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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.

A model of species interactions based on their use of shared resources was proposed in 1972 by Robert MacArthur and later expanded in an article (1980) and a book (1982) by David Tilman. This "resource-ratio theory" has been used to make a number of testable predictions about competition and community patterns. We reviewed 1,333 papers that cite Tilman's two publications to determine whether predictions of the resource-ratio theory have been adequately tested and to summarize their general conclusions. Most of the citations do not directly test the theory: only 26 studies provide well-designed tests of one or more predictions, resulting in 42 individual tests of predictions. Most of these tests were conducted in the laboratory or experimental microcosms and used primary producers in freshwater systems. Overall, the predictions of the resource-ratio theory were supported 75% of the time. One of the primary predictions of the model, that species dominance varies with the ratio of resource availabilities, was supported by 13 of 16 tests, but most other predictions have been insufficiently tested. We suggest that more experimental work in a variety of natural systems is seriously needed, especially studies designed to test predictions related to resource supply and consumption rates.

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The decay of community similarity with distance (distance decay) is reported for many taxa in a variety of geographic settings. However, the importance of scale, distance measure, ecoregions, and ecological transition zones to distance decay has not been thoroughly examined. The goal of our study was to test the effects of these factors on distance decay in two freshwater assemblages (benthic macroinvertebrates and fish) with differing dispersal abilities in small streams within the Patuxent River basin, Maryland, USA. The Patuxent basin contains a geologic Fall Line, an ecological transition zone separating the two main regions of the basin. For both assemblages, we examined distance decay in similarity at several extents: entire Patuxent, Piedmont sub-region, and Plains sub-region using both linear geographic and stream network distances. Decay patterns were observed across all extents and distances. At the Patuxent extent decay rates differed between linear and stream distance only for macroinvertebrates (linear !stream); with both distance measures, similarity in fish decayed faster than similarity in macroinvertebrates. Within the Plains, decay rates for macroinvertebrates were lower than at the Patuxent for both distance measures; no difference in decay rates for this assemblage were detected in the Piedmont. Decay rates of similarity for fish only differed (lower) from rates at the Patuxent when examined at the Piedmont extent with stream distance. Similarity for the subset of sites that were located in separate ecoregions decayed at a slower rate than similarity for the entire data set only for macroinvertebrates with linear distance, suggesting a weak effect of the transition zone on distance decay. Together, these results suggest multiple factors contribute to the distance decay pattern and therefore regional diversity patterns, suggesting conjoint examination of these factors will further our understanding of the mechanisms governing regional diversity patterns.

Summary1. I present a successional study of marine organisms on pen shells ( Atrina rigida ) at different regions of St Joe Bay, Florida. By incorporating measures of relative abundance and assembly time I show how the relationship between local and regional diversity develops through different successional stages. 2.The results showed that, with time, motile species richness increases significantly while evenness indices remain high and constant. Sessile species, on the other hand, increased in both species richness and evenness through time. 3. For the motile species, regions seem to remain different while local saturation is observed. These results suggest that this group is under species-sorting: species are mobile enough that recruits and adult dispersal within a region maintain differences among regions, while local communities are saturated. 4. For the sessile species, the local-regional relationship was unsaturated at all sampling dates with both untransformed and rarefied data. Regions are initially similar in community structure, then differ through time to become similar again at the last sampling date. This may reflect a priority effect: propagules that arrive at a shell may initially exert influence on the species composition on a shell, so that at intermediate sampling times regions differ in community structure. However, at the last sampling there were no differences detected among regions, suggesting that dispersal distances might be larger for this group of species. 5. These results suggest the following. (1) The degree of species saturation will depend on the successional stage of a community. (2) Incorporating species abundances (i.e. through rarefaction or other techniques) demonstrates the role of species commonness or rarity in determining patterns of community diversity at different scales. (3) Depending on the group of species studied, the size of the region will vary and will influence the local-regional dynamics: the perceived region for sessile species may be larger than for motile species.

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.. abstract: Fiddler crabs are highly sexually dimorphic. Males possess one small (minor) feeding claw and one greatly enlarged (major) claw; females possess two small claws. The major claw is used to attract mates and for burrow defense, but it is costly for the male to possess. We tested the hypothesis that the major claw also functions as a thermoregulatory structure, a function that would allow males to spend a greater amount of time at the surface, foraging and attracting potential mates. Fiddler crabs Uca panacea were exposed to a source of radiant heat and body temperatures were monitored.Four groups of crabs were tested: intact males, males with the minor claw removed, males with the major claw removed, and females. The body temperatures of males without the major claw increased more rapidly and reached higher values than did those of males with the major claw intact, but the results from these animals were similar to those of females. These results support the hypothesized thermoregulatory function of the major claw. The major claw may function as a heat sink, transferring heat away from the body and dissipating it into the air. Enhanced thermoregulatory ability provided by the major claw may partially ameliorate the energetic costs of possessing such a large claw.

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