conservation in theory

8/9/2019 Conservation in Theory

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Extinction Risk: Theoretical Considerations

I. Introduction A. Changes in the size of a wildlife population over time can have a wide range of

consequences depending on the species, the location, and many other factors.1. Growth of white-tailed deer (Odocoileus virginianus ) populations over the past 15years has caused changes in native vegetation structure and composition in mplaces, particularly in the upper Midwest.

2. On the other hand, the number of Florida panthers (Puma concolor ) has declined to the point that many biologists are skeptical that the species can avoidextinction.

B. Here we deal with the body of theory that has been developed to account for whhow populations become extinct. Most of that theory deals with extinction as aconsequence of low numbers, the various difficulties that a population can get i

when it is too small.II. Demographic problems contribute to the risk of extinction A. All populations experience some variation in their numbers over time, and som

this variation is predicable.1. If a population has a distinct breeding season (i.e., they rut in the fall and cal

the spring), each year after the females give birth to their offspring, the popuwill grow in size.

2. If there is a season (say, a tough winter), that is particularly hard on a popula the population abundance will decline.

B. However, we cannot forecast all the variation in abundance.1. Random, unpredictable variation in population abundance is called stochasti2. Two types of random variation a population may experience are demographi

environmental stochasticity.a. Demographic stochasticity.

1) Demographic stochasticity is random variation in things like sex ratiobirth rates, and death rates.

2) Wildlife populations are comprised of individuals that are not exactly 3) As a result, often more or fewer individuals die or are born than would

expected. For example, if the female black bears (Ursus americanus ) in apopulation with demographic stochasticity produce a mean of 2 offsprper year, some females in the population may have no offspring, somehave 1 offspring, and some may have 2, 3, or even 4 offspring. Thisvariation is random from year-to-year.

4) Demographic stochasticity occurs within a population even if theenvironment does not vary. Here is an example of demographicstochasticity in a group of mountain gorillas (Gorilla beringei ).


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a) Female gorillas breed every 3.5-4.5 years; therefore, the meanfecundity rate = 0.25, which means you can expect 1 female mougorilla to give birth to 1 offspring every 4 years.

b) If 3 of 4 females in a group give birth during the same year, and th4 th female gives birth the following year, there may be 3 years bef the next offspring is born.

c) If the mean fecundity rate is 0.25 and there are 4 females in thegroup, we would expect 1 offspring per year. However, as we havalready seen, we can have multiple births in 1 year followed by seyears with no births.

d) Likewise, several or no individuals may die during the same year to chance.

e) Having more infants born or more individuals die than expected wchange the mean growth rate of the population. If a population is

very small, demographic variation can have very large consequennamely, if the population is small and the death rate is high, thepopulation could go extinct.

b. Environmental stochasticity.1) Unpredictable variation in the environment, or environmental

stochasticity, also creates variation in population growth rate over time2) Although some environmental variation is predictable, like the changi

seasons, variation between different years tends to be less predictable.3) Rainfall, temperature, snow pack amounts and conditions, frequency a

intensity of storms, humidity, and amount of cloud cover can all vary

between years, and these weather conditions directly affect the amounfood available each year.4) During bad years, birth rates may drop and death rates may increase.

together, low birth rates and high death rates can substantially decreaspopulation size.

3. In a large population with a wide distribution, stochastic effects generally ar too harmful. However, if a population is very small, and/or is found only insmall area, demographic and environmental stochasticity combined or alonedrive a population to the brink of extinction. Here is the idea in different tera. A large population is usually able to recover from bad years or catastroph

because some individuals usually survive to reproduce in future years. Fexample, if 90% of a population of 3,000 animals is killed during acatastrophe, say a wildfire, 300 individuals would remain to continue toreproduce.

b. However, if the same catastrophe kills 90% of a population of 30 individuonly 3 individuals would remain. If 2 are male and 1 is an old non-reprod


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female, the population will become extinct. This is what happened to theheath hen on Marthas Vineyard in the early 20 th century.

III. Genetic problems also contribute to the risk of extinction A. Another problem faced by small populations is the loss of genetic variation at b

the individual and population level.1. Individuals have genetic variation.

a. All diploid organisms are defined as having 2 copies of each gene. Each called an allele.

b. One copy is inherited from each parent.c. Although each allele contains information about the same trait, they may

be identical.d. When an individual has corresponding alleles that are different, this indiv

possesses genetic variation for that trait.2. Populations also possess genetic variation.

a.

This variation occurs when individuals in the same population have differin their genetic makeup.b. A population with little genetic variation is less likely to survive environm

change because all individuals may be equally susceptible. For example,geneticists fear that the lack of genetic variation in cheetah ( Acinonyx jubatus )populations makes them more vulnerable to introduced diseases such as fleukemia.

3. Genetic variation is maintained primarily by mutation and by gene flow fromother localities in which different genes have a selective advantage.

B. Influences of population size on genetic variation.1. In small populations, chance events can significantly alter allele frequenciesalteration is called genetic drift.

a. If only a few individuals are drawn at random to form the next generationalleles they carry are not likely to be in the same proportions as alleles in gene pool from which they were drawn.

b. Two important causes of genetic drift are bottlenecks and founder effects1) Bottlenecks.

a) Even organisms that normally have large populations may pass throoccasional periods when only a small number of individuals surviv

b) During these population bottlenecks, genetic variation can be lost bchance.

c) Populations in nature pass through bottlenecks for many differentreasons. For example, predators may reduce populations of their p to very small sizes.

2) Founder effects.a) When a species expands into new regions, a small number of pione

individuals may start populations.


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b) These pioneers are not likely to have all the alleles found in their sopopulation. Even if they do, the allele frequencies are likely to difffrom those in the source population.

c) The situation is equivalent to that for a large population reduced bybottleneck, but rather than a small surviving population, there is a sfounding population.

d) This type of genetic drift is called a founder effect.e) Examples follow.

Because many plant species reproduce sexually by self-fertilizatsingle seedan extreme example of a founder effect, may start new plant population.

Population geneticists studied the genetic composition of afounding population whenDrosophila subobscura , a well-studiedEuropean species of fruit fly, was discovered near Puerto Montt

Chile, in 1978 and at Port Townsend, Washington, in 1982. The founding insects probably reached Chile and the U.S. froEurope aboard the same ship, because both populations aregenetically very similar.

As expected from a small founding population, only a small pof the total genetic variability found in Europe reached the Americas.

Geneticists estimate that a least 10, but no more than 100, fliinitially arrived in the New World.

2. Small populations lose genetic variation over time by inbreeding.

a. If a population is small enough, there are not many mates available from to choose. Eventually, individuals mate with others that are closely relateb. The more closely related individuals are, the more genes they share.c. If individuals with the same genes mate, the resulting offspring may inhe

identical alleles.d. In very inbred populations, many individuals have identical copies of har

genes, often resulting in reduced fertility and unhealthy offspring. Forexample, evidence has been found of reduced sperm fertility of inbred lioand cheetahs.

e. The problems associated with low genetic diversity due to inbreeding areinbreeding depression.

3. The extinction risk of a small population may be affected by the genetic hea the population.a. From a management perspective, to protect the long-term viability of a

population, we need to ensure that there are enough individuals to protectfrom the loss of genetic variation.


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b. If a population is below the threshold necessary to maintain genetic varia the fitness of the individuals in the already small population may decreas

c. With lower fecundities and/or higher mortalities, it will be increasinglydifficult for the small population to recover.

d. To maximize the amount of variation maintained in a population, zoos anconservation organizations have organized gene banks.

IV. Allee effects and extinction vortexes A. Another factor that may push populations toward extinction is the Allee effect.

1. At very low population densities, individuals may not have enough contact wother individuals of the population.

2. It can be difficult for individuals to find mating partners, causing the reprodurate and the population growth rate to drop.

B. Once a population becomes too small to sustain itself, it may enter an extinctionvortex.

1. An extinction vortex is very much like a whirlpool or a black hole; once you to be pulled toward its center, it becomes harder and harder to escape.

2. As populations fall below a critical threshold, it becomes too sensitive toenvironmental and demographic stochasticity to maintain stable numbers.

3. Populations in this downward spiral cannot change its course without outsidinterference and management. (An extinction vortex is a little like hypotherUnless a person suffering from hypothermia receives heat from an external s their body temperature will continue to drop until they die.)

C. How large must a population be to sustain itself and avoid the problems faced bsmall populations?

1. This is a difficult question with no clear answers but probably varies someweach species.2. Some biologists speculate that populations should have at least 1,000 -10,00

individuals to persist long term; others estimate that populations should realhave at least 100,000 individuals or more to persist indefinitely.

V. Population viability analysis A. One way to determine the long-term sustainability of a population is to perform

population viability analysis (PVA).1. A PVA is a model used to predict the probability of a population declining o

becoming extinct.2. If a population is not likely to survive in its present condition, PVA can be u

determine which aspects of the population (i.e., fecundity, mortality, or habitquality) will be the most sensitive to conservation efforts.

B. Effective population size.1. Small populations may be in jeopardy of losing genetic variation via genetic

and with it, their ability to respond to environmental change.


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2. Thus, biologists are keenly interested in the question of how large a populatimust be in order to stave off the loss of genetic diversity.

3. This question is difficult to address, however, because not every member of population contributes to the genetic structure of the population in the same a. Mating systems may prevent some individuals from mating with others, a

there may be age-specific differences in reproductive output.b. Because of this, simple estimates of population density do not provide en

information to evaluate the vulnerability of the population to genetic lossc. One way to overcome this difficulty is to calculate the genetic effective

population size, denoted Ne.1) The EPS is the size of an idealized population that has the same amou

genetic drift as the natural population under study.2) An idealized population is one in which the number of reproducing ma

and females is the same (1:1 sex ratio), random mating occurs among

and females (that is, every male has an equal probability of mating witevery female), the rate of emigration and immigration is constant, andwhich there is no age structure. Usually Ne < N, though this is not always the case.

3) The EPS depends on the relative reproductive success of males and feand the variance in that success.

4) Thus, to determine Ne, we calculate separate effective population sizes fomales and females as follows:

1K

V K

1KNN

m

mm

mm

males

+

=

,

1K

V K1KNN

f

f f

f f females

+

=

where:

Nm and Nf = the numbers of breeding males and females, respectively,Km and Kf = the average numbers of offspring produced by males andfemales, respectively, in their lifetime, andV m and V f = the variance in the number of offspring produced by malesand females, respectively.


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a) Inspection of these equations reveals how the variance in the numboffspring produced influences the EPS of males and females.

b) For a given number of breeding males, Nm, producing an averagenumber of offspring Km, the EPS Nmaleswill decrease as the variance inoffspring V m decreases. For example, for a population with Nm = 10,and average number of offspring Km = 2, the effective male populationsize, Nmales, will be 7.6 if V m = 1, but 4.7 if V m =2.

5) The separate values for males and females given in the equations abovbe combined in the following way to calculate the overall EPS:

1.-N

1N

14eNfemalesmales

+=

Clearly, the number, reproductive output, and variance of reproductiveoutput of males and females interact to determine the EPS of a popula

4. How large should the EPS be in order to maintain genetic diversity in the facgenetic drift? It has been suggested that an effective population size of at lea500 is required, though it is likely that the minimum number will vary widelamong populations having sex ratios, age structures, and social arrangement

5. Related to the concept of EPS is that of minimum viable population size (MVa. The MVP is the smallest population that can persist for a specified time, u

taken to be 1000 years.b. The MVP is dependent both on the demographics of the population and t

genetic diversity within the population.VI. How to prevent extinction

A. Of the 12 examples of extinction or steep decline mentioned in chapter 15 of thprobably the most important is modification or destruction of habitat.

B. The first step in averting extinction is to recognize the problem.C. The second step is to discover how the population got into its present mess.

1. Is the cause of decline a single factor or a combination of factors?2. Are those factors still operating?3. If so can they be nullified?

D. Once a decline in a species is recognized and the causes are determined, the thiis to treat the problem.1. Sometimes all it takes is a legislative change such as a ban on hunting (as wi

Canadian muskoxen).2. More usually, active management (e.g., predator control and captive breedin

the Lord Howe Island woodhen) is necessary.


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3. The management actions needed to reverse the fortunes of a declining specieseldom more than conventional management techniques unless a species is idesperate straits.

4. Ex-situ techniques preserve and amplify a population of an endangered specieoutside its natural habitat. Thereafter it can be reintroduced. The Lord HowIsland woodhen and the Arabian oryx are good examples.

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