Geoscience Reference
In-Depth Information
(1993), and Remmert (1994) provided a historical perspective of how the field
developed. In this chapter I discuss procedures to develop useful viability
analyses. Specifically, statistical methods to estimate the variance components
needed to develop a PVA, the need to incorporate individual heterogeneity
into a PVA, and the need to incorporate the sampling variance of parameter
estimates used in a PVA are discussed.
Qualitative Observations About Population Persistence
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Qualitatively, population biologists know a considerable amount about what
allows populations to persist. Some generalities about population persistence
(Ruggiero et al. 1994) are as follows:
•
Connected habitats are better than disjointed habitats.
• Suitable habitats in close proximity to one another are better than widely
separated habitats.
•
Late stages of forest development are often better than younger stages.
•
Larger habitat areas are better than smaller areas.
• Populations with higher reproductive rates are more secure than those with
lower reproductive rates.
• Environmental conditions that reduce carrying capacity or increase vari-
ance in the growth rates of populations decrease persistence probabilities.
This list should be taken as a general set of principles, but you should rec-
ognize that exceptions occur often. In the following section, I discuss these
generalities in more detail and suggest contradictions that occur.
GENERALITIES
Typically, recovery plans for an endangered species try to create multiple pop-
ulations of the species, so that a single catastrophe will not wipe out the entire
species, and increase the size of each population so that genetic, demographic,
and normal environmental uncertainties are less threatening (Meffe and Car-
roll 1994). However, Hess (1993) argued that connected populations can have
