Chapter 20

Chapter 17 Answers
Populations and Communities
Visual Understanding
Figure 17.2
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.
Figure 17.33b
This graph predicts the equilibrium number of species on an island according to the island’s size
and distance from the mainland. Briefly explain the relationship.
The closer an island is to the mainland the higher the number of species that are liable to
colonize the island. Two islands of the same size and different distances from the mainland
would have greater numbers of species that colonized and survived on the closer island.
The larger an island is the greater the likelihood of more complex habitat types, and the easier it
will be for a colonizing species to survive and reproduce and become successful. Smaller islands
have a higher possibility for extinctions because of the lack of complex habitat and numbers of
different niches.
These factors work together to determine the relative numbers of successful colonizing species.
Closer, larger islands have more colonizations than islands farther away, and more habitats to
accommodate the new species than smaller islands, and so larger, closer islands tend to have
more surviving species, and fewer extinctions.
Challenge Questions
Population Dynamics
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 and
competition and some types of predation from opportunistic predators, certain diseases that pass
easily from individual to individual in crowded populations. Density-independent factors might
include food resources, freezes, floods, fires.
How Competition Shapes Communities
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 Black-throated 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.
How Coevolution Shapes Communities
Briefly discuss and give examples of the three types of symbiosis.
Commensalism involves relationships where 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, receiving 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.
Community Stability
Your friend, Susan, asks you why the rain forests are so diverse, when most other biomes are
not. Explain the reasons for high or low biodiversity of an area.
Large ecosystems contain more variety in the habitats, and so can contain more species. Areas
along the equator have longer growing seasons because they do not have the short day length
winter season that is characteristic of the rest of the planet. Finally, equatorial regions have
more climate stability over many hundreds of thousands of years; no glaciers during the ice
ages, for instances. For all these reasons the huge areas of tropical rain forests have, over the
millennia, developed into the most diverse biome, with the largest number of species, on our