Geoscience Reference
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on population viability of the snail kite (
Rostrhamus sociabilis
) and suggested
that this source of variation is important in persistence.
CONTRADICTIONS
Few empirical data are available to support the generalities just mentioned, but
exceptions exist. Berger (1990) addressed the issue of MVP by asking how
long different-sized populations persist. He presented demographic and
weather data spanning up to 70 years for 122 bighorn sheep (
Ovis canadensis
)
populations in southwestern North America. His analyses revealed that 100
percent of the populations with fewer than 50 individuals went extinct within
50 years, populations with more than 100 individuals persisted for up to 70
years, and the rapid loss of populations was not likely to be caused by food
shortages, severe weather, predation, or interspecific competition. Thus, 50
individuals, even in the short term of 50 years, are not a minimum viable pop-
ulation size for bighorn sheep. However, Krausman et al. (1993) questioned
this result because they know of populations of 50 or less in Arizona that have
persisted for more than 50 years.
Pimm et al. (1988) and Diamond and Pimm (1993) examined the risks of
extinction of breeding land birds on 16 British islands in terms of population
size and species attributes. Tracy and George (1992) extended the analysis to
include attributes of the environment, as well as species characteristics, as
potential determinants of the risk of extinction. Tracy and George (1992) con-
cluded that the ability of current models to predict the risk of extinction of
particular species on particular island is very limited. They suggested that
models should include more specific information about the species and envi-
ronment to develop useful predictions of extinction probabilities. Haila and
Hanski (1993) criticized the data of Pimm et al. (1988) as not directly relating
to extinctions because the small groups of birds breeding in any given year on
single islands were not populations in a meaningful sense. Although this criti-
cism may be valid, most of the “populations” that conservation biologists
study are questionable. Thus results of the analysis by Tracy and George
(1992) do contribute useful information because the populations they studied
are representative of populations to which PVA techniques are applied. Specif-
ically, small populations of small-bodied birds on oceanic islands (more iso-
lated) are more likely to go extinct than are large populations of large-bodied
birds on less isolated (channel) islands. However, interaction of body size with
type of island (channel vs. oceanic) indicated that body size influences time to
extinction differently depending on the type of island. The results of Tracy and
