Extinction and Small Populations – the fewer you are the more problems you have.

Small populations are generally at a greater risk of extinction than large populations. They are subject to rapid declines in numbers for three main reasons:

loss of genetic variability and related problems of inbreeding and genetic drift
demographic fluctuations due to random variations in birth and death rate
environmental fluctuations due to variation in predation, competition, disease and food supply; and natural catastrophes that occur at irregular intervals, such as fires, floods, volcanic eruptions, storms and droughts.

The figures below illustrate the rapid increase in probability of extinction for bird species as the number of breeding pairs decreases; and bighorn sheep as the population size decreases.

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        Figure.  Probability of Survival versus Time for Bighorn Sheep Populations.  N = Population Size.

Loss of Genetic Variability

If you remember your basic biology, an allele is a different form of a gene. So we have, for instance, basically two alleles for eye color-–one for blue eyes and one for brown eyes. If you have both a blue and a brown eyed allele in your genetic makeup you are heterozygous for that trait; while if you have either all blue or all brown eye color alleles you are homozygous. In a population of a given species, heterozygosity can result in greater fitness for the overall population through greater rates of growth, survival and reproduction. Why? Because having two or more different forms of a gene in a population will result in greater flexibility in dealing with the environment (this is why some individuals may be resistant to a disease which kills other members of the same species). It is also important to note that a healthy allele received from one parent may mask a nonfunctioning allele received from another parent.

The greater fitness or flexibility characteristic of heterozygous individuals is sometimes referred to as hybrid vigor.


Genetic Drift

If an allele occurs at a low frequency in a small population it has a significant probability of being lost in each generation.  The gradual loss of rare alleles from a population changes the overall genotype (pool of available genes) of the population and is referred to as Genetic Drift.

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Example: a "rare" allele occurs in 5% of all genes present.

If the population equals 1000 individuals then 100 copies of the allele are present in the population (1000 individuals X 2 copies per individual (diploid species) X .05) = 100 copies of allele.

But if the population consists of only 10 individuals there will only be 1 copy of the allele present in the population (10 X 2 X .05) = 1.

It is important to note that there is an added problem here – the total population is not a truly significant number because all the animals in the population will not be involved in breeding (juveniles, old, ill, small, sterile or weak individuals will not breed). Instead, we must consider what is called the Effective Population Size (Ne), which can simply be defined as the number of individuals in the population able to breed.

The Effective Population Size is often approximately 10% of the overall population ( Ne = 0.1N ). For example, in a study of alligators (N=1000) only 10 animals were found to be of the right age and health to breed. In this case Ne = .01N !

When alleles are lost through genetic drift the population suffers a loss of overall genetic variability. New alleles may be introduced to the population by either mutation or through the immigration of individuals from other populations of the same species in order to balance the loss due to genetic drift. The rates at which genetic variability is added to a population by these two processes differ greatly. Note that in the figure below a small amount of immigration (2 to 5 individuals per generation) will serve to maintain the level of heterozygosity. Mutation rates, on the other hand, range from about .001 to .0001 mutations per gene per generation, and as such would be totally insignificant in a small population. As noted in the figure below, the mutation rate would have to be between 10X and 100X greater than normal in order for mutation alone to preserve the heterozygosity of a small population.

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Reduction of genetic variability in a population can lead to any or all of the following:

inbreeding depression
loss of evolutionary flexibility
outbreeding depression


Inbreeding Depression

Most natural populations have mechanisms inplace which prevent inbreeding. These mechanisms include natural dispersal, unique individuals odors and sensory cues which prevent inbreeding. However, in very small populations these mechanisms can fail – a type of Allee Effect, in which there is a loss of social structure in a population below a certain minimum population size. Other types of Allee Effects include the inability of pack predators to bring down prey when the pack is too small, the inability of herds to find food or offer mutual defense when the herd size is too small, and the inability of individuals to find mates in widely dispersed populations.

When inbreeding results, the population suffers further from higher mortality among offspring, fewer offspring, and weak and sterile offspring which will have low mating success. For example, in Sweden a small (less than 40 adults), isolated population of adders (Vipera berus) apparently suffered from inbreeding depression which resulted in small liter size, a high proportion of deformed and stillborn offspring, and low overall genetic heterozygosity. When a number of adult males were imported into the population the number and health of offspring increased.

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In general, inbreeding depression is due to the expression of harmful alleles when they occur in homozygous individuals.

Another example of inbreeding depression is that of the Gray Wolves of Isle Royale, Canada. The population was established by 1 breeding pair of wolves around 1949. By 1980 the population had grown to approximately 50 individuals, but by 1990 it had dropped to 14. Few pups were sighted by investigators who also noted that females were not breeding. Further study indicated that the population had probably had no more than 2 to 3 breeding pairs of wolves for from 5 to 7 generations. Based on previous studies, there was an expected loss of from 39 to 65% of the populations genetic variation, which was later confirmed at 50%. All the wolves were as genetically similar as siblings and were probably descended from a single mother. The decline in the wolves was attributed to inbreeding depression, with the lack of reproducing females attributed to the unwillingness of the wolves to mate with close relatives. It is important to note that this example is considered to be controversial in that recent reports indicate that the population has rebounded, leading some to suggest that the original population crash may have been due to some cause other than inbreeding depression.

Example: The Florida Panther ( Felis concolor corgi). This subspecies of mountain lion has suffered an extreme reduction in population size due to habitat fragmentation, with numerous lions killed by cars. The current population is less than 50, and the results of inbreeding are clearly illustrated in the figure below which compares the panther’s testicle size, sperm motility, sperm abnormality, etc. to other non-inbred subspecies.

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Outbreeding Depression

Outbreeding depression is caused by the interbreeding of different subspecies ( = races ) leading to diminished fitness of offspring. Generally, there are strong ecological, behavioral, physiological and morphological isolating mechanisms which prevent the interbreeding of subspecies. However, in cases of very small populations outbreeding will occur as another type of Allee Effect.

Example: Ibex (Capra ibex)

This species was extirpated in the High Tatra Mountains of Czechoslovakia in the late 19th Century, but was later restored with new stocks which included two other subspecies (C. hircus and C. nubiana), both of which come from warmer climates. Capra ibex usually mated in midwinter so that the young would be born during the relatively mild months of April and May. The cross of C. ibex with C. hircus and C. nubiana resulted in summer mating (the time when the warmer climate subspecies usually mated) with juveniles born in the winter months. The harsh winter conditions led to the deaths of the juveniles.

A second example of outbreeding depression also occurred in Czechoslovakia when the large Siberian Roebuck was crossed with the smaller native species. Females of the native species died during childbirth because the fetuses were too large.

Loss of Evolutionary Flexibility

The loss of genetic variability in a small population may limit its ability to respond to new conditions and long term changes in the environment such as pollution, new diseases, and global climate change.

Further problems for small populations

Unequal Sex Ratio

A population may consist of unequal numbers of males and females due to chance, selective mortality, or harvesting of only one sex by humans. Also, social systems within a population may result in a de facto unequal sex ration, such as in elephant seals where there is a single dominant male within the herd, or in African Wild Dog packs where only the dominant female will give birth to pups.

Unequal sex ratios will directly affect the Effective Population Size (Ne). For instance, in a population of a monogamous goose species consisting of 20 males and 6 females, only 12 individuals (6 males and 6 females) will be mating, so the Ne = 12, not 26.

In general we can use the equation

Ne = (4NmNf)/(Nm+Nf) where Nm = number of males, and Nf = number of Females.

The effects of progressively greater inequality of sex ratio in a small population is illustrated in the graph below.

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Population Fluctuation and Bottlenecks

In some species the population size varies greatly from generation to generation, and the average Effective Population Size will be greatly affected by years with very low population size.

     Ne is here determined by the equation:

Ne = t/(1/N1+1/N2+1/N3+…+1/Nt) , where Nt equals the population size for the year t.

In this case a single generation with either a very large or a very small population will have a major effect on Ne. Let’s use an example of five years with N values of 100, 100, 10, 100, and 100. Our average population is 82, but our Ne is only 36. If it had not been for the single low population size year, our Ne would have equaled 100. As a result, that single low population year can result in a loss of rare alleles, and a decline in heterozygosity.

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     This reduced heterozygosity is due to what is called the Founder Effect, a case in which only a few individuals establish a new population.   Note in the graph above that when Ne is less than 10 there is a rapid loss in the percentage of original heterozygosity retained by the population.

     As we have seen above, a single year with a significantly reduced population results in a very depressed  Ne, which can result in a genetic bottleneck due to the founder effect.

Example: Lions (Panthera leo) of the Ngorongoro Crater, Tanzania.

     This population totaled roughly 60 to 75 lions until 1962, when an outbreak of biting flies severely decimated the population, leaving only 9 females and 1 male alive.  7 additional males migrates into the relatively isolated area in 1964.  Despite this influx, and an increase in population to 75-125 animals, the crater lions still exhibit evidence of inbreeding depression due to a genetic bottleneck.   Symptoms include reduced genetic viability, high levels of sperm abnormality, and a reduced reproductive rate (relative to the nearby Serengeti lion population). 

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Example: The cheetah (Acinonyx jubatus) also displays evidence of a genetic bottleneck in the past.  Studies indicate that the cheetah displays very low levels of genetic variability in two isolated populations.  These low levels of genetic variability are about what would be expected after 10 to 20 generations of inbreeding.   Consequences of this loss of heterozygosity in the Cheetah include: a male sperm count only about 10% that of related felid species, over 70% of sperm are aberrant in some way, and an unusual vulnerability to disease in captivity (loss of genetic flexibility).

Demographic Variation

     The concept of demographic variation based on stochastic concepts simply means that chance events can affect the population.  What happens if all offspring of a given year are male, sterile, or still born?   Obviously, in a large population the probability of such events is vanishingly small; but in a small population this probability becomes much greater.  This concept becomes particularly important for species with highly variable birth and death rates (annual plants, short-lived insects), and those populations with low growth rates (consider recovery time from some natural disaster).

Environmental Variation and Catastrophes

     While demographic variation affects only the individual, environmental variation (again, stochastic events) affect all the individuals in the population.  While we may tend to think in terms of disasters (floods, fires, etc.), this actually includes all random variation in both the biological and physical environment.  Consider the figure below.  In this case, the

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rabbit population relies on the plant population for food, for which it competes with the deer population.  It is preyed upon by the fox population, and can be weakened or killed by parasites and disease causing organisms.  

     Each of these populations is subject to natural random variation in size.  A year of abundant rainfall will be good for the plants which will grow copiously, supplying plenty of food for the rabbits and deer which would serve to reduce competition.  However, a very wet year may also lead to abundant parasites and disease carrying organisms which could adversely affect the rabbit population.  A cool wet year may also make the rabbits more susceptible to disease, etc.  A major increase in the fox population would be bad for the rabbits for obvious reasons.    An increase in the deer population would lead to increased food competition.   Random climatic variation must also be considered.  What would be the effects of a short, very dry summer; or a long extremely cold winter?  What about an ice storm?   Natural catastrophes are totally unpredictable from the viewpoint of most organisms other than man (and very often even we find them unpredictable), and can kill up to 70 to 90% of a population in a given area. 

     Overall, the effects of environmental variation will have a much greater effect on the population than will demographic variation.  As you can see in the figure below, the minimum viable population is greater for low levels of environmental variation than it is for demographic variation alone.  Further, as the level of environmental variation increases, so does the minimum viable population required to insure species survival.

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     Finally, consider that the problems faced by small populations do not occur as isolated events, but can act in an additive fashion.   The overall effect is similar to the vortex of water when a sink or bath tub is being drained.   Far from the vortex the current is weak, but as you get closer to the vortex the current becomes stronger, and harder to escape.  

     Small populations face the danger of entering an Extinction Vortex when the population size drops below a minimum number (see Figure below).   At this point, many of the factors discussed above begin to work additively (genetic drift leads to inbreeding which leads to demographic variation (sterility, weakness, inflexibility) which leads to more genetic drift, etc.), and the species is unable to avoid extinction.


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