Introduction

Over the last few lectures we've talked about the stochastic threats to persistence in small populations. We identified four classes of threats and some of their qualitative properties:

1. Demographic stochasticity - Unlikely to be a problem in populations with more than 50-100 individuals

2. Environmental stochasticity - Likely to be a problem unless population sizer are on the order of 1000-10,000

3. Natural catastrophes - No single populations can ever be large enough to buffer against natural catastrophes

4. Genetic stochasticity - Not a problem in populations with $N_e$ greater than a few hundred. Therefore, not likely to be a problem in populations large enough to buffer environmental stochasticity

These general guidelines are useful, but suppose you're asked to design a recovery plan for the northern spotted owl.¹ How do you go about determining

• How many breeding pairs are necessary to provide a reasonable chance of long-term survival?

• What are the prospects for increasing the number of breeding pairs?

• What management manipulation will are most necessary to prevent extinction?

• What are the most critical stages of the life-cycle, i.e., have the largest impact on population dynamics?

Recall that the models we've discussed so far are based on very general assumptions. To answer these questions for any specific species, perhaps even for any population, a demographic model describing the dynamics of that species or population is necessary. The approach a conservation biologist takes to answering is (or should be) the same that a population ecologist takes to answering those questions. There are, however, two significant differences.

1. Population biologists choose a particular species for study, at least in part, because they think they that the species they have chosen will allow them to address broad, general issues of conceptual importance in population biology. Conservation biologists have the species chosen for them by circumstances - the circumstances of endangerment.

   One consequence of this difference is that it may be much more difficult for conservation biologists than for population biologists to get complete demographic information.² Population biologists choose species that allow them to get the information they need. Conservation biologists have to figure out how to get the information they need from species that were chosen for them.

2. Population biologists are often satisfied with discovering the factors that limit population size, population growth rate, or species distribution. Conservation biologists use that information to project the fate of populations/species and to decide among management strategies.

How do we do those projections? Well, we've noted over the last couple of lectures that there are many stochastic processes that affect population size. We can't just measure birth and death rates and make a simple projection. We have to take account of the variability in birth and death rates. Two principles guide our approach.

1. Any finite population will eventually go extinct.

2. We cannot predict population size with certainty. We can only specify the probability of particular outcomes.