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How different characteristics of host-plants affect the extinction risk in moths?
Marko Nieminen
Department of Zoology, Division of Ecology, P.O. Box 17 (P. Rautatiekatu 13), FIN-00014
University of Helsinki
Many moth species are very tightly connected to a certain host-plant species. Even though the
different host-plant attributes may be crucially important for the existence of moths and
butterflies, the effects on different species are not known. I used an incidence function model
(Hanski 1994, J. Anim. Ecol. 63:15 1-162) to study these effects on the extinction probability of
moth species. I fitted the model to moth occupancy data from the Rauma archipelago (Itämies
1982, Aquilo Ser. Zool. 21:13-52) by non-linear regression (maximum likelihood estimation) and
studied the relationships between estimated x-values of moth species and different host-plant
attributes. The x-value describes how fast the extinction probability decreases with increasing
island area (A) and, at the same time, with increasing population size. The inferences which can
be made from the x-values are based on the following ideas and assumptions. Population density
is assumed to be independent of A, but the population size is directly dependent on A, and, thus,
the extinction probability should decrease with increasing A. If x > 1, then the expected time to
population extinction increases exponentially beyond some critical patch size and the extinction
becomes very unlikely. Therefore, in these species, only small populations have a great risk of
extinction (due to demographic stochasticity). If x < 1, then there is no such critical limit in patch
size and even large populations have a considerable risk of extinction. Consequently, the x-values
of different moth species reflect the strength of environmental stochasticity towards them, and
the value of x decreases as a rule with increasing importance of environmental stochasticity. The
properties of the host-plants are very likely an important part of that stochasticity. I will present
an analysis on what are the most important host plant characteristics in this context.