The Chowdhury Ecosystem Model: A Test of the Latitude-Niche Breadth Hypothesis

Our latest paper has now been accepted for publication by the French ecological journal View et Milieu. It is the latest in a series of three that uses the Chowdhury Ecosystem Model (see Appendix 1) to examine an important problem in evolutionary ecology, i.e., the niche breadth-latitude hypothesis, according to which greater species numbers in the tropics are correlated with narrower niches. The Chowdhury Ecosystem Model permits simulations over evolutionary (millions of years) and much shorter ecological time scales. It is agent based, i.e., each individual is treated separately with its own random birth and death, instead of by a differential equation describing how the total number of individuals changes with time. Sometimes, differential equations can give qualitatively wrong results compared to the more realistic individual treatment. In physics, such agent-based simulations have been used for half a century as “Molecular Dynamics” or “Monte Carlo”. Monte Carlo simulations are also important tools in biology.

The Abstract of the paper follows.

Habitat width along a latitudinal gradient
D. Stauffer, C. Schulze, K. Rohde

We use the Chowdhury ecosystem model, one of the most complex agentbased ecological models, to test the latitude-niche breadth hypothesis, with regard to habitat width, i.e., whether tropical species generally have narrower habitats than high latitude ones. In two previous studies using the Chowdhury Model, we have shown that simulations result in faster speciation in the tropics and in latitudinal diversity gradients, that the complexity of foodwebs increases with time and at higher rates in the tropics (Rohde & Stauffer 2005), and that latitudinal ranges of species are greater in the tropics, contradicting Rapoport’s rule (Stauffer & Rohde 2006). In this paper we show that the Chowdhury Model does not support the latitude-niche breadth hypothesis for the niche dimension habitat width: habitats, measured by comparing species numbers in small and large areas at a particular locality, are generally wider and not narrower in the tropics. This hypothesis cannot, therefore, give a causal explanation of latitudinal gradients in species diversity

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