| Many biogeographers place a boundary between Californian and Oregonian Provinces near Point Conception, California (34(DEGREES)27'N, 120(DEGREES)28'W). I report on a field study of the scale and causes of this distributional boundary.; I collected gastropods, algae, barnacles, chitons, and anemones from rocky shores at 40 study sites, located along nearly 400 km of coastline around Pt. Conception. I found 21 species whose ranges reportedly ended in the study area. The range boundaries of these species were highly aggregated near Pt. Conception: 11 fell within 10 km of coast extending east (downcoast) from Pt. Conception. A broad "Transition Zone" between provinces is usually attributed to gradients that do not adequately explain the small size of the distributional break indicated by this study. There is evidence that this narrow transition zone coincides with a convergence of coastal water masses. It is unclear whether littoral species indicate the physiological and biological characteristics of the converging waters, their motions, or both.; Ecological field experiments are seldom used to investigate biogeographical patterns. Explanations for range boundaries often assume that habitats just beyond a species range are unsuitable. I investigated the suitability of such habitats for a northern, littoral barnacle, Semibalanus cariosus, by transplanting it to two sites beyond its ordinary range, with control transplants to sites that it occupies naturally. Large barnacles in the extralimital (northern) transplants survived and reproduced as well as, or better than those in the controls (southern). Results on the survival of small barnacles were ambiguous. Extralimital barnacles grew more slowly and delayed release of nauplii, relative to the controls; these results were unexpected for a northern barnacle transplanted into warmer water. The transplants failed to show that the southern sites are unsuitable, although experiments with settled barnacles cannot address the role of larval supply, habitat selection, or early mortality.; Dramatic morphological changes that accompany development in many benthic species are paralleled by extraordinary changes in ecological relations; barnacles, with sessile, benthic adults and planktic larvae, are classical examples. I suggest that the distinction between benthic and planktic processes may be useful in developing testable theories on limits to the geographic distribution of such species. (Abstract shortened with permission of author.)... |
