Chapter 42: Populations
Modified from Principles of Life: Hillis, Sadava, Hill and Price.
42.1 Populations Are Patchy in Space and Dynamic over Time
1. You have been live-trapping, marking and releasing voles in a 1-hectare plot for several months. Based on your field work, you calculate the population density of the study plot to be 150 voles per hectare. The total area occupied by the population is 4 hectares. What is the total population size?
It is usually impossible to count all the individuals in a population, so researchers typically measure population density and then multiply it by the area occupied by the population to calculate the total population size. In this example, 150 voles per hectare x 4 hectares = 600 voles for the total population size.
2. Timber rattlesnakes are found in the eastern half of the United States: north to southern Maine, south to northern Florida, and west to central Texas. This species of snake prefers forested areas, particularly those containing woody rocky ledges with southern exposure for basking. Areas with high rodent densities are ideal. Roads pose great risk and agricultural fields are avoided. Explain the following concepts and how they apply to the species: geographic range, habitat, and habitat patch (habitat island).
A species’ geographic range is the region where it is found. For timber rattlesnakes, the range is the eastern half of the United States. Within a geographic range, a species may be restricted to a particular type of environment, which is called its habitat. The habitat of the Timber Rattlesnake is forested areas with rocky ledges and high rodent densities. Sometimes suitable habitat occurs in patches surrounded by unsuitable habitat. Roads and agricultural areas may fragment the forested habitat of timber rattlesnakes into patches (islands) of suitable habitat.
42.2Births Increase and Deaths Decrease Population Size
3. An ecologist who has been monitoring a cohort of Couch’s spadefoot toads in a locality in California describes the total population size as declining. What does this tell you about the per capita birth rates and death rates of the study cohort?
If the total population size is described as declining, then the per capita death rate must exceed the per capita birth rate of the cohort.
4. Why is it unusual for population densities to remain unchanged over time?
It is unusual for population densities to remain unchanged over time because this situation would occur only when the number of births exactly equals the number of deaths.
42.3 Life Histories Determine Population Growth Rates
5. Predict how increased survivorship and increased fecundity would influence per capita growth rate.
High survivorship and higher fecundity would typically result in a higher per capita growth rate.
6. Many small mammals produce large litters every few weeks during the breeding season. Given this aspect of their life history, what would you expect regarding adult survivorship? This type of adult survivorship is an example of what phenomenon?
Species of small mammals that invest heavily in reproduction often do so at the expense of survivorship, so we would predict short life expectancies in these species. The negative relationship between reproduction and survival is an example of a life-history tradeoff.
42.4 Populations Grow Multiplicatively, but the Multiplier Can Change
7. Why is the per capita growth rate described as density-dependent?
A population density increases and resources become scarcer, birth rates decline and death rates increase. In other words, r decreases as the population becomes more crowded. Thus, the per capita growth rate is said to be density dependent.
8. Just as the per capita growth rate of a population is described as density-dependent, so too can other factors that influence population size. Of the following three factors, which would you describe as density-dependent: disease, weather and starvation?
Disease and starvation would be considered density dependent factors because they have a greater impact as conditions become more crowded. Weather is not a density-dependent factor.
42.5 Immigration and Emigration Affect Population Dynamics
9. How does the division of some populations into discrete subpopulations relate to the possible “rescue” of a subpopulation from extinction? How would barriers to immigration influence the potential for rescue?
Rescue occurs if a subpopulation that has undergone a large decline is saved from extinction by the immigration of individuals from other subpopulations. As one would predict, barriers to immigration reduce the potential for rescue.
10. Metapopulation A of crustaceans has eight subpopulations, each with more than 50 individuals. Metapopulation B of crustaceans has four subpopulations, each with fewer than 30 individuals. In the face of environmental disturbance, which metapopulation would likely go extinct sooner and why?
Metapopulation B will likely go extinct sooner because the time to extinction is shorter for the metapopulation with fewer and smaller subpopulations.
42.6 Ecology Provides Tools for Conserving and Managing Populations
11. What are dispersal corridors, and how have experiments demonstrated that they help species persist in patchy environments?
Dispersal corridors are relatively thin strips of habitat of a particular type that connect patches of the same type of habitat. Their importance in permitting individuals to disperse from one patch to another was demonstrated in experiments describe in Figure 42.11 of the textbook.
Additional Resources: use the web addresses listed
- Animated Tutorial 42.1 Multiplicative Population Growth Simulation
- Animated Tutorial 42.2 Density Dependent Population Growth Simulation
- Animated Tutorial 42.3 Metapopulation Simulation
- Animated Tutorial 42.4 Habitat Corridors
- Activity 42.1 Population Growth