COOPERATION AND COMPETITION ALONG SMOOTH ENVIRONMENTAL GRADIENTS

This paper explores the ecological consequences of competition and coop- eration, and the resultant abrupt species zonation that could occur along smooth environ- mental 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 intra- specific recruitment enables juveniles to use the shelter of established individuals as set- tlement (or germination) sites. A single-species model incorporating both forms of coop- eration 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.