Ecologists have long questioned why fluctuating populations tend to persist
rather than go extinct. Populations that persist indefinitely are regulated by
mechanisms that cause demographic density dependence, which works to bound
fluctuation above zero. In a series of studies, I have sought to determine the processes
and mechanisms that regulate local populations of coral-reef fish. In the Exuma Keys,
Bahamas, fairy basslets (Gramma loreto) live in aggregations on the undersides of
coral-reef ledges. These aggregations often constitute local populations because
movement between aggregations is rare. The largest individuals occupy prime feeding
positions near the front of ledges and force smaller individuals remain near the back
where they have lower feeding rates. Based on these initial observations, I designed
two experimental studies of the demographic consequences of variation in basslet
density. In the first study, I manipulated the density of newly-settled fish to explore the
effects of high recruitment on population size. Populations with experimentally
elevated recruitment converged in density with unmanipulated populations, primarily
due to density-dependent mortality. I found no evidence that density dependence was
caused by intraspecific competition; rather it appeared to be due to a short-term
behavioral response by predators (aggregative and/or type 3 functional response). In a
second study, I manipulated the densities of adults among populations with a standard
average density of newly-settled fish. Two measures indicated that the intensity of
competition increased at higher densities of adults, which likely made small fish more
susceptible to predation, thereby causing density-dependent mortality. Long-term
observations indicated that basslet populations were regulated at temporal scales
exceeding two generations. At Lizard Island on the Great Barrier Reef, I also
examined how patterns of recruitment of coral-reef fishes were modified across a range
of natural recruit densities in the presence and absence of resident predators. Predators
decreased recruitment and increased mortality for all species, but these effects varied
considerably among species. The results of each of these studies stress the importance
of both competitive and predatory mechanisms in modifying patterns of abundance
established at the time of larval settlement, as well as regulating local population size. / Graduation date: 2002
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/32387 |
| Date | 11 September 2001 |
| Creators | Webster, Michael Scott |
| Contributors | Hixon, Mark A. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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