Altitudinal size clines, species richness and population density: case studies in Orthoptera

Abstract

We examined size clines for various external body structures in two grasshopper species (Chorthippus vagans and Oedipoda miniata) and one katydid species (Poecilimon birandi), along an altitudinal gradient ranging from sea level to 1,980 m in southwest Anatolia (Turkey). In O. miniata and P. birandi all measured structures (body, tegmina, pronotum, and hind femur) were smallest at the highest altitude for both sexes. Hence, in these species, different structures covaried in the same direction, with increasing altitude. In contrast, there was no clear relationship between altitude and size in C. vagans, and different structures covaried in different directions in males and females (i.e., some structures became larger at higher elevations, whereas others became smaller, and this differed with sex). For some other C. vagans traits, there was no significant intraspecies variation. Hence, O. miniata and P. birandi followed the converse Bergmann's Rule, whereas there was no consistent pattern for C. vagans. For C. vagans, small body size was not associated with either local population density (measured as relative abundance) or local species richness, suggesting that neither intra- nor interspecific competition determined body size for this species. For O. miniata, the smallest individuals were found at the site with the highest grasshopper diversity, suggesting that interspecific competition could have influenced O. miniata body sizes at this altitude. However, the largest O. miniata individuals generally occurred at the sites with the highest relative abundance of O miniata. Hence, in this species, body sizes were generally larger at sites with high population densities — consistent with sites that were most favorable and so able to support high densities of O miniata also producing the largest individuals of this species. Observational data on P. birandi similarly suggested that, for this katydid, there is little relationship between local diversity/abundance and altitudinal size-clines.