Chapter 24 Lecture Enrichment Ideas
Use local examples to describe how any two species in the same community must occupy at least slightly different niches. In the U.S. midwest, corn has many insect herbivores: corn earworm, corn stem borer, corn root worm. How can they all survive in the same field?
Discuss how organisms that have a host-parasite relationship must actually coevolve. Why would there necessarily be a relationship between the phylogenies worked out for loons and loon lice? What is the fate of a loon louse if its only host goes extinct? One way to help determine species relationships, such as among birds, is to examine the relationship of the parasites that baby birds get from their parents in the nest.
Discuss disease as a parasitic relationship between the host and the disease-producing organism, and consider the "new diseases," such as AIDS, which kill the host, as opposed to a disease such as a cold, which does not.
The evolution of viral and bacterial diseases can also be explained as a case of cycling. The numbers of indivduals who come down with the current strain of influenza peaks, then most are resistant until another strain evolves. Explain how this differs in time and mechanism from predator-prey cycling.
Frederick Clements saw the changes in vegetation that occurred in Nebraska following a tornado or prairie fire, but he was the first to see a pattern to it and call the phenomenon "succession." His error was in considering the community a "superorganism" that acted as a group. This might be a good place to briefly discuss the recent Gaia hypothesis that is sometimes used to promote the whole world as a superorganism. Currently scientists are debating the same issue: is the whole more than the sum of its parts?
Successional changes are often in response to human-made disturbance. Indeed, there is little virgin timberland or prairie left in the United States and nearly all of Europe and China represents surfaces that have been intensely cultivated for centuries. Animals likewise are heavily responsible for the sequence of succession, through plowing of soil, distribution of seeds, etc. In agriculture, the term "climax" is being replaced with the term "potential natural community" to reflect the difference between the current disturbed community and what would potentially be present in the absence of the human activity, without alluding to a superorganism effect.
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