Spatial Variability

At one time, the ecosystem was considered to be the interacting community and abiotic conditions of a site. This view gradually has expanded to incorporate the spatial pattern of interacting component communities at a landscape or watershed level (see Chapter 9). Patches within a landscape or watershed are integrated by disturbance dynamics and interact through the movement of organisms, energy, and matter (see Chapter 7). For example, the stream continuum concept (Vannote et al. 1980) integrates the various stream sections that mutually influence each other. Downstream ecosystems are influenced by inputs from upstream, but the upstream ecosystems are influenced by organisms returning materials from downstream (e.g., Pringle 1997). Soils represent substantial storage of carbon and nutrients in some patches but may contain little carbon and nutrients in adjacent patches connected by water flux. Riparian zones (flood-plains) connect terrestrial and aquatic ecosystems. Periodic flooding and emerging arthropods move sediments and nutrients from the aquatic system to the terrestrial system; runoff and falling litter and terrestrial arthropods move sediments and nutrients from the terrestrial to the aquatic system (Cloe and Garman 1996,Wipfli 1997).The structure of riparian and upslope vegetation influence the interception and flow of precipitation (rain and snow) into streams (Post and Jones 2001). The structure of ecosystems at a stream continuum or landscape scale may have important consequences for recovery from disturbances by affecting proximity of population sources and sinks. Patches representing various stages of recovery from disturbance provide the sources of energy and matter (including colonists) for succession in disturbed patches. Important members of some trophic levels, especially migratory herbivores, birds, and anadromous fish, often are concentrated seasonally at particular locations along migratory routes. Social insects may forage long distances from their colonies, integrating patches through pollination, seed dispersal, or other interactions. Such aggregations add spatial complexity to trophic structure.

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