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...

Urban areas often contain sizeable pockets of degraded lamg such as inactive landfill~ that could be reclaimed as wildlife habitat and as connecting links to enhance remnant natural area£ In the northeastern U.S., many such lands fail to undergo natural succession to woodlaru~ instead retaining a weedy, herbaceous cover for many year~ We hypothesize that seed dispersal is a limiting factor, and that a form of secondary succession could be stimulated by introducing clusters of trees and shrubs to attract avian seed dlsperser~ As a direct tesg we censused a 1.5-ha experimental plantation on the Fresh Kills Landfill (Staten Island, New York) one year after installation, in search of evidence that the plantation was spreading or increasing in dimity. The 17planted specie~ many from coastal scrub forests native to this region, were surviving well but contributed almost no seedlings to the area~ in part because only 2096 of the installed trees or shrubs were reprodactiv~ Of the 1079 woody seedlings foun~ 95 96 came from sources outside the plantatior~, most ( 71 ~6 ) were from fleshy-fruttea~ bird-dispersed plants from nearby woodland fringes. Although the restoration planting itself had not begun to produce seedling~ it did function as a site for attracting dlsperser~ who enriched the young community with 20 new specie~ One-fourth of ail new recruits were from nine additional wind-dispersed specie~ Locations with a high ratio of trees to shrubs had proportionately more recruit& indicating that plant size contributed to disperser attractiorL The density of new recruits of each species was dependent on distance from the nearest potential seed source. Introducing native species with the capacity to attract avian dispersers may be the key to success of many restoration program.~ Restablecimiento del bosque en una clausura: Rdpida adici6n de especies por aves dispersoras Resumen: Areas urbanas usualmente contienen nucleos aislados de tama~o conslderabl~ de tierras degradadas, como vertederos pt~blicos inactivos que pueden ser reclamados como hdbitat para vlda silvestr~ y como vlnculos de conecci6n para ampllar dreas naturales remanente~ En el Noreste de Estados Unidos muchas de estas tierras fracasan en el proceso natural de sucesiOn hacla bosque~ en vez retienen pot muchos a~os una cubierta h~qadcea de maleza£ Nuestra hip6tesis es que la dispersi6n de las semillas es un factor limitante. Una forma de sucesi6n secundat~ puede ser simulada introduciendo conglomerados de drboles y arbusto& para atrear aves dispersoras de semillax Como test dtrecto nosotros sensamos 1.5-ha de una plantaci6n experi. mental en el vertedero p~blico de "'Fresh Kills" (Staten Island, New York) un a~o despuds de la instalaci6n, en la bt~squeda de evtdencia que demuestre que la plantaci6n fue dispersada o increment6 en diverslda~ Las 17 especiesplantada~ muchas de arbustos costeros nativos de la regi6n, sobrevivieron bien, pero, prdcticament¢ no contribuyeron en semiilas en el dreag en parte porque s6lamente el 2096 de los drboles o arbustos instala...

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Substitutive (replacement) and partial additive experimental designs, with their underlying models, remain the two most popular techniques in weed-crop competition studies, despite considerable criticism of these approaches in the recent literature. We review standard designs for two-species competition experiments and demonstrate the advantages of a reciprocal yield model applied to data from an additive series experiment, using mixtures of Japanese millet and tomato. A traditional replacement series analysis failed to provide a general model of competition among these two species over several total plant densities, while an application of a reciprocal yield (inverse linear) model to the same data was successful. This technique allows evaluation of the influences of both weed on crop and crop on weed, as well as the partitioning of net competition effects into intra- and interspecific components. One Japanese millet plant was competitively equivalent to 3.7 tomato plants, as measured by effects on tomato biomass, while one tomato plant was equivalent to 0.14 Japanese millet plants, as measured by effects on millet biomass. Skewness of per plant biomass distribution is shown to be a result but not an unambiguous measure of competition. Expansion of a reciprocal yield model to mixtures of more than two species is illustrated using three species of duckweed. While some caution is recommended, the reciprocal yield analysis applied to data from appropriately designed experiments is a substantial improvement over more traditional methods.

Floristic composition and soil characteristics (moisture, pH, nutrient contents) in abandoned upland rice paddies of different ages were analyzed to clarify the regenerative aspects of succession as a tool for habitat restoration. The study sites represented five seral stages: newly abandoned paddy fields; successional paddy fields abandoned for 3, 7, and 10 years; and a 50‐year‐old Alnus japonica forest. A vegetation sere was apparent in changes of dominant plant species in the order Alopecurus aequalis var. amurensis (annual grass), Aneilema keisak (annual forb), Juncus effusus var. decipiens (rush), Salix koriyanagi (willow), and Alnus japonica (alder) communities. These temporal stages resemble the spatial zonation of vegetation in local riparian floodplain ecosystems, indicating a hydrosere, with soil moisture decreasing over time. Age distributions and life forms of the dominant plant species support a “tolerance” model of secondary succession, in which the established species persist into later successional stages. Persistence of earlier colonizers led to a net cumulative increase in species richness and a more even distribution of species cover with increasing field age. Between 10 and 50 years, vegetation stabilizes as an alder community. Soil moisture content decreased steadily with paddy field age after an initial rise immediately after their abandonment, whereas pools of organic matter, N, P, K, Ca, and Mg, increased with field age. The pace and direction of recovery of native vegetation and natural soil properties in these abandoned rice paddies resembled classic old field succession, a form of secondary succession that often serves as a template for guiding restoration efforts. Active intervention, in particular dismantling artificial levees, could accelerate the recovery process, but natural habitat recovery generally appears sufficiently robust to achieve “passive” restoration of this rare community without intervention.