Dominant species are most competitive in exploiting

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5. Another hypothesis is that they are most successful at avoiding predators 6. Invasive species , typically introduced to a new environment by humans , often lack predators or disease F. Keystone Species 1. Keystone species exert strong control on a community by their ecological roles, or niches 2. In contrast to dominant species, they are not necessarily abundant in a community 3. Field studies of sea stars exhibit their role as a keystone species in intertidal communities 4. Observation of sea otter populations and their predation shows how otters affect ocean communities G. Foundation Species (Ecosystem “Engineers”) Foundation species (ecosystem “engineers”) cause physical changes in the environment that affect community structure For example, beaver dams can transform landscapes on a very large scale Some foundation species act as facilitators that have positive effects on survival and reproduction of some other species in the community Bottom-Up and Top-Down Controls The bottom-up model of community organization proposes a unidirectional influence from lower to higher trophic levels In this case, presence or absence of mineral nutrients determines community structure, including abundance of primary producers The top-down model , also called the trophic cascade model, proposes that control comes from the trophic level above In this case, predators control herbivores, which in turn control primary producers Long-term experimental studies have shown that communities vary in their relative degree of bottom-up to top-down control Pollution can affect community dynamics Biomanipulation can help restore polluted communities Concept 54.3: Disturbance influences species diversity and composition Decades ago, most ecologists favored the view that communities are in a state of equilibrium This view was supported by F. E. Clements who suggested that species in a climax community function as a superorganism Other ecologists, including A. G. Tansley and H. A. Gleason , challenged whether communities were at equilibrium Recent evidence of change has led to a nonequilibrium model , which describes communities as constantly changing after being buffeted by disturbances Characterizing Disturbance A disturbance is an event that changes a community, removes organisms from it, and alters resource availability Fire is a significant disturbance in most terrestrial ecosystems It is often a necessity in some communities The intermediate disturbance hypothesis suggests that moderate levels of disturbance can foster greater diversity than either high or low levels of disturbance High levels of disturbance exclude many slow-growing species Low levels of disturbance allow dominant species to exclude less competitive species The large-scale fire in Yellowstone National Park in 1988 demonstrated that communities can often
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