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.

Different components of an ecosystem can respond in very different ways to habitat fragmentation. An archipelago of patches, representing different levels of fragmentation, was arrayed within a successional field and studied over a period of 6 years. Ecosystem processes (soil mineralization and plant succession) did not vary with the degree of subdivision, nor did most measures of plant and animal community diversity. However, fragmentation affected vertebrate population dynamics and distributional patterns as well as the population persistence of clonal plant species. The results highlight the dangers of relying on broad community measures in lieu of detailed population analyses in studies of fragmented habitats.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. In spatially heterogeneous habitats, plant community change may reflect spatially localized population-level processes that are sensitive to the size of an average habitat patch. However, local species turnover can also be determined by initial conditions and large-scale processes, in which case patch size effects may be overridden. To examine the role of patch size in directing secondary succession, we subdivided a newly abandoned agricultural field into an array of experimental patches (32, 288, and 5000 m2, grouped to sample equivalent portions of the field), and have thereafter censused the resident plant and animal communities at regular intervals. Here we report results from the first 6 yr of studies on the changing vascular plant community in an experimentally fragmented landscape. The general course of change in all patches followed a trajectory typical of old-field succession, toward increasing dominance by longer lived and larger plant species. The same group of species that dominated at the start of the study continued to dominate after 6 yr, although in very different proportional abundances. Larger patches were more species rich than their smaller counterparts, and had a higher proportion of nonshared species, but the additional species were transient and low in abundance. Spatial heterogeneity in vegetation, measured as local community dissimilarity, increased in all patches but to a lesser extent in the largest patches, where censuses of nearby permanent quadrats indicated less divergence over time. At a population level, the strongest effect of patch size was that local populations of clonal species were more prone to disappear from the smallest patches. Nevertheless, summary measures of temporal community change did not reflect significant differences in localized species turnover. We conclude that patch size does not markedly affect the rate or pattern of early secondary succession, at the scales imposed in our experiment.

Studies of biological invasions indicate that natural recruitment of new species can occur as a “nucleation” phenomenon, in which scattered colonization foci spread and coalesce. Ecological reclamation of damaged lands might make use of this potential for enhanced natural dispersal, by inoculating sites with multiple small plantings to attract animal dispersers and other mutualists from nearby remnants of natural habitat. We conducted an experimental test of this proposition. On a 6‐ha section of an abandoned municipal landfill in the New Jersey Meadowlands, we installed 16 clusters of 21 trees and shrubs in an array of fenced plots. Clusters contained seven native species known to: (1) attract bird dispersers to introduce propagules from remnants of off‐site habitat; (2) contribute propagules by virtue of high reproductive output and clonal growth; and (3) accelerate woodland succession on open, degraded habitats. Average plant size was varied, with half the plots receiving larger trees and shrubs, to test whether woody plant size would enhance any attractive function. An additional eight empty plots were studied to estimate background rates of recruitment and to test for a fencing effect. Site preparation included the addition of 90 cm of fresh substrate, including organic matter, and a cover crop of annual grasses. Recruitment of woody plants inside and surrounding the experimental plots was examined for five years, and results were compared on the basis of treatment and recruitment mode (avian, wind, or clonal dispersal). Woody plant recruitment into experimental plots was rapid and substantial, primarily via dispersal from natural sources. Plots with larger plants attracted significantly more recruits at the outset, but this difference diminished over time. Fall seed rain samples yielded a mean estimate of 426 seeds/m2 within plots. However, size distributions of recruiting woody species increasingly shifted toward larger individuals each year. Experimental manipulations that opened seed beds for woody plant recruitment had short‐lived effects, indicating a narrow window of opportunity for establishment. Spread of the planted species themselves was generally weak, although clonal growth contributed substantially to spread on the margins of plots. Most recruitment outside experimental plots was from external sources. A strong proximity component was found for bird‐dispersed recruits, which were highly clustered near planted plots, with the highest densities near source populations on the site margin. Wind‐dispersed trees and shrubs, by contrast, were not associated with planted plots and were concentrated near one corner of the site. Discounting plot interiors, total recruitment density for the site after 5 yr was ∼800 woody stems/ha, 36% via avian dispersal, 10% via clonal spread, and the remainder via wind‐borne propagules. New recruits represented 26 woody plant species, all but four from external sources, and only five common species contributed more than a few recruits. We conclude that techniques for man...