Modelling the joint effects of predator and prey diversity on total prey biomass

Abstract

Summary Niche differentiation is widely thought to promote both coexistence of many species within a trophic level, and higher total biomass of those species. However, the joint effects of changes in diversity within multiple trophic levels are unknown. The potential for niche differences to promote high diversity and biomass within a trophic level may depend on the diversity and composition of adjacent trophic levels. Using a simple mechanistic model, I examine the joint consequences of changes in diversity on two adjacent trophic levels (‘predators’ and ‘prey’) for the total biomass of prey. Niche differences among prey reflect a trade‐off between ability to compete for resources and ability to avoid/sustain predation. Niche differences among predators reflect differences in ability to consume different prey species. Total prey biomass is a complex, joint function of prey and predator diversity. In particular, transgressive overyielding (when a diverse polyculture of prey outperforms any prey monoculture) can occur for some parameter values when a specialist predator mediates prey coexistence, and for a different set of parameter values when a specialist and a generalist predator together mediate prey coexistence, but is impossible when a generalist predator alone mediates prey coexistence. Increased initial predator diversity tends to reduce total prey biomass even when predators cannot coexist, due to a ‘selection’ effect for the most effective predator. Predators coexist only in prey polycultures. Coexisting predators can, but do not necessarily, reduce total prey biomass to a lower level than could any predator alone. The niche differences that permit coexistence strongly constrain the rank order of total prey biomass across all prey and predator combinations. Knowledge of the key trade‐offs that promote coexistence within food webs can allow powerful qualitative predictions of the effects of species loss on ecosystem‐level properties such as total biomass.