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The causes of nomadism, migration, and decline in vertebrates are debated issues in the ecological sciences. Literature suggests nomadism may arise in species that specialize in granivory, nectivory, or the utilization of rodent outbreaks. Migration is thought to arise as a result of the exploitation of certain scarce or variable food resources. 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.
A fresh perspective regarding the causes for species nomadism, migration, and decline is being investigated utilizing the ideas within the Textural Discontinuity Hypothesis. The Textural Discontinuity Hypothesis stems from complex systems analysis and posits that body mass distributions form aggregations within ecological systems, and that those body mass aggregations reflect discontinuous distributions of resources. Additionally scientists have posited that species at the edges of body mass aggregations may be exposed to highly variable resources. Literature indicates nomadic and declining bird species populations occur at the edges of body mass aggregations more frequently than expected. Migratory bird species also may be located at the edges of body mass aggregations more frequently than expected. The morphological spacing of species within aggregations may yield clues regarding species interactions. The distribution of species within a body mass aggregation would have low variance if species within an aggregation interact with each other strongly – morphological overdispersion has been documented in many animal communities and reflects strong competitive interactions among species.
I analyzed nomadism, migration, and decline in South African birds using an information-theoretic approach. I assembled a series of plausible models based upon suggested or theoretically predictive characteristics. Additionally, I used a series of Monte Carlo simulations to investigate the distribution of species within aggregations, in terms of body mass.
Results 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 there was no single model supported, and often many models were in the confidence set, providing only weak inference. Within body mass aggregations, there is more variance among species than null expectations, thus with my dataset morphological overdispersion is not present within body mass aggregations. Nomadism, migration, and decline are complex phenomena which incorporate different species characteristics, perhaps explaining why such debate still exists over the causes of these phenomena.