Abundance and distribution of desert annuals: are spatial and temporal patterns related?

1 Spatial distribution-abundance relationships have been intensively studie d. Workers have now begun to study temporal relationships but little is kno wn about how these are related to spatial patterns. We examined these relat ionships both over space and time using 15 years of data from permanent qua drats in the Chihuahuan Desert. 2 Winter and summer annual communities were analysed. Within single years, the species in each community exhibited a positive relationship between dis tribution and abundance at two spatial scares, 0.25-ha plots and 0.25-m(2) quadrats. Over 15 years, the species in each community also exhibited a pos itive relationship between abundance and the number of years in which they were present. Species frequency distributions, both in space and over time, were scale-dependent. 3 Over space, the frequency distribution was bimodal at large (plot) scales , but unimodal at small (quadrat) scales in both communities. Over time, th e frequency distribution was bimodal for winter annuals but unimodal for su mmer annuals, a difference we attribute to the marked difference between th e two communities in the temporal consistency of rank abundance of species. We conclude that both niche and metapopulation models can be used in expla ining the distribution patterns of species but at different scales. The fac tors and processes in these models are not mutually exclusive in determinin g the overall abundance and distributions of these species in space and ove r time.