• Winemiller (1990) deleted “weak” trophic links from one set of food webs that he described for fish communities in Venezuela (see fig. 17.3). What was his criterion for designating weak interactions? Earlier, Paine (1980) suggested that ecologists could learn something by focusing on “strong” links in communities. How did Paine’s criterion for determining a strong link differ from Winemiller’s?
• What is a keystone species? Paine (1966, 1969) experimented with two sea stars that act as keystone species in their intertidal communities along the west coast of North America and in New Zealand. Describe how the intertidal communities in these two areas are similar.
• Explain how the experiments of Lubchenco (1978) showed that feeding preferences, population density, and competitive relations among food species all potentially contribute to the influences of “keystone” consumers on the structure of communities. What refinements did the work of Lubchenco add to the keystone species hypothesis?
• When Power (1990) excluded predaceous steelhead and large roach from her river sites, the density of herbivorous insect larvae (chironomids) decreased. Use the food web described by Power to explain this response.
• Using Tscharntke’s food web (1992) shown in figure 17.5, predict which species would be most affected if you excluded the bird at the top of the web, Cyanistes caeruleus. What species would be affected less? Assume that C. caeruleus is a keystone species in this community.
• Some paleontologists have proposed that overhunting caused the extinction of many large North American mammals at the end of the Pleistocene about 11,000 and 10,000 years ago. The hunters implicated by paleontologists were a newly arrived predatory species, Homo sapiens. Offer arguments for and against this hypothesis.
• All the keystone species work we have discussed in chapter 17 has concerned the influences of animals on the structure of communities. How could other groups of organisms, such as parasites and pathogens, act as keystone species?
  
  

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Answers The feeding variations between fresh water species was explained by Kir Wine miller 1990 The food web examined resulted to a complex structure He involved a strong tropic networks as a measure ... View the full answer