Ecological succession How communities and ecosystems change

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 Clearcut Fire

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 (a) Before a controlled burn. A prairie that has not burned for several years has a high propor- tion 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. Figure 53.21a–c

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. (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. Figure 53.22a, b

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 On a sand dune On a cliff After a glacier retreats On a new volcanic island

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 environment for followers (b) Dryas stage (c) Spruce stage (d) Nitrogen fixation by Dryas and alder increases the soil nitrogen content. Soil nitrogen (g/m2) Successional stage Pioneer Dryas Alder Spruce 10 20 30 40 50 60 (a) Pioneer stage, with fireweed dominant

Succession

Ecological Succession Succession after the Yellowstone fires. Ecological Succession Ecological succession is the set of changes in community composition that occur over time in a new or disturbed community. 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

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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

Pioneer species Species, typically plants, that are able to colonize barren ground Must disperse easily Must live on poor soils So: pioneer species are hardy, with adaptations such as long roots, symbiotic with nitrogen-fixing bacteria