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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. © 2017 Elsevier B.V.

Body mass has important physiological and ecological implications. The high degree of variability in herbivore mammals' body mass is linked to differences in their gut anatomy, which, in turn, has important effects on their feeding habits. We aimed to untangle the effects of Cope's and Bergmann's rule on the evolution of body size in even-toed hoofed mammals (Artiodactyla, Mammalia). By using variation partitioning techniques and direct sister species comparisons, we estimated the potential effects of latitude, time and phylogeny on body size variation in Cenozoic Artiodactyls. We tested whether species tend to be larger when living at higher latitudes as compared to their phylogenetically closest relative, whether closely-related species living in sympatry tend to diverge more in body size, and whether recent species tend to be larger than their older relatives. Our results showed that both latitudinal and temporal trends in Artiodactyla body size are evident throughout the Cenozoic, and became particularly intense during the coldest part of it. Artiodactyls tend to be larger than their close relatives when living at higher latitudes, in keeping with Bergmann's rule. Yet, this relationship is weakened by the strong tendency for sister species to diverge in body size when living nearby. We also found a clear tendency for late Artiodactyl species to be larger, and live for shorter, than their ancestors.Thus, there is strong evidence that the evolution of body size in even-toed hoofed mammals was influenced by climatic changes, and by an overall tendency to grow larger, mostly in coincidence with the vegetation shift from forested to grassland biomes which characterized the second half (Neogene) of the Cenozoic. Although Cope's and Bergmann's rule in mammals are often portrayed as having climate change as a common explanation, our data suggest they are mechanistically separate phenomena. © 2015 Elsevier B.V.