Theoretical studies suggest that fluctuations in availability of resources should lead to related fluctuations in the intensity of competition, and that significant competition should occur only in years of severe resource scarcity. This prediction has been supported by a number of studies conducted in animal populations, but has never been tested in plant populations. In this study, we performed neighbor removal experiments in conjunction with rainfall manipulation experiments, to test how fluctuations in rainfall, the major limiting resource for desert annuals, affect the intensity of competition in populations of the desert annual Stipa capensis. In order to test for the effect of spatial heterogeneity on the relationships between rainfall and competition intensity, populations were studied in three types of habitats: slopes, depressions, and wadis. Competition intensity was quantified, for each combination of habitat and rainfall, as the percentage of potential seed yield per germinating plant which was prevented due to competitive effects. Intensities of competition in the studied populations varied considerably between habitats, between seasons, and between rainfall manipulation treatments. In the most favorable habitat, the wadi, competition was always intense, independent of rainfall conditions. In the drier, slope and depressions habitats, natural and experimental changes in the amount of the yearly rainfall had considerable effects on competition intensity, but most pronounced competition was associated with high resource availability, a pattern which is opposite to the pattern that was predicted and documented for animal populations. The overall results suggest that competitive effects may have critical influence on the population dynamics of the species studied. This contradicts previous hypotheses on the relative importance of competition in desert plant populations.
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This research was supported in part by a grant from the National Council for Research and Development, Israel and the G.S.F., Miinchen, F.R.G. The authors would like to