Geoscience Reference
In-Depth Information
but are capable of reaching large population sizes within these patches. Nun-
ney and Campbell (1993) noted that demographic models and genetic mod-
els both have resulted in similar estimates of minimum viable population size,
but that the ideal spatial arrangement of reserves remains an issue.
Lande (1995) suggests that genetic mutations may affect fitness, so ignor-
ing genetic effects results in underestimates of viability. Mutation can affect
the persistence of small populations by causing inbreeding depression, by
maintaining potentially adaptive genetic variation in quantitative characters,
and through the erosion of fitness by accumulation of mildly detrimental
mutations. Populations of 5,000 or more are required to maintain evolution-
ary viability. Theoretical results suggest that the risk of extinction caused by
the fixation of mildly detrimental mutations may be comparable in impor-
tance to environmental stochasticity and could substantially decrease the long-
term viability of populations with effective sizes as large as a few thousand
(Lande 1995). If these results are correct, determining minimum viable popu-
lation numbers for most endangered species is an exercise in futility because
almost all of these populations are already below 5,000.
Conservation biologists would like to have rules to evaluate persistence
(Boyce 1992), such as the magical Franklin-Soulé number of 500 (Franklin
1980; Soulé 1980) that is the effective population size (
N
e
) to maintain genetic
variability in quantitative characters. Unfortunately, these rules lack the realism
to be useful. The Franklin-Soulé number was derived from simple genetic
models and hence lacks the essential features of a PVA model discussed here.
Attempts with simplistic models such as Mangel and Tier (1994) and Tomiuk
and Loeschcke (1994) also do not provide defensible results because of the
lack of attention to the biology of the species and the stochastic environ-
ment in which the population exists. Until conservation biologists do good
experimental studies to evaluate population persistence empirically, I question
the usefulness of the rules and simplistic models suggested in some of the
literature.
PVA can be viewed as a heuristic tool to explore the dynamics of an endan-
gered population but not as a predictive tool. PVA could be used to identify
variables to which the population may be sensitive and to investigate the rela-
tive benefits of alternative kinds of management. Some readers will argue that
in this context, the absolute reliability of the model estimates of extinction
probability, or time to extinction, matters much less than the extent to which
risk is affected by different demographic and environmental variables. I dis-
agree because conclusions from PVA so strongly depend on which sources of
variation are included in the model and their relative magnitudes. For exam-
ple, the importance of demographic variation is stressed in PVA because it
