The effect of landscape structure on community self-organization and critical biodiversity

Critical biodiversity has been defined as the level of species richness at which communities are most susceptible to disturbance, where even small perturbations resulting from the introduction or extinction of a single species may trigger a mass extinction event. Beyond this threshold, it has been hypothesized that ordered communities with well-defined spatial structure will spontaneously form; these ordered communities are predicted to be resilient to small perturbations such that mass extinctions will no longer occur. We adopted a complex systems approach to explore how landscape pattern affected the critical biodiversity threshold (S-c) and the ability of communities to self-organize in heterogeneous random and fractal landscapes representing a gradient of spatial contagion. Communities that evolved in random and clumped fractal (H = 1.0) landscapes attained nearly the same average species richness (random S = 22, fractal S = 20.5), but the range of variation in community species richness was 3 x greater in random landscapes (random CV = 66%, fractal CV = 21%). Some communities that formed on random landscapes collapsed completely and never recovered, whereas complete system collapse never occurred on landscapes with a high degree of spatial contagion (clumped fractal). Nevertheless, spatial contagion initially enhanced the susceptibility of communities to mass extinction, and thus the critical biodiversity threshold was higher in landscapes with high spatial contagion (random S-c = 15; clumped fractal S-c 20). In other words, a greater number of species was ultimately required to buffer communities from the small perturbations that occasionally triggered mass extinctions on these highly ordered landscapes. The likelihood of attaining this critical biodiversity was also affected by landscape structure. Communities on clumped fractal landscapes evolved to (or got stuck at) the critical biodiversity threshold, whereas communities with an intermediate degree of order (H = 0.5) generally evolved beyond this point and attained a high level of species richness. Spatial structure is not a prerequisite for the emergence of community structure, but organized communities are inevitable in highly structured (ordered) landscapes. Order begets order and this order ultimately enhances system stability and the susceptibility of the system to mass extinction. (C) 2004 Elsevier B.V. All rights reserved.