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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?

Group Projects

• Use the search functions for the IUCN Red List of Threatened Species to explore a group of species that interests you (e.g., amphibians, plants, or reptiles). Select 100 species (each member in your group can do 20 species). If you have a smaller group, divide only 50 (as opposed to 100) species amongst your group. Make a record of the current level of risk for each species (e.g., extinct in wild, critically endangered, endangered, vulnerable, near threatened, least concern) and the level of risk when first put on the Red List (the history section provides this). What fraction of species have moved from a higher to a lower risk category, indicating an improved conservation status? What fraction of species have moved in the opposite direction, indicating a worsening situation? What fraction have stayed the same? By looking at other attributes reported in the Red List database for your sample of 100 species, can you find any traits or situations associated with improvements in the threat status of the species versus becoming even more imperiled?

• Select a species of conservation concern that interests you, perhaps because it lives in your area. If you were to conduct a population viability analysis for this species, what information and data would you ideally want to have? Perform a literature search to see if a PVA has yet been performed for your species. If so, evaluate the analysis. If not, perform an additional search to see whether you can locate data that would be useful in developing a PVA. Evaluate these data.

Useful Websites

• Chicago Wilderness Habitat Project describes a community-based volunteer monitoring network for Chicago area that covers a wide variety of taxa, ranging from bird, to frogs, to insects and trees. http://www.habitatproject.org/index.html

• IUCN Red List of Threatened Species can be searched by risk category, taxonomic group, geographic region, habitat types, threat types, etc. http://www.iucnredlist.org

• NatureWatch hosts environmental monitoring programs that rely on volunteers to monitor and report data on frogs, plants, ice or worms. https://www.naturewatch.ca/

• North American Breeding Bird Survey provides raw data and analyses concerning population size and trends for more than 400 bird species. http://www.pwrc.usgs.gov/BBS

• Program Mark is free software that uses data on marked individuals to estimate population size, survival, and other parameters. http://warnercnr.colostate.edu/~gwhite/mark/mark.htm

• U.S. Geological Survey Patuxent Wildlife Research Center maintains a public library of computer programs and software for estimating the size of populations from mark-recapture data or from distance survey methods. http://www.mbr-pwrc.usgs.gov/software.html

• VORTEX Population Viability Analysis Software, developed by Bob Lacey of the Chicago Zoological Society, is available free of charge. http://www.vortex9.org/vortex.html

Suggested Readings for In-class Discussion

• Crone EE, Ellis MM, Morris WF, Stanley A, Bell T, et al. (2013) Ability of matrix models to explain the past and predict the future of plant populations. Conserv Biol 27: 968-978.

• Frankham R, Bradshaw CJA, Brook BW (2014) Genetics in conservation management: Revised recommendations for the 50/500 rules, Red List criteria and population viability analyses. Biol Conserv 170: 56-63. http://www.sciencedirect.com/science/article/pii/S0006320713004576 (open access)

• McCaffery RM, Maxell BA (2010) Decreased winter severity increases viability of a montane frog population. PNAS 107: 8644-8649. http://www.pnas.org/content/107/19/8644.short (open access)

• Zeigler SL, Che-Castaldo JP, Neel MC (2013) Actual and potential use of population viability analyses in recovery of plant species listed under the US Endangered Species Act. Conserv Biol 27: 1265-1278.