In an experimental test of plant community invasibility, we introduced seeds of a native ruderal, California poppy (Eschscholzia californica), at fixed density into experimental plots in a California winter annual grassland. Each of the 42 plots, which ranged in size from 2 m2 to 32 m2, had been studied for 4 yr previous to the introduction, with the common observation that a subset of plots of each size consistently held more species than others. It was primarily in these more species—rich plots that establishment and reproduction by the experimental invader occurred. Success of the invader per plot, measured as the total number of plants germinating, producing seeds, or perennating, varied with plot size, but the statistical contribution of plot size was secondary to that of local species number. Contributing variables were the extent of small mammal disturbance (positive) and the degree to which a single resident plant species (in particular, Bromus diandrus) dominated a plot (negative). In contrast to theories of competitive exclusion via niche partitioning, species—rich plots were more invasible.

Habitat subdivision by geography or human activity may be an important determinant of regional species richness. Cumulative species-area relationships for vertebrates, land plants, and insects on island archipelagoes show that collections of small islands generally harbor more species than comparable areas composed of one or a few large islands. The effect of the degree of habitat subdivision in increasing species richness appears to increase with the distance from potential sources of colonists. Mountaintop biotas show no clear differences between species richness on large alpine areas and collections of smaller peaks. National park faunas generally have more species in collections of small parks than in the larger parks. In all cases where a consistent effect of subdivision is observed, the more subdivided collection of islands or isolates contains more species. To the degree that these data provide guidance for establishing nature reserves, they suggest that increasing the numbers of reserves may be an important component of conservation strategies.

Biotic communities inhabiting collections of insular habitat patches often exhibit compositional patterns described as "nested subsets". In nested biotas, the assemblages of species in relatively depauperate sites comprise successive subsets of species in relatively richer sites. In theory, nestedness may result from selective extinction, selective colonization, or other mechanisms, such as nested habitats. Allopatric speciation is expected to reduce nestedness. Previous studies, based largely on comparisons between land-bridge and oceanic archipelagos, have emphasized the role of selective extinction. However, colonization could also be important in generating strong patterns of nestedness. We apply a recently published index of nestedness to more than 50 island biogeographic data sets, and examine the roles of colonization, extinction, endemism, and, to a limited extent, habitat variability on the degree on nestedness. Most data sets exhibit a significant degree of nestedness, although there is no general tendency for land-bridge biotas to appear more nested than oceanic ones. Endemic species are shown to generally reduce nestedness. Comparisons between groups of non-endemic species differing in overwater or inter-patch dispersal ability indicate that superior dispersers generally exhibit a greater degree of nestedness than poorer dispersers, a result opposite that expected if colonization were a less predictable process than extinction. These results suggest that frequent colonization is likely to enhance nestedness, thereby increasing the compositional overlap among insular biotas. The prevalence of selective extinction in natural communities remains in question. The importance of colonization in generating and maintaining nested subsets suggests that (1) minimum critical areas will be difficult to determine from patterns of species distributions on islands; (2) multiple conservation sites are likely to be required to preserve communities in subdivided landscapes; and (3) management of dispersal processes may be as important to preserving species and communities as is minimizing extinctions.

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We monitored settlement of sea urchins (Strongylocentrotus spp.) and crabs (primarily Cancer spp.) and concurrent physical variable:; at Bodega Marine Laboratory (BML) and Salt Point (northern California) fromApril through August 1992.lJpwelling favorable winds led to lower temperatures, higher salinities, and lower subsurface pressures at BMI,, while periods of relaxation from upwelling typically caused a lagged reversal of each of these trends. This response to relaxation was due to a combination of alongshore and onshore flow of near-surface water at this site. Crab settlement was significantly correlated with the rate of temperature increase, corresponding to relaxation events. In contrast, sea urchins settled primarily during an unusual relaxation event which poss,ibly involved remote forcing. Sea urchin and crab settlement were negatively correlated. Possible reasons include residence in different water masses, different biomechanical characteristics, and predation by crab larvae on sea urchin larvae. The time-course of settlement at Salt Point was similar to that at BML for bloth species groups.Although transport and nearshore oceanographic conditions have been a focus of settlement investigations since the early days of larval ecology (Young 1990), many questions remain regarding physical oceanographic influences on the transport and settlement of marine invertebrate larvae in the coastal ocean. 'The specific transport mechanisms underlying successful settlement have been identified for only a few species (e.g. Shanks 1986; Roughgarden et al. 1988; Farrell et al. 199 1). Several recent studies have focused specifically on the role of variable coastal winds in larval transport and settlement (e.g. Johnson et al. 1986; Hobbs et al. 1!)92). In this study, we Acknowledgments