Ricklefs, R. E. (2010). Evolutionary diversification, coevolution between populations and their antagonists, and the filling of niche space. Proceedings of the National Academy of Sciences, 107(4), 1265–1272. http://doi.org/10.1073/pnas.0913626107
It is difficult to think about ecological niches without considering the consequences for species coexistence and biodiversity. Stemming from this is the idea of “niche filling”, in that a finite niche exists and because one species is already “filling” it, another cannot persist in the same niche at the same geographical location. This has led to the theory of an equilibrium number of niche spaces, whereby diversification in one clade is balanced by a decrease in diversity in other clades. Ricklefs tested this hypothesis by analyzing several datasets of bird clade diversity and range sizes, predicting that if the hypothesis holds true, the total niche space per clade would scale with the species diversity. His results found that this was not the case, predicting this independence of niche space may be due to higher clade overlap and smaller niche space for individual species within high-diversity clades. The constraint on niche space, Ricklefs proposes, may be caused by the coevolution of pathogens. As pathogens co-evolve with their host, they keep the niche space of one particular species from expanding too broadly, thereby allowing for a higher diversity of closely related species. In the field of niche theory, the inclusion of pathogens is novel, as the ‘boundaries’ of niche space are conventionally defined by competition interactions or resource limitation. The inclusion of both pathogens and co-evolutionary dynamics in the defining of a species niche space represents an important, although somewhat daunting, step towards a further understanding of niche theory. Ricklef’s theory is based on the idea that ‘diversity begets diversity’ evolutionarily and that pathogens are host-specific and respond to the co-evolutionary arms race, otherwise known as the Red Queen Hypothesis, by host switching, and I am doubtful how often this is seen in nature.