Biology 100/101
Lecture 5: Ecosystems in Time (Succession)

"To think about":
Question 5, page 870

Answers to many of these questions can be found on the "Answers to End-of-Chapter Questions" page at the text website.

Key Terms:

succession	climax community	pioneer species
disturbance	primary succession	secondary succession
disclimax	environmental change	soil formation
prescribed burning

Ecological Succession

From the Latin, succedere, to follow after

• Succession is the change in structure of the biological community over time.

• Pioneer species colonize site following disturbance.

• Pioneer species tend to change the environment favoring the growth of other species that could not have colonized the same site earlier.
  • soil stabilization
  • soil nutrient enrichment (organic matter and biological nitrogen fixation)
  • increased moisture holding capacity
  • change in light availability
  • change in temperature

• Communities change through time in response to many influences:
  • climate change
  • disturbances of all scales
  • invasion of species from other areas (exotics)
    Zebra mussels and their spread
    Purple loosestrife

• Species (or life forms) tend to replace one another in a somewhat predictable manner until a stable climax community is established.

What is Primary Succession?

• The processes involved in changing an area from one lacking any community (no plants, no animals, no insects, no seeds, etc.) to one consisting of individuals, populations, communities, and ecosystems. CAUTION!!The text definition is misleading.

• Growth substrate has no organic matter, only mineral material such as sand, bare rock, gravel from glacial outwash, volcanic ash and lava (CAUTION!! different from text).

• Primary succession starts WITHOUT SOIL.

• Pioneer plants, such as lichens and mosses, become established on the bare mineral substrate from spores or seeds that are blown, washed, or carried in by animals.

• Examples of Primary Succession (See page 856, Fig. 42.3 in text as well)
  • Bare rock Rocks erode, sand accumulates, decomposition adds organic matter, and voila! soil. As soil continues to form, larger (and more woody) plants appear.

  • Volcanic eruption Following the eruption, there is no life in the blast zone. Eventually, however, pioneer species of plants do establish themselves, beginning the course of succession.

  • Glacier and Glacial retreat

  • Man-made structures

What is secondary succession? How does it differ from primary succession?

• Secondary succession is the reconstruction of an ecosystem following a disturbance that damages or removes all or part of the previous community, but leaves the SOIL intact.

• Faster than primary succession.

• Succession results in the transition of the original pioneer species to climax community.

• Pioneer species become established from seeds, spores or plant parts surviving in the soil or from vegetation in surrounding undisturbed areas.

• Examples of Secondary Succession
  • Floods,
  • tornadoes
  • fires- Prescribed burning to maintain prairies in a state of disclimax.
  • Plowing also fig. 42.5, p.857 in text-"Weeds" are pioneer species. Plowing keeps land in a continual state of disclimax.

• Pioneer species colonize first.

• Newly arriving species alter the conditions.

• New species of plants displace existing plants.

• Animals come in with or after the plants they need to survive.

• With infrequent disturbance a climax community will become established.

• Disturbances start the process of succession.

Take me home.