Wendy L. Reed, Mark E. Clark North Dakota State University
Density dependent regulation of population size has been a long-time focus of ecological and evolutionary studies, however, the mechanisms underlying density dependence have not been resolved. In density dependent regulation, as population size increases either the birth rate of the population decreases or the death rate of the population increases. Because populations consist of individuals, changes in the birth or death rates must result from changes in individuals. That is, changes in the birth or death rates result from changes in the morphology, behavior or physiology of individuals in the population. No population can increase indefinitely, so mechanisms for density dependent population growth must be manifested in individuals at some point. Unfortunately detecting this pattern in free-living populations is problematic in large part because population size is difficult to experimentally manipulate. However, when the density of individuals in a given area increases, social interactions among individuals increase, and these behavioral interactions are amenable to experimental manipulation and can have far-reaching physiological effects on individual reproductive success.
This study will test the effects of pre-natal behavioral interactions among adults on the post-natal physiology of offspring. A population of American coots (Fulica americana) will be used as a model system to explore this potential density-dependent mechanism. When population densities increase in an area, behavioral interactions among breeding pairs increase because territories are crowded together and intrusions are more frequent. Therefore, simulated increases in population density will be performed by placing captive adult coots in the territories of free-living, breeding adult coots just prior to egg laying. Following the experimental intrusion and egg laying, egg constituents (primarily hormones present in the egg yolk) will be analyzed. Changes in egg physiology may act as a conduit for density-dependent feedback in the population because the early developmental period is especially sensitive to physiological perturbations. The structure and function of hormone systems are similar in most vertebrates, and the results from this study will provide a novel way to approach general processes regulating their populations. Ultimately understanding the biological mechanisms regulating these populations is the key to conservation and management. A central focus of this study is graduate, undergraduate, and community education. Field studies are conducted in rural North Dakota and depend upon local support and involvement, which is fostered by direct educational outreach to the citizens and landowners in these communities.
