The uncertain role of diversity dependence in species diversification and the need to incorporate time-varying carrying capacities

Abstract

One contribution of 11 to a theme issue 'The regulators of biodiversity in deep time'. There is no agreement among palaeobiologists or biologists as to whether, or to what extent, there are limits on diversification and species numbers. Here, we posit that part of the disagreement stems from: (i) the lack of explicit criteria for defining the relevant species pools, which may be defined phylogenetically, ecologically or geographically; (ii) assumptions that must be made when extrapolating from population-level logistic growth to macro-evolutionary diversification; and (iii) too much emphasis being placed on fixed carrying capacities, rather than taking into account the opportunities for increased species richness on evolutionary timescales, for example, owing to increased biologically available energy, increased habitat complexity and the ability of many clades to better extract resources from the environment, or to broaden their resource base. Thus, we argue that a more effective way of assessing the evidence for and against the ideas of bound versus unbound diversification is through appropriate definition of the relevant species pools, and through explicit modelling of diversity-dependent diversification with time-varying carrying capacities. Here, we show that time-varying carrying capacities, either increases or decreases, can be accommodated through changing intrinsic diversification rates (diversity-independent effects), or changing the effects of crowding (diversity-dependent effects).

Five palaeobiological laws needed to understand the evolution of the living biota

“…The first is through Malthusian reasoning; unchecked exponential growth, when origination exceeds extinction, quickly runs afoul of limited resources, and so is unsustainable in the long term; that is, with limited resources the origination rate cannot exceed the extinction rate for very long, and even as new resources become available, exponential growth will quickly turn that new resource into a constraint on diversification, even if the resource pool is growing 25 (unless it is growing exponentially). Thus, as resource limitation is encountered, the instantaneous origination and extinction rates will quickly converge, leading to a convergence in the long-term averages.…”

Sex and the shifting biodiversity dynamics of marine animals in deep time

“…T he fossil record provides direct, historical evidence that can help constrain controls on biodiversification, such as the relative roles of diversity-dependent and diversity-independent processes (1)(2)(3). Statistical analyses of the well-preserved fossil record of marine animals support diversity dependence, albeit with a shifting value of equilibrium diversity (4-6, but see refs.…”

The challenges to inferring the regulators of biodiversity in deep time

“…Verbal interpretations of a diversity dependence mechanism built into a dynamic carrying capacity exist [40,88], but an environmental regulator of such dynamic limits has not been identified statistically. Marshall & Quental [87] argue that an appropriate definition of the species pool and the consideration of time-variable carrying capacities could reconcile evidence of bounded versus unbounded diversification. The authors propose a diversitydependent modelling framework with a carrying capacity varying through time as a result of changing either intrinsic diversification rates or the strength of the diversity-dependent effect.…”

“…Developing this line of thought in this volume, Marshall & Quental [87] explore the hotly debated question of limits on diversification [40,79]. Verbal interpretations of a diversity dependence mechanism built into a dynamic carrying capacity exist [40,88], but an environmental regulator of such dynamic limits has not been identified statistically.…”