Local extinctions of obligate frugivores and patch size reduction disrupt the structure of seed dispersal networks

Abstract

A central problem in ecology is to understand how human impacts affect plant-animal interactions that lead to effective seed dispersal services for plant communities. Seed dispersal services are the outcome of plant-frugivore interactions that often form local networks of interacting species. Recent work has shown that some frugivorous bird species are more critical to network organization than others. Here, we explore how patch size and the potential local extinctions of obligate frugivorous birds affect the reorganization of seed dispersal networks. We examined the structure of 20 empirical seed dispersal networks documented across tropical avian assemblages occupying widely variable habitat patch sizes, a surrogate of the amount of remaining habitat. Networks within small forest patches consistently supported both lower plant and bird species richness. Forest patch size was positively associated with nestedness, indicating that reductions in patch size disrupted the nested organization of seed dispersal networks. Obligate frugivores, especially large-bodied species, were almost entirely absent from small forest patches. Analysis at the species level showed that obligate frugivores formed the core of interacting species, connecting species within a given seed dispersal network. Our combined results revealed that patch size reduction erodes frugivorous bird diversity, thereby affecting the integrity of seed dispersal networks. We highlight the importance of conserving large forest patches to maintain tropical forest functionality.

Fragmented tropical forests lose mutualistic plant–animal interactions

“…Earlier studies have illustrated that individual interactions within networks can be gained or lost as a response to habitat alterations (de Assis Bomfim et al, ), even when changes in the species composition remain indistinguishable (Nielsen & Totland, ). Therefore, structural changes in the network, namely rewiring, could mitigate some direct and short‐term effects of fragmentation on tree seed dispersal by the replacement of extinct interaction links.…”

“…Finally, we computed for each site the proportion of interactions that were provided by the keystone seed dispersers identified by the model‐based approach (KEY) and by the traditional approach. We chose these four interaction metrics (CON, SA, KEY, NODF) because they represent key characteristics of the seed dispersal network structure and can offer complementary information on fragmentation effects on the interactions that tropical trees heavily depend on (de Assis Bomfim et al, ; Delmas et al, ; Vidal et al, ).…”

“…We note that these regression models do not account for possibly spatially autocorrelated residuals, and thus they should be considered to examine realized patterns of covariation between fragmentation and seed dispersal rather than causal links between these. Finally, species richness has been found to be an important driver of species interaction numbers (de Assis Bomfim et al, ). Thus, we assessed the relationships between the posterior means of interaction metrics and species richness in each prediction site with linear regressions.…”

“…In forest ecosystems, frugivorous animals, particularly birds and large mammals, are the most important seed dispersers (Fleming & Kress, ; Sebastián‐González, ), but they face rapid decline due to forest fragmentation and defaunation (de Assis Bomfim, Guimarães, Peres, Carvalho, & Cazetta, ; Galetti et al, ; Nagy‐Reis et al, ). Large mammals can disperse a magnitude of large seeds over long distances (Vidal, Pires, & Guimarães, ), whereas small‐ and medium‐sized birds are limited to small seeds due to trait matching (Bender et al, ), but tend to be more abundant and thus interact more frequently and with a higher number of plant species.…”

The underappreciated role of rodent generalists in fungal spore dispersal networks

“…Identifying the characteristics of species that are relatively more important within dispersal networks is essential for understanding patterns in network structure (Sebastián‐González ). Dietary specialization is commonly among the best predictors of a species’ importance within seed dispersal networks, with species that consume fruits as their primarily food source contributing the greatest to seed dispersal (Pigot et al , Sebastián‐González , de Assis Bomfim et al ). However, species with more generalized diets have occasionally been shown to be the most important dispersers, particularly if they can move seeds long distances (Donatti et al , Escribano‐Avila et al ) or if they occur in habitats not occupied by specialists (Carlo and Morales ).…”