One Species at a Time

Habitat destruction is a leading cause of endangerment and extinction, and it follows that the primary requirement for species recovery is restoration of habitat. The ESA requires the Department of Interior to designate critical habitats for endangered species, the areas in need of protection for the survival of the species. The amount of critical habitat that is designated depends upon political as well as biological factors. Federal designation of a critical habitat results in restriction of the human activities that can take place there—thus, landowners are usually interested in keeping their lands out of this designation. If landowners are politically powerful, they can exert their influence on elected officials and have profound effects on the recovery plan for a particular endangered species.

The biological part of a critical habitat designation includes a study of the habitat requirements of the endangered species and setting a population goal for it. The Department of Interior's critical habitat designation has to include enough area to support the recovery population. The designation of critical habitat has an extra benefit as well—protection of this habitat can protect dozens of other less well-known species that may be approaching endangerment.

The recovery plans for both the Lost River and the shortnose suckers sets a short-term goal of one stable population made up of at least 500 individuals for each unique stock of suckers. To understand why at least this many individuals of a species are required to protect the species from extinction, we need to review some of the special problems of small populations.

Growth and Catastrophe in Small Populations A species' growth rate is influenced by how long the species takes to reproduce, how often it reproduces, the number of offspring produced each time, and the death rate of individuals under ideal conditions. (Calculation of growth rate is discussed in Chapter 15.) For instance, species that reproduce slowly take longer to grow in number than species that reproduce quickly. Thus the growth rate of an endangered species influences how rapidly it can attain a target population size. Shortnose and Lost River suckers have relatively high growth rates and will meet their population goals quickly if the environment is ideal (Figure 14.15a). For more slow-growing species, such as the California condor (Figure 14.15b), populations may take decades to

(a) Lost River sucker

(a) Lost River sucker

(b) California condor

Figure 14.15 The effect of growth rate on species recovery. (a) This graph illustrates the rapid growth of a hypothetical population of quickly reproducing Lost River suckers. (b) The slow growth rate of the California condor has made the recovery of this species a long process. Today, nearly 30 years after recovery efforts began, the population of wild condors is still only in the dozens. Two wild populations of 150 condors each must be established for the bird to be removed from endangered status.

Figure 14.15 The effect of growth rate on species recovery. (a) This graph illustrates the rapid growth of a hypothetical population of quickly reproducing Lost River suckers. (b) The slow growth rate of the California condor has made the recovery of this species a long process. Today, nearly 30 years after recovery efforts began, the population of wild condors is still only in the dozens. Two wild populations of 150 condors each must be established for the bird to be removed from endangered status.

Figure 14.16 A victim of small population size. The heath hen was once abundant throughout the eastern United States. Although it was protected when its population was nearly 50 individuals, a series of unexpected disasters caused its extinction.

Figure 14.16 A victim of small population size. The heath hen was once abundant throughout the eastern United States. Although it was protected when its population was nearly 50 individuals, a series of unexpected disasters caused its extinction.

recover. The rate of a species' recovery is important because the longer a population remains small, the more it is at risk of experiencing a catastrophic environmental event that could eliminate it entirely. The story of the heath hen is a classic example of the dangers facing small populations.

The heath hen was a small relative of the prairie chicken that lived on the East Coast of the United States (Figure 14.16) and was a favorite game bird of early European settlers. Prior to the American Revolution, the heath hen was found from Maine to Virginia. Increased settlement resulted in loss of habitat and increased hunting, noticeably lowering heath hen populations by the time of the Revolutionary War. In the 1870s, the only heath hens that were left occupied a tiny island called Martha's Vineyard off the coast of Cape Cod in Massachusetts. Human development on the island further reduced the suitable habitat for heath hen breeding, and in 1907 there were only 50 heath hens left on Martha's Vineyard. A 1,600-acre sanctuary was established for their protection the following year.

The sanctuary seemed to be successful—the original 50 heath hens reproduced rapidly and there were 2,000 individuals on Martha's Vineyard by 1915. Unexpectedly, a fire in 1916 wiped out much of the habitat that the birds used for breeding. In addition, the next winter was unusually harsh and food was scarce, and an influx of goshawks, predatory birds that preyed on the heath hens, reduced the population further. Finally, many of the remaining heath hens fell victim to a poultry disease brought to the island by domestic turkeys. There were only 14 heath hens left by 1927, and most of them were males. The last living heath hen was seen on March 11, 1932. He died that year.

Why did the heath hen become extinct? The last birds were wiped out by a series of relatively common and entirely natural events: fire, starvation, predation, and disease. The heath hen's continued existence as a species would not have been so vulnerable to these occurrences if the population size had not been severely reduced by habitat loss and overhunting. A small population is very vulnerable to normal fluctuations in its numbers, which are the consequence of disease and disasters. A population of 1,000 individuals can survive a population drop of 100; the same fluctuation dooms a population that starts with only 100 individuals. In the case of the heath hen, even when hunting and habitat destruction were halted, the species' survival was still extremely precarious.

The population goal of 500 individuals for both species of suckers in Upper Klamath Lake is still quite small, but in the short term it will help these fish avoid the same fate as the heath hen.

Genetic Variability and Survival Small populations of endangered species can still be protected from the fate that befell the heath hen. Having additional populations of the species at sites other than Martha's Vineyard would have nearly eliminated the risk that all members of the population would be exposed to the same series of environmental disasters. This is the rationale behind placing captive populations of endangered species at several different sites. For instance, the captive whooping crane population is located at the U.S. National Biological Service's Patuxent Wildlife Research Center in Maryland, the International Crane Foundation in Wisconsin, the Calgary Zoo in Canada, and the Audubon Center for Endangered Species Research in New Orleans. However, if endangered species populations remain small in number, they are subject to a subtler but potentially equally damaging disaster—the loss of genetic variability.

A species' genetic variability is the sum of all of the alleles and their distribution within the species. Differences among alleles produce the variety of traits within a population. For example, the gene that determines your blood type comes in three different forms, and the combination of alleles that you possess determines whether your blood type is O, A, B, or AB. Thus, a population containing all three blood-type alleles contains more genetic variability (for this gene) than a population that contains only two alleles.

The loss of genetic variability in a population is a problem for two reasons: (1) On an individual level, low genetic variability leads to low fitness; and (2) on a population level, rapid loss of genetic variability may lead to extinction.

Individual Genetic Variability As we discussed in Chapter 9, fitness refers to an individual's ability to survive and reproduce in a given set of environmental conditions. There are two reasons that high genetic variability on an individual level increases fitness. We will use an analogy to illustrate the costs of low genetic variability in individuals. First, imagine that you could own only two sets of footwear (Figure 14.17a). If both pairs are dressy shoes, you might be prepared to meet a potential employer, but if you had to walk across campus to your job interview

(a) Heterozygote has higher fitness than either homozygote.

(b) Heterozygote masks the deleterious allele.

Homozygote: Relatively low fitness mi

Homozygote: Relatively low fitness

Homozygote: Relatively low fitness

Heterozygote: Relatively high fitness

(b) Heterozygote masks the deleterious allele.

Homozygote: Relatively high fitness
Homozygote: Relatively low fitness

Figure 14.17 The benefits of heterozygosity. In this analogy, each pair of shoes represents an allele. (a) Heterozygotes may better prepared for a diversity of life experiences than homozygotes. (b) Heterozygotes may have higher fitness than some homozygotes because certain alleles are deleterious and recessive. In this case, homozygotes for the normal allele also have higher fitness than homozygotes for the recessive allele.

Heterozygote: Relatively high fitness

Heterozygote: Relatively high fitness

Figure 14.17 The benefits of heterozygosity. In this analogy, each pair of shoes represents an allele. (a) Heterozygotes may better prepared for a diversity of life experiences than homozygotes. (b) Heterozygotes may have higher fitness than some homozygotes because certain alleles are deleterious and recessive. In this case, homozygotes for the normal allele also have higher fitness than homozygotes for the recessive allele.

in a snowstorm, you would be pretty uncomfortable. If you own two sets of winter boots, your feet will always be protected from the cold, but you would look pretty silly at a dinner party. However, if you own both dress shoes and winter boots, you are ready for slush and snow as well as a nice date. In a way, individuals experience the same advantages when they carry two different alleles for a gene—that is, when they are heterozygous. If a protein produced by each al-lele works best in different environments, heterozygous individuals are able to function efficiently over a wider range of conditions.

The second reason that high individual genetic variability increases fitness is that, in many cases, one allele for a gene is deleterious—that is, it produces a protein that is not very functional. In our shoe analogy, this might be sneakers with blown-out toes. If you have these sneakers and dress shoes, at least you have one pair of shoes that covers your feet (Figure 14.17b). In the case of a deleterious allele, a heterozygous individual still carries one functional copy of the gene. Genetic variability can help mask the effects of deleterious alleles, because the functional allele is dominant—that is, it tends to drown out the deleterious allele (see Chapter 4). An individual who is homozygous (carries two identical copies of a gene) for the deleterious allele will have low fitness—in our analogy, two pairs of blown-out sneakers and nothing else. When individuals are heterozygous for many genes, the cumulative effect is often greater fitness relative to individuals who are homozygous for many genes.

In a small population, where mates are more likely to be related to each other than in a very large population simply because there are fewer mates to choose from, heterozygosity declines. When related individuals mate—known as inbreeding—the chance that their offspring will be homozygous for any allele (one that both parents inherited from a shared ancestor) is relatively high. The negative effect of homozygosity on fitness is known as inbreeding depression. This is seen in humans as well as other species—numerous studies consistently show that the children of first cousins have higher mortality rates (thus, lower fitness) than children of unrelated parents. In a population of an endangered species, the low rates of survival and reproduction associated with high rates of inbreeding can seriously hamper its ability to recover from endangerment.

We should note that in some populations, inbreeding does not lead to lower fitness. This appears to be the case in populations in which inbreeding has been historically common. Here, deleterious alleles have been "purged" from the population because inbreeding exposes these alleles to natural selection (that is, allows them to be expressed in homozygotes). Because these homozygotes have low fitness, the alleles they carry are rarer in subsequent generations and are lost over time. However, this appears to be a relatively rare occurrence—in an examination of 25 captive mammal species, only one showed clear evidence that deleterious alleles had been purged. For most species, inbreeding seems to be a significant threat to survival.

Genetic Variability in Populations Small populations also lose genetic variability as a result of genetic drift, a change in the frequency of an allele in a population occurring simply by chance. Genetic drift was discussed as a process for causing evolutionary change in Chapter 10. However, genetic drift in a small population can have detrimental consequences.

Imagine a population in which the frequency of blood-type allele A is 1%— that is, only one out of every 100 blood-type genes in the population is the A form (we use the symbol ia). In a population of 20,000 individuals, we calculate the total number of ia alleles as follows:

total number of blood-type alleles in population =

total population x 2 alleles/person

total number of ia alleles =

total number of alleles in population x frequency of allele ia

If a few of the individuals who carry the ia allele die accidentally before they reproduce, the number of copies of the allele drops slightly in the next generation of 20,000 people, say to 385 out of 40,000 alleles. The chance occurrences that led to this drop result in a new allele frequency:

frequency of ia alleles in population =

total number of ia alleles / total number of blood-type alleles in population lUc = °.°096

The change in frequency from 1% to 0.96% is the result of genetic drift.

A change in allele frequency of 0.04% is relatively minor. There will still be hundreds of individuals who carry the ia allele. It is not unlikely that in a subsequent generation, a few individuals carrying allele ia will have an unusually large number of offspring, thus increasing the allele's frequency in the next generation.

Now imagine the effects of genetic drift on a small population. In a population of only 200 individuals and with an ia frequency at 1%, only four of the individuals in the population carry the allele. If two of these individuals fail to pass it on, the frequency will drop to 0.5%. Another chance occurrence in the following generation could completely eliminate the two remaining ia alleles from the population. Thus, genetic drift occurs more rapidly in small populations and is much more likely to result in the complete loss of alleles (Figure 14.18). Typically, the alleles that are lost via genetic drift have little current effect on fitness—after all, if the protein produced by the allele significantly

Figure 14.18 Genetic drift affects small populations more than large populations. In this graph, each line represents the average of 25 computer simulations of genetic drift for a given population size. After 100 generations, a population of 500 individuals still contains 90% of its genetic variability. In contrast, a population of 20 individuals has less than 5% of its original variability.

Figure 14.18 Genetic drift affects small populations more than large populations. In this graph, each line represents the average of 25 computer simulations of genetic drift for a given population size. After 100 generations, a population of 500 individuals still contains 90% of its genetic variability. In contrast, a population of 20 individuals has less than 5% of its original variability.

increased fitness, natural selection should result in the allele increasing in the population. However, many alleles that appear to be neutral with respect to fitness in one environment may have positive fitness in another environment. For example, there is some evidence that individuals with type A blood are more resistant to cholera and bubonic plague than people with blood-type O or B. Therefore, possessing the ia allele may be neutral relative to other blood-type alleles in areas where these diseases are rare, but it could be an advantage where the diseases are common.

Populations with low levels of genetic variability have an insecure future for two reasons. First, when alleles are lost, the level of inbreeding depression in a population increases, which means lower reproduction and higher death rates, leading to smaller populations that are susceptible to all of the other problems of small populations. Second, populations with low genetic variability may be at risk of extinction because they cannot respond to changes in the environment. When few alleles are available for any given gene, it is possible that no individuals in a population possess a trait that allows them to survive an environmental challenge. For example, if blood-type A really does protect against some infectious diseases, a population with no individuals carrying the IA allele could have no survivors after exposure to bubonic plague.

As always, there are some exceptions to the "rule" described above. For example, widespread hunting of northern elephant seals in the 1890s reduced the population to 20 individuals—this probably wiped out much of the genetic variation in the species. However, elephant seal populations have rebounded to include about 150,000 individuals today. Although genetic variability is quite low in this species today, elephant seals continue to thrive, but there are many more examples of the costs of low genetic variability. The Irish potato is perhaps the most dramatic example of this cost.

Potatoes were a staple crop of rural Irish populations until the 1850s—a healthy adult man consumed about 10 potatoes, or 14 pounds, each day. Although the population of Irish potatoes was high, it had remarkably low genetic variability for two reasons. First, potatoes are not native to Ireland (in fact, they originated in South America), meaning the crop was limited to few varieties that were originally imported—and the majority of potatoes grown on the island were of a single variety, called Lumper for its bumpy shape. Second, new potato plants are grown from potatoes produced by the previous year's plants, and thus are genetically identical to their parents. This agricultural practice ensured that all of the potatoes in a given plot had identical alleles for every gene. All of the available evidence indicates that the genetic variability of potatoes grown in Ireland during the nineteenth century was extremely low.

When the organism that causes potato blight arrived in Ireland in September 1845, nearly all of the planted potatoes became infected and rotted in the fields. The few potatoes that had escaped the initial infection were used to plant the following year's crops. Some varieties of potatoes in South America carry alleles that allow them to resist potato blight and escape an infestation unaffected. However, apparently very few or no Irish potatoes carried these alleles, and in 1846 the entire Irish potato crop failed. As a result of this failure and another in 1848, along with harsh policies instituted by the ruling British government in Ireland, nearly 1.5 million Irish peasants died of starvation and disease and another 1 million left home for North America.

Irish potatoes descended from a small group of plants that were missing the allele for blight resistance, so even an enormous population of these plants could not escape the catastrophe caused by this disease. Similarly, since small populations lose genetic variability rapidly via genetic drift, keeping endangered species from declining to very small population levels may be critical for avoiding a similar genetic disaster. This is why preserving adequate numbers of Lost River and shortnose suckers, even at the expense of crop production in the Klamath Basin, is such a high priority if we wish to save the species from extinction.

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