo Forest Reserve, from June to July 2005, using scan sampling. Abundance and species richness of avian frugivores were compared between two compartments (N3 and W22 last logged in 1952 and 1997 respectively). Two 1‐ha plots were established in each compartment from which two C. gomphophylla trees selected for observation. A total of 203 avian frugivores comprising 17 species, visited the eight C. gomphophylla trees. Avian frugivore abundance and richness were significantly higher in N3 than W22 (χ2 = 5.83, χ2 = 0.03, P < 0.05). Fischer's alpha diversity index also indicated a higher avian frugivore (frugivory?) in N3 than that in W22. The Sorensen's similarity index showed that species composition between the two compartments was moderately similar. The diversity of forest specialists and generalists was not significantly different in the two compartments (F = 0.3451, P = 0.082 and F = 0.368, P = 0.553). Our results stress the significance of logging intensity on avian feeding guilds and confirm that forests which have had enough recovery time are better habitats for avian frugivore assemblages.

Logging and forest edges reduce redundancy in plant–frugivore networks in an old‐growth European forest

Summary Seed dispersal by frugivores is the basis for regeneration of fleshy‐fruited plants in forest ecosystems. Previous studies have reported a decrease in forest specialist frugivores due to logging and forest edges. Forest generalists appear less sensitive and may even increase at forest edges. Such changes in the abundance of frugivores may have consequences for consumer/resource ratios and competition in plant–frugivore networks. Optimal foraging theory predicts an increase in dietary specialization of animals at low consumer/resource ratios due to reduced competition. A decrease in forest specialists in logged forests should cause decreased consumer/resource ratios, increased dietary specialization and reduced redundancy, whereas an increased abundance of forest generalists at edges may compensate for a loss of specialists. In Europe's last old‐growth lowland forest (Białowieża, Eastern Poland), we recorded fruit removal by frugivores from fleshy‐fruited plant species in the interior and at edges of logged and old‐growth forests for 2 consecutive years. The abundance of forest generalists increased at forest edges, whereas specialists were unaffected. Conversely, logging resulted in a decrease in abundance of forest specialists but had no effect on the abundance of generalists. Accordingly, consumer/resource ratios increased from interior to edges and were reduced in the interior of logged forests compared with the interior of old‐growth forests. As predicted by optimal foraging theory, a decrease in consumer/resource ratios coincided with increased dietary specialization and a loss of redundancy in the interior of logged forests. Despite low dietary specialization, redundancy was reduced at forest edges as forest generalists dominated plant–frugivore interactions. Synthesis. We show that a shift in frugivore assemblages at forest edges and increased dietary specialization of frugivores in the interior of logged forests involved a loss of redundancy compared with continuous old‐growth forests. This suggests that seed dispersal services in secondary forest habitats depend on an impoverished subset of dispersal vectors and may suffer reduced adaptive potential to changing environmental conditions. Thus, our study highlights the value of old‐growth forests for the conservation of frugivore‐mediated seed dispersal processes.

Seasonal fluctuations of resource abundance and avian feeding guilds across forest–farmland boundaries in tropical Africa

Seasonal fluctuations in climatic factors are expected to increase in future decades. However, little is known about the response of tropical species communities to seasonal fluctuations in climate and resource availability, particularly across different habitat types. We examined the relationship between spatio‐temporal fluctuations in the abundance of fruits and invertebrates and two avian feeding guilds, i.e. frugivores and insectivores, in forest and farmland habitats in western Kenya. Fruits and invertebrates fluctuated substantially throughout the year, but seasonal fluctuations were asynchronous between the two habitat types. Species richness and total abundance of frugivores and insectivores also fluctuated strongly and were closely related to the abundance of their respective resources. Frugivore species richness fluctuated anti‐cyclical in forest and farmland habitats, suggesting that several frugivorous species tracked fruit resources across habitat boundaries. In contrast, insectivorous bird richness fluctuated synchronously in the two habitat types, suggesting a lack of local‐scale movements across habitat boundaries. We conclude that bird communities strongly respond to seasonal fluctuations in resource availability, but responses differ between feeding guilds. While frugivores seem to respond flexibly to seasonal fluctuations, for instance by tracking fruit resources across habitat boundaries, insectivorous birds appear to be more susceptible to the expected increase in seasonal fluctuations in resource availability.

Shifts in community structure of tropical trees and avian frugivores in forests recovering from past logging

Logging is one of the most pervasive threats to biodiversity in tropical forests. In this study, we concurrently examined the responses of the avian frugivore community and the community of fruiting trees to past logging in a tropical forest in northeastern India. We predicted that the abundance of, and visitation by large-bodied frugivores would be lower in logged forests, resulting in reduced seed dispersal and recruitment of biotically dispersed species (especially large-seeded species). Data were collected from two logged and unlogged sites using (a) belt transects for estimating avian frugivore abundance (b) fruiting tree watches to quantify frugivore visitation and (c) vegetation plots to characterize recruitment. Our results show that differences in the abundance of bird species between logged and unlogged sites was correlated with body mass, with larger species being scarcer and smaller species more abundant in logged areas. Correspondingly, visitation rates by large avian frugivores was lower in logged compared with unlogged sites. Finally, biotically dispersed tree species, especially large-seeded species, had lower recruitment in logged forests, potentially due to lowered visitation rates and decreased fruit removal by large bodied avian frugivores. In the long term, these differences in recruitment may result in a tree community shift towards small-seeded biotically dispersed species and abiotically dispersed species in logged areas.