Biotic And Abiotic Perturbations

Submitted By atreamy
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“Almost all organisms on earth live in environments that have been altered, often drastically, by humans.” (Rohr et al. 2006) Organisms must have the ability to behave under these conditions as their success or failure in altered environments is determined by the way they react to these modifications. While hundreds of species are suffering dramatically from these environmental changes, others, such as “urbanized and pest species, are doing better than ever.” (Sih et al. 2010) Within the scope of this paper, a closer look at a study of the “response of fish introduction and subsequent disappearance of amphibians, in naturally fishless alpine lakes” (Knapp et al. 2001), will be reviewed.
“The question of how natural communities respond to biotic and abiotic perturbations is central to many of today’s most pressing ecological problems, including the maintenance of biodiversity in the face of escalating habitat alteration and the invasion of ecosystems by exotic species.” (Knapp et al. 2001) “Over the past 50 years, ecosystems have changed more than at any previous time in the history of our species.” (M.A., 2005) Prior this study, only short term studies had been conducted which created several limitations. Short-term studies were found to be not long enough in accurately “capturing only the transient dynamics that characterize food webs following a perturbation.” (Yodzis 1998) These perturbations of whole lakes were only conducted in one direction, solely when a species was added or removed. Finally, the studies were conducted at only a single site or pair of sites. This created difficulty in generalizing from the specific response of one site to how the other sites might react to the same perturbation. In this case, the introduction of trout species into naturally fishless lakes.
It was not until this study that a long term study approach had been taken. This was to assist in gaining perspective and generate a better understanding “on the long-term community response to fish introductions and the capacity of these communities to recover after fish disappearance.”(Knapp et al. 2001) While it is not necessarily clear what the underlying hypothesis of this experiment was, it can be inferred that the results were to better understand the effects of habitat fragmentation on the mountain yellow-legged frog by the introduction of trout into alpine lakes in the Sierra Nevada.
Study Area
The area of this study was located in the Sierra Nevada of eastern California. It may be of interest to note that this area encompasses both part of the John Muir Wilderness and Kings Canyon National Park. Lakes were chosen due to similar physical and chemical characteristics. This was due to their “common glacial origin and their location in watershed dominated by intrusive igneous bedrock.” (Melack et al. 1985) The chosen lakes were small, oligotrophic (relatively low in plant nutrients and containing abundant oxygen in the deeper parts), depauperate in biodiversity (lacking in numbers or variety of species), and cold (ice free for only four months per year).
The entire area of study was naturally fishless due to numerous barriers to upstream movement of fishes. (Knapp 1996) Between the early- and mid-1900s, 60% of these lakes were stocked with golden, rainbow, brook, and brown trout, in order to create recreational fishing areas.
Fish Surveys
Visual encounter surveys; which can be used to determine the species richness of an area, and gillnets; which is when a fishing net is hung vertically so that fish are trapped by their gills, to verify the presence and/or absence of trout in each lake. In locations where the bottom of lakes could be seen, the count was taken visually.
Amphibian Surveys
The mountain yellow-legged frog was the most common amphibian species through the visual encounter survey of these lakes. Pacific tree frogs and the Yosemite toad were also found but were considered to be “too rare” in the allotted lakes