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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.