Dispersal among local communities can have a variety of effects on species composition and diversity at local and regional scales. Local conditions (e.g., resource and predator densities) can have independent effects, as well as interact with dispersal, to alter these patterns. Based on metacommunity models, we predicted that local diversity would show a unimodal relationship with dispersal frequency. We manipulated dispersal frequencies, resource levels, and the presence of predators (mosquito larvae) among communities found in the water-filled leaves of the pitcher plant Sarracenia purpurea. Diversity and abundance of species of the middle trophic level, protozoa and rotifers, were measured. Increased dispersal frequencies significantly increased regional species richness and protozoan abundance while decreasing the variance among local communities. Dispersal frequency interacted with predation at the local community scale to produce patterns of diversity consistent with the model. When predators were absent, we found a unimodal relationship between dispersal frequency and diversity, and when predators were present, there was a flat relationship. Intermediate dispersal frequencies maintained some species in the inquiline communities by offsetting extinction rates. Local community composition and the degree of connectivity between communities are both important for understanding species diversity patterns at local and regional scales.

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A commonly observed phenomenon in plant communities is that the addition of a limiting resource leads to an increase in productivity and a decrease in species diversity. We tested the hypothesis that the mechanism underlying this pattern is a disproportionate increase in mortality of smaller or shade—intolerant species in more productive sites caused by reduction of light levels. We added water and/or one of three nutrients (nitrogen, phosphorus, and potassium) to a 1st—old—field community dominated by weedy annuals and measured effect on productivity, species composition, diversity, and light levels after one growing season. Diversity was not clearly related to productivity in this experiment. Watering increased productivity, but, contrary to expectations, had no effect on density of surviving plants, species diversity, or abundance of low—growing species. Almost all the increase in biomass with watering was due to a positive response by Ambrosia artemisiifolia, an upright annual that was the most common species in the canopy in all treatments. The addition of nitrogen had only a small positive effect on productivity, but strongly decreased density of surviving plants, species diversity, and abundance of most low—growing species. Only Ambrosia increased in abundance with nitrogen addition. The phophorus and potassium additions had little effect on the community. We suggest that the high mortality and low diversity in the nitrogen addition plots, but not in the more productive watered plots, was due to limitation by nitrogen earlier than limitation by water during the growing season. The consequence was earlier canopy closure and greater mortality due to light limitation.

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Mouquet, N., Munguia, P., Kneitel, J. M. and Miller, T. E. 2003. Community assembly time and the relationship between local and regional species richness. -Oikos 103: 618-626.Many previous studies have assumed that a linear relationship between local and regional species richness indicates that communities are limited by regional processes, while a saturating relationship suggests that species interactions restrict local richness. We show theoretically that the relationship between local and regional richness changes in a consistent fashion with assembly time in interacting communities. Communities show saturation in their early assembly stages because only a subset of the regional pool may colonize a locality. At intermediate assembly times, communities will appear unsaturated until significant competitive exclusion occurs. Finally, when communities reach equilibrium, we found saturation as a result of resource competition resulting in the dominance of a limited number of species. We show that habitat size and species fecundity are important in determining the time needed for the community to reach equilibrium and thus affect the relationship between local and regional species richness. Our results suggest the number of coexisting species is a function of local and regional processes whose relative influences might vary over time and that research using the relationship between local and regional species richness to infer mechanisms limiting species richness must have knowledge of the assembly time of the community.

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If it were possible to promote or suspend the formation of specifically sensitized lymphocytes (activated T cells), the problems of achieving a sustained attack on tumor cells and microbial parasites, or of preventing graft rejection, might be largely overcome. Methods for manipulating the immune response for such purposes have been proposed from time to time (1-3), but progress has been slow because so little is known of how cell-mediated immunity is normally regulated. Although much has been learned about the allied problem cf what controls the formation of antibodies (4), almost nothing is known about the mechanism that regulates the production and function of the cells which mediate delayed-type hypersensitivity (DTH).I While studying the tumor-suppressive activity of Mycobacterium boris BCG it was observed that lymphoid tissues which were under the stimtflatory influence of a BCG infection were capable of a much more vigorous response to a second antigen (5). Both cellular and humoral immunity to sheep red blood cells (SRBC) were augmented, as evidenced by higher and more sustained levels (DTH) and increased numbers of plaque-forming cells (PFC) in responding lymph nodes. Since D T H does not usually appear unless special conditions of immunization are used, these findings suggested that the formation of activated T cells is normally restricted by an inhibitory mechanism that does not operate properly in lymphoid tissues infected with BCG.Miller et al. (5) have shown that mice given a subcutaneous injection of SRBC in saline develop a poorly sustained state of hypersensitivity which conforms to all of the established criteria by which D T H is recognized, including its mediation by 0-bearing lymphocytes. It was therefore possible to study the mechanism which regulates T-cell activity in the absence of any influence from adjuvants such as were used by Nelson and Mildenhall (6) when they, too, showed that mice develop classical D T H in response to SRBC.

Competitive interactions involve two separate processes: the effect of a species on others in the community and the response of a species to all others. Five species from a 1 st-yr field were investigated to determine if there is any correlation between competitive effect and response and if the patterns of effect and response can explain the relative success of each species in the full community. Effect and response were measured by observing the growth of individuals in all possible monocultures and two-species mixtures, using ambient densities (Year 1) and a range of densities (Year 2) for each species. Both monocultures and two-species mixtures were obtained by removing unwanted individuals at emergence, leaving naturally emerging individuals of the desired species.Competitive effect and response were found to be inversely correlated. The inverse correlation lead to a hierarchy of competitive ability, with Ambrosia artemisiifolia being the competitive dominant, followed by Agropyron repens, Plantago lanceolata, and finally the competitive subordinates Chenopodium album, Lepid!um campestre (used in Year 1), and Trifolium repens (used in Year 2). The interactions were generally asymmetric, e.g., Ambrosia artemisiifolia had a large suppressive effect on the other species and demonstrated no response to their presence. The hierarchy and a lack of specificity of the interactions suggest that all the species are limited by, and competing for, the same resource or resources. The hierarchy of competitive ability appears to be a major factor in determining the abundance of each species in the full community.