• Robyn Redekopp

    Robyn Redekopp
    Univ. of Washington
    Researched the diverse effects of predators to their ecosystems.

    From June until September of 2004 I worked as an intern at the Alaska Fisheries Science Center in Seattle, Washington. Most of my time was spent doing research for Dr. Charles W. Fowler, leader of the Systemic Management Studies Program at the Center's National Marine Mammal Laboratory. This research covered the diverse effects of predators on their ecosystems. When a predator is added to an ecosystem, the common belief is that the population of their prey automatically decreases, but studies have shown the relationship to be much more dynamic, diverse, and complicated than this (Pimm 1991, Sih et al. 1985).

    In my research on predator-prey relationships, I reviewed approximately 100 papers published in various scientific journals. The papers covered various different experiments on the effects of predators on their prey populations (none that were performed specifically for the purpose of my research). Sixty-one data points were gathered from 21 papers. All of the information extracted was collected into a Microsoft Excel file containing data on the species involved, the length and location of the study, and the prey population numbers when the predator was either added or removed. Numbers were entered as the ratio (R) of the prey population affected by predation to the prey population free of the effects of predation.

    The results of my research (Fig. 1) show a broad range in the change in prey population in reaction to predators. In 5 of the 61 cases the

  • prey populations were not affected by the presence or absence of the predator (R=1). The effects of predators on prey populations were beneficial to the prey (prey populations were larger in presence of the predator) for about one-third of the cases; one case showed a prey population that was twenty-fold larger in the presence of the predator than in its absence. For the remainder of the cases (about 56%) the prey population was smaller in the presence of the predator. Thus, nearly half of the prey populations either benefited (increased, colored bars to the right of 0.0 in the bottom panel of Figure 1) from the effects of predation or were unaffected.

    Explanations for why prey population would increase involve the complexity of ecosystems. For example, many predators feed on a

  • variety of prey species. Some of these prey may themselves be predatory on other species in the diet of the initial predator - an intermediate predator. When the initial predator is present, there would be (on average) more suppression of the intermediate species, leading to increases in some of the populations of prey it shares with the initial predator.

    Another potential explanation may play a role, even in combination with the above. Predators feeding on several species can relieve some prey from competition by reducing the populations of their competitors.

    There are probably other factors that contribute to observed increases in prey populations - empirical observations that may otherwise seem counterintuitive.

    The rest of my time as an intern at the Alaska Fisheries Science Center was spent compiling a list of the predators, parasites, and diseases that effect individual prey/host species. The list consists of 244 prey species, collected from 104 books. The results show that smaller bodied species fall prey to more predators than do larger prey species. I also spent 2 weeks on the NOAA Ship Miller Freeman near Kodiak, Alaska, performing acoustics research. My duties included sorting, weighing, and measuring catches, removing otoliths and stomach samples, and participating in various oceanographic studies.

    My 10 weeks working for NOAA were overall very educational. My previous familiarity with Excel was broadened and I was exposed to two other software packages. I learned about some of the failings of conventional forms of management and ways that they can be fixed. Also on the Miller Freeman, my knowledge of the biology of fishes was increased, and I was introduced to oceanographic research - something I enjoyed very much.

  • References:

    Pimm, S.L.
    1991. The balance of nature? Ecological issues in the conservation of species in communities. The University of Chicago Press, Chicago, IL. 434pp.

    Sih, A., P.H. Crowley, M.A. McPeek, J.W. Petranka, and K. Strohmeir. 1985. Predation, competition and prey communities: a review of field experiments. Annual Reviews of Ecology and Systematics 16:269-311.