Introduction

In the last few lectures we identified four categories of stochastic threats to small populations¹ So far we have focused on those stochastic threats that directly affect the numbers of individuals in a population.

1. Demographic stochasticity - not much of a problem in populations with more than 50-100 reproductive individuals

2. Environmental stochasticity - requires large populations, on the order of 1000-10,000 individuals to give a reasonable chance of long-term survival.

3. Natural catastrophes - if catastrophes occur at an appreciable frequency and if they eliminate a large fraction of the population, no single population can survive over the long term.

Before starting our discussion of how to use what we've learned in the management of threatened species, we have one more stochastic threat to discuss: genetic stochasticity or genetic drift.

As I mentioned in my introductory lecture, the early literature in conservation biology - at least the early literature defined as the conservation biology literature of the early 1980's - was dominated by studies of the threats to small populations, and genetic threats were among those most prominently identified. What exactly are those threats? Well, they can be divided into two basic categories

1. Short-term effects on individual viability and fecundity - these have a direct effect on demographics of the population, potentially making it even more susceptible to extinction than in would be if these factors could be ignored

2. Long-term effects on the ability of populations to respond adaptively to environmental change - even if genetic stochasticity has no immediate effect on the ability of populations to survive, it may potentially have some effect on their ability to respond when environmental change occurs, whether we are talking about global climate change or the introduction of a new pathogen

Before we talk about the possible effects of genetic changes on short-term and long-term survival, it is necessary first to talk about the types of genetic changes that happen in small populations. Only then can we explore their effects.