Species richness and speciation rates for all terrestrial animals emerge from a synthesis of ecological theories

The total number of species on earth and the rate at which new species are
created are fundamental questions for ecology, evolution and conservation.
These questions have typically been approached separately, despite their
obvious interconnection. In this study we approach both questions in
conjunction, for all terrestrial animals. To do this, we combine two
previously unconnected bodies of theory: general ecosystem models and
individual based ecological neutral theory. General ecosystem models
provide us with estimated numbers of individual organisms, separated by
functional group and body size. Neutral theory, applied within a guild of
functionally similar individuals, connects species richness, speciation
rate and number of individual organisms. In combination, for terrestrial
endotherms where species numbers are known, they provide us with estimates
for speciation rates as a function of body size and diet class.
Extrapolating the same rates to guilds of ectotherms enables us to
estimate the species richness of those groups, including species yet to be
described. We find that speciation rates per species per million years
decrease with increasing body size. Rates are also higher for carnivores
compared to omnivores or herbivores of the same body size. Our estimate
for the total number of terrestrial species of animals is in the range
1.03-2.92 million species, a value consistent with estimates from previous
studies, despite having used a fundamentally new approach. Perhaps what is
most remarkable about these results is that they have been obtained using
only limited data from larger endotherms and their speciation rates, with
the predictive process being based on mechanistic theory. This work
illustrates the potential of a new approach to classic eco-evolutionary
questions, while also adding weight to existing predictions. As we now
face an era of dramatic biological change, new methods will be needed to
mechanistically model global biodiversity at the species and individual
organism level. This will be a huge challenge but the combination of
general ecosystem models and neutral theory that we introduce here is a
way to tractably achieve it.