COOPERATION AND COMPETITION ALONG SMOOTH ENVIRONMENTAL GRADIENTS

This paper explores the ecological consequences of competition and coo peration, and the resultant abrupt species zonation that could occur a long smooth environmental stress gradients. We present one-and two-spe cies models of sessile organisms incorporating competition for space a nd two forms of cooperation: habitat amelioration and positive recruit ment. In these models, habitat amelioration reduces an organism's mort ality rate when it is surrounded by other organisms (''safety-in-numbe rs''), 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 envi ronmental gradient. In a two-species model the same mechanisms may lea d to sharp boundaries in species composition if, as empirical studies suggest, a trade-off exists between colonization and cooperation abili ties (e.g., strong cooperators are poor colonizers). Positive interspe cific recruitment obscures the interspecific boundaries.