When the world turned brown: Over browsing, apparent competition and the “Miocene Transformation”

Background/Question/Methods

The Miocene (23.5- 5.8 Ma) was a key epoch during which the mostly forested, terrestrial biomes of the early Cenozoic were transformed into the more open forest and grassland biomes characteristic of the Pliocene- Quaternary Period of the late Cenozoic. Though most of the plants and mammalian lineages that dominated terrestrial biomes during the Quaternary were already present during the Eocene-Oligocene epochs, the events that enabled these assemblages to dominate came to fruition during the Miocene (the “Miocene Transformation”). Current theories suggest that climate change was the main factor driving this transformation. Yet the evidence shows that the decline in forested habitat, the decrease in the number of browsing species, and the increases in the diversities of mixed-feeding and grazing species already began when the world temperature was rather stable. I propose an alternative, biotic-based hypothesis, which suggests that two factors, (a) over-browsing of the forest resulting in intensive competition for food between the giant browsers and the newly evolved mixed-feeding giant Proboscidea, and (b) the appearance of habitat generalist carnivores, mainly felids, which exerted apparent competition between small to large size forest-dwelling browsers and grazing, open grassland, species, were mainly responsible for the Miocene transformation.

Results/Conclusions

The size race between mammalian carnivores and herbivores that started at the late Paleocene (60 Ma) has stabilized 45 Ma when carnivores reached their maximal, physiologically possible, size of 500-1000kg and herbivores size reached 8-18 ton, a size that made these mega-herbivores almost immune to predation. This size ratio of about 1:10 was kept ever since resulting it first browsers and then mix-feeders over-browsing forest and control the trees turning the forested habitat from its “green” HSS state (where plant are resource limited) to a “brown” Fretwellian state (where plants are controlled by herbivores). This resulted in first opening of the forested biomes and then enhancing the spread of grassland biomes and gave advantage to habitat generalist mix-feeders and carnivores enabling them to replace their forest specialist competitors. The rich and divers browsers fauna of the Miocene was driven to extinction, by being caught between intensive food competition by mixed-feeders and over-exploitation by habitat generalist carnivores. The simultaneous development of a cooler, more seasonal climate may have enhanced this process, but its effect was probably secondary to the deadly combination of increased browser diversity with decreased forest cover that characterized the Miocene transformation.