Lecture 6

The Main Points

• To summarize where we have been and where we are going, I presented a model that suggests a set of variables (Growth, immigration, mortality, and emigration) that produce the changes we observe in community structure across space and in three different time scales (day-to-day/year-to-year; intervals of disturbance; climatic change).

• The boxes in the model represent "assemblages" of species that have come to live in particular spots within the "continuum" of environmental gradients that make up the world. We have discussed the changes in these assemblages as one moves across space and considered the problems of ordination in the resulting vegetation continuum.

• What we will be doing next is to consider the changes in these assemblages through time. Namely, the changes in presence and relative abundance of species.
  

  Here are the three time scales"

  1. Day to Day/Year to Year scales are associated with the small, continuous changes due to Natality and Growth, Immigration of individuals to resident populations and the arrival of new species, Mortality, and Emigration (mainly animals).

  2. Disturbance brings about longer-term and larger changes in the "structure "of the community due to high rates of Immigration, Seed Bank Germination, and localized Extinction of some non-resistant forms. After a rapid re-colonization and reappearance of resistant forms, the Day to Day and Year to Year processes are the usual causes of change.

  3. Climatic Changes are the very long time scales. Besides the changes caused by the former processes, we may see, in particular, Low Rates of Immigration of "New" forms, Extinction of some resident forms, and would expect to see Natural Selection and ultimately, Evolution of some long term resident forms.

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