Deck 23: Global Ecology

Question 1

How are the influences of El Niño and La Niña related to the concepts of top-down versus bottom-up control of populations, communities, and ecosystems (p. 406)?

Answer 1

The El Niño Southern Oscillation is a phenomenon occurring on a large-scale due to variations in the temperature and pressure over the Pacific and Indian Oceans. El Niño is characterized by a low pressure and a warm temperature than average over the tropical eastern Pacific Ocean. The western pacific is simultaneously characterized by a high pressure and a low temperature. These conditions produce a high rainfall over the North and South America but drought over the western Pacific region. The reverse condition has been observed in La Niña.
The effects of the physical and chemical factors such as nutrients and temperature on the ecosystem are called bottom-up controls. The effects of consumers on ecosystem are called as top-down controls. The bottom-up and the top-down controls control the primary production of ecosystem.
The community or population may be affected by the following:
• Bottom-up controls such as food availability.
• Top-down controls such as predation and disease.
El Niño and La Niña influence the global ecology. They affect the marine and terrestrial organisms. El Niño and La Niña acts as bottom-up control by affecting precipitation and primary productivity in the North and South America and the western Pacific. This east-west pattern shown by El Niño and La Niña is also shown by the bottom-up controls of the ecosystem.
El Niño and La Niña have been found to create conditions similar to the trophic cascade caused in top-down control by consumers. The trophic cascade involves the influence of the consumers at various trophic levels in the food chains and the food webs.

Question 2

How does the example of El Niño and the Great Salt Lake confound the concepts of top-down and bottom-up control?

Answer 2

El Niño has been found to influence the climate over the global level. It has been reported to have influenced the salinity of the Great Salt Lake.
Increased precipitation due to El Niño led to a decrease in the salinity of the lake causing drastic changes in the lake community.
Low salinity have been observed to cause decline in the salt-tolerant species and allowed invasion of the predaceous insects. These insects in turn reduced the population of the herbaceous zooplankton. The reduction in rate of grazing caused increase in the production of phytoplankton and hence primary productivity.
The effects of the physical and chemical factors such as nutrients and temperature on the ecosystem are called bottom-up controls. The effects of consumers on ecosystem are called as top-down controls. The bottom-up and the top-down controls control the primary production of the ecosystem.
The consumers are proposed to affect the primary production in the ecosystem through the trophic cascades. Their activity has been found to reduce the primary productivity in the ecosystem.
However, the effect of El Niño was reverse on the Great Salt Lake. El Niño led to a trophic cascade that led to increase in the primary productivity. Precipitation is considered as a bottom-up control that had induced top-down control in the form of consumer-mediated trophic cascade.
Thus, El Niño and the Great Salt Lake mix up the concepts of top-down and bottom-up control.

Question 3

The example of El Niño and the Great Salt Lake might lead us to what general conclusion concerning the concepts of top-down and bottom-up control?

Answer 3

El Niño has been found to influence the climate over the global level. It has been reported to have influenced the salinity of the Great Salt Lake.
Increased precipitation due to El Niño led to a decrease in the salinity of the lake causing drastic changes in the lake community.
Low salinity have been observed to cause a decline in the salt-tolerant species and allowed invasion of the predaceous insects. These insects in turn reduced the population of the herbaceous zooplankton. The reduction in rate of grazing caused increase in the production of phytoplankton and hence primary productivity.
The effects of the physical and chemical factors such as nutrients, and temperature on the ecosystem are called bottom-up controls.
The effects of consumers on ecosystem are called as top-down controls.
The bottom-up and the top-down controls control the primary production of the ecosystem.
The consumers are proposed to affect the primary production in the ecosystem through trophic cascades. Their activity has been found to reduce primary productivity in the ecosystem.
However, the effect of El Niño was reverse on the Great Salt Lake. El Niño led to a trophic cascade that led to increase in the primary productivity.
This example leads to a conclusion that the concepts of bottom-up and top-down control are applicable at the level of community, ecosystem, or population level. Such processes can be strongly influenced and controlled by the global climatic systems such as El Niño. Also the response of ecosystems to global climatic patterns is influenced by the concepts of bottom-up and top-down control operating at the ecosystem level.

Question 4

How might human-induced alterations to the global nitrogen cycle impact aquatic ecosystems (see chap­ters 3, 18, and 19, pp. 70-71, 403-406, and 434-35)?

Question 5

When and why is it often necessary to narrow a search of the research literature?

Question 6

How might human-induced alterations to the global nitrogen cycle affect terrestrial ecosystems (see chap­ters 15, 16, and 19, pp. 339-41, 369-70, and 432-33)?

Question 7

Ecologists are now challenged to study global ecology. The apparent role played by humans in changing the global envi­ronment makes it imperative that we understand the workings of the earth as a global system. However, this study requires approaches that are significantly different from those that can be applied to traditional areas of ecological study. Historically, much of ecology focused on small areas and short-term stud­ies. What are some of the main differences between global ecology and, for instance, the study of interspecific competition (see chapter 13) or forest succession (see chapter 20)? How will these differences affect the design of studies at the global scale?

Question 8

Why is reducing forest area through deforestation a fundamental threat to biodiversity (see chapter 22)?

Question 9

When and why may it be important to broaden a literature search?

Question 10

How does fragmentation of habitat as a result of human changes in land cover threaten populations (see chapters 4, 10, and 21, pp. 94-5, 228-30, and 472-75)?

Question 11

Geologists, atmospheric scientists, and oceanographers have been conducting global-scale studies for some time. What role will information from these disciplines play in the study of global ecology? Why will global ecological studies generally be pursued by interdisciplinary teams? How can ecologists play a useful role in global studies?

Question 12

Why is the ecological impact of deforestation always greater than the area of forest removed?

Question 13

What changes in sea surface temperatures and atmospheric pressures over the Pacific Ocean accompany El Niño? What physical changes accompany La Niña? How do El Niño and La Niña affect precipitation in North America, South America, and Australia?

Question 14

What can we conclude from the evidence summarized by figures 23.20 to 23.23?
Figure 23.20 A 160,000-year record of atmospheric CO2 concentrations and temperature change (data from Barnola et al. 1987).
Figure 23.23 The Suess effect (data from Bacastow and Keeling 1974).

Question 15

Review evidence that the El Niño Southern Oscillation signif­icantly influences populations around the globe. Much of our discussion in chapter 23 focused on the effects of the El Niño Southern Oscillation on populations. Considering our discus­sions in chapters 18 and 19 of physical controls on rates of ter­restrial primary production and decomposition, how does the El Niño Southern Oscillation likely affect these ecosystem processes in Australia or the American Southwest? How would you test your ideas?

Question 16

What aspects of global warming are widely supported by available evidence?

Question 17

In chapter 23, we briefly discussed how humans have more than doubled the quantity of fixed nitrogen cycling through the biosphere. In chapter 15 we reviewed studies by Nancy Johnson (1993) on the effects of fertilization on the mutualistic relationship between mycorrhizal fungi and grasses. The increases in fixed nitrogen cycling through the biosphere, par­ticularly that portion deposited by rain, are analogous to a global-scale fertilization experiment. Reasoning from the results of Johnson's study, how should increased fixed nitrogen supplies affect the relationship between mycorrhizal fungi and their plant partners? How would you test your ideas?

Question 18

Are there uncertainties remaining regarding global warming?

Question 19

As we saw in chapters 18 and 19, nitrogen availability seems to control the rates of several ecosystem processes. How should nitrogen enrichment affect rates of primary production and decomposition in terrestrial, freshwater, and marine envi­ronments? How could you test your ideas? What role might geographic comparisons play in your studies?

Question 20

Why may the history of CFCs in the atmosphere in the years following the Montreal Protocol offer encour­agement as humanity strives to reverse the modern buildup of atmospheric CO ₂?

Question 21

Ecologists predict that global diversity is threatened by land use change and by the reductions in habitat area and the fragmen­tation that accompany land use change. Vitousek (1994) sug­gested that land use change may be the greatest current threat to biological diversity (see fig. 23.3). What role do studies of diversity on islands and species area relationships on continents (see chapter 22) play in these predictions?
Figure 23.3 Some causes and potential consequences of global environmental change (data from Vitousek 1994).

Question 22

Skole and Tucker (1993) documented the rate and extent of recent deforestation in the Amazon Basin in Brazil. This is a prominent example of the land cover changes that likely threaten biological diversity. However, as we saw in chapter 16, Bush, Piperno, and Colinvaux (1989) documented agricultural activity in New World tropical forests beginning at least 6,000 years ago. What makes present-day deforestation in the Amazon Basin different from this historical activity? What does the long history of agriculture in the Amazon Basin sug­gest about the potential for coexistence of agriculture and biological diversity there?

Question 23

Review the long-term atmospheric CO ₂record as revealed by studies of air trapped in ice cores. What is the evidence that burning of fossil fuels is responsible for recent increases in atmospheric CO ₂concentrations?

Question 24

What evidence is there that variation in atmospheric CO ₂concentration is linked to variation in global temperatures? In recent years the governments of most countries of the world have been working hard to develop international agreements to regulate CO ₂emissions. Why are these governments con­cerned? How might rapid changes in global temperatures lead to the extinction of large numbers of species? How might changes in global temperatures affect agriculture around the world?