Abstract
Species have been commonly hypothesized to have high population densities in geographic areas which correspond to either the centre of the species geographic range or climatic niche (abundant–centre hypothesis). However, there is mixed empirical support for this relationship, and little theoretical underpinning. We simulate a species spreading across a set of replicated artificial landscapes to examine the expected level of support for abundant–centre relationships in geographic and niche space. Species niche constraints were modeled as a single axis which was related directly to population growth rates. We found strong evidence for abundant–centre relationships when populations follow deterministic growth, dispersal is high, environmental noise is absent and intraspecific competition is low. However, the incorporation of ecological realism reduced the detectability of abundant–centre relationships considerably. Our results suggest that even in carefully constructed artificial landscapes designed to demonstrate abundant–centre dynamics, the incorporation of small amounts of demographic stochasticity, environmental heterogeneity or landscape structure can strongly influence the relationship between species population density and distance to species geographic range or niche centre. While some simulated relationships were of comparable strength to common empirical support for abundant–centre relationships, our results suggest that these relationships are expected to be fairly variable and weak.
Highlights
According to the abundant–centre hypothesis, species should have higher population density in the centre of their geographic distribution (Brown 1984, Sagarin and Gaines 2002)
We explore the extent to which abundant–centre relationships are sensitive to non-equilibrial population dynamics by incorporating demographic stochasticity into simulations of species spatial population dynamics, using both a deterministic and stochastic Ricker model to examine spatial population dynamics
We considered population dynamics within each cell to be determined by a discrete single species Ricker model (Eq 1), where population size in the timestep (Nt+1) is determined by the population size at time t (Nt) and the per capita population growth rate R, discounted by the effects of intraspecific competition
Summary
According to the abundant–centre hypothesis, species should have higher population density in the centre of their geographic distribution (Brown 1984, Sagarin and Gaines 2002). Populations are assumed to be well connected throughout the geographic range, be at their carrying capacity and in equilibrium with the environment. Framing the abundant–centre hypothesis in niche space does not address many of the abundant–centre assumptions. This version of the hypothesis focuses on the fundamental niche (Hutchinson 1957), niche is typically estimated from occurrence data (implicitly linking the species niche and geographic distribution; Pulliam 2000) and producing something closer to the realized niche (Soberón 2007). Population density is assumed to be equilibrial and unaffected by dispersal or competition (Pironon et al 2016)
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