Ingvar N. Nilsson

CAN VERTEBRATE PREDATORS REGULATE THEIR PREY

Whether vertebrate predators can regulate their prey or not has long been a controversial question. At the one extreme it has been claimed that predators have no impact on prey numbers but consume only a doomed surplus (Errington 1946), and at the other that predators strongly interact with their prey causing either stable equilibria or cycles (Tanner 1975; Keith et al. 1977). However,.there are no field studies showing a regulatory effect of predation among vertebrates. Here we report on such a study.

Breeding Bird Community Densities and Species Richness in Lakes

Multiple regression analysis revealed that breeding bird community densities in eleven south Swedish lakes were positively correlated both with total phosphorus concentrations of the water and with the percentage of fen area. Eutrophic lakes support more species than oligotrophic ones, but a man-made increase in productivity does not lead to higher species richness in formerly oligotrophic lakes. Species number correlates positively with pH and shore development. About the same species number occurs in single lakes and in similar sized parts of large lakes. It is suggested that lakes, in northern Europe, appear to the birds as any other major continental habitat. The slopes of species-area relations for lakes are computed for vascular plants, invertebrates, fish, and birds and reasons for the low slopes (z < 0.21) are discussed.

Turnover of vascular plant species on small islands in lake Mckeln, South Sweden 1976?1980

SummaryIn 1976, 1978 and 1980 plant species were recorded on 41 forested islands. Apparent species turnover was much higher than true turnover, as was confirmed by recording the age of trees and shrubs in 1980. Extinction rate was negatively correlated with island area and the number of immigrations declined with distance to the mailand. No clear differences in turnover were found for species with different dispersal systems. The proportion of the species populations on an island that consisted of only one individual declined with increasing island area. Conversely, species with 75 or more individuals made up about 17% of the floras on the smallest but about 32% on the largest islands. The results of this study suggest that MacArthur and Wilsons theory of island biogeography is applicable to plants.

Species Richness and Dispersal of Vascular Plants to Islands in Lake Mockeln, Southern Sweden

Plant species richness and other variables were analyzed on 41 islands and 2 mainland plots. The species—area coefficient was .30, but was higher (.73) for strictly terrestrial and lower (0.13) for shore species. In a stepwise multiple regression analysis, 85% of the variation in species richness was accounted for by loge surface area, percent of the ground covered by stones, and foliage, respectively. The average height of the trees correlated positively with the number of strictly terrestrial species but negatively with shore species. The difference between the number of strictly terrestrial species, predicted from the multiple regression equation, and that observed was proportionally larger on smaller islands. There was evidence that plant species carried by animals dispersed less efficiently to the islands. The flora contained proportionally fewer of these species with decreasing island area. The converse was true for water—dispersed species and some wind—dispersed (those with pappi). These observations are consistent with the hypothesis of higher extinction rates on small islands. A dynamic equilibrium in species number does not necessitate an equilibrium in the proportions of species in different dispersal categories.

Seasonal Changes in Food of the Long-Eared Owl in Southern Sweden

The total number of prey categories included in the diet was higher in winter and thus showed an opposite seasonal pattern as compared with the number of prey species forming the main food. Calculated preference indices were high for the field vole in autumn and winter, and for the wood mouse in summer, which may be explained by changed profitability to the owl of the habitats. Predictions from optimal foraging theory are discussed in relation to the field data.