Chapter 7 The Perils of Small Populations

Discussion Questions

1. If the endangered pygmy rabbit (Brachylagus idahoensis) and the threatened blue whale (Balaenoptera musculus) both went through population bottlenecks that lasted for 10 years, for which of the species would you expect more severe loss of genetic variation due to the bottleneck? Why?
2. Imagine a population with non-overlapping generations (in other words, the parental generation reproduces and dies off immediately). In one particularly bad year, only five offspring are produced. What is the probability that the population will become extinct because all the offspring are of only one sex (either male or female)? Hint: Create a table similar to Table 7.2, in which the outcome of interest is the sex of each offspring.
3. Use the appropriate equations for effective population size to determine which of the following populations will lose heterozygosity more quickly: a population (starting at generation 0) with a 9:1 sex ratio that remains constant at 240 individuals for four additional generations, or a population with a 1:1 sex ratio that starts at 240 individuals in generation 0, crashes to 20 for one generation, and recovers to 240 for the second, third, and fourth generations.
4. When might you expect a translocation of a few individuals to result in genetic rescue? Are there circumstances when such a translocation might do more harm than good?
5. If a species will likely require human management and intervention forever, should its persistence be considered a conservation success or failure?
6. Imagine that you can invest $1 million per year in conservation activities for one (and only one) of the following three species, each of which has only a 1% chance of survival over the next century if you do not immediately act on its behalf: (a) a leopard that with your investment of $1 million per year could recover to the point of having a 20% chance of survival over the next 100 years, but will never number more than 10 to 20 individuals; (b) a flowering plant that with your investment of $1 million per year could recover to the point of having a 50% chance of survival over the next 100 years and, if it does survive, will achieve large flowering populations of historic size and splendor; or (c) a species of nudibranch (a marine mollusk) that with your investment of $1 million per year could recover to the point of having a 95% chance of survival over the next century—but in only one location, a small coastal bay. Decide how you will invest your budget, and defend your choice.
7. Imagine that a population has low genetic diversity and the individuals in this population have low fitness. How might you determine whether the low fitness is caused by a lack of genetic diversity?

Group Projects

• An Allee effect can occur if small population size disrupts the functioning of social groups. Drawing on your natural history knowledge, identify one species from each of the following animal groups that you might expect to suffer the Allee effect at low populations, and briefly mention the reasoning behind your selection: carnivores, mammalian grazers, birds, fish, primates.
  
  
• Recovery plans prepared by the U.S. Fish and Wildlife Service for threatened and endangered species are available online. Randomly select 20 such recovery plans for endangered species (which tend to have smaller populations than threatened species), and summarize to what extent the plans include genetic risks as a concern. Determine whether any management approaches are discussed as a response to the genetic problems of these small populations. Defend your thinking about whether the level of attention given to genetic issues in these plans is appropriate.

Useful Websites

• 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


• Radiolab (WNYC) produced an excellent audio story about efforts to conserve the Kirtland's Warbler. http://www.radiolab.org/story/91723-weighing-good-intentions/


• U.S. Fish and Wildlife Service provides information on threatened and endangered species listed under the U.S. Endangered Species Act. http://www.fws.gov/endangered/species/ Recovery plans can be found at http://www.fws.gov/endangered/species/recovery-plans.html

Suggested Readings for In-class Discussion

• Hostetler JA, Onorato DP, Jansen D, Oli MK (2013) A cat's tale: The impact of genetic restoration on Florida panther population dynamics and persistence. J Anim Ecol 82: 608-620.
  
  
• Miller CR, Waits LP (2003) The history of effective population size and genetic diversity in the Yellowstone grizzly (Ursus arctos): Implications for conservation. PNAS 100: 4334-4339. http://www.pnas.org/content/100/7/4334.short (open access)
  
  
• Palomares F, Godoy JA, Lόpez-Bao JV, Rodríguez A, Roques S, et al. (2012) Possible extinction vortex for a population of Iberian lynx on the verge of extirpation. Conserv Biol 26: 689-697.
  
  
• Scott JM, Goble DD, Wiens JA, Wilcove DS, Bean M, Male T (2005) Recovery of imperiled species under the Endangered Species Act: The need for a new approach. Front Ecol Environ 3: 383-389.
  
  
• Traill LW, Brook BW, Frankham RR, Bradshaw CJ (2010) Pragmatic population viability targets in a rapidly changing world. Biol Conserv 143: 28-34.