Terrestrial food web complexity in Amazonian forests decays with habitat loss

The conversion of natural ecosystems into human-modified landscapes (HMLs) is the main driver of biodiversity loss in terrestrial ecosystems.1,2,3 Even when species persist within habitat remnants, populations may become so small that ecological interactions are functionally lost, disrupting local interaction networks.4,5 To uncover the consequences of land use changes toward ecosystem functioning, we need to understand how changes in species richness and abundance in HMLs6,7,8 rearrange ecological networks. We used data from forest vertebrate surveys and combined modeling and network analysis to investigate how the structure of predator-prey networks was affected by habitat insularization induced by a hydroelectric reservoir in the Brazilian Amazonia.9 We found that network complexity, measured by interaction diversity, decayed non-linearly with decreasingly smaller forest area. Although on large forest islands (>100 ha) prey species were linked to 3–4 potential predators, they were linked to one or had no remaining predator on small islands. Using extinction simulations, we show that the variation in network structure cannot be explained by abundance-related extinction risk or prey availability. Our findings show that habitat loss may result in an abrupt disruption of terrestrial predator-prey networks, generating low-complexity ecosystems that may not retain functionality. Release from predation on some small islands may produce cascading effects over plants that accelerate forest degradation, whereas predator spillover on others may result in overexploited prey populations. Our analyses highlight that in addition to maintaining diversity, protecting large continuous forests is required for the persistence of interaction networks and related ecosystem functions. [Display omitted] •Functional extinctions have changed the fauna of land-bridge islands in the Amazon•The complexity of predator-prey networks decays with decreasing forest area•Several species have no potential predators or fewer prey on small forest islands•Forest loss in the Amazon generates simplified dysfunctional food webs Investigating the vertebrate fauna of islands formed by a reservoir in Amazonia, Pires et al. learn how interaction networks change with forest size. Their results indicate that abundance declines on small islands reduce network complexity and disrupt trophic structure. These results show that small forest remnants host dysfunctional food webs.