Age structure, residents, and transients of Miocene rodent communities.

The age structures of two successive rodent communities are studied on the basis of a rich record from well-dated Miocene sections (17-10 Ma) in north-central Spain. Community age is defined as the mean of the residence times of the community members at the time of the locality age. Community ages are negatively correlated with the numbers of community members. These members are divided into residents (with continuous membership times > or =1.54 million years) and transients (with membership times <1.54 million years). During episodes of species loss, there is a preferential disappearance of transients while residents are retained, a pattern referred to as the "seniority rule." The residents define the studied communities. They are associated with early successional stages of vegetation, and transients are associated with later stages. Under stable conditions, early arrivals in succession are "transient" and replaced by competitive later arrivals. The reversed roles of transients and residents in the studied fossil record are explained by assuming high degrees of disturbance. We view the system within the context of nonequilibrium metapopulation theory, in which competitively superior species become transients because of their dependence on ephemeral late successional habitats.