Page 662 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

626 part IV Soils, ecosystems, and Biomes
(a) Mount St. Helens, 2008.
▲Figure 19.19 The pace of change in the region of Mount
St. Helens. [(a) ISS astronaut photo, naSa/gSFC; (a) all 1983 photos by robert Christopherson; (b) all 1999 photos by Bobbé Christopherson.]
(b) Pre-eruption landscape north of (c) Post-eruption landscape in same volcano, 1979. region, 1980.
(d) Repeat photography of post-eruption recovery at Meta Lake, 1983 and 1999.
47°
46°
OREGON
45°
123°
123° Olympia 122° 47°
Mt. Rainier
Mt. St. Helens National Volcanic Monument
Mt. St. Helens
0 30
WASHINGTON
Mt. Adams
46°
60 KILOMETRES
Portland
stages, eventually reaching a mature state with a pre- dictable climax community—a stable, self-sustaining assemblage of species that would remain until the next major disturbance. However, contemporary biogeogra- phy and ecology assume that disturbances constantly disrupt the sequence and that a community may never reach some climax stage. Mature communities are in a state of constant adaptation—a dynamic equilibrium— sometimes with a lag time in their adjustment to environmental changes. Scientists now know that successional processes are driven by a dynamic set of interactions with sometimes unpredictable outcomes.
Disturbance often occurs in discrete spatial units across the landscape, creating habitats, or patches, at dif- ferent successional stages. The concept of patch dynam- ics refers to the interactions between and within this mosaic of habitats, which add to complexity across the landscape. The overall biodiversity of an ecosystem is, in part, the result of such patch dynamics.
aquatic succession Aquatic ecosystems occur in lakes, estuaries, wetlands, and along shorelines, and the com- munities in these systems also undergo succession. For
example, lakes and ponds exhibit successional stages as they fill with sediment and nutrients and as aquatic plants take root and grow. The plant growth captures more sediment and adds organic debris to the system (Figure 19.20). This gradual enrichment in water bodies is known as eutrophication (from the Greek eutrophos, meaning “well nourished”).
In moist climates, a lake will develop a floating mat of vegetation that grows outward from the shore to form a bog. Cattails and other marsh plants become established, and partially decomposed organic material accumulates in the basin, with additional vegetation bordering the re- maining lake surface. Vegetation and soil and a meadow may fill in as water is displaced; willow trees follow, and perhaps cottonwood trees; eventually, the lake may evolve into a forest community. Thus, when viewed across geologic time, a lake or pond is really a temporary feature on the landscape.
The stages in lake succession are named for their nutrient levels: oligotrophic (low nutrients), mesotrophic (medium nutrients), and eutrophic (high nutrients). Greater primary productivity and resultant decreases in water transparency mark each stage, so that photosynthesis
(e) Repeat photography showing secondary succession, 1983 and 1999.
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