Trophic cascades and plant-frugivore mutualisms: Ecological consequences of rewilding apex predators on Mediterranean ecosystems

Abstract

Trophic cascades are powerful mechanisms structuring ecosystems across food webs, ultimately affecting primary producers. Apex predators play key roles in ecosystems and can trigger cascading effects across lower trophic levels. Therefore, given the recent recovery of large carnivores worldwide through active and passive rewilding, assessing the potential ecological effects of reintroducing top-order predators in ecosystems is paramount to gain insights on the functioning of trophic webs and the correct enforcement of conservation measures. Plant-animal interactions can be altered by the novel presence of apex predators, potentially affecting relevant functions that subordinate species maintain in ecosystems. For instance, mutualistic and antagonistic interactions such as zoochory or herbivory can be affected by trophic cascades, with contrasting consequences for the growth, survival and reproduction of many plant populations. This is especially important in defaunated ecosystems, where some opportunistic frugivores play the role of legitimate seed dispersers for many large-fruited plants due to the historical or recent extinction of frugivore megafauna. Such is the case of frugivorous mesocarnivores, which are functionally unique seed dispersers for many Mediterranean large fleshy-fruited plants, but whose abundance and behaviour is often controlled by larger predators via intra-guild competition. Small mammals also play a double role in trophic webs, acting as prey for subordinate carnivores but also as seed predators for many plants. Thus, the quantity (i.e. number of dispersed seeds) and the quality (e.g., post-dispersal seed survival, seedling survival) of the seed dispersal service that mammal frugivores provide to plants may be modified under predation risk across the successive post-dispersal stages, ultimately affecting plant demography, community composition and forest regeneration processes. The current global-change scenario demands research on how human-mediated ecological changes affect ecosystem functioning and the entire dynamics of food webs.