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Chapter 21 Answers
Populations and Communities
Visual Understanding
1. Speculate on what happens with populations that follow the exponential growth model.
As the numbers increase, the population passes the carrying capacity of the area. Some
limiting factor will come into play: the amount of available food, nesting space, sunlight,
and so on. The population will either need to shift to a different resource, leave the area
to find resources, or have a massive die-off of individuals.
2. Figure 21.9
How would the niche change for Chthamalus if Semibalanus were removed from the
community? How would the niche change for Semibalanus if Chthamalus were removed
from the community?
If Semibalanus were removed from the community, Chthamalus could expand into the
deeper areas because the entire vertical surface is a fundamental niche for this species
and only the presence of another is preventing it from exploiting the deeper areas.
If Chthamalus were removed from the community, this would not have a major effect on
Semibalanus because its fundamental niche and realized niche are the same.
Challenge Questions
1. Give at least two real examples, from your own area, of limiting factors on a
population that are density-dependent factors, and two that are density-independent
factors on the same population.
Answers will vary. The density-dependent factors will probably include hormonal
responses, competition and some types of predation from opportunistic predators, and
certain diseases that pass easily from individual to individual in crowded populations.
Density-independent factors might include food resources, freezes, floods, fires.
2. Use the terms from this section: interspecific competition, fundamental niche, realized
niche, niche overlap, competitive exclusion, and resource partitioning to discuss Robert
MacArthur’s warbler research.
Answers will vary. Here are two possible versions.
1. Five species of warblers live in one area. The assumption is that the fundamental
niche of each species could overlap significantly with the other species, resulting in
interspecific competition. Because there is a very large resource with a lot of different
microhabitats available within it the 5 species have apparently gone through some
resource partitioning which leaves each species with a portion of the entire habitat,
minimizing competition. There may still be some competitive exclusion occurring,
according to the diagram in the text, which would limit the realized niche of at least some
species.
2. Robert MacArthur studied warblers and found that 5 species, all insectivores, of
essentially the same size, all co-existed in the same habitat. He knew that there must be
interspecific competition for resources. The fundamental niche of some of the species
does still overlap; for example, the Blackburnian warbler niche overlaps the Blackthroated green warbler, and also overlaps the Cape May warbler. The realized niche of
each, however, may be a reduced amount of the tree wherever the other species are
present. This competitive exclusion from a portion of the fundamental niche is one
example of how organisms engage in resource partitioning in order to minimize
interspecific competition.
3. Briefly discuss and give examples of the three types of symbiosis.
Commensalism involves relationships in which one organism benefits and the other
organism is not affected either positively or negatively as far as scientists can tell. One
example is the tropical fishes, such as the clownfish, that can live among the tentacles of
the sea anemone.
Mutualism benefits both species. Many examples have a species of ant as one of the
partners. Ants partner with aphids, caring for the aphids and repelling predators, and
receive aphid honeydew as a food source in return.
Parasitism benefits one of the species at the expense of the second species. The brood
parasites cuckoos and cowbirds lay their eggs, in secret, in the nests of other birds, one
egg to a nest. The large baby usually kills the babies that belong in the nest, leaving the
small parents (usually warblers, bluebirds, or pipits) to feed and raise one very large
child that was, literally, left on their doorstep. There are also many internal and external
parasites such as lice, parasitic wasps, and tapeworms.
4. Many U.S. communities struggle with issues of deer overpopulation. Explain how
human activities have created this situation.
In most places, habitat loss and extermination of natural predators have allowed the deer
population to become overcrowded for the habitat available. Further, human housing
developments now exist on land that was previously used by deer, and the deer have been
forced to inhabitat smaller and smaller areas. The deer are forced to forage in people’s
yards and gardens, creating a “nuisance” to homeowners.