The spread of a reinvading species: range expansion in the California sea otter

The spread of the California Enhydra lutris population provides excellent material for studying the spread of an invading species because the data are not confounded by spread in multiple spatial dimensions. Data on the range expansion of California sea otter is analysed within the context of a classical mathematical model incorporating growth, diffusion and advection. Classical theory predicts that population fronts will form, that asymptotic rates of advance will be constant with homogeneous habitat, and that these asymptotic rates are given by twice the square root of the product of 2 factors, the intrinsic rate of increase and the diffusion coefficient. These patterns were observed in the historical data for range expansion of the otter. The central remaining problem is explaining the observed differences in the rate of spread between the northern and southern fronts. Analysis suggests that advection plays, at most, a minor role in those differences and that estimated differences in the diffusion coefficients are sufficient to account for observed patterns. Habitat-dependent differences in mortality seem to present the major competing hypothesis, and both factors may contribute to the observed differences. -from Authors