Speciation-rate dependence in species–area relationships

Abstract

The general tendency for species number (S) to increase with sampled area (A) constitutes one of the most robust empirical laws of ecology, quantified by speciesarea relationships (SAR). In many ecosystems, SAR curves display a power-law dependence, S ∝ A z . The exponent z is always less than one but shows significant variation in different ecosystems. We study the multitype voter model as one of the simplest models able to reproduce SAR similar to those observed in real ecosystems in terms of basic ecological processes such as birth, dispersal and speciation. Within the model, the species-area exponent z depends on the dimensionless speciation rate ν, even though the detailed dependence is still matter of controversy. We present extensive numerical simulations in a broad range of speciation rates from ν = 10 −3 down to ν = 10 −11 , where the model reproduces values of the exponent observed in nature. In particular, we show that the inverse of the species-area exponent linearly depends on the logarithm of ν. Further, we compare the model outcomes with field data collected from previous studies, for which we separate the effect of the speciation rate from that of the different species lifespans. We find a good linear relationship between inverse exponents and logarithm of species lifespans. However, the slope sets bounds on the speciation rates that can hardly be justified on evolutionary basis, suggesting that additional effects should be taken into account to consistently interpret the observed exponents.

The case for ecological neutral theory

“…It also showed the importance of long-distance dispersal in the system because without this, the gradient of the most frequently observed second phase was too shallow on logarithmic space [59,73]. Furthermore, long-distance dispersal events can stand in for speciation in this model [49], which might be true much more generally.…”

The Unified Neutral Theory of Biodiversity and Biogeography at Age Ten

“…This was first shown for intermediate dispersal distances in relatively small communities [99], then more generally for all but the smallest dispersal distances in an infinite community [26]. This model is qualitatively correct, but the fit to empirical data is relatively poor and cannot be resolved even with small speciation rates [27]. A good fit to data (including an appreciable region of the SAR that follows the power law) requires not only smaller speciation rates, but also 'fat-tailed' dispersal kernels [28] that exhibit a greater probability of dispersal over long distances and are generally considered more realistic than the alternatives [29] (Figure I).…”

“…If a new factor introduced to the model produces a dramatic improvement in its performance, then it is likely that the new factor is important. For example, fits of species-area curves by neutral models were not realistic [26,27] until long-distance dispersal was incorporated [28] (Box 3). This supports the general concept that long-distance dispersal is important [29].…”

Extinction debt and the role of static and dynamical fragmentation on biodiversity

“…In this scenario the species-area curve exhibits two distinct regimes. The first for small areas in which the growth of species richness with area can be approximated by a power-law and the second one that takes place at large areas, known as the linear regime (Campos et al, 2012;Durrett and Levin, 1996;Pigolotti and Cencini, 2009). The remaining curves in the panel correspond to species number as the effective area is reduced due to habitat loss.…”