The Coexistence of Species in Fractal Landscapes

The degree of spatial dependence in a landscape can be modeled by varying the landscape's fractal dimension, D. I simulated landscapes in which D varied independently of environmental variability. Individuals of 10 species with modes placed evenly along the dominant environmental gradient were randomly placed on these landscapes and were allowed to reproduce, disperse, and compete. The results demonstrate that the effect of a landscape's environmental variability on species coexistence is affected by the degree of spatial dependence in a complex manner. Increasing the fractal dimension (decreasing spatial dependence) allows more species to exist per microsite and per landscape. However, extremely high fractal dimensions cause fewer species to coexist on the landscape scale. Observed habitat breadths are increased (and beta diversity is decreased) as a function of D. I suggest that these patterns can be explained by habitat area, the mass effect, ecological equivalency, and global biological constraints.

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