Predicting Future Extinction and Factors That Make Species Endangered

Dramatic statements regarding the loss of biodiversity have become commonplace in both the scientific literature and the popular media. However, if you look closely at these claims, you will see that, although they are consistently pessimistic, they do not seem to agree about exactly how bad the current extinction crisis is. Estimating extinction rates is complicated and involves a series of assumptions and educated guesses. Rather than attempting to count actual extinctions, most researchers use indirect methods to estimate extinction rates. Habitat destruction is widely considered to be the major cause of extinction and endangerment of species. Unlike other major causes of endangerment, such as pollution, exploitation, introduced predators, and disease, habitat destruction (typically deforestation for the terrestrial environment) is relatively easy to quantify over vast land areas using satellite imagery. Given an estimate for the rate of habitat loss, researchers might then ask 'how many species are expected to become extinct with each million hectares of habitat destroyed?' To answer this question, one must know something about the relationship between habitat area and species diversity. Species-area curves show the relationship between the diversity of species and the physical size of an island, habitat patch, or sampling space. The general pattern is that species initially accumulate rapidly as area increases, but the rate at which new species are added gradually declines, until eventually all of the species present in a region are accounted for. This saturating relationship between diversity and area is one of the most robust patterns in all of ecology. Species-area curves have been successfully applied to many different taxa in a wide variety of settings and terrestrial habitats. The implication of the general shape of this relationship is that there will initially be few consequences of habitat destruction (i.e., a small percent of species will become extinct), but as the process continues and more and more habitat is destroyed, the impact on biodiversity becomes more extreme (Figure 1).

Area

Figure 1 A species-area curve (S = cAz), where S is the number of species in habitat of area A, and c and z represent constants that depend on the type of species being considered and the type of habitat involved. In the curve shown c = 4 and z = 0.3. Using such a curve, one can estimate extinction of species associated with any fraction of habitat loss.

Area

Figure 1 A species-area curve (S = cAz), where S is the number of species in habitat of area A, and c and z represent constants that depend on the type of species being considered and the type of habitat involved. In the curve shown c = 4 and z = 0.3. Using such a curve, one can estimate extinction of species associated with any fraction of habitat loss.

The relationship between species diversity, S, and area, A, is described by the equation

S = cAz where c is a constant reflecting the number of species in a habitat of area A, and z is the rate of species accumulation as A increases. Estimates of z are crucial if one wants to predict the impact of habitat destruction on diversity. Larger z values are typical of true oceanic islands and for species with relatively limited movement, while smaller values are common for virtual islands that really are areas embedded within a continuous landscape (such as parks surrounded by agriculture) and for highly mobile species.

Although we may not be sure of the value of z in the species-area curve, the idea that habitat conversion or destruction causes loss of species is well-documented. This means that even without having data on particular species, ecologists can anticipate that species associated with particular habitats will be endangered if too much habitat is lost. For example, even though there are not counts of plants and animals declining where forests are logged, we can still be sure that some ofthe forest residents are endangered by habitat loss. The mechanism by which habitat loss endangers species is reduction in population size. If there is less habitat for a species, then that species must have fewer individuals. A small population size endangers a species for several reasons which we discuss below. But first it is worth recognizing that the tendency to use the phrases 'habitat loss' and 'habitat destruction' can be misleading. Habitat does not disappear - it is converted to a different type of landscape, such as farmland or logged forests with remnant patches of woodland left standing. Historical deforestation events have generally not produced the magnitude of extinction one would expect from a simple application of species-area curves. Between 90% and 99% of Puerto Rico was logged, yet only 10% of the resident terrestrial bird species in that forested habitat went extinct. Similarly at least 90% of the forests in eastern United States were logged, yet only three forest-dwelling birds went extinct. Although these percentages are qualified by the awareness that we have only recently been able to adequately track whole taxa such as birds for extinctions, it is still true that the risk of extinction can be tempered by the quality of converted habitat, which may not always be as uninhabitable as a species-area curve implies.

Population size is the best indicator of a species' endangerment. First, small populations are at greater risk of extinction because they are more susceptible to chance misfortune such as failing to mate successfully or to survive a winter in spite of there being an average likelihood that individuals in the population should succeed. For instance, if we flip a coin 100 times we will probably get roughly 50 'heads' and 50 'tails'. But if we flip a coin only ten times, there is a greater chance of getting a more lop-sided result, such as 10 'tails' - and if tails meant a chance death, then clearly a small population size is a great disadvantage. These effects are referred to as demographic stochasticity and usually apply to populations of reproductive females substantially less than 100. Of course, environments fluctuate due to droughts, heat waves, or unusually harsh winters, and species we normally think of as being much more numerous than 100 can be driven to unusually low numbers by temporarily poor environmental conditions, and then become at risk of extinction due to demographic stochasticity. Mathematical models of population extinction in fluctuating environments suggest that populations smaller than 1000 are likely to be at substantial risk because of a combination of fluctuating environments and demographic stochasticity. A second unfortunate attribute of small populations is that they usually occupy small areas. Any time a population is restricted to a small area it is at risk ofbeing exterminated by one catastrophic event such as a wildfire or a large flood.

For some species there is a positive feedback whereby a decline in population size also causes a predictable (as opposed to chance) decline in survival or reproductive success of the remaining individuals. When reductions in population catalyze further population decline, it is called an Allee effect. Allee effects can occur in certain species that experience difficulty in finding a mate when at low density. Imagine, for example, the difficulties that a whale might have in finding a mate if there are only a few hundred whales left throughout an enormous ocean. One reason the Northern Right whale is not recovering, even though all hunting on it has been halted, is that mates cannot locate one another in the vast ocean. Allee effects have been well documented in certain small populations of plants that rely on insect pollinators such as bees. Bees often specialize on visiting abundant plant species because they can more efficiently gather pollen and nectar if they keep visiting the same common flower; as a result a plant species that becomes too scarce may be neglected by pollinators and hence fail to reproduce. Allee effects can also arise when social interactions become impaired at low population size. For example, animal species that rely on cooperative group hunting may be incapable of bringing down large prey if the group becomes too small.

Lastly, small population size can imperil a species because of genetic effects. Individuals in small populations may have no choice but to mate with related individuals. Mating with kin is called inbreeding, and inbreeding can severely reduce the survival or reproductive fitness of any offspring that are produced (a phenomenon called inbreeding depression). Inbreeding tends to increase homozygosity because two related individuals are more likely to share the same version of a gene than are two unrelated individuals. Offspring that are homozygous for many genes may be worse off than individuals that are highly heterozygous for a couple of reasons. First, heterozygosity (having two different alleles for a particular gene) can be beneficial if the two alleles result in the production of two different versions of a protein. For certain genes, producing multiple versions of proteins can be correlated with greater physiological flexibility or improved immune system function. Homozygotes lose this advantage. Second, many harmful versions of genes are recessive or masked by the presence of a healthy version of the gene inherited from the other parent. But an individual that is homozygous for a harmful allele will express the genetic disorder. Examples of human genetic disorders caused by recessive alleles include cystic fibrosis, tay-sachs, and phenylketonuria. Both of these mechanisms can cause reduced fitness of inbred offspring, and reduced fitness of individuals will translate into reduced population growth and increased risk of extinction for the population as a whole.

Even in the absence of inbreeding, small populations tend to lose genetic diversity over time. Unique alleles (versions of genes) are lost due to genetic drift. Moreover, mutations, which are the source of new alleles, are rare and therefore fewer new alleles are generated in small populations. This loss of genetic diversity can be devastating to a population. Unlike natural selection, which increases the frequency of adaptive or beneficial alleles, genetic drift is completely random with respect to adap-tiveness. In other words, either good or bad versions of genes may be lost, just by chance, and the odds of these random losses are higher when a population is small.

The rate at which genetic diversity is lost from a population does not depend on the total population size, but rather on what is called the effective population size. Effective populations are always less than the actual population because they refer to a concept called an ideal population. An ideal population is one with a 50:50 sex ratio, every individual being equally successful at mating and reproducing, and the same population size year-after-year. A population that departs from these ideal conditions has an effective population that is lower by an amount that is roughly proportional to the degree to which the ideal conditions are violated. Population genetics theory provides models for estimating effective population size given information on how a population departs from the ideal conditions. The reason the effective population size of any species is inevitably less than the numerical abundance of species is because sex ratios commonly depart from 50:50, the number of matings and offspring per parent is usually variable in wild populations, and populations in nature fluctuate. The important point is that species with small effective population size lose genetic diversity far more rapidly than species with large effective populations. Although small populations are always more at risk than larger populations, species that are represented by chronically small populations often do not show severe genetic inbreeding. These 'naturally' and chronically small populations may have evolved mating systems and genetic mechanisms that have reduced inbreeding depression in comparison to large populations that are suddenly driven to low numbers by human activities. This distinction between chronically small populations and newly small populations is one reason that criteria for identifying which species are at risk typically factor in trends in abundance and recent declines in addition to population size.

When talking about endangered species it is important to note that anything that makes populations small imperils a species. Even if the cause of a species decline is halted because fishing or hunting has been halted, or the last remaining habitat is protected, a small population may still be doomed to extinction. Once populations shrink below a certain threshold level they enter an extinction vortex in which inbreeding depression, chance misfortune, disrupted social systems, and reduced genetic variability all conspire against their persistence and make the situation worse with each new generation. In those circumstances captive breeding may be the only hope. In short, small population size equals an endangered species, and declining populations can also equal an endangered species because prolonged declines will inevitably produce small populations. One key question is how small does a population have to be to be endangered? The answer depends on the mating system (do a few males get all the females.? how uneven is reproductive success?) and rate of reproduction. Certainly any population below 1000 is at risk of extinction, and populations as high as 10 000 can be highly endangered if most of the individuals are males or too young or too old to reproduce.

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