Relationship between reported cases of human Lyme disease in 1996 and species richness of ground-dwelling birds (A), small mammals (B), and lizards (C) in the eastern United States. GA, Georgia; MC, Mid-Atlantic states; ME, Maine; NC, North Carolina; NE, New England states; NY, New York; PA, Pennsylvania; SC, South Carolina; VA, Virginia; and WV, West Virginia. From Ostfeld and Keesing (2000) with permission from Conservation Biology. Please see extended permission list pg 571.
Community structure changes over a range of time scales, from annual to decadal to millenial time periods. Temporal patterns of community organization and their sensitivity to environmental changes can indicate their stability to anthropogenic changes.
Community structure changes on annual time scales as population sizes respond to environmental conditions. Changes in resource quality, competition, and predation lead to population irruptions of some species and local extinction of others, thereby affecting their interactions with other species and leading to changes in community structure.
Ecological succession, the sequential stages of community development on newly exposed or disturbed sites, is one of the best documented ecological phenomena and has provided a unifying concept that integrates species life history strategies, population behavior, community dynamics, and ecosystem processes. Early successional communities usually are dominated by relatively generalized ruderal species with high mobility and rapid reproductive rates. Later succes-sional stages are increasingly dominated by species that are more specialized, are less mobile, and have lower reproductive capacities. Although most studies of succession have focused on plants, insects show successional patterns associated with changes in vegetation, and the relatively rapid heterotrophic succession in decomposing wood and animal carcasses has contributed much to successional theory.
A number of factors influence successional pathways. Local substrate conditions can restrict initial colonists to those from the surrounding species pool that can become established on distinct substrates, such as serpentine, volcanic, or water-saturated soils. The composition of the initial community, including survivors of the previous disturbance and colonists, can affect the success of subsequent colonists. Subsequent disturbances and animal activity can affect suc-cessional pathways. Animals, including insects, create germination sites for colonists and suppress some host species, thereby facilitating, inhibiting, or reversing succession. In fact, animal activity often may account for vegetation changes that have been attributed to plant senescence.
Several models of succession have augmented the early model of succession as a process of facilitated community development, in which earlier stages create conditions more conducive to successive stages. In some cases, all the eventual dominants are present in the initial community, and succession reflects differential development time and longevity among species (i.e., the tolerance model). Some successional stages are able to competitively exclude later colonists, the inhibition model. Succession may advance beyond such stages as a result of plant injury or death from subsequent disturbances or animal activity.
Paleoecological research indicates that species interactions and community structures have been relatively consistent over evolutionary time. However, the communities occupying particular sites have changed over these time periods as the environmental conditions of the site have changed.
The relationship between species or functional diversity and community or ecosystem stability has been highly controversial. Much of the discussion reflects different definitions of diversity and stability. Stability can be seen to have two major components: resistance to change and resilience following perturbation. Succession is the expression of resilience. Although much evidence indicates that a particular community composition or structure may not be replaced at a site, indicating instability at the local level, the structure of communities at a landscape scale ensures that disturbed sites are near population sources and that component communities are maintained within a shifting landscape mosaic, indicating stability at the landscape or regional level.
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