Wade B. Worthen

Community composition and nested-subset analyses: basic descriptors for community ecology.

Three primary descriptors of community structure are the number, identities (composition), and abundances of species therein. Over the past 35 years, most attention has focused on relationships involving number (species-area and species-energy relationships) or abundance distributions (broken-stick, geometric, log-normal, core-satellite, etc.). Composition patterns have been underemphasized, even though several tools for addressing particular non-random patterns in species composition are available. One non-random pattern in community composition is nested subset structure. A community has a nested subset structure if the species found in depauperate replicates are also found in progressively more species-rich assemblages. In this review, the problems of failing to consider composition patterns like nestedness are described, using species-area relationships and the SLOSS debate (single large or several small reserves) as an example. In addition, nestedness analyses are promoted as: 1) important descriptive tools for determining whether a community has this common non-random pattern of species composition, and 2) as important investigative tools for suggesting mechanisms potentially structuring a community.

Community Structure and Environmental Stress: Desiccation Promotes Nestedness in Mycophagous Fly Communities

In a previous field experiment, communities of mycophagous flies that emerged from Agaricus bisporus mushrooms exhibited a nested-subset pattern related to mushroom size. A phorid species and Leucophenga varia (Drosophilidae) emerged from small, medium, and large mushrooms, but other drosophilid species were usually limited to large mushrooms. Here, we conducted two experiments and one field survey to determine whether this nestedness pattern is caused by nested desiccation tolerances among species, coupled with the mediating effects of mushroom size. In the first experiment, 20 first instar larvae of either Drosophila tripunctata or D. putrida were added to A. bisporus mushrooms that weighed 5 g, 10 g, or 20 g. Mushrooms were placed in environmental chambers at different temperatures for 5 d (maxima of 25°C, 30°C, or 35°C). Both mushroom size and temperature had significant direct effects on the proportion of larvae completing development; larvae in small mushrooms are more susceptible to desiccation stress than larvae in large mushrooms. In a second experiment, we examined the effect of desiccation stress on community structure. Woodland mushrooms collected in the field were cut in half; the halves were placed at different incubation temperatures (maxima of either 25°C or 35°C) for 5 d, and emerging flies were counted and sorted by species. The halves incubated at 35°C produced significantly nested communities, the halves incubated at 25°C did not. Correlations between fly abundance and mushroom mass were also more significant at higher temperatures. These results were consistent with the hypothesis that desiccation stress, differentially mediated by mushroom size, drives nested subset structure in these communities. In a four-week field survey, however, nestedness was not related to mean temperature or rainfall. We contend that small habitats are less resistant to changes in environmental conditions than larger habitats, so the intensity of environmental stress will correlate with habitat size. So, some initially variable and non-nested communities might decay to a nested-subset pattern of species composition as a result of differential habitat buffering under stressful environmental conditions.

Nestedness in assemblages of metazoan ecto- and endoparasites of marine fish

Assemblages of metazoan ectoparasites of 79 species and gastrointestinal helminths of eight species of marine fishes were analysed to examine whether nestedness is related to sample size, abundance, species richness, and prevalence of infection, and whether the use of z-scores or Monte Carlo simulations yields different results. No significant differences in the number of nested assemblages were found with the two methods, and neither sample size nor abundance, but prevalence of infection of ectoparasites was correlated with nestedness. Species richness was significantly correlated with nestedness only when fish species with fewer than three parasite species were not excluded. Differential colonisation probabilities are the most likely cause of nestedness.

Nested subset analyses of colonization-dominated communities: metazoan ectoparasites of marine fishes

Communities have a nested subset structure if the species comprising depauperate assemblages represent non-random subsets of progressively richer communities. Nestedness results from differential extinction probabilities, differential dispersal and colonization probabilities, nested environmental tolerances, or facilitated succession. Previous research suggests that the frequency of significant nestedness, and the degree of nestedness as measured by a standardized index, C, should be greater for extinction-dominated communities than colonization-dominated communities. To test these hypotheses, we conducted nestedness analyses on 38 communities of metazoan gill and head ectoparasites infecting different species of marine fishes. Extinction, environmental tolerances and niche space, and interspecific interactions do not significantly influence the structure of these ectoparasite communities. As such, they provide an appropriate system in which to examine the frequency and degree of nestedness in colonization-dominated assemblages relative to extinction-structured communities. Using two common analyses, we found that nestedness in these ectoparasite communities was rare (1 of 38 communities after a conservative Bonferroni correction for multiple comparisons). However, the mean standardized nestedness score (C = 0.483 ± 0.243) was not significantly different from the mean nestedness score of extinction-structured communities reported in the literature. Therefore, although these communities do seem less nested than extinction-dominated communities (from the frequency analysis), the C statistic is unable to distinguish these groups. Also, neither the frequency of significant nestedness nor mean standardized C scores were related to mean water temperature, host habitat (benthic, pelagic, or benthopelagic), host trophic level (herbivore, planktivore, predator, or omnivore), or whether or not the host is a schooling species. These results confirm previous conclusions stating that these communities are largely random, unstructured assemblages.

Nested subset structure of larval mycophagous fly assemblages: nestedness in a non-island system

Nested subset structure has been studied in archipelagoes and fragmented habitats, and has been attributed to differential colonization and extinction rates among species and nested environmental tolerances. In this experiment, we tested for nestedness in assemblages of mycophagous fly larvae. Twenty mushrooms in each of three size classes (4.8–6.0 g, 10–15 g, 21–32 g) were placed on moist potting soil in experimental cups. The cups were placed in oak and pine forests in Greenville, S.C., USA for 5 days, where they were available to ovipositing flies. Upon collection, the mushrooms were incubated in the laboratory for 3 weeks and all emerging flies were sorted by species, counted, and weighed. A random placement analysis was conducted to determine whether the species richness pattern was a sampling artifact of the species abundance distributions. The actual species richness pattern did not conform to the random placement model; most mushrooms contained significantly fewer species than predicted by random sampling. The communities were strongly nested as measured by two different indices, and the nestedness pattern was related to mushroom size. Small mushrooms usually produced no flies or a single species, Dohrniphora sp. (Phoridae). Medium and large mushrooms typically produced more species-rich communities that usually contained the phorid and Drosophila putrida, D. tripunctata, and Leucophenga varia. This core guild was nested within a more diverse assemblage that included D. falleni, Mycodrosophila dimidiata, a muscid, and two Leptocera sp. (sphaeroceridae). These patterns are tentatively explained in the context of nested desiccation tolerances, mediated by differences in mushroom size.

HIGHER-ORDER INTERACTIONS AND INDIRECT EFFECTS: A RESOLUTION USING LABORATORY DROSOPHILA COMMUNITIES

The importance of higher-order interactions to community structure has been debated for decades. We attempt to resolve the conflict by placing higher-order interactions in their appropriate functional relationship with indirect effects. Indirect effects are changes in the outcome of a pairwise relationship caused by other species. These changes can be mediated through changes in species densities (altering the frequency of encounters) or through a change in the nature of the relationship itself (such as a change in the per capita competition coefficients in Lotka-Volterra models). These latter forms of indirect effects, limited by their specific mechanistic definition, are higher-order effects. They can be resolved empirically only if density effects can be accounted for, and this additional analytical requirement may explain their scarcity. We also test the competitive relationships among mycophagous drosophilids and describe an indirect effect. However, there was no evidence of a significant higher-order effect; the nonadditive effect is apparently mediated through changes in species densities. Previous laboratory studies of multispecies Drosophila communities failed to document nonadditive effects. We suggest that the multigenerational approach of earlier studies may obscure indirect effects that could be important to natural communities.

Phenotypic and demographic variability among patches of Maienthenaum canadense (Desf.) in central New Jersey, and the use of self-incompatibility for clone discrimination

Dept. of Biological Sciences and Bureau of Biological Research, Rutgers-The State University of New Jersey, P.O. Box 1059, Piscataway, NJ 08854 WORTHEN, W. B. AND E. W. STILES. (Dept. Biol. Sciences, Rutgers-The State University of NewJersey, P.O. Box 1059, Piscataway, NJ 08854) Phenotypic and demographic variability among patches of Maianthemum canadense (Desf.) in central NewJersey, and the use of self-incompatibility for clone discrimination. Bull. Torrey Bot. Club 113:398-405. 1986.-Phenotypic variability, documented for nine patches of Maianthemum canadense (Desf.) at three sites in central New Jersey, did not correspond to presumed environmental differences between sites or correlate with differences in ramet density. More significant differences were found among patches than among sites for ramets/m2, percentage of ramets flowering/M2, percentage of flowering ramets fruiting/M2, flowers/inflorescence, and the per cent hot-water soluble carbohydrates in the fruit pulp. Fruit nutrient characteristics and flowers/inflorescence measures had very low variances within patches, and seem more likely to have been influenced by genetic differences among clones than environmental differences among habitats. Hand-pollination crosses of Maianthemum canadense were made within a patch and between patches. Incompatibility was complete for within-patch crosses, suggesting genetic identity of patch examined. The phenotypic variability described among patches may have a genetic basis.

Pollen-limited fruit set in isolated patches of Maianthemum canadense Desf. in New Jersey'

WORTHEN, W. B. AND E. W. STILES (Department of Biological Sciences and Bureau of Biological Research, Rutgers-The State University of New Jersey, Piscataway, NJ 08855-1059). Pollen-limited fruit set in isolated patches of Maianthemum canadense Desf. in New Jersey. Bull. Torrey Bot. Club 115: 299-305. 1988.-We examined the effects of pollen addition and raceme density on fruit set in seven isolated patches of Maianthemum canadense Desf. (Liliaceae; False Lily-of-the-Valley) in central New Jersey. We removed racemes from two of four quadrants in each patch to create distinct high and low density sectors. We hand pollinated all the racemes in four 1-m quadrats (1/quadrant) within each patch, and compared fruit set in these quadrats with fruit set in eight naturally pollinated control quadrats (2/quadrant). We recorded the percentage of racemes that bore fruit and the mean number of fruits/infructescence in each quadrat, and analyzed these data with three-factor analyses of variance (patch, density, and pollination-treatment effects). There was a significant effect of pollen addition on fruit set. In four patches, hand pollination significantly increased both the percent of racemes that bore fruit and the number of fruits/infructescence. We conclude that fruit set in these patches of M. canadense was pollen limited. A previous study reported that fruit set in M. canadense was resource limited, but our results suggest that fruit set in different patches may be limited by different factors. The clonal architecture of a patch and the relatedness and distance between mating plants may influence these patterns. There were no significant density effects, in any patch, on fruit set in hand pollinated or naturally pollinated quadrats. Although this suggests that plant density (and inferentially, competition for resources) did not limit fruit set, clonal growth patterns may confound this conclusion.

Complex Interactions between Biotic and Abiotic Factors: Effects on Mycophagous Fly Communities

We measured the direct and interactive effects of larval density, ant predation, and soil moisture on larval survival and community structure of mycophagous flies. First instar larvae of a brown-eyed Drosophila tripunctata mutant were added to Agaricus bisporus mushrooms (10, 20 or 40 larvae/mushroom). The mushrooms were placed on dry, moist, or wet soil in ant-access and ant-exclusion treatment cups. The cups were placed in a woodland for 5 days where they were available to native flies for oviposition. We used ANOVA and MANOVA to measure the effects of our independent variables on the percentage of mutant larvae completing development and the number and diversity of native metamorphs. Ants visited wet soils (30% visited) more frequently than moist (20% visited) or dry soils (10% visited), and had stronger negative effects on larval survivorship and native metamorph abundance in wet and moist microenvironments

Temperature Tolerance in Three Mycophagous Drosophila Species: Relationships with Community Structure

Patterns of host use at several spatial and temporal scales suggest that temperature tolerance may influence the structure of mycophagous fly communities. For instance, Drosophila putrida is more abundant than D. falleni and D. tripunctata in dry sites, at dry periods, and in small, desiccation-prone mushrooms. In this experiment, we acclimated flies from these species at different temperatures (15°C, 20°C, 25°C, and 30°C) for 5 d, and then measured their responses to acute thermal stress (increasing temperature 0.5°C/min). ANOVA were used to describe the effects of sex, species, and acclimation temperature on: 1) survivorship after 5 d, and 2) Critical Thermal Maxima (CTMax). Critical Thermal Maximum was measured as the temperature at which 50% of the flies in a sample were incapable of righting themselves. Survivorship was uniformly high for all species from 15°C to 25°C. However, at 30°C, D. putrida survival (74%) was significantly higher than either D. falleni (39%) or D. tripunctata (23%). When averaged across all acclimation temperatures, D. putrida also had a higher CTMax than the other species (40.7°C, compared to 40.2°C for D. falleni and 39.9°C for D. tripunctata). D. putrida was the only species with a CTMax that increased significantly after acclimation at 30°C. Thus, D. putrida was more tolerant to chronic and acute thermal stress than the other species, and was the only species to show an adaptive physiological response to high temperature exposure. These species differences are consistent with spatial and temporal abundance patterns, and may contribute to patterns in community structure.