Chapter 56

1. Subsets of species are used to describe communities because it is impractical to account for every species in a community. There are at least two reasons for this: (1) communities can vary in spatial and temporal scale, making it hard to designate boundaries around them, and (2) it is impossible to identify every species in a community because many are not described. For example, many bacteria, small microbes, and small invertebrates are hard to see, let alone identify.

   The community subset used for frogs and toads on Mount St. Helens was a taxonomic one—they all belong to an amphibian community.

2. Nontrophic interactions, such as competition and facilitation, are not depicted in the food web.

3. Pond A

   	Abundance	Proportion (pi)	ln (pi)	pi ln(pi)
   Pacific tree frog	6	0.3	–1.20	–0.36
   Western toad	8	0.4	–0.92	–0.37
   Northern red-legged frog	4	0.2	–1.61	–0.32
   Cascades frog	2	0.1	–2.30	–0.23
   H				1.28

   The new Shannon index value for pond A is H = 1.28. Pond B, with H = 1.39, still has slightly higher species diversity than Pond A.

1. The most important factor limiting species membership in the local community is dispersal and immigration. Seven species did not pass through the species supply filter, while 4 species and 3 species, respectively, did not pass through the abiotic and biotic filters.

2. Given that the species supply filter has the most potential to exclude species from a local community, the best management action would be to exclude, as much as possible, non-native species from entering the mountain in the first place.

1. Elk would be released from the direct interaction of predation by wolves and would increase in abundance. The increase in elk would result in more grazing pressure on aspens, causing them to decline. Thus, fewer wolves would have indirect effects for aspen. With fewer aspens, there would be other indirect effects of the lack of wolf predation: fewer snowshoe hare, beaver, and other rodents that depend on aspens for food. Fewer small herbivores could affect other carnivores in the system such as coyote, raven, and short-tailed weasels.

2. Beavers most closely fit the ecosystem engineering species definition because they are able to create, modify, and/or maintain physical habitat by cutting down (and killing) trees and using them to dam streams and create ponds and wetlands that provide habitat for themselves and other species. Beavers might also be considered a keystone species because their effect is large relative to their size or abundance, but keystone species are thought to mostly act through food webs by creating trophic cascades. Beavers would not be considered foundation species because foundation species have large effects on the communities as a consequence of their large size or great abundance.

3. The lottery model poses that in communities where species use similar limiting resources and have similar effects on one another, the element of equal chance for all individuals to obtain those resources determines coexistence. The model assumes that when resources are made available, they are used at random by individuals of different species that happen to be in the “right place at the right time.” As long as all individuals have similar chances of obtaining resources (or “winning the lottery”) and no clear advantage in population growth, then their presence in the community should be maintained by chance events that free up resources for individuals competing for those resources.

1. Disturbance is an abiotic event that may physically or chemically injure or even kill some individuals, creating opportunities for other individuals to grow and/or reproduce. For example, hurricanes, forest fires, and volcanic eruptions are disturbances. Stress occurs when some abiotic factor reduces the growth, reproduction, and ultimately survival of some individuals. For example, drought or extreme cold can cause stress in organisms.

2. A-59

   Some biotic factors that induce change in communities include species interactions, such as predation, competition, or disease. Others include physical damage by organisms, such as trampling or digging.

3. False. Primary succession is controlled by facilitative and inhibitory processes acting together over time. The progress of succession depends on the early colonists, each of which can facilitate and inhibit other colonizing species. For example, in Glacier Bay Dryas and alders allowed spruce trees to become established, but later in succession, competition from spruce trees led to the decline of early successional species.

4. An alternative state, or regime shift, occurs when a disturbance or stress causes a community to follow a different successional trajectory, leading to an alternative community state. Examples include the extirpation and then reintroduction of wolves to aspen forests of Yellowstone National Park, the presence or absence of sea otters in kelp forests along the west coast of North America, and the effects of beavers on wetlands in Minnesota.

1. Costa Rican farmers often grow multiple crops together (such as corn and sweet potatoes) to reduce the number of insect pests that attack the plants. For example, parasitoid wasps are attracted to corn pollen and attack sweet potato pests, thus benefitting the sweet potato crop. Likewise, the corn plants receive pollination services from the wasps.

   Growing polycultures could be a good strategy under drought conditions if each species performs differently under low water conditions. For example, if one species performs better than another, overall productivity could still be high compared with a condition in which only one species, or monoculture, that did not respond favorably to drought.

1. 	Pumice Plain				Blowdown Zone				Tephra-fall Zone				Reference
   Small mammal species	Proportion (pi)	ln (pi)	piln(pi)		Proportion (pi)	ln (pi)	piln(pi)		Proportion (pi)	ln (pi)	piln(pi)		Proportion (pi) ln (pi)	ln (pi)	piln(pi)
   Deer mouse (Peromyscus maniculatus)	1.00	0	0		0.20	–1.61	–0.32		0.25	–1.39	–0.35		0.10	–2.30	–0.23
   Yellow-pine chipmunk (Tamias amoenus)	0				0.40	–0.92	–0.37		0				0
   Cascade golden-mantled ground squirrel (Spermophilus saturatus)	0				0.05	–3.00	–0.15		0				0
   Creeping vole (Microtus oregoni)	0				0.10	–2.30	–0.23		0				0
   Shrew mole (Neurotrichus gibbsii)	0				0.05	–3.00	–0.15		0				0
   Trowbridge’s mole (Sorex trowbridgii)	0				0.10	–2.30	–0.23		0				0.05	–3.00	–0.15
   Montane shrew (Sorex monticolus)	0				0.10	–2.30	–0.23		0.15	–1.90	–0.28		0.10	–2.30	–0.23
   Southern red-backed vole (Clethrionomys gapperi)	0				0				0.45	–0.80	–0.36		0.65	–0.43	–0.28
   Townsend’s chipmunk (Tamias townsendii)	0				0				0.05	–3.00	–0.15		0
   Ermine (Mustela erminea)	0				0				0.05	–3.00	–0.15		0
   Northern flying squirrel (Glaucomys sabrinus)	0				0				0				0.05	–3.00	–0.15
   Vagrant shrew (Sorex vagrans)	0				0				0				0.05	–3.00	–0.15
   Northern water shrew (Sorex palustris)	0				0				0				0.05	–3.00	–0.15
   H			0				1.68				1.44				1.04

2. The Pumice Plain had the lowest species diversity (H = 0) and the Blowdown Zone had the highest species diversity (H = 1.68). The Pumice Plain had the lowest species richness (1 species) compared to the Blowdown Zone, which had the highest species richness (7 species). The Reference and the Tephra-fall Zone both had the same species richness (6 species).

   

   The species diversity data for small mammals seem to fit the intermediate disturbance hypothesis well. As the degree of disturbance experienced by the four community types increases, so does species diversity of small mammals in those communities, up to a point—the Blowdown Zone. Species diversity then declines under the extreme conditions of the Pumice Plain.

   The pattern of species diversity seen in small mammals in the Tephra-fall and Blowdown Zones could be determined by the variety of different habitats and resources that became available to them in the secondary successional communities that developed after the eruption. Compared with the reference area, these two communities likely foster more species and at a higher relative abundance because of these new resources. In the primary successional community of the Pumice Plain, though, the habitats and resources are not nearly as diverse or abundant and thus cannot support more than one small species.

3. The deer mouse (Peromyscus maniculatus) is the only species present in all four communities. This suggests that the deer mouse has a life history that allows it to live in primary, secondary, and climax successional communities. It is likely able to disperse widely, growth quickly, and reproduce often—all characteristics of an early successional, r-strategist species. The deer mouse is also likely to be an opportunistic and generalist species, living in a variety of habitats and feeding on a variety of food items.

4. The Tephra-fall and reference area communities have more species in common than the Blowdown Zone, but all three communities have their own characteristic small mammal species assemblages. Given that each community is represented by a different successional stage, with vegetation characteristic of secondary and climax successional communities, it makes sense that the species composition of the small mammals inhabiting those communities will reflect these differences.

1. Corals are foundation species because their effects on the coral-reef community are due to their relatively large size and abundance. They are also ecosystem engineering species; as they grow, their skeletons physically modify the environment, increasing habitat for other reef species.

2. The genera Acropora and Orbicella would be most important in building reef structure, and Porites and Agaricia would be least important. Acropora and Orbicella have the highest rugosity, indicating greater complexity, which results in a diversity of reef habitats. They also have the highest calcification rates, indicating that they add reef structure more rapidly than the other two genera.

3. Because of the rapid decline in both reef rugosity and rate of calcification, the genera Acropora and Orbicella are likely declining most rapidly. These are the genera most responsible for the large size and complexity of the reef. The loss of these genera would affect reef function by reducing the number of habitats and thus the number and abundance of many species in the reef community.

4. The reef does not appear to be regaining its original functionality, at least in the time span of the model. Although the coral cover increases from 10 to 45 percent, neither rugosity nor calcification rate shows a corresponding increase. Rugosity varies only slightly over the time span, ending with a slight upward trend. Calcification rate increases near the middle of the time period before showing a continuous downward trend. The low levels of both factors, compared with their very high levels in the original healthy reef (seen at the start of model i), indicate a much lower functionality. The low functionality may be a matter of timing; a reef’s function can be compromised when foundation species are lost (as shown in model i), but because the foundation species (massive corals such as Acropora and Orbicella) are replaced slowly, it takes much longer for the reef to recover. The lower functionality probably indicates that foundation species are being replaced by smaller species such as Porites and Agaricia, both of which have lower rugosity and calcification rates. Their effect may be significant in the short term because of their rapid reproductive rate, but less than that of Acropora and Orbicella because they do not contribute significantly to reef building.

5. Yes, both model coral-reef communities are undergoing succession. For model ii, the recovery could be considered secondary succession, given that it is restoring its structure after a major disturbance (loss of most of its coral structure). According to the concept of alternative states, under similar environmental conditions, succession can result in different assemblages of organisms. That is, the new community does not return to its original state. In model ii, the coral community does not reach the same level of rugosity or calcification as the starting conditions of model i, indicating that either different species are present and/or their abundance has changed. The lower levels of rugosity and calcification will result in fewer habitats and lower biodiversity. Given more time, as the reef continues to undergo succession, more massive, slower-growing corals may again predominate, and the reef may be able to support higher species diversity. But the possibility that the coral-reef communities show hysteresis, or the inability to recover to the original community, is also a possibility if the effects of climate change intervene.