Lecture 8

Topics

• How can we visualize what is happening to cause the changes we see in communities?

• The central observations, the assumptions and the first inferences of Island Biogeography.

The Main Points

• You should read pp.549-550, 594-599 in Ricklefs to support this lecture.

• Communities always seem a little less abstract to me when I begin working with the actual "species" that make up an assemblage. Granted, the concept of the species is a further abstraction to this already complicated business, but at least we can begin to think about particular organisms and what they are doing.

• We wont't be working very much with the species concept per se, but you may want to read through the Species Definitions that are part of the Study Guide.

• In 1967, Robert MacArthur and E.O. Wilson published Island Biogeography, a book with some ideas about how species might become established in new environments. While the central metaphore concerns the colonization of islands, ecologists have not felt constrained to limit the theoretical motif to oceanic islands. Indeed, the principles have been tested in insular environments such as streams, ponds, lakes, mountain-tops, wilderness areas and caves. It remains to be seen how generally applicable the theory will be.

• Why Study Islands? MacArthur and Wilson provide some insights:
  • Simpler than a continent or an ocean

  • Visibly discrete object that can be named

  • Resident populations can be directly associated with the island.

  • Advantages to the evolutionary studies
    1. Multiplicity
    2. Variable in Size, Shape, Isolation
    3. Variable in ecological factors.
       

  • Insularity is a universal feature of biogeography. Parallels in:
    1. Streams
    2. Ponds
    3. Caves
    4. Gallery forests
    5. Broken Woodlands

• Species-Area Curves
  1. Area itself seldom has a direct effect on a species presence. More often, area allows a large enough sample of habitats, which control species occurrence.

  2. "division of area by 10 [in going from one island to the next] divides the fauna by 2." Darlington (1957) on the hertofauna of the West Indies. (Island Biogeography Fig 2)

  3. S = CA^z where: S = number of species A = Area C = a variable dependent on taxon and region z = slope of the relationship; varies little among taxa or with a given taxon in different parts of the world. Most values cluster in the range 0.2 - 0.35 Within islands or continents 0.12 - 0.17

• The Species Equilibrium (Study Figs. 29-7 to 29-12)
  1. Immigration and Extinction Rates produce a kind of equilibrium

  2. Immigration here is the rate of arrival of new species. It is high at first as the first colonists arrive. As more species arrive from the mainland source, immigration decreases rapidly and becomes 0 when all species from the source have arrived.

  3. Extinction increases as more species colonize the island and begin to expand. Competition for space and food plus the instability of small population sizes contributes to the rapid increase in extinction.