Abundance of tree species along a moisture gradient in the Santa Catalina Mountains of southeastern Arizona. The species' patterns of abundance are independent of each other. Thus, community composition changes continuously along the gradient.
Why do species exhibit different patterns of response to change in moisture? Answer: There are many possible reasons. Each species may have its own adaptive characteristics. Moreover, species may respond differently to the presence of other species, including competitors, predators, and parasites.
Change in community composition across an ecotone. The plant communities on normal and serpentine soils are greatly different, and the transition from one community to another occurs over a short distance.
Why is there a sharp transition between the two community types? Answer: The different soil types require very different adaptations, and thus different species are adapted to each soil type.
Character displacement in Darwin's finches. These two species of finches (genus Geospiza) have bills of similar size when allopatric, but different size when sympatric.
Why do populations have different bill sizes when they occur with other species? Answer: When the two species live on different islands they most likely consume similar foods and so their beaks are similar in size. When they occur in the same area, their niches overlap and there would be great competition for the same resources. Under these circumstances, one species would usually outcompete the other driving it to extinction. But what often happens is that selection will favor a splitting up of the resources, called resource partitioning, and under these circumstances, each species may become more "specialized" for its portion of the niche, resulting in different morphologies, called character displacement.
Detecting interspecific competition. This experiment tests how removal of kangaroo rats affects the population size of other rodents. Immediately after kangaroo rats were removed, the number of other rodents increased relative to the enclosures that still contained kangaroo rats. Notice that population sizes (as estimated by number of captures) changed in synchrony in the two treatments, probably reflecting changes in the weather.
Why are there more individuals of other rodent species when kangaroo rats are excluded? Answer: The kangaroo rats competed with all the other rodent species for resources, keeping the size of other rodent populations smaller. In the absence of competition when the kangaroo rats were removed, there were more resources available which allowed the other rodent populations to increase in size.
Predator-prey in the microscopic world. When the predatory Didinium is added to a Paramecium population, the numbers of Didinium initially rise, while the numbers of Paramecium steadily fall. When the Paramecium population is depleted, however, the Didinium individuals also die.
Can you think of any ways this experiment could be changed so that Paramecium might not go extinct? Answer: This could be accomplished in a variety of ways. One option would be to provide refuges to give some Paramecium a way of escaping the predators. Another option would be to include predators of the Didinium, which would limit their populations (see Ecosystem chapter).
Change in ant population size after the removal of rodents. Ants initially increased in population size relative to ants in the two enclosures from which rodents weren't removed, but then these ant populations declined.
Why do ant populations increase and then decrease in the absence of rodents? Answer: At first, the number of small seeds available to ants increases due to the
absence of rodents. However, over time, plants that produce large seeds
outcompete plants that produce small seeds, and thus fewer small seeds are
produced and available to ants; hence, ant populations decline.
1). The entire range of factors an organism is able to exploit in its environment is its
b). realized niche.
c). fundamental niche.
2). The phenomenon known as character displacement is associated with
a). sympatric species.
b). allopatric species.
c). competitive exclusion.
d). primary succession.
3). As the number of individuals of the predatory species increases, the prey population
d). first increases, and then begins to decrease.
4). In order for mimicry to be effective in protecting a species from predation, it must
a). occur in a palatable species that looks like a distasteful species.
b). have cryptic coloration.
c). occur such that mimics look and act like models.
d). occur in only poisonous or dangerous species.
5). Brightly colored poison-dart frogs are an example of
a). Batesian mimicry.
b). Müllerian mimicry.
c). cryptic coloration.
d). aposematic coloration.
6). Which of the following is an example of commensalism?
a). a tapeworm living in the gut of its host
b). a clownfish living among the tentacles of a sea anemone
c). a lichen
d). bees feeding on nectar from a flower
7). A parasite that feeds on its host from inside is a(n)
8). A species that interacts with many other organisms in its community in critical ways can be called a
b). primary succession species.
c). secondary succession species.
d). keystone species.
9). Succession that occurs on abandoned agricultural fields is best described as
b). primary succession.
c). secondary succession.
d). prairie succession.
10). Lichen growing on the surface of rocks provides an example of
d). secondary succession.
Test Your Visual Understanding
1). The graph above shows three general stages of succession indicated by the labels a, b, and c that occurred after the receding of a glacier at Glacier Bay, Alaska. Using the concepts of tolerance, facilitation, and inhibition, explain what is happening at each stage of succession.
Answer: Stage a corresponds to tolerance, where weedy r-selected species (the mosses) that are tolerant of the harsh conditions. After the glacier receded, the substrate was barren, covered with rocks. Mosses, and similarly lichens, are some of only a few species that are able to inhabit abiotic, barren substrates. They introduce nitrogen and other minerals into the soil that forms and permits other species of plants to grow.
Stage b corresponds to facilitation, where the mosses elevated the levels of nitrogen to the point where other plant species could survive. The alder plants moved in and further altered the environment, lowering the pH of the soil. The continuous changing of the environment allowed still more species to live in the community.
Stage c corresponds to inhibition, where the alders change the soil in such a way as to favor larger trees of spruce and hemlock. Eventually, the trees outcompeted the alders for soil and other forms of vegetation gradually replaced the alders.
Apply Your Knowledge
1). Through genetic engineering, bacterial toxin genes from a soil bacterium, Bacillus thuringiensis (Bt), have been inserted into corn plants, making them resistant to certain insects. The corn plants produce the toxin that kills insects but doesn't harm the plant or people. However, because the corn pollen, which also contains the toxins, could spread to neighboring milkweed plants, there was a concern that monarch butterflies, which are killed by the toxin, could be at risk. The lethal dose of toxin consists of 2500 pollen grains/in2 of milkweed leaf. Milkweed plants within a field of corn contain on average 500 pollen grains/in2 of milkweed leaf. Assume that the level of pollen decreases by 60% with every 10 feet. How much pollen will reach milkweed plants that are 10 feet from the cornfield? 20 feet from the cornfield?
Answer: Milkweed plants in the cornfield contain 500 pollen grains/in2 of milkweed leaf. If the rate of dispersion drops by 60% for every 10 feet then only 40% make it onto a plant 10 feet away. Therefore, a milkweed plant that is 10 feet away from the cornfield will receive 500 x 0.4 or 200 pollen grains/in2 of milkweed leaf. A milkweed plant that is 20 feet away will receive 500 x 0.4 x 0.4 or 80 pollen grains/in2 of milkweed leaf.