29.9: Conservation of Small Populations

Small population sizes put a species at extreme risk of extinction due to a lack of variation, and a consequent decrease in adaptability. This weakens the chances of survival under pressures such as climate change, competition from other species, or new diseases. Large populations are more likely to survive pressures such as these, as such populations are more likely to harbor individuals that have genetic variants that are adaptive under new stresses. Small populations are much less likely to have such variation.

Modern genomic techniques can identify homozygosity in deleterious genes that is caused by inbreeding. This happens when closely-related organisms produce offspring; the offspring have a higher chance of receiving two identical deleterious alleles. For example, the wolves in Isle Royale National Park went through an extreme population reduction caused by a disease outbreak, which led to increased inbreeding. The wolf population has continued to decline, at one point containing only two wolves.

Whole-genome sequencing allowed researchers to identify the lineage of the remaining wolves on Isle Royale, which showed sibling-sibling and parent-offspring matings in the small population based on patterns of chromosomal inheritance. Analysis of gene sequences showed deleterious single-nucleotide polymorphisms (SNPs) within functional genes that reduce the fitness of these wolves. These mutations explain physical characteristics that can be seen in Isle Royale wolves, such as malformations in their spines and rib cages.

Park rangers are bringing in wolves from outside the park to establish healthy genetic diversity on Isle Royale. Given a healthier, larger population, the wolves should be able to thrive on Isle Royale. Rangers also track the wolves on the island with GPS collars to determine if their conservation methods are effective.

The priority for conservation biology is the preservation of biological diversity. To accomplish this goal, scientists must identify populations that are endangered with extinction. Small populations, such as the Florida panthers, are especially at risk because they have low genetic diversity, making it difficult for them to resist disease or adapt to changes in their environment.

These cats previously ranged from Florida to Louisiana along the Gulf Coast. Due to land development, their habitat was reduced to a small area in Southern Florida. By 1973, there were only around 25 panthers left. Such a small population led to inbreeding, which caused kinked tails, heart defects, and other genetic abnormalities that reduced their reproductive capacity.

Once a population reaches a critically small size, it can experience what is called an extinction vortex, a circular chain of events that build on one another to further decrease population size, leading to extinction. It works this way. A small population is prone to inbreeding, or mating with close relatives. As a result, there will be less genetic diversity in the population. Lower genetic diversity leads to lower survival and reproduction rates in the population over time, further decreasing the population size to the point of extinction.

One way to prevent an extinction vortex is to import individuals from other populations. For example, conservation biologists introduced female panthers from Texas into the Florida population to increase its size and genetic diversity. Today, there are about 125 Florida panthers due to these conservation efforts. Although the population has grown, conservation is still required. GPS collars are used to track individual panthers, allowing conservationists to monitor the health of the population.