The present study is based on a 14-yr time series of light trap data of moths (Lepidoptera) species in an undisturbed alder swamp forest in Central Europe. Vegetation change was followed on five pennanent plots. The biomass of all plant species in the portion of the forest surrounding the light trap was determined. As the feeding specificity of all the moth species was known, we were not only able to categorize moth species according to their potential feeding specificity, but were also able to estimate total food supply and its diversity in the studied ecosystem. In monophagous species, moth population size, characterized by mean annual catch, was positively correlated with food supply, whereas in oligo- and polyphagous species the correlation was close to zero. Although the ability of plant species to support a monophagous moth is positively dependent on its biomass, there are some relatively rare plant species supporting monophagous moths, and some common species not supporting monophagous moths. Directional changes were found both in plant and moth species; directional changes in moth species are more pronounced than changes in plant cover. There is no evidence suggesting that directional changes in moth population sizes are caused by the changes in food supply. Pronounced directional changes were found mainly in species not confined to the closed alder forest and are related to changes in wider areas, in some cases with known changes in species distribution. Non-directional year-to-year variability, described by Lloyd's index, is best explained by the potential population growth rate of a moth species estimated on the basis of species fecundity. Correlation of Lloyd's index with number of food plant species is significantly positive, but low (r = 0.25); no significant differences were found between categories of feeding specificity. This shows that the dependence of population variability on species feeding specificity, both potential and expressed as number of food plants in the ecosystem, is weak.

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