Demographic stochasticity does not predict persistence of gecko populations
Authored by K Wiegand, C Wissel, K Henle, SD Sarre, T Stephan, R Brandl
Date Published: 2001
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Abstract
We present a population viability model for an arboreal gecko (Oedura
reticulata). This gecko needs a habitat of smooth-barked Eucalyptus
woodlands. In Western Australia its distribution has declined
dramatically, largely through clearance of woodlands, but populations
persist within woodland remnants. Evidence from extensive field data
suggests that the gecko was formerly distributed through much of the
original eucalypt woodlands, and that geckos show little movement
between patches. The populations in all woodland remnants seem to be
isolated. We ask whether the present distribution of the gecko across
remnants could have been produced solely by the extinction of
populations through demographic stochasticity.
To test this possibility, we developed a stochastic, individual-based
model including environmental stochasticity and estimated the percentage
of extinct populations of different size from known field
characteristics and the time span since the clearing of the woodland.
The model predicted a relationship between remnant size and gecko
persistence, driven by demographic stochasticity, that is qualitatively
similar to the observed pattern. Despite extensive testing, however, we
found that the model predicted an incidence function much too optimistic
for the observed distribution of populations in small remnants. This
discrepancy between field data and our model is due to a series of
implicit assumptions. Thus, our modeling exercise sheds light on the
procedures commonly applied to population viability analyses of single
populations of endangered species. The implicit assumptions involved in
such models make many predictions vague. We suggest that for the study
of declining species like O. reticulata it is essential to adequately
test extinction models and therefore population viability analyses.
Tags
fragmented landscapes
habitat
Extinction
Size
Biology
Viability analysis
Western australian wheatbelt
Oedura-reticulata
Gehyra-variegata
Gekkonidae