RISKS OF RARITY: DEMOGRAPHIC RISKS - 1

Corresponding Readings in Primack, Richard B. Essentials of Conservation Biology.
Chapter 11: pages 279-283 & 301-308

Introduction:

The evil quartet, along with other threats such as pollution and climate change, threaten to drive species to extinction. However, while these threats can make a species very rare, do they necessarily cause extinction? Why don’t species "hang on" in low numbers? Why do some species recover once a threat such as hunting is ended (the sea otter), while others plummet to extinction (the passenger pigeon)? What are the risks associated with being rare? 

A Case History:

• How one species went extinct, in detail. The heath hen, an eastern relative of prairie chicken, was so common and stupid they were captured by hand by early settlers as food. Much hunted, and by 1870 only 200 individuals remained. Further decline, then a reserve established, and recovery occurred, then a string of "bad luck" wiped it out.

What Causes an Extinction?

The demise of a species occurs when the last survivors fail to replace themselves, thus a particular species becomes extinct one or a few individuals at a time, and is gone forever when the last individual dies.

Lest this all seem too obvious and trivial, consider the following questions:

• what causes one rare species to become rarer still, and eventually go extinct, while another rare species may undergo a spectacular recovery. Passenger pigeon vs. sea otter
• both conservation dollars and staff are limited. Like the triage systems of hospitals in wartime, can we identify populations too hopeless to save, others that will recover on their own, and concentrate our efforts on those where the difference between success and failure depends on us.
• suppose a species on the brink of extinction is brought into a captive breeding program. How many should you rear before releasing this small population back into its habitat ?

All of these questions have at their core the following two ideas:

• How does the chance of recovery or persistence vary with population size?
• What factors need to be taken into account in answering this question for a particular species? Is the answer the same for pandas and Kirtland's warbler?)

 The answers are believed to lie in an understanding of population dynamics and population genetics. We begin with the former.

The Dynamics of Population Growth:

 A. exponential and sigmoid growth, r and K:

If you release a small number of individuals in a favorable and bountiful habitat, they will multiply in numbers. A graph of numbers against time is J-shaped, and is referred to as exponential growth

 In unchecked exponential growth, numbers increase without limit to infinity. Clearly this is impossible for real populations, and so an upper limit must be reached because space, food, and other resources become limiting, or perhaps predators increase in response to an abundant prey population. An s-shaped curve describes growth with limits.

 This model has only two parameters:

• "r", the intrinsic growth rate, describes how fast the population grows in the unchecked, or exponential, phase.
• "K", the carrying capacity of the environment, acts as a ceiling on numbers, determining the equilibrium population that a given habitat can support.  

B. density dependence vs. density independence

Although our focus is on rare populations, it is useful to understand what controls the numbers of natural populations in general. The s-shaped model states that as populations reach the limit of available resources, growth slows. The potential for population growth depends on density.

A second view holds that the real world is more irregular. In times of good weather and in habitats of abundant resources, populations grow until conditions change. Irregular checks to population growth due to droughts, storms, poor seasons etc happen often enough to keep populations from exploding to outrageous densities. Because the occurrence of such events is unrelated to how rare or crowded the population is, we call such effects density-independent.

Evidence from nature suggests that populations often behave according to expectations under density-dependence. Results also depend somewhat on the organisms you study. Density-dependence is more prevalent in larger, slow-reproducing vertebrates with territoriality and other population-regulating behavior. In contrast, short-lived, fast-reproducing little beasties, such as insects appear more likely to fluctuate with changing environmental conditions.

 Next we shall consider the conservation implications of these principles.

Transparencies: 1. Goshawk J-shaped growth curve, 2. Ringnicked pheasant J-shaped growth curve, 3. S-shaped growthof yeast cells in test tube, 4. S-shaped growth of sheep in South Australia, 5. Annual abundances of lemmings in Alaska, 6. Birth rates, death rates and density dependence, 7. Long term population change shows density dependence, 8. Relationship between body size and generation time.

Links:

• Global Rank System: Learn about a ranking system used by the Natural Heritage Program that quickly communicates global rarity and ecosystems.
• San Francsico Garter Snake: this is a rare and endangered species.
• The International Rhino Foundation: this is a web cite for a non-profit corporation whose sole purpose is dedicated to the conservation of five species of rhinos.