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Transcript
Ecological succession How communities and ecosystems change Balance of nature • Is there a “balance of nature”? • What does that even mean? – Suggests “stability” – Suggests “lack of change” • Do ecosystems stay the same? – Or are they constantly changing? – May be hard to tell because human time span is short Constant change or stability? • Do ecosystems stay the same? • Or, they constantly changing? • Human time scales are short, may not notice change Community Change or Stability Communities are constantly changing, they are in nonequilibrium – many are in some state of recovery from disturbance Drought Flood Fire Clearcut What Is Disturbance? • A disturbance – Is an event that changes a community – Removes organisms from a community – Alters resource availability Fire • A major disturbance in most terrestrial ecosystems – Is often a necessity in some communities Figure 53.21a–c (a) Before a controlled burn. A prairie that has not burned for several years has a high proportion of detritus (dead grass). (b) During the burn. The detritus serves as fuel for fires. (c) After the burn. Approximately one month after the controlled burn, virtually all of the biomass in this prairie is living. Yellowstone fires, 1988 • Communities can often respond very rapidly to a massive disturbance (a) Soon after fire. As this photo taken soon after the fire shows, the burn left a patchy landscape. Note the unburned trees in the distance. Figure 53.22a, b ( b) One year after fire. This photo of the same general area taken the following year indicates how rapidly the community began to recover. A variety of herbaceous plants, different from those in the former forest, cover the ground. Ecological succession • Changes in community composition over time – In a disturbed community = secondary succession – In a new community = primary succession Succession • Secondary – More common – Where an ecosystem previously existed – Examples? • • • • Flood Storm Earthquake After volcano destroys existing ecosystem Succession • Primary – Occurs where no ecosystem existed before or where no trace remains • • • • On a sand dune On a cliff After a glacier retreats On a new volcanic island • Where rocks are laid bare by erosion Pioneer species • Species, typically plants or lichens, that are able to colonize bare ground • Must disperse easily • Must live on poor soils – Hardy organisms, with adaptations such as long roots, or symbiotic with nitrogen-fixing bacteria Pioneer species • Lichens are important • Composed of 2 different species: – Fungus – absorbs nutrients and holds water – Alga – photosynthesis • On rock, lichen can grab hold and begin to break the rock Succession • Moraines in Glacier Bay, Alaska – Follows a predictable pattern of change in vegetation and soil characteristics - Some species improve (a) Pioneer stage, with fireweed dominant environment for followers (b) Dryas stage 60 Soil nitrogen (g/m2) 50 40 30 20 10 0 Pioneer Dryas Alder Spruce Successional stage (d) Nitrogen fixation by Dryas and alder increases the soil nitrogen content. (c) Spruce stage Ecological Succession Ecological succession is the set of changes in community composition that occur over time in a new or disturbed community. Succession after the Yellowstone fires. Succession at Mt. St. Helens. Retreating Glaciers at Glacier Bay Alaska Make It a Natural Laboratory for Studying Primary Succession Primary succession occurs when organisms colonize a barren environment. Primary Succession at Glacier Bay, Alaska A climax community is the stable community at the final stage of succession. Succession Succession shows some general trends that include: 1) Biomass increase over time. 2) An increase in the number and proportion of longer-lived species. 3) Increased species diversity. Succession on Mt. St. Helens – another site of intense study. Mt. St. Helens - 1976 Mt. St. Helens – 1980 eruption: May 18, 1980 Meadow vegetation immediately after eruption 1980 1981 1982 1985 1989 1992 1994 1981 1985 1989 1992 1994 1998 Ecological Succession Eastern US Yellowstone wolves Dying aspen grove Streamside elk grazing Hey, what’s that noise? Yellowstone Ecosystem Lunch grew back! • Beaver dams – Can transform landscapes on a very large scale Figure 53.18