Relative abundance distributions reveal constraints on tetrapod community diversity

Previous efforts to understand the pace of species diversification through time have disagreed over whether species diversity is unbounded or limited by a carrying capacity. Tetrapods (limbed vertebrates) have frequently been employed as a case-study in examinations of this issue, with studies of diversity being used to support both sides. Here, we examine the shape of relative abundance distributions (RADs) within terrestrial tetrapod communities between the Carboniferous and the Eocene to assess their ecological structure of tetrapod communities, assessing whether the RADs are consistent with “simple” models characteristic of finite resources distributed among species, or more “complex” models where more ecological processes permit greater functional diversity. Our results indicate that terrestrial tetrapod communities have remained consistent in the shape of their RADs since the establishment of ecological limits in the late Carboniferous, albeit with evidence of an increase in global tetrapod diversity at the Cretaceous/Paleogene boundary. Throughout much of the interval studied, the RADs of tetrapod communities best fit “simple” models indicative of finite resources. This contrasts with findings for the marine realm, where a substantial increase in community complexity is found following the end-Permian mass extinction. These results indicate that terrestrial and marine ecosystems are governed by different limits on their ecological complexity, with tetrapod communities being governed by finite ecological limits.