Living with the elephant in the room: Top-down control in Eurasian large mammal diversity over the last 22 million years

Mammalian megaherbivores are an essential component of modern day ecosystems. They control a large share of primary productivity, modify habitats by clearing dense vegetation, and keep other herbivores populations small. When rare, small herbivores additionally suffer increased extinction risk by predation. Since megaherbivores are almost immune to predation, it was suggested that the detrimental effect of megaherbivores on smaller competitors has the additional consequence to reduce the prey biomass available to predatory species, thereby increasing their extinction risk. This form of indirect competition between guilds of species is known as apparent competition. These mechanisms are all forms of top-down control on ecosystem functioning, whereby species up on the food chain control diversity at lower levels. We tested these competing hypotheses on a large compilation of Neogene to Recent Old World mammals. We found evidence in favor of apparent competition. However, direct competition effects by both megaherbivores and carnivores on small prey are even more pervasive. Our results suggest that megaherbivores have been a dominant component of the mammal diversity over time and space during the last 22 million years in Eurasia.