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

Abstract

Summary 1 Spatial distribution–abundance relationships have been intensively studied. Workers have now begun to study temporal relationships but little is known about how these are related to spatial patterns. We examined these relationships both over space and time using 15 years of data from permanent quadrats 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 distribution and abundance at two spatial scales, 0.25‐ha plots and 0.25‐m2 quadrats. Over 15 years, the species in each community also exhibited a positive 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, the frequency distribution was bimodal for winter annuals but unimodal for summer annuals, a difference we attribute to the marked difference between the two communities in the temporal consistency of rank abundance of species. We conclude that both niche and metapopulation models can be used in explaining the distribution patterns of species but at different scales. The factors and processes in these models are not mutually exclusive in determining the overall abundance and distributions of these species in space and over time.