Resource unpredictability promotes species diversity and coexistence in an avian scavenger guild: a field experiment

Chance per se plays a key role in ecology and evolution, e. g., genetic mutation, resource spatiotemporal unpredictability. In community ecology, chance is recognized as a key factor in community assemblage, but less is known about its role in intraguild processes leading to species coexistence. Here we study the relevance of resource unpredictability per se as a promoter of intraguild positive interspecific interactions and as a biodiversity enhancer in an Old World avian scavenger guild, which has evolved to feed upon spatially and temporally unpredictable resources, i.e., carcasses. We performed a large-scale field experiment in which 58 carcasses were disposed of and observed until complete consumption, either in continuously active supplementary feeding stations (predictable carcasses) or disposed of at random in the field (unpredictable carcasses). Richness of scavenger species was similar at unpredictable and predictable carcasses, but their relative abundances were highly uneven at predictable carcasses leading to higher scavenger diversity (Shannon index) at unpredictable carcasses. Facilitatory interspecific processes only occurred at unpredictable resources but were disrupted in predictable conditions because the dominant specialist species (in our case, the Griffon Vulture Gyps fulvus) arrived earlier and in larger numbers, monopolizing the resource. Small, endangered scavengers congregated at supplementary feeding stations but profited less compared to unpredictable carcasses, suggesting that they could constitute an ecological trap. Our findings offer new insights into the relevance of unpredictability of trophic resources in promoting both positive facilitatory interspecific interactions and species diversity and thus maintaining the function of guilds. Finally, the preservation of randomness in resource availability and the processes associated with its exploitation should be a major goal of conservation strategies aimed to preserve scavenger guilds evolved under naturally unpredictable trophic resources.