Lecture 12 - Problems of Small Populations

No population lasts forever. Changing climate, succession, disease, and a range of unusual events ultimately ______

lead every population to the same fate: extinction.

The real questions to consider are whether a population goes extinct sooner rather than later, what factors cause the extinction, and whether other populations of the same species will continue elsewhere:

Will a population of African lions last for more than 1000 years and go extinct only after a change in climate, or will the population go extinct after 10 years because of introduced disease and hunting by humans?

An ideal conservation plan for an endangered species would protect as many individuals as possible with the greatest possible area of high-quality, protected habitat:

In practical terms, planners, land managers, politicians, and wildlife biologists often attempt to achieve realistic goals, guided by general principles.

In case, they need to know how much longleaf pine habitat a red-cockaded woodpecker population requires to persist.

Is it necessary to protect habitat containing 50, 500, 5000, or 50,000, or more individuals to ensure the survival of the species?

If the goal is persistence for a greater number of years, then a _______

larger minimum viable population (MVP) size is needed.

In a groundbreaking paper, Shaffer (1981) defined the number of individuals necessary to ensure the long- term survival of a species as the minimum viable population:

“A minimum viable population for any given species in any given habitat is the smallest isolated population having a 99% chance to remaining extant for 1000 years despite the foreseeable effects of demographic, environmental, and genetic stochasticity, and natural catastrophes.”

A population’s ability to adapt to a changing environment depends on genetic variability, which occurs as a result of ______

individuals’ having different alleles – different forms of the same gene.

New alleles arise in a population either by ________

random mutations or through the migration of individuals from other populations.

This random process of allele frequency change is known as genetic drift, and it is a separate process from changes in allele frequency caused by natural selection.

When an allele occurs at a low frequency in a small population, it has a significant probability of being lost in each generation.

Genetic variability is lost randomly over time through genetic drift.

Because of genetic drift, small populations lose genetic variation more rapidly than large populations. Some small populations may lack any genetic variation.

Mating among closely related individuals, which occurs in small populations, often results in lower reproductive success and weak offspring.

Mating between unrelated individuals of the same species often results in offspring with a high fitness as measured by survival or high reproduction. Mating among close relatives leads to low fitness or inbreeding depression. Mating between individuals in widely different populations or even different species sometimes results in lowered fitness or outbreeding depression.

_________: Low genetic diversity (extreme homozygosity). A variety of mechanisms prevents inbreeding, mating among close relatives, in most natural populations.

Inbreeding Depression

________: heterozygous, but not necessarily adapted to the local environment. Individuals unable to find mates within their own species (or population) may mate with individuals of related species – the resulting offspring face outbreeding depression

Outbreeding Depression

The effective population size Ne will be much smaller that the total population size N when there is great variation in reproductive output, an unequal sex ratio, or population fluctuations and bottlenecks.

Unequal sex ration:

• Ne = [4(Nf X Nm)] / (Nf + Nm)

Variation in reproductive output.

Population fluctuations and bottlenecks.

• Ne = t / (1/N1 + 1/N2 + ….. + 1/Nt)

Managing for genetic variation.

The effective population size (Ne) declines when the number of males and females in a breeding population (N) of 100 individuals is increasingly unequal.

Random fluctuations in birth and death rates, disruption of social behavior following decreased population density, and environmental stochasticity all contribute to instability in the population size, often leading to local extinction.

Random variation, or stochasticity, in the environment can cause variation in the population size of species.

For example, the population of an endangered butterfly species might be affected by fluctuations in the abundance of its food plants and the number of its predators.

Intensive management is often required to prevent small populations from declining further in size and going extinct.

The smaller a population becomes, the more vulnerable it is to further demographic variation, environmental variation, and genetic factors that tend to lower reproduction, increase mortality rates, and so reduce population size even more, driving the population to extinction.

An important implication of the extinction vortex is that addressing the original cause of population decline may not be sufficient to recover a threatened population.

Once a population drops below a certain size, it enters an extinction vortex in which the factors that affect small populations tend to drive its size progressively lower.