current projects
Prehistoric Extinction of Birds on Oceanic IslandsQuestion: What ecological characteristics put native bird species at risk of extinction over 1500 years ago on the remote Hawaiian archipelago? Data Studied: Mean body size, dietary and ecological information, and phylogenetic lineage of all known indigenous, non-migratory land and freshwater bird species of the five largest Hawaiian Islands. Extinct species were divided into "prehistoric" and "historic" extinction categories based on the timing of their latest occurrence. Analysis: I compared the ecological selectivity of the pre-European extinction to later "historic" extinctions using Kolmogorov-Smirnov tests. Size-bias was measured by logistic regression. I compared body mass distributions of prehistoric victims, historic victims, and extant species as well as foraging guilds and lineages. Regression trees were used to predict probability of prehistoric and historic extinction based on five ecological predictor variables (body size, feeding guild, endemicity, nest location, and flightlessness). Conclusions: Prehistoric extinctions showed a strong bias toward larger body sizes and flightless, ground-nesting species. Many small, specialized species also disappeared, implicating a suite of human impacts. In contrast, medium-sized species showed the highest extinction rates among historic extinctions.
Interspecific pairwise relationships among body length, clutch size, and latitude: anatomy of a macroecological paradox in birds
Aim: It is generally thought that ecogeographic and allometric rules can be detected if the underlying mechanisms responsible for them are operating. Here, we examine this assumption in a system of three interrelated variables (i.e., body size, clutch size, and latitudinal distribution in birds) with attention to the variation observed within each of the three variables. Methods: We used linear regression to detect significant correlations between pairwise combinations of variables at the genus, family, and order levels. Independent contrast analyses were performed on four clades with available phylogenies, and the evolutionary heritability of each variable was quantified. Results: The latitudinal increase in body size (Bergmann's Rule), the negative allometry of clutch size (Calder's Rule), and the latitudinal increase in clutch size (Lack's Rule) were all detected with increasing frequency toward higher taxonomic levels. Evolutionary heritability of all three variables decreased with taxonomic level, which led to an increase in the variability of the variables at higher taxonomic levels. Independent contrasts yielded fewer significant results than linear regressions, detecting Lack's Rule in three of four clades, Bergmann's Rule in one clade, and Calder's Rule in one clade. Main Conclusions: Overall, the three relationships formed a triangle of interrelated relationships where indirect effects oppose direct effects. Generalizing from our study, failure to detect ecogeographic rules and allometries does not prove a lack of underlying process. The process may be present but overridden by a more powerful conflicting process. Within a system of interrelated variables, the likelihood of detecting a ''rule'' is influenced by the amount of variation inherent in the variables.
Mean-Variance scaling in Body Size
We examined body size variability in a number of diverse taxonomic groups in order to determine the extent to which passive and driven mechanisms govern body size evolution. Using global body size data on Birds, Mammals, Elasmobranchs, Cephalopods, and Hydrozoan medusae, we calculated the mean and variance of body size within each genus and family and compared these values to those generated by a passive model of body size evolution. We found that the mean and variance of body sizes across the animal kingdom exhibit a power-law scaling relationship with an exponent of approximately 2, which is consistent with a random multiplicative process of body size change. This power-law scaling is reminiscent of Taylor's Power Law, which is also based on underlying multiplicative processes. However, real differences in the relative variability of body size do exist between groups of species; thus, we suggest that a mean-variance scaling relation for body size with an exponent of 2 is best viewed as a null expectation with respect to phylogenetic, morphological and selection constraints, and that meaningful differences between groups may be measured relative to this prediction.
Past Projects
Craig McClain, Gary Rosenberg and I tested the generality of the island rule-- a graded trend from gigantism in small-bodied species to dwarfism in large-bodied species-- in the deep sea, a non-insular, resource constrained system. Our paper, "The island rule and the evolution of body size in the deep sea," appeared in the Journal of Biogeography. Body size measurements of deep and shallow-water gastropods were obtained from the Malacolog v3.3.3 database. We then conducted regression analyses of the body size changes of deep-sea gastropod species relative to their shallow-water congeners. Our results indicate that, consistent with the island rule, gastropod genera with small-bodied shallow-water species have significantly larger deep-sea representatives, while the opposite is true for genera that are large-bodied in shallow water. Bathymetric body size clines within the deep sea are also consistent with predictions based on the island rule. Like islands, the deep sea is characterized by low absolute food availability, leading us to hypothesize that the island rule is a result of selection on body size in a resource constrained environment. Body size of deep-sea species tends to converge on an optimal size for their particular ecological strategy and habitat.
Top-down and Bottom-up Regulation of the Grassland Arthropod Community
At Hendrix College, Matt Moran and I investigated the relative strengths of top-down and bottom-up processes acting in an Arkansas grassland. We also studied the effects of fire timing on grassland plants and arthropods.Our paper, "Seasonal Variation in Top-down and Bottom-up Processes in a Grassland Arthropod Community," was published in July 2003 in Oecologia.