DYNAMICS OF GEOGRAPHIC RANGE WITHIN MARINE ANIMAL GENERA: RELATIVE IMPORTANCE OF SPECIES RANGE AND NUMBER OF SPECIES
Geographic ranges of genera reflect ranges of individual species as well as the number of species in the genus. To explore the relative importance of these factors, we have analyzed distributional data on marine animal genera from the Paleobiology Database. We used regression analysis to determine the strength of the effects of species-level ranges and species richness. If the collection of species ranges within a genus is characterized by its average (mean or median) and by its maximum, the maximum species range is the stronger determinant of genus range, which stands to reason given that most species are narrowly distributed. Greater still, however, is the effect of the number of species in the genus. The relative importance of these factors is consistent whether we consider variation among coeval genera or changes within genera from one stratigraphic stage to the next. Moreover, there is a basic symmetry in the expansion and contraction of genus ranges; the effects of species range and species richness have about the same strength for genera that are expanding and those that are contracting. The effect of adding one species to a genus is the same as the effect of increasing maximum species range by about 1000 km.
Changes through time in the maximum constituent species range for a given genus may reflect interval-to-interval changes in the roster of species that are maximally ranging, or actual changes in the range of individual species. If we consider all instances in which a species is sampled in two successive stages, we find that in about 20% of these cases the species is the most widespread member of the genus in both stages, a frequency about twice that expected if the identities of maximally ranging species were randomized. In contrast to the data at large, for the subset of genera in which the most widespread species is the same in successive stages, changes in maximal species range are more important than changes in the number of species.