Scale breaks (discontinuities) in attributes of animal communities (such as body masses) correlate strongly with a set of poorly understood biological phenomena that seem to mix contrasting attributes. These phenomena include invasion, extinction (high species turnover), increased population variability, migration and nomadism. The clustering of these phenomena at predictable scale breaks suggests that variability in resource distribution or availability is greatest at scale breaks. Location at scale breaks affords species great opportunity, but also potential crisis. Complex behaviors such as migration and rapid adaptation leading to speciation may evolve most efficiently and commonly at scale breaks, where there is the greatest potential reward, although with the highest potential cost.
The causes of nomadism, migration, and decline in vertebrates are poorly understood. Literature suggests nomadism may arise in species that specialize in granivory, nectivory, or the utilization of rodent outbreaks. Literature also suggests that species become migratory because they exploit certain scarce or variable food types. Species decline is hypothesized to be the result of many different factors as well; large species, island species and specialists may be more prone to decline.
GoalsTo determine the distribution of functional groups within and across scales, the association of measures of biotic variability in vertebrates (e.g., invasions, extinctions, nomadism, migration) with discontinuities in body mass distributions, and cross-scale analyses of patterns in body mass distributions from local to hemispheric scales. Data sets examined so far include four different taxa in two different ecosystems.
This project is complete with results compiled into a master's thesis. Results of analyses suggest that a combination of species characteristics, including the distance to the edge of a body mass aggregation, explain the complex phenomena of nomadism, migration and decline. Generally no single predictive model was supported. However, results suggest that position in which a species exists within a body mass distribution can aid in predicting nomadic and migratory bird species. Almost all models tested show that as a species approaches the edges of a body mass aggregation, it has a greater likelihood of being nomadic or migratory. Models testing for species decline, however, yield less robust results and instead indicate that larger body masses of species indicate a declining population. However, these results may be due in part to human observation of declining species being biased towards larger species.