Temporal Changes in Populations and Species Diversity in a California Rodent Community

Abstract

Population trends and their proximate effects on the species diversity of a rodent community were examined on a stand of coastal sage scrub in southern California, from the summer of 1968 to the autumn of 1969. The rodents comprising the fauna were Dipodomys agilis agilis, Perognathus fallax fallax, P. longimembris pacificus, Peromyscus eremicus fraterculus, P. maniculatus gambellii, P. californicns insignis, Reithrodontomys megalotis longicaudus, Neotoma lepida intermedia , and Neotoma fuscipes macrotis . Live-trapping was done at monthly intervals for a period of 15 months. Individuals collected were examined for reproductive condition, marked and released. Periods of population increase, decrease, or stability were accompanied by variable patterns of survivorship and addition of residents and transients. During periods of population decrease the rodents exhibited reduced survival with the exception of P. maniculatus , which showed increased survival. For the increase phase of the rodent populations, survival was equal to, or greater than, mean survival for the entire period of study; however, the addition of new residents and transients was often high ( P. maniculatus and N. lepida ). The temporal changes in these population components led to asynchronous fluctuations in population densities of the rodents, which yielded the major changes in species diversity. This is further emphasized by examining the correlation of population numbers between the six commonest species. Only six of the 15 pair-wise correlations were significant. Rodent species diversity was measured by employing Brillouin's information formula. Of the three components of diversity—relative abundance, species number, and sample size—a significant relationship was found between species richness and diversity ( r = +0.910), and sample size and diversity ( r = +0.566). Although uncorrelated with diversity, evenness was important, as some of the diversity changes between months were due to shifts in evenness and not changes in species numbers. The fluctuations in the relative abundance of species and the variable presence of the rarer species led to the temporal fluctuations in species diversity.