Chapter 4 Review Questions

F (see “Ecological Succession”)

F (see “Changing Ecosystems”)

F (see “Changing Ecosystems”)

F (see “Ecological Succession”)

T (see “Ecological Succession”)

T (see “Effects of Human Activities”)

T (see “Synergism”)

F (see “Feedback”)

T (see “Evolution, Speciation, and Extinction”)

F (see “Impacts of Global Change”)

Ecological succession is a relatively slow process. It involves the gradual replacement of one assemblage of species by another as environmental conditions change over time. Some of these changes are created by the species themselves and others occur more indirectly.

Primary succession is the colonization of a previously unvegetated surface, such as when a glacier retreats or a landslide removes all traces of the vegetation of the previous ecosystem. Primary colonizers begin the process, and the community proceeds through several seral stages (e.g., lichens, herbs, shrubs, trees) until a mature community with relatively little change in species results. This “climax community” may be in a state of dynamic equilibrium, although ecologists disagree on what this means in terms of stability. The nature of the climax may be determined by climatic conditions (climatic climax), or by geological factors such as soil type (edaphic climax).

Succession can also occur on land with soil that has been disturbed so that colonization begins anew. This “secondary” succession occurs after fires, after fields being ploughed, and so on. The first colonizers tend to be “weedy” herbs, but otherwise the process is the same. Cyclic succession is when succession gets interrupted by disturbance at some point during the progression and is “reset” back to an earlier seral stage. This may happen due to localized fire or treefall, for example, and leads to the development of ecotones: communities that are a blend of their adjacent communities with higher biodiversity.

(see “Ecological Succession”)

Phenology is the way in which climate affects the seasonal patterns of plants and animals. Bloom time for plants, for example, is influenced by temperature, and rising temperatures associated with global climate change will be reflected in the timing of flowers. These changes in phenology can lead to mismatches in the timing of dependent species. Seabirds that rely on zooplankton prey are particularly vulnerable to these mismatches, as zooplankton are short-lived species with a limited window of peak abundance. Spring migration is also occurring earlier and fall migration later for many species. Some species will be able to adapt to these rapid changes; others will not. Notwithstanding such rapid adaptations (and undoubtedly there will be many more surprises), extinctions will occur because some species are incapable of adjusting at such rapid rates.

(see “Impacts of Global Change”)

The intermediate disturbance hypothesis indicates that ecosystems subject to moderate disturbance generally maintain higher levels of diversity compared to ecosystems with either low or high levels of disturbance.

(see “Indicators of Immature and Mature Ecosystems”)

The productivity of early successional phases is often higher than later phases; nutrient cycling is often more rapid; overall biodiversity tends to be reduced; the species most adversely affected are often highly specialized ones at higher trophic levels; and the species that benefit most are usually pioneer species (weeds and pests) that have broad ranges of tolerance and efficient reproductive strategies for wide dispersal.

(see “Effects of Human Activities")

(see “Indicators of Immature and Mature Ecosystems”, Table 4.1)

Inertia the tendency of a natural system to resist change, the ability of an ecosystem to withstand perturbation. Resilience is the ability of an ecosystem to recover (return to its original state) after a disturbance.

(see “Changing Ecosystems”)

The mussel, a native of the Black and Caspian seas, was introduced to the Great Lakes from the ballast of freighters in the mid-1980s. Evidence from Europe indicated that the species was an aggressive colonizer, able to displace most native species. Densities as high as 30,000 per square metre have been recorded as testament to the mussel’s ability to grow on itself. In a short period of time, it displaced 13 species from Lake St Clair and caused the near-extinction of 10 species in western Lake Erie. The mussels also colonize spawning sites for other fish, with, as yet, undetermined impact on their populations. The mussels are filter-feeders that remove phytoplankton from the water, thereby affecting all the species higher in the food chain, such as walleye, bass, trout, and perch. The linking factor seems to be the collapse of the deep-water amphipod Dipoeria, a major food source for whitefish. Mussels do not remove all species of phytoplankton equally. This is creating problems with blooms of blue-green algae, such as the toxic Microcystis aeruginosa, that are not ingested by the mussels. Some scientists believe that the algae may be primarily responsible for Lake Erie’s 500 to 1,000 km² dead zone, which had mostly been attributed to chemical pollutants. The mussel has now spread through the Great Lakes, where it appears capable of colonizing any hard surface.

(see “Invasive Alien Species”)

The sea otter is a large seagoing weasel of the outer coasts, flourishing in giant kelp beds and bearing a valuable pelt that was valued by Indigenous and European hunters alike. The British, seeking trading goods to barter with the Chinese in exchange for tea, discovered that sea otter pelts were in great demand in China and thus made every effort to ensure that the west coast became British (rather than Spanish or Russian!) Columbia. The otters were easy to catch, and Russian, American, and Spanish hunters, aided by local Indigenous populations, finished off what the British had begun. Within 40 years, populations were reduced from more than half a million to 1,000 to 2,000. On the coast of British Columbia, it is likely that they were completely extirpated. Otters play a critical role in controlling sea urchin populations. Sea urchins are voracious eaters of kelp, which provides food and habitat for many other species. When overgrazed by sea urchins, this productive habitat disappears. Otter populations, through their control of the urchin population, are therefore critical to maintaining the productivity of the entire community, right up to bald eagle populations.

(see “Species Removal”)

Positive feedback loops are when change in a system provides conditions that further exacerbate that change. Negative feedback loops are when a change provides conditions within the ecosystem that ultimately moderate or reduce that change.

(see “Feedback”)

The number of individuals in a species is known as the When calculated on the basis of a certain area, such as the number of sea otters per hectare, it becomes population density. Changes in population characteristics are known as population dynamics. Populations change as a result of the balance among the factors promoting population growth and those promoting reduction. The most common response is through adjustments in the birth and/or death rates.

(see “Population Growth”)

Carrying capacity is the maximum number of individuals of a given species that can be sustained in a given area indefinitely, provided a stable supply of resources exists. Carrying capacity varies with changes in the ecosystem.

(see “Population Growth”)

Organisms that demonstrate the kind of S-shaped growth curve of Figure 4.8 are density-dependent, and as the population density increases, the rate of growth decreases. In other words, the larger the population, the lower the growth rate. This view is in accord with the equilibrium view of ecosystems discussed earlier. Some organisms, however, are density-independent, and the population operates with a positive feedback loop—the more individuals in the population, the more that are born, and the population grows at an increasing rate to demonstrate a J-shaped curve. At some point, this population meets environmental resistance, causing the population to crash back to, or below, the carrying capacity.

(see “Population Growth”)

(see “Population Growth”, Table 4.2)

Peary caribou exist north of the 74th parallel by digging under the snow to feed on vegetation. In 1974–5, heavy snows and freezing rains led to high mortality as the herd starved to death, unable to reach their food source. Unfortunately, these are the very same weather conditions predicted to become more common as a result of global climate change.

(see “Population Growth”)

Through over-hunting, humans have been causing extinction for thousands of years. More recently, destruction and conversion of habitats by humans has become the major driver of biodiversity loss worldwide. Finally, climate change may lead to the extinction of up to 50 per cent of Earth’s terrestrial species by 2050. This is particularly true because human impacts are greatest in tropical biodiversity hotspots.

(see “Evolution, Speciation, and Extinction”)

Unfortunately, the temporal and spatial scales of ecosystem change are often so great that they are difficult to observe in the human lifespan. This limitation has been recognized as the shifting baselines phenomenon, where every generation sets the baseline for change at the beginning of its own existence, rather than from the beginning of human-induced change. Global climate change will place considerable stress on many species in terms of their limits of tolerance. This will lead to changes in range and abundance, and some species will become extinct. Climate change will also influence the functioning of ecosystems, the characteristic ways in which energy and chemicals flow through the plants, herbivores, carnivores, and soil organisms that are the living components of ecosystems, as described earlier in this chapter. Productivity will change; in some places it will increase, in others decline. Food webs will be disrupted as predators and prey react differently to the changing conditions. When faced with such dynamic ecosystem changes, we must use equally dynamic thinking to confront the challenges of the future.

(see “Implications”)

One of the main pressures for contemporary evolution is human harvesting of prey populations. In established fisheries, once fish enter targeted age classes, predation by humans occurs at rates two to three times higher than that of natural predators, often exceeding 50 per cent. Fisheries have hence selected for the survival of certain fish, usually small fish. These smaller fish produce fewer and less viable eggs, leading scientists to formulate the Big Old Fat Fecund Female Fish (BOFFFF) hypothesis. These old fish are irreplaceable in that they not only contain more eggs but also proportionately more viable eggs. As BOFFFFs are harvested, fish populations have increasingly greater difficulty in achieving replacement population and a negative feedback loop sets in.

(see “Evolution, Speciation, and Extinction”)

Restoration ecology has developed as a field of study and practice to help repair environmental damage. The goals of these efforts vary enormously—from merely stabilizing an area with a self-maintaining cover of vegetation, as in the reclamation of many industrial sites, to efforts to restore areas to their pre-disturbance condition. One of the common difficulties in the latter situation is ascertaining the nature of the original ecosystem. Restoration ecologists are now concentrating more on trying to restore natural processes in an area rather than reintroducing components. However, ecological restoration is challenging and costly, and widespread agreement exists that it is better to avoid degrading ecosystems in the first place rather than to try restoring them afterward.

(see “Ecological Restoration”)