Defaunation effects on plant recruitment depend on size matching and size trade-offs in seed-dispersal networks

Abstract

Defaunation by humans causes a loss of large animals in many ecosystems globally. Recent work has emphasized the consequences of downsizing in animal communities for ecosystem functioning. However, no study so far has integrated network theory and life-history trade-offs to mechanistically evaluate the functional consequences of defaunation in plant-animal networks. Here, we simulated an avian seed-dispersal network and its derived ecosystem function seedling recruitment to assess the relative importance of different size-related mechanisms. Specifically, we considered size matching (between bird size and seed size) and size trade-offs, which are driven by differences in plant or animal species abundance (negative size-quantity relationship) as well as in recruitment probability and disperser quality (positive size-quality relationship). Defaunation led to impoverished seedling communities in terms of diversity and seed size, but only if models accounted for size matching. In addition, size trade-off in plants, in concert with size matching, provoked rapid decays in seedling abundance in response to defaunation. These results underscore a disproportional importance of large animals for ecosystem functions. Downsizing in ecological networks will have severe consequences for ecosystem functioning, especially in interaction networks that are structured by size matching between plants and animals.

Predicting the non‐linear collapse of plant–frugivore networks due to habitat loss

“…We used the minimum percentage of forest cover where each species was recorded as a direct assessment of species sensitivity to habitat loss. However, it would be interesting to investigate other sequences of extinctions built as functions of different variables that have been associated with extinction proneness, such as body size (Donoso et al 2017). Regarding particular species traits, another issue to be better explored in future studies is the phylogenetic relationship among species that go extinct (Rezende et al 2007).…”

“…This is because different species vary in the way they respond to habitat loss, with several studies now showing the existence of losers and winners in humanmodified ladscapes (Laurance et al 2006, Banks-Leite et al 2012, Tabarelli et al 2012). In the case of birds, a variety of traits, such as body size, habitat and diet specialization, migratory status and generation length, have been associated to extinction proneness (Newbold et al 2013, Donoso et al 2017). However, extinction risk is often a synergistic function of both intrinsic species traits and the nature of threat (Dirzo et al 2014).…”

The distribution of plants and seed dispersers in response to habitat fragmentation in an artificial island archipelago

“…Large‐bodied mammals and birds are critical for the maintenance of intact plant communities because they contribute to the efficient seed dispersal of large‐seeded plants and help to mitigate the impact of negative density dependence (Caughlin et al., ). Loss of large seed dispersers may result in a shift in the seed rain and seed bank towards plant species with small seeds (de Melo, Dirzo, & Tabarelli, ; Harrison et al., ), leading to limited recruitment of large‐seeded plants (Cramer, Mesquita, & Williamson, ; Magnago et al., ) and eventually decreasing the community level seed size (Donoso, Schleuning, García, & Fründ, ; Osuri & Sankaran, ). This is a rapid process, with population seed sizes of a species decreasing markedly even within a time span less than 100 years (Galetti et al., ).…”

Complementary Effects of Species Abundances and Ecological Neighborhood on the Occurrence of Fruit-Frugivore Interactions

“…A stronger role of neutral mechanisms in predicting pairwise interactions was expected from our system, a typical temperate environment transformed by post-glacial human activity, usually composed of few and mostly generalist species, and in which all the bird species under study are congeners (i.e., they might exhibit less trait variability than those presented by González-Castro et al, 2015). However, trait matching could be expected to be a pivotal mechanism in those cases in which trait variation among species is larger, and where coevolutionary or long-term processes of ecological fitting have been operating (e.g., in tropical contexts; Dehling et al, 2014;Bender et al, 2017;Donoso et al, 2017). Nontheless, incorporating interaction identity in our model was essential since it would have revealed any possible preference between species.…”

“…Secondly, there is a deterministic process driven by a matching of traits (e.g., phenology, morphology, behavior, etc.) that finally permits the interaction between cooccurring species (Maglianesi et al, 2014;Gonzalez and Loiselle, 2016;Donoso et al, 2017). Although both mechanisms are not mutually exclusive, neutral processes are usually expected from systems dominated by generalist species (e.g., González-Castro et al, 2015), whereas niche-driven processes are expected to be more relevant in those systems where high biodiversity involves a large trait variation among species (e.g., in tropical contexts, Bender et al, 2017).…”