Abstract
Among closely related species with overlapping fundamental niches usually one excludes the other(s), unless they differ in their realised niches. In some instances, however, niche overlap is inevitable. One example are European water frogs of the Rana lessonae / R. ridibunda / R. esculenta-complex, where the hybrid R. esculenta is reproductively dependent on one or the other of the two parental species. Hence, it has to live in close sympatry with them, but species and sex ratios in such mixed populations vary widely among ponds. In this paper we investigate the spatial and temporal distribution of the three species by comparing nine ponds with different ecological conditions and frog proportions. All three species occurred in all ponds, but in significantly different absolute and relative numbers. R. lessonae proportions are higher in smaller, more structured ponds with rich vegetation under water, while R. ridibunda dominates in larger, less structured ponds with little vegetation under water. The hybrid R. esculenta is intermediate in its distribution; it occurs in larger ponds than R. lessonae, but in more vegetated ones than R. ridibunda. The sex ratios of the three species differ also within the ponds. In R. esculenta, the proportion of females decreased with increasing fluctuation in water temperature. The observed spatial distribution of adults is best explained through 81 species differences in the habitat-related development and survival of their progeny,
which is known from several experiments. Temporal changes in species proportions
were not related to pond characteristics. This, however, is not surprising, as year-to21 year changes in pond features were small, and strong site fidelity, combined with high annual survival, creates substantial temporal autocorrelation between population compositions of successive years.
Holenweg Peter, A K