Cooperation and Competition Along Smooth Environmental Gradients

This paper explores the ecological consequences of competition and cooperation, and the resultant abrupt species zonation that could occur along smooth environmental stress gradients. We present one- and two-species models of sessile organisms incorporating competition for space and two forms of cooperation: habitat amelioration and positive recruitment. In these models, habitat amelioration reduces an organism’s mortality rate when it is surrounded by other organisms (“safety-in-numbers”), and positive intraspecific recruitment enables juveniles to use the shelter of established individuals as settlement (or germination) sites. A single-species model incorporating both forms of cooperation may exhibit sharp discontinuities in population density along an environmental gradient. In a two-species model the same mechanisms may lead to sharp boundaries in species composition if, as empirical studies suggest, a trade-off exists between colonization and cooperation abilities (e.g., strong cooperators are poor colonizers). Positive interspecific recruitment obscures the interspecific boundaries.