UBC Theses and Dissertations
UBC Theses and Dissertations
Shifted community states in four marine ecosystems : some potential mechanisms Okey, Thomas Anthony
The rigorous demonstration of truly stable alternate community states is elusive in marine ecosystems and might remain so for some time. Examples of marine community states that have shifted are nevertheless increasingly conspicuous. The growing concern over these altered community states is often related to questions of persistence and reversibility, especially when these shifted states are considered to be degraded. I used empirically-based trophic models and direct empirical field studies to explore the potential of particular hypothesized mechanisms to generate and maintain alternate community states in four marine ecosystems: a Galapagos rocky reef, Prince William Sound, Alaska, the West Florida Continental Shelf, and coral reefs of the Spermonde Archipelago or Southwest Sulawesi, Indonesia. Construction and analysis of an Ecopath with Ecosim (EwE) model of a Galapagos rocky reef indicated that the unsustainable fishery targeting the holothurian Stichopus fuscus can, by itself, trigger the replacement of previously diverse reef platform communities with Aiptasia sp. anemone barrens. Construction and analysis of a Prince William Sound, Alaska EwE model indicated that severe disturbances such as the Exxon Valdez oil spill can shift a marine biotic community to an alternate state that persists in a stable manner for decades, and that the character of such disturbances, in terms of their breadth and community-level life-history signatures, might strongly influence whether a community shifts to an alternate state. In addition, a search for keystone species provided a whole-system approach to identifying species or functional groups whose depletion or removal might have the most severe consequences for community structure, and the most potential for mediating shifts. Construction and analysis of a West Florida Shelf EwE model indicated that increased sea floor shading by coastal phytoplankton (i.e., resulting from nutrient runoff pollution) can cause broad shifts in this continental shelf community by shading benthic primary producers, which support much of the overall shelf community. An empirical field study of monsoons and runoff in Southwest Sulawesi, Indonesia indicates some of the mechanisms involved in shifting tropical reefs from coral-dominated to algae-dominated systems. These contributions feature some newly emerging approaches for gaining insights into marine communities and for developing hypotheses that can be more rigorously evaluated in the future. None of these examples, however, are comprehensive or strictly falsificationist by themselves. The continued integration of these emerging community/ecosystem modeling approaches with direct empirical studies should vastly increase the potential for distinguishing the relative roles of natural and anthropogenic forces in shaping marine communities. The goal of the first steps described here was to identify particular mechanisms in each example that have the potential to generate or maintain community shifts.
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