Chapter 8 Population Size, Trend, and Viability

Discussion Questions

  1. You have been tasked with estimating the number of individuals of the threatened San Benito evening primrose (Camissonia benitensis) in a 400-acre reserve. This plant seems to grow only on serpentine soils, and the reserve has several small patches of serpentine soil. The adult plant is small and can be hard to spot among surrounding vegetation. In addition, the seeds of this species can remain viable in the soil for at least 20 years. How might you go about estimating the total size of the San Benito evening primrose at the reserve?
  2. You work for a conservation organization in Indonesia. You started out focusing on orangutans but now are interested in another five species that seem to be at risk due to deforestation. Under what circumstances would you forgo conducting a PVA for each of these additional five species?
  3. Explain the logic underlying the Lincoln–Petersen mark-recapture approach to estimating population size. Imagine applying this method to a species of your choosing. For this species, which of the major assumptions of the Lincoln–Petersen method are most likely to be violated? Will these violations tend to result in an underestimate or an overestimate of the true population size?
  4. A population experiences a "good year" with a 3% increase (λ = 1.03), a so-so year with a 1% increase (λ = 1.01), and a bad year with a 5% decline (λ = 0.95). What is the realized rate of population increase over the three-year period? If this rate holds over a longer period, what is the geometric mean?
  5. You have studied a population of abalone. You decide to divide the individuals into four classes: larvae, small, medium, and large. In any given year, a larva has a 70% chance of settling and becoming a small abalone (otherwise it dies); 60% of small abalones remain in the small size class, 15% grow into the medium size class, and none grow all the way from the small to large class in single year; 50% of medium-size abalones remain in the medium class, and 10% become large; large abalones have a 20% chance of survival; and, on average, a large abalone produces 27 larvae. Draw a life-cycle diagram for this population. Indicate all possible transitions with arrows, and place a numerical value next to each arrow. What is the probability that an individual abalone in the small size class will die within a year? What is the probability that an individual currently in the small size class could reach the large size class in just two years? What is the probability that an individual currently in the small size class will die in either of the next two years? Now construct a demographic matrix corresponding to the life-cycle diagram. Choose any starting population vector, and project the population out until λ stabilizes to within ±0.001 (this is easily done with a computer spreadsheet program such as Excel). What value of λ did you obtain, and what does this mean for the long-term change in abalone numbers?
  6. Suppose you work for an organization that manages harvests of a fish population. The number of fish has been declining for more than 20 years, and in response your organization has severely restricted harvests. This year the fish population suddenly increased, and the public is pressuring your organization to increase the allowable harvest in response. How would you respond to this pressure, and how would you explain the possible role of stochasticity in the sizes of fish populations?

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