A reanalysis of competing hypotheses for the spread of the California sea otter.

From 1938 to 1972, the range of California sea otters (Enhydra lutris nereis) expanded with the northern and southern fronts spreading at rates of approximately 1.4 km/yr and 3.1 km/yr, respectively. J. A. Lubina and S. A. Levin proposed the following three factors to explain the large disparity in spread rates: (1) regional differences in dispersal; (2) regional differences in population growth; and (3) advection due to the known presence of a southerly flowing offshore current. While Lubina and Levin used a reaction-diffusion framework to argue for large differences in dispersal, our approach uses a stage-structured integrodifference matrix model to show that relatively minor differences in survival provide a more parsimonious explanation for the disparity in spread rates; especially if the survival rates between the northern and southern groups differ in more than one life stage. The argument is made that many of the present estimates for otter survival rates span intervals wide enough to explain the different spread rates--even more so in the likely case that advection plays at least a minor role in otter movement.