There is a long discussion whether species richness is continuously expanding or if there are some limits given by environmental conditions. The finding that diversity patterns at large spatial scales are tightly related to climate and largely independent of particular evolutionary histories of clades has led to the idea that energy availability or environmental productivity limits species richness. However, there is little consensus as to the exact processes driving this species-energy relationship. The most straightforward explanation is the more-individuals hypothesis (MIH) which states that higher energy availability promotes a higher total number of individuals in a community, which consequently increases species richness by allowing for a greater number of species with viable populations. We review and evaluate the reliability of various predictions of the MIH that have been tested, showing that empirical support for the MIH is mixed, partially due to the lack of proper formalization of the MIH and consequent confusion as to its exact predictions. We then formulate a theory of biodiversity dynamics formalizing the ideas implicitly included in the MIH, explicitly addressing the relationships between energy availability E, total number of individuals J, and species richness S, as well as the process of speciation, colonization and extinction.
Results/Conclusions
Our theory does not assume that species necessarily have the same access to resources, and thus comprises a wide range of processes and species interaction, assuming these interactions are constrained by E and encompassed by the functions that relate species origination and extinction rates to population sizes. In contrast to other theories of diversity dynamics, we assume that although J constrains the dynamic of S, species richness may in turn positively affect J as new species utilize niches which are not used when the community is less species-rich. Such dynamics lead to stable equilibrium of species richness (i.e. carrying capacity) driven by diversity-dependence due to negative relationship between S and population sizes (and thus extinction rates), and modulated by resources, population fluctuations, and species origination rates. The theory provides unique predictions concerning species richness patterns related to productivity, environmental stability and the factors affecting speciation rate (e.g. temperature). We test these predictions using multiple datasets, showing that the theory is able to predict a wide spectrum of non-trivial patterns, and thus it represents a useful general framework for understanding diversity dynamics and diversity patterns across space, time, and taxa.