Population Viability Analysis
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happens to be the simplest source of variation to model, generally requiring
only the assumption of binomial variation. Temporal variation has received
less emphasis because it is a more difficult to obtain estimates of the temporal
variance of population parameters. Individual heterogeneity has received no
attention because this source of variation is by far the most difficult source to
quantify, particularly for survival rates. The only way to draw valid inferences
about the importance of various sources of stochasticity affecting a population
is to have reasonably good estimates of these parameters. Simplistic PVA models based on few or no data lead to simplistic and unreliable answers. Without
data, why would you expect anything else?
Until rigorous experimental work can be conducted, conservation biologists should borrow information from game species, where long-term studies
have been done that will provide estimates of temporal and spatial variation
and individual heterogeneity. Rules that predict temporal variation in survival
as a function of weather, or individual variation in survival as a function of
body characteristics, provide alternative sources of data. For at least some game
species, data exist to develop such rules. Furthermore, these kinds of data
probably will never be available for many endangered species; the opportunity
to collect such data was lost with the decline of the population to current
(threatened) levels. Thus I suggest the use of surrogate species to help meet the
data needs of realistic models of persistence. Taxonomically related species may
provide information, although species in the same ecological guild may also
provide information on temporal and spatial variation. Note the distinction
between using estimates of the temporal and spatial variation and individual
heterogeneity from a related species and using estimates of survival and
recruitment from a surrogate species. Estimates of survival and recruitment
from a stable or increasing population would obviously be inappropriate for a
species with a declining population.
Conclusion
In summary, most estimates of population viability are nearly useless because
one or more of the following mistakes or omissions are made in developing a
model to estimate persistence. By listing omissions, this list suggests the essential ingredients to develop a useful PVA.
• Few or no data are available to estimate basic parameters in the population
model, with almost all the parameter estimates just guesses. The resulting estimate of persistence is therefore strictly a guess.
325
happens to be the simplest source of variation to model, generally requiring
only the assumption of binomial variation. Temporal variation has received
less emphasis because it is a more difficult to obtain estimates of the temporal
variance of population parameters. Individual heterogeneity has received no
attention because this source of variation is by far the most difficult source to
quantify, particularly for survival rates. The only way to draw valid inferences
about the importance of various sources of stochasticity affecting a population
is to have reasonably good estimates of these parameters. Simplistic PVA models based on few or no data lead to simplistic and unreliable answers. Without
data, why would you expect anything else?
Until rigorous experimental work can be conducted, conservation biologists should borrow information from game species, where long-term studies
have been done that will provide estimates of temporal and spatial variation
and individual heterogeneity. Rules that predict temporal variation in survival
as a function of weather, or individual variation in survival as a function of
body characteristics, provide alternative sources of data. For at least some game
species, data exist to develop such rules. Furthermore, these kinds of data
probably will never be available for many endangered species; the opportunity
to collect such data was lost with the decline of the population to current
(threatened) levels. Thus I suggest the use of surrogate species to help meet the
data needs of realistic models of persistence. Taxonomically related species may
provide information, although species in the same ecological guild may also
provide information on temporal and spatial variation. Note the distinction
between using estimates of the temporal and spatial variation and individual
heterogeneity from a related species and using estimates of survival and
recruitment from a surrogate species. Estimates of survival and recruitment
from a stable or increasing population would obviously be inappropriate for a
species with a declining population.
Conclusion
In summary, most estimates of population viability are nearly useless because
one or more of the following mistakes or omissions are made in developing a
model to estimate persistence. By listing omissions, this list suggests the essential ingredients to develop a useful PVA.
• Few or no data are available to estimate basic parameters in the population
model, with almost all the parameter estimates just guesses. The resulting estimate of persistence is therefore strictly a guess.