Concept 42.6: Ecology Provides Tools for Conserving and Managing Populations

For millennia, humans have tried to maintain or increase populations of desirable or useful species and reduce populations of species they consider undesirable. Such efforts are most successful if they are based on knowledge of how those populations grow and what determines their densities.

Knowledge of life histories helps us manage populations

Knowing the life history of a species helps us identify those life stages that are most important for the species’ reproduction and survival, and hence for its population growth rate. Here are a few examples of how this knowledge can be applied.

Conserving Endangered Species

Populations of Edith’s checkerspot butterfly in the San Francisco Bay area are threatened with extinction. Larval survival is critical to the population dynamics of these butterflies (see Figure 42.1). Temporal and spatial variation in butterfly density is tied to the availability of two larval food plants, California plantain (Plantago erecta) and purple owl’s clover (Orthocarpus densiflorus), which are found only on serpentine soils. Maintaining healthy populations of these plants is critical for conservation of the butterflies, but the low-growing Plantago and Orthocarpus are suppressed by tall introduced grasses. Grazing by cattle can control the invasive grasses, and grazing is an important strategy for conserving Euphydryas editha. This example shows that achieving conservation goals is often compatible with human use of the environment.

Managing Fisheries

The black rockfish (Sebastes melanops) is an important game fish that lives off the Pacific coast of North America. Rockfish grow continually throughout their lives. As is true of many animals, larger females produce more eggs than smaller females. Larger females also provision their eggs with larger oil droplets. Larger droplets provide more energy to the newly hatched larvae, allowing them to grow faster and survive better than larvae with smaller oil droplets.

These life-history characteristics have important implications for rockfish populations. Because fishermen prefer to catch big fish, intensive fishing off the Oregon coast between 1996 and 1999 reduced the average age of female rockfish from 9.5 to 6.5 years. Thus in 1999, females were, on average, smaller than in 1996. This change decreased the average number of eggs produced by females in the population and reduced the average growth rate of larvae by about 50 percent, causing a rapid decline in population density. Because a relatively small number of large females can produce enough eggs to maintain the population, one strategy for maintaining rockfish without shutting fishing down completely is to set aside a few no-fishing areas where some females are protected and can grow to large sizes.

Reducing Disease Risk

Because adult black-legged ticks feed and mate primarily on large mammals—white-tailed deer for example—it seems logical that controlling deer densities would reduce the abundance of tick nymphs, which present the greatest risk of transmitting Lyme disease to humans. Surprisingly, experimental reductions in deer density have had little effect on subsequent nymph abundance. Studies of the tick’s life history (see Figure 42.3) indicate that the success of larval ticks in obtaining a blood meal, not the number of them that hatch from eggs, has the greatest effect on the subsequent abundance of nymphs. As is true of rockfish, a few adult female ticks can produce enough eggs to maintain populations. Hence controlling the abundance of larval hosts (small mammals) is a more effective strategy for reducing tick populations than controlling the abundance of adult hosts (deer and other large mammals).

Knowledge of metapopulation dynamics helps us conserve species

Conservation biology strives to avoid the extinction of species. From studies of extinction rates, we know that the risk of extinction for a species with a metapopulation structure is affected by the number and average size of its subpopulations, and by rates of dispersal among them. Conservation planners therefore begin with an inventory of remaining areas of natural habitat and an evaluation of the risks to subpopulations in those areas. They then devise ways to protect as many habitat patches as possible, giving priority to those with the largest area because large patches potentially support the largest and most genetically diverse subpopulations. Planners also evaluate the quality of the patches, as measured by their carrying capacity for the species (often estimated from population densities), and develop ways to maintain their quality.

Finally, planners must consider opportunities for individuals to move among subpopulations (FIGURE 42.11). For some species (such as Edith’s checkerspot butterfly, which can fly through unsuitable habitat to reach a new serpentine outcrop), simple proximity of patches determines rates of dispersal and recolonization. For those species, patches that are distant from one another can be connected by a series of intervening patches that serve as “stepping stones.” For other species, however, a continuous area of habitat through which individuals can move, called a dispersal corridor, is needed to connect subpopulations (see Figure 42.11). Dispersal corridors can sometimes be created by maintaining vegetation along roadsides, fence lines, or streams, or by creating bridges or underpasses that allow individuals to circumvent roads or other barriers to movement (FIGURE 42.12).

Investigation

HYPOTHESIS

Subpopulations of a metapopulation are more likely to persist if there are no barriers to dispersal.

METHOD

  1. On seven replicate moss-covered boulders, scrape off the continuous cover of moss to create a cluster of 13 moss patches surrrounded by bare rock. A large central 50 × 50 cm moss “mainland” (M) is surrounded by 12 small patches of moss, each 10 cm2 in area. In the “insular” treatment (I), the patches are surrounded by bare rock (which is inhospitable to small moss-dwelling arthropods and is thus a barrier to dispersal). In the “corridor” treatment (C), the patches are connected to the mainland by a 7- by 2-cm strip of live moss. In the “broken-corridor” treatment (B), the configuration is the same as in the “corridor” treatment, except that the moss strip is cut by a 2-cm strip of bare rock.
  2. After 6 months, determine the number of small arthropod species present in each of the patches.
    Figure 42.11: Habitat Corridors Can “Rescue” Subpopulations from Extinction Data from the experiments summarized here suggest that corridors between patches of habitat for small arthropod species increase the chances of dispersal, and thus of subpopulation persistence.a

RESULTS

Patches connected to the mainland by corridors retained as many species as did the larger mainland patch to which they were connected. Significantly fewer species remained in the broken-corridor and insular treatments. Error bars indicate 1 standard error of the mean; bars with different letters are statistically different from one another.

CONCLUSION

Barriers to dispersal reduce the number of subpopulations that persist in a metapopulation.

ANALYZE THE DATA

  1. What percentage of the species present in the mainland patch was lost, on average, in the corridor patches?
  2. What percentage of the species present in the mainland patch was lost, on average, in the insular and broken-corridor patches? What do these percentages tell you about the average risk of extinction for a subpopulation in a small patch?
  3. The insular patches were two-thirds the size of the broken-corridor patches, because the latter contained 5 cm2 of habitat in the corridor in addition to the 10 cm2 in the patch itself. Did this difference in patch size affect subpopulation persistence?
  4. What effect is patch size by itself expected to have on the persistence of a given species in a patch? Do you see evidence for this effect in the data?

Go to LaunchPad for discussion and relevant links for all INVESTIGATION figures.

aA. Gonzalez and E. J. Chaneton. 2002. Journal of Animal Ecology 71: 594–602.

Go to ANIMATED TUTORIAL 42.4 Habitat Corridors

PoL2e.com/at42.4

Figure 42.12: A Corridor for Large Mammals An overpass above the Trans-Canada Highway in Banff National Park, Alberta. Bears, elk, deer, and other large mammals use such overpasses as corridors to move between areas that would otherwise be effectively isolated from one another by the highway with its speeding traffic.

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LINK

Strategies for conserving communities of interacting species are discussed in Concept 44.5

CHECKpoint CONCEPT 42.6

  • Describe three elements of a conservation plan for a species that has a metapopulation structure.
  • What similarities do you see between the rockfish and black-legged tick examples summarized above, in terms of the factors that will determine whether a population grows or shrinks?
  • Conservation plans for marine species such as rockfish often lack a corridor component. Why?

You may have noticed that this chapter has focused on singlespecies populations, but it has also mentioned interactions among species—rodents interacting with acorns and ticks, ticks with hosts and bacteria, butterflies with food plants and invasive plants. These ubiquitous interactions between species, which play important roles in their population dynamics, will be the subject of the next two chapters.

Question 42.2

How does understanding the population ecology of disease vectors help us combat infectious diseases?

ANSWER How frequently people come into contact with a dangerous pathogen depends on its abundance and distribution and on those of any disease vectors that transmit it to humans. Population ecology helps us understand what determines the abundance and distribution of pathogens and their vectors, and hence allows us to devise ways of controlling their abundance or avoiding contact with them.

By studying the survivorship and fecundity of the blacklegged tick throughout its life history (Concept 42.3), Rick Ostfeld learned that it is the abundance of hosts for larvae and nymphs, rather than weather or the abundance of hosts for adult ticks, that determines tick abundance. Furthermore, Ostfeld ascertained that ticks become infected with Borrelia when they feed on infected hosts, and that small mammals are the most effective reservoirs for Borrelia. Long-term studies of rodent population dynamics had already shown that rodent abundance is positively correlated with acorn availability (Concept 42.4).

This knowledge suggests a simple and quick way to reduce the probability of contracting Lyme disease (Concept 42.6). Acorn production can be used to predict areas that are likely to become infested with disease-carrying ticks, and various measures can be taken to reduce the risk of human contact with those ticks. These measures include boosting public education efforts for several years after a big acorn crop, posting warning signs in high-risk areas, and alerting healthcare specialists whose patients live or work in high-risk areas. A longer-term possibility that has yet to be assessed is to reduce the fraction of ticks that carry Borrelia by augmenting populations of hosts that do not harbor Borrelia (such as lizards and birds) in the host community. Ticks that bite these hosts will not become infected with Borrelia.

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