General Ecology
Spring 2002
Review Problem Answers--Unit 3
Biodiversity measures Succession Global diversity patterns Ecosystems Island Biogeography Intermediate disturbance Equilibrium/non-equilibrium Conservation biology Nature of Communities
1. The popular idea of the "balance of nature" gives people the idea that communities are in equilibrium and that species tend to maintain their roles for the good of the community. Equilibrium is tenuous at best and the good of the community is only true when it is also in the individual's best interest. However, sometimes mechanisms can "balance" out...as in the balance between colonization rates and competitive exclusion rates resulting in highest diversity at intermediate disturbance frequencies. Or similarly, the balance between tolerating low resource levels and competitive exclusion resulting in highest species diversity at intermediate productivity levels.
2. The rivet model presumes that each species plays a role in maintaining the community as a functioning unit. As species are lost, the community becomes less and less functional until it collapses completely. The redundancy model presumes that several species play the same role interchangably and that as a species are lost, another can move in to take its place, but the ability of communities to withstand additional changes is weakened with each species loss.
3. A keystone species is a species whose presence or absence controls the ability of many other species to persist in the community. Pisaster eats mussels, the dominant competitor among rocky intertidal invertebrates. With Pisaster present, competition from mussels is reduced, allowing many other species to occupy space on the rocky substrate. Without Pisaster, mussels competitively exclude at least half of the other sessile invertebrate species. Sea otters are another example of a keystone predator in that they eat urchins which otherwise wipe out young kelp. With otters present, there is a kelp forest that supports a high diversity of other species. Without otters, there is no kelp forest regeneration, and consequently, a loss of the many other species that depend on the kelp.
4. The index of similarity is 2z/(x+y), where x and y are the numbers of species in the two communities and z is the number of species in common between the two communities. In the two Point Defiance communities, there are 9 and 7 species, with 5 of those in common. That leads to an index of similarity of 2*5/(9+7) = 0.625.
Diversity Measures
5. Estuarian bird communities
a. Estuary 1 has the highest richness
b. Estuary 2 has the lowest diversity.6. Intertidal invertebrate community
a. The species richness is 3; the Shannon-Weiner diversity is H' = 0.267 (using log) or 0.615 (using ln).
b. An increase in Pisaster would not change richness, but would increase diversity.
c. An increase in Balanus would not change richness, but would decrease diversity.Island Biogeography
7. MacArthur and Wilson's Island Biogeography model is called an equilibrium model because it is based on the equilibrium between immigration and extinction.
a. It predicts that large near islands should have higher immigration and lower extinction rates (= higher species numbers) than small far islands.b. It predicts that small near islands should have both higher immigration and higher extinction rates (= higher turnover rates) than large far islands.
8. Distant oceanic islands have fewer species than the same area would on a mainland because...
Equilibrium view: lower immigration rates can't offset the given extinction rate on the island whereas they can on the mainland.Non-equilibrium view: low immigration rates have been unable to fill all the available habitats on the island. Extinction plays no part. Immigration has yet to bring the species number to equilibrium.
Intermediate Disturbance Hypothesis
9. The intermediate disturbance hypothesis proposes that species diversity should be highest with intermediate frequencies of disturbance because of two opposing mechanisms. At very high disturbance frequencies, only the most rapidly dispersing species can colonize before another disturbance. More species can colonize as disturbance frequency drops. At very low disturbance frequencies, the most competitive species have time to exclude most of the other species and cause low species diversity. With more frequent disturbances, competitive exclusion has less time to occur.
10. The bromeliads growing on the intermediate branches should have the highest diversity according to the intermediate disturbance hypothesis. To see if the mechanism is consistent with this hypothesis, one would have to follow the succession of species in undisturbed bromeliads and then look for a mixture of early, mid, and late successional species in the intermediate branch bromeliads.
Succession
11. The three mechanisms for succession differ in their assumptions about how earlier successional species affect the performance of later successional species. Therefore, testing among these hypotheses involves measuring the performance of later successional species with and without the earlier successional species present.
12. Initially in succession, only rapidly dispersed species will be present. Over time, more and more species disperse and invade the area, increasing diversity. However, as time continues, the competitively dominant species will exclude more and more species, resulting in a smaller and smaller species diversity.
13. Senecio succession
a. There is essentially no effect on Senecio biomass of removing Epilobium in year 2 or in year 3. Both P-values are above 0.05 and the biomass numbers are very similar to those of controls.b. Again, there is no effect on Senecio biomass of removing all other vegetation in either year 2 or year 3. Again, the P-values are high and the biomass numbers similar to controls.
c. Basically, other vegetation has no effect on Senecio. The third year decline must be caused by something else.
d. It seems that Senecio causes its own abundance to crash in year 3.
e. There is no evidence for competition from other species causing the displacement of Senecio in the successional sequence. A much more likely hypothesis, in light of the data, is that Senecio does something to make its site less suitable for itself, and it is something that other plants do not do. One possibility is that Senecio exudes some toxic compound into soil that causes autotoxicity. Another possibility is that Senecio harbors some pathogen that becomes abundant in the soil after a year.
Global Patterns in Species Richness
14. The productivity hypothesis doesn't work very well because the highest productivity should result in a lower diversity than an intermediate productivity (remember the diversity-productivity curve). (This hypothesis has been salvaged by proponents by observing that nutrients are rather scarce in tropical soils resulting in intermediate productivity and high diversity.)
The predictability and specialization hypothesis only works if the tropics are more predictable in their resource availabilities. Seasonal scarcity of resources in the temperate zones comes about in winter. But seasonal scarcity of resources occurs in the tropics in drought-deciduous habitats, and yet diversity is still high.
The time hypothesis seems to work well, but has only a couple of observations to corroborate it. The first observation is that numerous species are continually expanding their ranges northward, presumably after the retreat of the last glaciation. The other observation is that Europe has a lower species diversity than North America. This has been used as corroboration for the evolutionary age hypothesis because Europe's mountain ranges run east-west and when species were migrating south in front of the advancing glaciers, they came up against the mountain ranges in Europe and were eliminated. In North America, species had no barriers to southerly retreat and could re-occupy their former ranges when the glaciers receded. It would be a nice addition to the evidence to find a geologic record that showed low diversity just after the retreat of glaciers and increasing diversity during interglacial periods. However, that kind of geologic record has not yet been discovered.
15. Hypothesis for plants: Increased nutrients allow increased biomass. This causes competition for light, and competitive exclusion. Therefore, diversity would decrease.
Hypothesis for insects: Increased plant biomass provides increased food supply for the insects and, if increased biomass provides increased structural complexity, would also provide the insects with more microhabitats. Therefore, diversity would increase. (If, on the other hand, increased abundance of the competitively dominant plant species allowed large populations of a specialist insect species or a competitively dominant insect species to develop, insect diversity could decrease.)
Equilibrium vs. Non-equilibrium
16. Ecologists use the term stability to describe the ability of communities to retain their species in the face of disturbance or regain their species after disturbance. The idea that species diversity increases stability is controversial because arguments and models have supported both a positive and a negative association between diversity stability. Recent experimental data from Tilman suggests that total biomass of a grassland plant community is more stable as species diversity increases, but that individual species biomass is less stable as species diversity increases.
17. The fundamental difference between equilibrium and non-equilibrium views is the role of variation, usually environmentally induced. In the equilibrium view, this variation is unimportant and should be ignored where possible. In the non-equilibrium view, this variation is what allows the existence of the mechanisms that structure the community and is fundamentally important.
18. "Top-down control" means that consumers reduce the population of their prey below carrying capacity so that competition does not limit the population. "Bottom-up control" means that resources limit the size of a population through competition. Bottom-up control fits more easily into the equilibrium context because consumer population reduction represents a kind of disturbance. However, some would argue that consumption is just as much an equilibrium phenomenon as is competition. The concept of top-down vs. bottom up can help predict how altering fishing pressure might affect lower trophic levels.
Ecosystem Ecology
19. Gross primary productivity is the total amount of carbon fixed through photosynthesis. Net primary productivity is the amount of fixed carbon left for growth and reproduction after respiration.
20. Trophic pyramids
a. Ecosystem 2 is more likely to represent mammals, whereas ecosystem 1 is more likely to represent fish. The transfer rates are higher in ecosystem 1 (more of the standing crop from one trophic level is transferred to the next), which is more typical in an ecosystem with exothermic herbivores and predators.b. The energy transfer rate from primary producers to primary consumers is always lower than the transfer rate from primary consumers to secondary consumers because the primary consumers are herbivores--turning plant biomass into animal biomass--whereas the secondary consumers are carnivores--turning animal biomass into animal biomass. Animal biomass has a higher digestibility for animals than does plant biomass.
21. A community with high net primary productivity can support more trophic levels than a community with low primary productivity because there is more energy and biomass to be transferred to higher levels. For example, if we assume that only about 10% of energy or biomass is transferred to each successive trophic level, at some level there won't be enough energy available to support an additional population of consumers. This will happen at a lower trophic level if the starting energy/biomass supply is smaller than if it is bigger.
Aquatic systems can often support more trophic levels than terrestrial systems because most aquatic vertebrate consumers are ectothermic rather than endothermic. The reverse is generally true in terrestrial systems. Because ectothermic consumers lose less of their energy to heat generation, there is more energy available to pass to the next trophic level, allowing more levels to be supported.
22. Zooplankton come up to the surface at night to feed on phytoplankton. The phytoplankton must remain in the surface waters (in the photic zone) to carry on photosynthesis in the day.
23. Most of the world's nitrogen is in the atmosphere as N2, which is unavailable to plants. Nitrogen becomes available to plants when it is "fixed" (N2 --> NH4+) through either abiotic or biotic means. Nitrogen becomes available to animals when they eat plants.
24. In unpolluted lakes, phosphorus is typically limiting, which allows both green and blue-green algae to persist. If nitrogen becomes limiting, blue-green algae have a competitive advantage because they can fix atmospheric nitrogen. Before they were eliminated from laundry detergents, phosphates in runoff water entered lakes where the additional phosphorus made nitrogen limiting. This gave blue-green algae a competitive advantage. In addition, blue-green algae are a less-preferred food for most invertebrate and many fish consumers because of their gelatinous sheaths. Between these two advantages, blue-green algae formed "blooms" on lakes when their populations exploded. As individuals died and entered the detritus, microbial consumption depleted much of the lower waters of the lake of oxygen, triggering massive fish kills and continued oxygen depletion. This series of events was prevented in most lakes by the reduction in phosphorus runoff.
Conservation Biology
25. The 100 km2 reserves may be too small for some species that have large home ranges and they will have more edges. More edges means both that more of the reserve is marginal habitat and that there will be more invasions of non-native species.
26. If a species is already in low numbers, low genetic variability can lead to 1) inbreeding depression from the expression of deleterious recessive alleles, 2) a lowered ability of the population to track environmental changes, and 3) lowered disease/parasite resistance.
27. The minimum viable population (MVP) is that population size that will ensure at some acceptable level of risk that the population will persist for a specified time. Longer lifespans decrease MVPs, populations with higher variation in growth rates require larger MVPs.
28. Area required increases at higher trophic levels (plants < herbivores < carnivores). Area required also increases as body size among carnivores increases.
29. With small populations, demographic fluctuations (variations in births and deaths), environmental fluctuations, lower genetic heterogeneity (can reduce ability of population to evolve), catastrophes might wipe out the entire population.
30. Mountain goats are probably not native to the Olympic mountains, at least not in the recent geologic past. The glacier that formed Puget Sound isolated the Olympic peninsula from the Cascades and there is no good evidence that mountain goats existed in the Olympics at that time, although there are conflicting opinions. Mountain goats were released into the Olympics for sport hunting in the early part of the 20th century. Since then, they have become established and their numbers have grown. However, there is controversy over the numbers because it is very difficult to count these rather elusive herbivores. The main controversy is over the amount of damage that the mountain goats are doing to the native and endemic plant species. The goats eat these plants, they paw the ground and kill them, and they maintain dust wallows that initiate erosion and degradation of sensitive meadows. Olympic national park officials want to rid the park of the mountain goats, but animal rights advocates argue that both the numbers and the damage estimates are exaggerated. Park officials are considering several options to remove the goats, from airlifting them, to sterilizing them, to shooting them, but have to balance financial constraints with public relations.
31. Genes for herbicide resistance, for Bt toxin to incorporate herbivore resistance, and nutritional genes have all been incorporated into various crop plants. Incorporating herbivore resistance genes into crop plants can allow growers to minimize or eliminate their use of insecticides to eliminate herbivores. This reduces the environmental impact of spraying and the energy required in applying the spray. However, the gene product can be carried by pollen to nearby wild plants, which then could become toxic to unintended insects. In addition, there is the potential that cross-breeding with wild plants will allow the gene for herbivore resistance to be incorporated into wild plant populations, causing deaths of unintended herbivore targets and selecting for herbivores with tolerance to the gene product.
32. Dams are one of the leading causes of death of young salmon making their way downstream to the ocean. Despite barging (taking salmon in trucks to the lower Columbia) there is a huge mortality. Coming back upstream, fish ladders help, but we really don't have good data on whether the dams interfere with returning adults. Dams have flooded a good deal of former spawning habitat. So there have been serious negative consequences of dams on fish. However, we don't know that removing the dams will be beneficial for fish in the short run. In addition to the scouring effects of the abrupt water release, which might damage the currently used spawning grounds, the sediments released and deposited have often been found to have high levels of heavy metals and other pollutants that could contaminate spawning grounds for years to come. So regardless of the economic arguments, of which there are many, there are solid ecological arguments to proceed only very cautiously, if at all, with dam removal.