Pattern analysis in successional communities - An approach for studying shifts in ecological interactions

Many ecological studies have addressed issues of vegetation spatial patterns in attempts to understand the processes generating them. We investigated changes in ecological processes during succession via the analysis of shrubs' spatial patterns in a system of linear sand dunes, an arid ecosystem located in the Negev Desert in Israel during three consecutive years. We hypothesized that spatial patterns change from clustered to regular as succession progresses due to changes in the relative importance of facilitation and competition in this environment. In this ecosystem communities of early successional stages are frequently disturbed by high rates of sand movement, whereas in later successional stages sand stability is high. We mapped in the field individual shrubs on high-resolution aerial photographs, and converted the digital images to a GIS data set. Using Ripley's K-function we analysed spatial patterns at three levels: the single-species level, among species and at the individual level, in three communities characterizing different successional stages. In the early successional communities we found clustered spatial patterns, in comparison with stable habitats where spatial patterns tended to be regular. We argue that these shifts in spatial patterns are indicative of the assumption that in this sand-dune system ecological interactions change from facilitation to competition as succession progresses. Further, we argue that these interactions operate in different spatial scales at the different successional stages, and that the study of these processes should be conducted at the spatial scales specific to each community.