BIO 321: Ecology, Review 1 KEY, Dr. Chris Paradise, Fall 2005
Answers in BOLD
Instructions: This review is worth 100 points (10% of course grade) and will be due on Wednesday, 9/28/05 in class. Late reviews will not be accepted. You may not consult any references or any other person while working on this review. Your signature at the bottom of the last page signifies that the work is yours alone, was completed in three hours or less, and is pledged under the Honor Code. When you begin you will have a maximum of three hours to complete the review!Confine your answers to the space below each question. Print legibly! Alternatively, the review may be done on the computer. You may type your answers to each question, and attach the printout to the review. For each question or part to a question, limit your answers to one concise paragraph, unless otherwise specified. Lengthy answers will be penalized the same as handwritten answers that spill out over the space given!
1. Discuss how two of the hypotheses of factors thought to enhance diversity at lower latitudes are actually interrelated (10 pts).
Diversity or richness is thought to be caused by many factors including productivity, the presence of predators, habitat structure, age of habitat, size of habitat, and more. These factors are often closely interrelated, and may be integrated to further explain the latitudinal diversity gradient. Take productivity and predators, for instance. At lower latitudes the climate is warmer and wetter, which allows for high productivity within the ecosystem. Potential evaoptranspiration (PET), a good measure of productivity, is positively correlated with temperature in an environment where water is not limiting. Therefore, at lower latitudes, where there is high temperature and abundant water, there is an increase in productivity. This increase in productivity is linked to an increase in species richness because there is more energy available to support more niches, and therefore, support more species. The more energy in the system, the higher up in trophic levels energy can flow, leading to larger populations of predators. Predation is a factor in determining the number of species because it limits competition within species, leads to small populations of prey, and leads to antipredator adaptations. The increase in productivity and predation at lower latitudes both work to increase species diversity. In a similar manner, productivity and habitat structure, productivity and age of habitat, and age and area can all be combined. One major point is that any two or more hypotheses put together can probably do a better job of explaining patterns of diversity than any one by itself.
2. Provide two examples of how an El Niño-Southern Oscillation (ENSO) event could affect energy flow in an ecosystem. One example should be from an aquatic ecosystem, and the other should be from a terrestrial ecosystem. Be specific (8 pts).
ENSO events result in dramatic changes in global climate, which can have large effects on energy flow within an ecosystem. Off the coast of Peru, for example, an ENSO event leads to decreased upwelling of cold, nutrient rich water. This limits the productivity of the coastal marine ecosystem because with fewer nutrients there is less primary productivity in marine algal populations. The reduction in primary productivity decreases the amount of energy available to zooplakton, the next trophic level. This results in a decrease in energy available to anchovies, which reduces their population size, and ultimately affects the availability to the human population. This same ENSO event can lead to an increase in productivity in the Galapagos Islands. An ENSO event causes unusually warm and wet weather. This increases the primary productivity of the plants in the terrestrial ecosystem, which leads to an increase in seeds and habitat heterogeneity. The increase in energy and available habitat lead to an increase in bird populations on the island. Thus, the changes in energy flow resulting from ENSO events can work to both increase and decrease the productivity of ecosystems, depending on where and how the climate changes.
3. Provide an example of how climate affects nutrient cycling in a forest. What themes of ecology are illustrated in your example (10 pts)?
Climate is defined as long-term precipitation and temperature patterns. In regions where there are warm and wet conditions, the rate of nutrient cycling is generally higher than in cooler or drier habitats for several reasons. The warm conditions increase the rate of decay of organic material by microbes, and therefore, increase the rate in which nutrients are returned to the soil. The wet conditions increase the amount of water in the soil, and therefore, can lead to more leaching of nutrients from the soil, or increase availability of dissolved nutrients to plants. An increase in water can also lead to an increase in weathering of rocks, which can increase the amount of nutrients available in the soil. The warm wet conditions increase primary productivity of plants, which increases nutrient cycling because nutrients are continuously taken up by plants for growth and reproduction. This increase in primary productivity also causes an increase in productivity at other trophic levels, which increases nutrient cycling through consumption and then defecation and decay. This example illustrates several themes in ecology including the importance of interactions, dynamic states, and variation in space and time. The interaction between trophic levels and between abiotic and biotic factors is important in determining the rate of nutrient cycling. Dynamic states are seen in the change in form of nutrients and in the processes that transform them. The varying availability of nutrients at different times and places based on climate, plants, and soil factors illustrate the importance of scale and variation in space and time. Disturbance may also be related to this - a drought can cause a decrease in nutrient cycling.
4. You’re a forester interested in the distribution of striped maple (Acer pensylvanicum) in mountain forests in western North Carolina as it is related to elevation. It generally occurs on the northern slopes, is an understory tree, and never dominates the forest. Seeds are wind-dispersed. Foresters have predicted that global climate change will shift the distribution of the striped maple northward or to higher elevations, and it is desirable to determine its current distribution and the factors that determine that distribution. Further, you suspect that the fire regime in the mountains will change due to global climate change, and the striped maple will be unable to adapt to these changes. Answer the following related questions (10 pts).
a. What life history factors (list two) would inhibit evolution to rapid environmental change?
Late age of maturity or first reproduction, low fecundity per reproductive season, long generation time or life span.
b. If the per capita growth rate of the maple population is small, how might the population respond post-fire?
If the per capita growth rate is small the population will not be able to increase population size rapidly after a major disturbance.
c. Describe briefly how the fire regime (frequency and intensity) could change as the air around a mountain system becomes warmer.
As the air becomes warmer it also becomes drier, which would result in drier vegetation and therefore a larger fuel load. A large fuel load increases the intensity of fires. Warm, dry conditions may also increase fire frequency because small sparks from lightning or other man made sources are more likely to start a fire. Therefore, both fire frequency and intensity would increase as air becomes warmer and drier.
d. Describe briefly one way that the theme of dynamism relates to this question.
A dynamic system is one that is constantly changing. Changes in global climate could result in large changes in the distribution and abundance of the striped maple. As temperatures get warmer, the striped maple changes it’s distribution by moving to northern latitudes and/or higher elevations. Likewise, with the increase in fire frequency and intensity due to warmer temperatures the abundance of striped maple may decrease because it cannot adapt quickly enough to these environmental changes.
5. List and describe two processes, in one sentence each, that affect the development of soils (6 pts).
Processes include weathering, erosion, decomposition, precipitation. Weathering is the breakdown of rock, due to physical or chemical processes. As rocks break down, they add nutrients or other elements to the soil. Erosion is the process of removing matter from an area, and this can cause a loss of soil. Deposition is the opposite, where matter is deposited to a particular area. The climate also affects the development of soil, because both temperature and precipitation affect decomposition and nutrient cycling within the soil and weathering of the bedrock that produces soil.
6. The picture below shows the relative sizes of the productivity and biomass of each of four trophic levels. How can the size of the biomass in Trophic Level 2 be larger than that of Trophic Level 1 (6 pts)?
Biomass is a measure of the dry weight of the organism and relates to productivity in that a more productive organism will accumulate a larger biomass. In this example the biomass of trophic level two could be greater than trophic level one because the organisms in trophic level two have a higher assimilation and consumption efficiency, and are therefore able to utilize the available energy more efficiently than trophic level one. This results in a relative increase in biomass compared to the lower trophic level.
7. According to the article by Schindler et al. (2003) on Pacific salmon, how do the changing population sizes of Pacific salmon, due to habitat alteration and over fishing, alter coastal terrestrial ecosystems (8 pts)?
Habitat alteration and overfishing have reduced the population of Pacific salmon. This reduction in population size is thought to alter the costal terrestrial ecosystem in several ways. First, it reduces food available to terrestrial consumers and decomposers that feed on salmon carcasses, juvenile salmon, and salmon eggs directly, including bears, birds, and various terrestrial invertebrates. Each of these populations may suffer a reduction in density or an increase in density-dependent competition as the salmon resources decline. However, this is speculative at this point, and most of these organisms have other resources upon which they can rely. The reduction in salmon population also reduces the amount of nutrients cycled into the costal riparian ecosystem, and this affects primary productivity of terrestrial plants. Schindler et al. described a decrease in tree productivity and diversity along rivers in low salmon years. This reduction in primary productivity also has an indirect effect on terrestrial herbivores, such as deer and moose, that feed on the plants. However, the effect on terrestrial ecosystems is difficult to measure because many terrestrial organisms are not directly reliant on salmon or salmon nutrients, and the effects of a loss of salmon resources may not affect recipient populations in a linear fashion, and therefore, the effect of decreased salmon populations may not be seen until it reaches a certain threshold level.
8. You’re an ecologist who’s hypothesized that the presence of a top predator enhances carbon cycling in warmer waters, due to an increased flow of matter and energy through the food web. You have decided to study this hypothesis by simulating ponds using cattle tanks. OUTLINE how you would set up this experiment. Include all experimental design parameters and constraints (e.g., numbers of replicates, treatments, assignment and placement of tanks, etc.) (10 pts).
In this experiment we are testing two factors: the presence or absence of a predator and the temperature of the water. Therefore in order to have a fully factorial experimental set up there will be 4 treatments: predator + warm water, no predator + warm water, predator + cold water, and no predator + cold water. The name of the predator and the actual temperature of the water are unimportant. In order to get an adequate comparison between treatments and understand variation within treatments, multiple replicates must be used - you must specify AT LEAST 3 replicates, but somewhere between 5 and 10 would be best. This means there will be a total of 4 x (no. of reps.) tanks. In order to reduce the effect of variation in the environment across the experimental set up, such as amount of sunlight and temperature, a blocking system could be used where one replicate of each treatment will be in a block, and each treatment replicate would be randomly placed within a block. To reduce the variation caused by timing, all of the treatments will be set up on the same day. The components that you add to the tanks could be specified in as much detail as you need, but it's most important to indicate that equal amounts of everything except the predator will be added to each tank to control for variation. For instance, you could mention that algae will be added in the same quantity to each of the cattle tanks, and it will be initially collected from the same pond to reduce variation in species composition. Predators should all be collected from the same population. To regulate temperature, if you use heaters you also need to add heaters to cold tanks to reduce any variation caused by the addition of the heater, but of course only turn them on in the warm tanks. Finally, carbon cycling could be measured in a variety of ways; by looking at primary productivity of the algae, changes in dissolved organic carbon, accumulation of biomass at higher trophic levels, or some other measure of energy flow in the system. Primary productivity could be measured by weighing the biomass of the algae that accumulates on a tile hanging in the tank. Photosynthesis could also be measured by looking at the carbon dioxide and oxygen levels.
9. What is the value of studying ecosystem energetics? In particular, discuss how we can study adaptation, interaction, and interconnections, and dynamics by using an energetics framework (10 pts).
Ecosystem energetics is valuable because energy flow determines many aspects of ecosystem structure and function. For example, the ecological efficiency of a particular species or trophic level determines the amount of energy available to the next trophic level. This interaction between trophic levels based on energy is important in determining population size, number of trophic levels, diversity, and number of predator species. Energetics is also related to adaptations of species. For example, ectotherms are adapted to use less energy and thus require less food, and may provide more energy to a higher trophic level than endotherms, which lose energy due to maintaining a constant temperature. Energetics is important in studying the interconnectedness of producers to consumers and decomposers. Without decomposers the energy in detritus would be unavailable to the system. Dynamic states can be discussed within an energetics framework by considering how changes in abiotic factors and latitude, for instance, can effect primary productivity, which affects the energy flowing throughout the ecosystem, and is thus also related to nutrient cycling. Thus, ecosystem energetics is the basis for many of the principle concepts of ecology.
10. Speculate on sequential hermaphroditism observed in a fish species that has a promiscuous mating system and is iteroparous (10 pts).
a. Assuming that the ancestral life history was one in which an individual was either male birth to death or female from birth to death, under what ecological or physiological conditions would sequential hermaphroditism have been favored by natural selection?
Sequential hermaphroditism would be favored in ecological conditions where maximizing reproductive potential at all life stages is important. This could be the case in a highly competitive ecosystem or one with limited resources, but where the limited resources are not clumped in distribution. In addition, potential mates must not be limiting, so there's no advantage to being territorial or having a monogamous or polygamous mating system. Physiologically, the fish needs to have a relatively simple reproductive anatomy and secondary sexual characteristics so that it wouldn’t be energetically expensive to switch between a male and female. If these, or related, ecological and physiological conditions are met then the advantage of increasing reproductive potential throughout the fish’s life would be favored by natural selection and the adaptation could evolve.
b. Based on a comparison of reproductive output of each sex when small vs. large, what sequence of sexes would you expect to evolve in this species?
A fish that has promiscuous mating and is interoparous would probably be most efficient as a sequential hermaphrodite if it started out as a male and then became female. In promiscuous mating systems access to females is not limited and small males would have as high a reproductive fitness as large males because they don't need to fight for territory or compete with other males for limited females. In addition, small males can produce just as much sperm (almost) as large males. However, females have a higher reproductive fitness when they are larger because they can produce more eggs - egg production is correlated with body size in fish, insects, etc. Therefore, for a fish to fully exploit its reproductive potential in this situation it would be beneficial for it to be a male when it is younger and smaller, where it would have a relatively high reproductive fitness in comparison to large males, and then become a female when it is older and larger and has a substantially higher reproductive fitness than small females.
11. Life table data for an annual plant, much like Eupatorium hyssopifolium or Daucus carota, are shown below. Answer the following questions related to the life table below. (Note: t = 0 refers to a 2003 census, t = 1 refers to 2004) (2 pts each = 6 pts).
a. List three life history traits of this species that you can glean from the data in the table.
The plant is an annual, thus it is semelparous, has a relatively late reproductive maturity as it doesn’t begin to produce offspring until late in its annual cycle. Mortality is high in younger age intervals, and fecundity is high.
b. Why does Σlxmx = (ΣFx)/a0 = R0 in this case?
Because this is a discrete population and there are no overlapping generations. All member of the original cohort die by day 362.
c. What is an application of life table analysis?
Life tables can tell you information about future population growth and dynamics. Life tables can provide information about life history strategies, they allow tracking of environmental disturbances or large cohorts, and they can be used to predict life expectancy.
12. Population Growth Models (3 pts each = 6 pts)
a. When a population grows according to the logistic model, at what population size is dN/dt the greatest?
At half of the carrying capacity the rate of population growth is the highest.
b. If a density-independent disturbance, like Hurricane Rita, wipes out 95% of two populations along the Texas coast, which would you predict would bounce back quicker – the opportunist or the competitor, and why?
The opportunist would rebound quicker because they are “r strategists” and have high rates of instantaneous population growth. The competitor is a “k strategist” and has lower rates of instantaneous population growth.
