Introduction

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It almost seems a truism to say it, but it's a simple fact that cannot be ignored when dealing with endangered species: Small populations are more likely to go extinct than large ones. Over the next couple of weeks we'll be talking in detail about the reasons for this, but it was recognized many years ago that when pouplations get too small they may be unsustainable - the Allee effect. Like so many other things in biology where we think that we've had an original idea, it turns out that Darwin [2, p. 109] already had the same insight:

Rarity $\dots$ is the precursor to extinction. We can $\dots$ see that any form represented by few individuals will, during fluctuations in the seasons or in the number of its enemies, run a good chance of utter extinction.

The peril small populations face may be either deterministic (the result of systematic forces that cause population decline, e.g., loss of pollinators, inability to find mates, inability to defend against predators) or stochastic (the result of random fluctuations that have no systematic direction).