Long-term and fine-scale coexistence of closely related species

Models of coexistence often assume that competitive exclusion takes place at fine scale, but that disturbances (non-equilibrium models) or heterogeneity (spatial models) are necessary for long-term coexistence. As an alternative, very slow exclusion among competitively equivalent species has been proposed, but questioned on the ground that in the long run even the smallest difference in competitive ability will express itself in the loss of species. In this paper we demonstrate long-term coexistence among closely-related mire plant species (notably Sphagnum mosses). In any bog in the boreal region, most species that can tolerate the nutrient-poor, acidic and partly anaerobic conditions are present. Thus, the ratio between the actual and regional species pool is 1 (or very close to 1), and selection of species from the regional pool seems hardly affected by interspecific competition or dispersal limitation. The evidence for coexistence comes from experiments and permanent plots (time scale 1-10 yrs), remapping (40-60 yrs) and stratigraphy (100-2000 yrs), and the spatial scale is cm-ha. At the cm-scale there were no cases of competitive exclusion, even when species were transplanted into each others microsites, and for 10 x 10-cm plots published stratigraphic data showed that combinations with 2-4 species lasted for 200-300 yrs on average, and in some cases almost 1000 yrs. Most temperate ecosystems will not remain without substantial changes in conditions for such a long time. Thus, exclusion among similar species can be so slow that it may well explain the coexistence for the whole "life-time" of a community.