BIO 321: Ecology, Review 2 KEY, Dr. Chris Paradise, Fall 2004

Instructions:  This review is worth 100 points (10% of course grade) and will be due on Wednesday, 9/22/04 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 four 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.      The hydrologic cycle can be viewed as an enormous solar-powered pump.  Outline the ways in which 2 other nutrient cycles are driven by this solar powered pump (6 pts).

Very few students read this question properly, and most of you addressed the role of the sun in powering the cycles.  The solar-powered pump is the hydrologic cycle, and movement of clouds from over ocean to over land and how the water gets back to the ocean are transfers that carry various other nutrients along..  Nutrients are dissolved in water vapor and droplets in clouds, water acts as a reservoir of certain inorganic or organic forms of various nutrients.  Water dissolves rocks and causes erosion of soils that contain certain nutrients, making them available to biota.

2.      Answer the following two questions regarding the carbon cycle (10 pts total).

a.      The atmospheric concentration of CO₂ does not appear to be rising as quickly as it might if all anthropogenic CO₂ were to remain in the atmosphere.  Describe the processes that cause flux of CO₂ between the atmosphere and other reservoirs?

Processes include photosynthesis, respiration, dissolution in water (ocean), combustion of fossil fuels, deforestation, and sedimentation (one process removed from atmosphere, but may count).

b.      What changes in potential energy are associated with the transformations you described in part “a”?

Energy transformations in biological systems typically link assimilatory and dissimilatory processes.  Photosynthesis, dissolution of a gas in a liquid, conversion to fossil fuels are processes that increase potential energy of C.  Respiration, burning of fossil fuels and decomposition of organic matter are processes that decrease potential energy.

3.      Relate the flux and regeneration of nutrients in the ocean to global climate and ocean circulation patterns (5 pts).

Global climate refers to patterns in rainfall, precipitation, and winds.  Wind patterns set up ocean circulation patterns and are responsible for mixing of water masses and upwelling.  Thermal patterns may cause stratification or vertical mixing of water masses.  Evaporation and precipitation move water from ocean to over land – rivers bring nutrients and water back to ocean.  Precipitation over ocean reduces salinity of surface waters, creating changes in density of water masses, possibly promoting mixing and regeneration. 

4.      Answer both of the following questions regarding the article by Neff et al. (2003) on the nitrogen cycle:

a.      Diagram the nitrogen cycle, including the major processes involved in transforming & transporting nitrogen.  Add in DON, and include three of the processes involved in its movement or formation (8 pts).

Here you must NAME the processes, not just draw arrows linking different forms or reservoirs.  The processes in the basic N cycle include fixation, nitrification, assimilation (uptake), ammonification, denitrification.  Three processes linking DON reservoir/form to other reservoirs/forms must be made.  Some of those processes include leaching, uptake, excretion/secretion, decomposition, and ammonification.

b.      Why might the nitrogen cycle short circuit be more prevalent in boreal, alpine, and tundra ecosystems?  (4 pts).

These ecosystems have short growing seasons, and during the growing season soils are quite moist.  Because of the cool temperatures, organic matter builds up and decays slowly.  There is little microbial activity to break down the organic matter and release inorganic nitrogen compounds (especially early on in the growing season), but there can be large fluxes of DON during snowmelt and during the growing season.  Plants that can take up labile DON compounds directly have a competitive, selective advantage over plants that can only take up nitrate or ammonia, and so we should not be surprised at this adaptation in boreal or tundra ecosystems.

5.      Consider a predator foraging for three different prey types that occur together in discrete patches.  The abundance of the 3 types varies within a patch, such that prey type A is more abundant that B, which is more abundant than C.  Their profitability also varies, with A being most profitable, then B, then C.  Additionally, some patches have higher total prey densities than other patches, that is, the patches are not of the same quality.  Answer the following 4 questions (4 pts each = 16 pts). 

a.      What factors does the predator assess in determining its diet breadth in this scenario?

Travel time, handling time, energy content, nutrient content, abundance (only of higher ranked prey).  The first two encapsulate profitability, or quality.  Quality by itself was insufficient to answer the question, as it is made up of other attributes that we discussed.

b.      Predict what would happen to both patch residence time and diet breadth if travel time between patches were to increase.

Patch residence time (PRT) will increase.  Diet breadth might increase IF the forager is assessing energy intake over the whole foraging bout (the increased travel time is taken into account) or if patch depletion occurs as foragers remain in a patch longer, breadth should increase as abundance of high ranking prey declines.  However breadth may stay the same depending on abundance of those high ranking prey.

c.      According to the marginal value theorem, why should residence time in a poor quality patch be shorter in the above scenario than in a poor quality patch in a habitat where all patches are of the same poor quality?

If all patches are of same poor quality it doesn’t pay to leave any one patch early, since the instantaneous rate of energy intake is not likely to be much higher in a new patch compared to the current one.  If there’s a likelihood of finding a higher quality patch, as in the current scenario, the forager will have a larger probability of increasing its instantaneous rate of energy intake by leaving a poor patch earlier.

d.      Consider a predator of the first predator that enters the habitat.  What would you predict to happen to patch residence time and diet breadth if the risk of predation were to increase only in high quality patches? 

If predation risk increases only in high quality patches, PRT is likely to decrease in those patches (it’s assumed the forager can tell which patches may contain a predator, so the first predator may stay in lower quality patches without predators longer).  Diet breadth may increase to make up for lost foraging time, especially if it spends most of its time in patches with lower overall abundance.  If the forager is really hungry, it may risk taking only highly profitable prey in a high quality patch.

6.      Cost/Benefit Analyses (12 pts).

a.      It is often found that eggs of many animals are actually smaller than the egg size that maximizes probability of survival.  This is likely due to the fact that… (underline or circle the correct choice)

Well, it turns out that none of these is correct, so I only graded your responses to b.  It was supposed to be choice ii, but it was supposed to say more intermediate sized eggs, not fewer.

b.      Discuss, in one sentence each, why you rejected the three choices in the previous question you did not choose.

i)        Producing large eggs decreases the survival of the adult female.  This may be true under some circumstances, but on its face, it is not necessarily true.  If a female produces lots of large eggs and uses up a lot of its energy, survival may decrease.

ii)      The highest fitness is achieved by producing fewer, intermediate sized eggs. It’s all relative, and it’s not clear here to what “fewer” is relative.  If it’s relative to many, small eggs, yes, it’s true.  That’s not the case here.

iii)    Many small eggs can lead to higher fitness of the offspring, since some will survive long enough to reproduce. Never true – the choice refers to fitness of the offspring, not the parents.  The few that survive will have higher fitness than those that don’t, but the ones that don’t survive will have 0 fitness.  The choice does not distinguish between those that survive and those that don’t.

iv)    The statement is not true – larger eggs are always favored by natural selection.  Not true; many of you thought of a variety of circumstances in which larger eggs are not favored.

c.      LIST the costs and benefits an organism experiences when it decides to wait and breed later in life.

Costs: dying before it ever gets to reproduce.

Benefits: growing to a larger size, being more experienced and mature, having a higher lifetime fecundity, becoming a better competitor for mates.

d.      In which environment would you predict natural selection favoring the evolution towards females having fewer, larger offspring – an environment with a highly variable risk of egg predation, or an environment with a low, constant risk of egg predation?  In one sentence, why?

Most of you agreed that a low, constant risk of egg predation would select for larger, fewer offspring.  It is dependent upon how variable and strong predation is in the variable environment.  If egg predation is rare, and weak when present, selection towards any change in egg size and number will be weak.  Under a constant pressure scenario, larger eggs might be either more likely to be found or more attractive or more likely to survive/evade predation.

7.      Relate fecundity to fitness, being sure to clearly indicate to what each of those terms refers (5 pts).

Fecundity is the number of offspring produced per reproductive episode (lifetime fecundity is number over entire lifetime).  Fitness is defined as reproductive success, or the genetic contribution of an individual to the next generation, or the number of offspring that survive to reproduce.  The last definition is key when comparing the two.  An organism can have high fecundity, but low fitness if few of the offspring survive to maturity.  Conversely, low fecundity can lead to low fitness (low survival) or high fitness (high survival).

8.      Population structure models.  Answer each of the following in one sentence each (3 pts each = 6 pts).

a.      How is the source-sink population structure model related to the metapopulation model?

It’s basically the same structure of metapopulation, but patch quality varies, leading to some subpopulations having a positive growth (and d-d effects) and some having negative growth.

b.      How is the source-sink population structure model related to the marginal value theorem (foraging in a patch)?

They both contain analogous ideas about patch quality.  Animals will leave high quality patches (or source patches) when instantaneous intake rate drops to a certain level or when density-dependent effects become strong.  Another way to consider the relationship is to consider the metapopulation in the source-sink model to be the prey populations upon which a forager is feeding.

9.      Life table analysis.  The life table below shows data collected to determine the growth rate of a population of an endangered land snail.  Researchers hypothesized that an age-structured population model would be most appropriate to estimate future population sizes of this endangered species.  Using data collected by other researchers and available in the primary literature, conservation biologists determined fecundity (b_x) and survival rate (s_x) schedules and assumed them to be constant.  They then censused the lone remaining population in NC in 2003, assessing number of individuals of each age class in that population.  Answer the following questions related to the life table below.  (Note: t = 0 refers to the 2003 census)

The following equations may be helpful to you:

R₀ = Sl_xb_x

T = Sxl_xb_x / Sl_xb_x

r_a = ln(R₀) / T

a.      Interpret the biological meaning of R₀. (3 pts) This is the net reproductive rate and is equal to the average number of offspring a female has over her entire life.

b.      Does this population appear to have a stable age distribution?  How is a SAD attained in any population? (3 pts)  It’s difficult to tell, but probably not yet, as some age classes are declining in proportion and others are increasing.  Given constant s_x and b_x schedules, a SAD will be achieved, but this population may not be there yet.

c.      List three life history traits you predict these snails to have based on data in the table?  (3 pts) Looks semelparous, late maturity (relative to lifespan), lots of offspring, little parental care (most likely), very little juvenile survival.

10. Population Growth Models (3 pts each = 9 pts)

a.      When a population grows according to the logistic model, at what population size is dN/dt the highest?  K/2, which occurs at the inflection point of the logistic curve.  However, the question asks what population size, so you must indicate that the population is at half carrying capacity.

b.      On what concept is density-dependence modeled in the logistic growth equation, and what is the biological meaning of this concept? Carrying capacity is the concept, and it refers to a maximum number of individuals of one species that a particular environment can hold at a particular time.  Explaining that density-dependence affects birth and death rates is not sufficient to answer the question – these things change as populations grow, but the concept is K.

c.      Show how the discrete population growth model is derived. The discrete model is N_t = N₀λ^t.  It’s derived from N_t+1 = N_t + BN_t – DN_t.  λ is set to equal (1+B-D).  Then the equation becomes N_t+1 = λN_t.  To derive the more general form, note that N₁ = N₀λ and N₂ = N₁λ, which is the same as N₂ = (N₀λ)*λ = N₀λ².

11. Outline of an essay (10 points). Select ONE question below, a or b.  Use the space on the next page to create an outline of how you would organize an essay on the topic, using specific examples.  This outline will form the basis of your 2^nd essay.

a.      What are the major effects of habitat fragmentation on populations?   Outline this question using specific, real examples for each type of effect identified.  You should identify 3 major effects on populations, comparing and contrasting these effects on populations of different species, using both animals and plants as your examples.

The causes of fragmentation are habitat destruction (logging, agricultural development, suburban sprawl), road-building, damming, and other forms of construction/habitat alteration.  Major effects of fragmentation include edge effects, reduced habitat size, increased risk of catastrophe, creation of a metapopulation structure in species not formerly in a metapopulation, longer distances between subpopulations, creation of barriers to dispersal.  Reduced habitat and new metapopulation structure lead to smaller population sizes, which can lead to reduced genetic variation.  Longer distances between subpopulations and barriers to dispersal also lead to reduced genetic variation.  Reduced genetic variation can lead to inbreeding depression and increased risk of local extinction (as can small population sizes).  These are mostly negative effects, but there are species that benefit from fragmentation, such as edge specialists, habitat generalists, and some non-native species.  This may lead to new or altered species interactions.  There are many other scenarios and chains of interactions among effects that you might explore in your essay.

If you outlined this question, you must include and describe some specific examples, with at least one drawn from a literature source.  The effects described above are changes in the habitat caused by fragmentation, which then lead to secondary effects on the organisms, populations, or communities.  For instance, fragmentation causes edge effect, which leads to effects on abiotic factors, which leads to effects on species.  Be sure to compare/contrast the effects you choose on different taxa that have different vagilities, habitat preferences, or mechanisms of dispersal.  For instance, many butterfly species prefer open, disturbed habitat and have high vagility, so some of those species might benefit from disturbance and fragmentation.  Interior forest butterflies, although they also have high vagility, might suffer adversely, due to their inability to cross long distances of open habitat to find new core forest habitat in which to breed.  Finally, you might consider commenting on the effects of fragmentation on overall biodiversity.

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