Geographical separation of two Ulex species at three spatial scales: does competition limit species' ranges?

Abstract

It is a common assumption that species' ranges are limited by their physiological tolerances to climatic factors, Biotic factors, such as competition, are rarely considered. We investigated the distributions of Ulex minor and U. gallii at three spatial scales from geographic ranges to individual heaths -to examine whether the species are negatively associated, as predicted by the hypothesis that ihe ranges of the species are limited by competition with each other. Distribution maps for the British Isles and France (100-400 km^ survey units) show the two species have largely separated, but slightly overlapping ranges. A region of range overlap on the heaths of Dorset, southern England was mapped using 4 ha survey squares. There was strong negative association between the species, and the heaths could be divided into zones where one species was dominant. There was some indication of edaphic differences between the V. m/Ro/--dominated zones and the V. gallii zones. The few heaths where the species co-occurred were surveyed using 4 m^ quadrats placed along transects. Usually one species was widespread over the heath, while the other occurred in patches. The species were strongly negatively associated in all transects. Therefore, the two species showed strong negative associations at three mapping scales. Apparent co-occurrences detected at one spatial scale largely disappeared when species were mapped at finer scales, emphasising the fractal nature of distributions. This provides evidence that the distributions of the two species are not independent and that they cannot coexist, and therefore that their ranges are limited by competition. Over their ranges, competitive superiority is probably determined by the climate. At the range boundaries in the region of overlap, climate is not important, but other physical factors such as edaphic conditions may determine the outcome of competition.J. M. Bullock (jmbul@ceh.ae.uk). R.

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