| The species-time relationship (STR) describes how the species richness of a community increases with the time span over which the community is observed. Understanding STRs is important for understanding the ecological processes underlying temporal turnover, and has implications for both theory and conservation. I have conducted a detailed analysis of the STR in an effort to more fully develop our understanding of this important pattern.; First, I analyzed species-time relationships for 521 breeding bird survey communities. I used a model of sampling effects to demonstrate that the increase in richness was not due exclusively to sampling. I estimated the time scale at which ecological processes became dominant over sampling effects using a two-phase model combining a sampling phase and either a power function or logarithmic ecological phase. These two-phase models performed significantly better than sampling alone and better than simple power and logarithmic functions.; Second, I gathered together 984 community time-series, representing 15 study areas and 9 taxonomic groups, and evaluated their STRs in order to assess the generality of the STR, its consistency across ecosystems and taxonomic groups, its functional form, and its relationship to local species richness. In general, STRs were surprisingly similar across major taxonomic groups and ecosystem types. STRs tended to be well fit by both power and logarithmic functions, and power function exponents typically ranged between 0.2 and 0.4. Communities with high richness tended to have lower STR exponents, suggesting that factors increasing richness may simultaneously decrease turnover in ecological systems.; Third, I evaluated underlying patterns in the abundance of individual species along the time-series, in order to understand how different population level patterns combined to generate observed STRs. Species demonstrated non-random aggregation of individuals through time. The majority of species demonstrated significant temporal autocorrelation in abundance. A statistical model incorporating aggregation performed better than a random placement model in describing turnover patterns. Randomizations suggest that including intraspecific temporal autocorrelation as well as aggregation improves the estimation of temporal turnover patterns. Finally, I present a review and synthesis of all work conducted on the STR, and the related species-time-area relationship, to date. |
