1.Of all the naturally occurring elements in the biosphere, why have the cycles of carbon, nitrogen, and phosphorus been so intensively studied by ecologists? (Hint: Think about the kinds of organic molecules of which these elements are constituents. Also think back to our discussions, in chapter 18, of the influences of nitrogen and phosphorus on rates of primary production.)
2. Parmenter and Lamarra (1991) studied decomposition of fish and waterfowl carrion in a freshwater marsh. During the course of their studies, they found that the soft tissues of both fish and waterfowl decomposed faster than the most rapidly decomposing plant tissues. Explain the rapid decomposition of these animal carcasses.
3. Review figure 18.2, in which Rosenzweig (1968) plotted the relationship between actual evapotranspiration and net primary production. How do you think that decomposition rates change across the same ecosystems? Using what you learned in
chapter 19, design an experiment to test your hypothesis.
4. Melillo, Aber, and Muratore (1982) suggested that soil fertility may influence the rate of decomposition in terrestrial ecosystems. Design an experiment to test this hypothesis. If you test for the effects of soil fertility, how will you control for the influences of temperature, moisture, and litter chemistry?
5. Many rivers around the world have been straightened and deepened to improve conditions for navigation. Side effects of these changes include increased average water velocity and decreased movement of water into shallow riverside environments such as eddies and marginal wetlands. What are the probable influences of these changes on nutrient spiraling length? Use the model of Newbold et al. (1983) in your discussion.
6.Likens and Bormann (1995) found that vegetation substantially influences the rate of nutrient loss from small stream catchments in the northern hardwood forest ecosystem. How do vegetative biomass and rates of primary production in these forests affect their capacity to regulate nutrient loss? How much do you think vegetation affects nutrient movements in desert ecosystems?
7. McNaughton, Ruess, and Seagle (1988) proposed that grazing by large mammals increases the rate of nitrogen cycling on the savannas of East Africa. Explain how passing through a large mammal could increase the rate of breakdown of plant biomass. How could the disappearance of the large mammals of East Africa affect nutrient cycling on the savanna?
8. The fynbos of South Africa is famous for the exceptional diversity of its plant community. Witkowski (1991) showed that invading Acacia are enriching the fynbos soil with nitrogen. How might enriching soil nitrogen affect nutrient cycling and primary production in this ecosystem? What mechanisms would likely produce your predicted changes?
9. Kauffman and his colleagues (1993) estimated that burning the tropical forest at their study site resulted in the loss of approximately 21 kg per hectare of phosphorus. This quantity is about 11% to 17% of the total pool of phosphorus. If total annual inputs of phosphorus to the ecosystem, mainly by rain and “dry fall,” amount to about 0.2 kg per hectare per year (Murphy and Lugo 1986), how long would it take these inputs to make up for a single agricultural burn such as that created by Kauffman and his colleagues? Assuming a constant rate of loss, how many burns would it take to totally exhaust existing supplies?
