Browsing by Subject "Population dynamics"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Ecological and anthropogenic drivers of giraffe (Giraffa camelopardalis tippelskirchi) population dynamics in the Serengeti
    (2014-04) Strauss, Megan Kate Louise
    Considerable changes in the abundance and demography of Serengeti National Park giraffes (Giraffa camelopardalis) provide insight into the drivers of ungulate population dynamics. When last systematically studied in the 1970s, the giraffe population was growing at a rate of 5-6% per year, in response to an increase in the biomass of palatable browse species. During three field seasons (2008-2010), I collected data on a significantly reduced population on giraffes: 5-7 times smaller than the 1970s peak. Interestingly, this drop in abundance corresponds to an increase in the density and cover of woody vegetation across the Serengeti. Using the Serengeti giraffe population as a case study, I explore the role of food supply, predation, parasites, and poaching on population dynamics. This investigation is facilitated by contemporary field studies, including vegetation surveys, aerial counts of giraffes, and regular observations of >900 individually identified giraffes. Recent observations are compared to results from the 1970s and combined with a range of other long-term data from the Serengeti (including data on lion kills from a >40-year dataset and data on poaching) to provide a multifaceted picture of the giraffe population and its environment. (I also devise a novel technique of studying lion claw marks on the skin of live giraffes as a measure of predation attempts.) My population estimates confirm that giraffe abundance has fallen significantly since the 1970s. Moreover, I find that recruitment and adult survival are lower now than in the 1970s. The data suggest that giraffe abundance in the Serengeti is currently limited by poaching and by a reduction in the relative dominance of palatable plant species brought about, in part, by the giraffe's own browsing behavior. Using a population model, I investigate how survival and fertility affect giraffe population growth and find evidence that population growth is most sensitive to adult survival. I recommend reducing the targeted poaching of adult giraffes as a conservation measure.
  • Thumbnail Image
    Item
    The role of predator removal and density-dependence on mallard production in northeastern North Dakota
    (2010-05) Amundson, Courtney Linda
    Density-dependence is a central tenet of wildlife population dynamics and harvest management, yet the pathways and mechanisms by which density-dependence operates remain unclear. In 1994, Delta Waterfowl Foundation began trapping meso-predators in northeastern North Dakota to determine if it was a viable technique for increasing waterfowl production. Subsequent research on replicated experimental sites found that trapping predators increased nest success by about 2-fold, and nest success was the most important factor regulating population growth in previous studies of midcontinent mallards. Given the philopatric nature of most waterfowl species, higher nest success on trapped sites was predicted to lead to 36% greater annual population growth on trapped sites versus controls, which should have led to doubling of pair densities approximately every two years. This backdrop provided me with the unique opportunity to examine the role of density-dependence on waterfowl production at a local scale in North Dakota. My dissertation focuses on the role of density-dependence and predator removal on mallard duckling survival, duckling body condition, and overall mallard productivity in northeastern North Dakota from 2006-2007. Contrary to predictions, predator removal had no beneficial effect on mallard duckling survival. Duckling density was weakly negatively associated with mallard duckling survival, but had no effect on duckling condition. Duckling survival, not nest success, was the most important factor influencing population growth and production and appeared to be functioning as the key limiting factor in our populations. Overall, predator removal did not lead to local population increases on trapped sites and added far fewer incremental ducks to the fall flight than originally predicted. The perils of duck hunting are great - especially for the duck. - Walter Cronkite.