Cohesion and survivorship of a rodent community during the past 4 million years in southwestern Kansas

Preliminary studies of rodent community evolution in the Meade Basin of sou thwestern Kansas do not support the hypothesis that all organismal communit ies are tightly bound, highly co-evolved units. With the Jaccard Index as a measure of similarity, rodent community composition appears to change cont inually through the 4.2 million year study period, thus supporting the clas sic 'open' community concept of Gleason (1926) and Whittaker (1975). This a lso logically suggests that many predator-prey systems are opportunistic, a lthough larger predators and prey may co-exist for longer periods if their species lifespans are longer. Survivorship analyses show that extinction ra tes are statistically similar for much of late Pliocene time, but escalate rapidly during the middle Pleistocene. Survivorship data are best fit by a linear model, providing empirical support for the idea that rodent communit y structure is determined primarily by the interplay of stochastic climatic and biotic influences. An exponential distribution (straight line) on a se mi-log survivorship plot in real time may indicate either a dynamic (Red Qu een) or stable (evolutionarily stable strategy) equilibrium. The 'smoking g un for Red Queen competitive co-evolution in a study system will be data de monstrating that species in a community evolve in the presence of guild ass ociates, combined with independent climatic data identifying a pacemaker fo r continual, albeit stochastic, environmental change during the study perio d. Preliminary data from the Meade Basin rodent community support a 'weak' version (Ridley, 1993) of the Red Queen hypothesis.