The effects of wind speed, competition, and body size on perch height selection in a guild of Libellulidae species (Odonata)

For eleven species of sympatric libellulids, male mean mass was positively correlated with wing aspect ratio, wing loading, and mean perch height. We tested the hypo theses that perch height selection was governed by interspecific competition or biomechanical responses to increased wind speed at higher perches. Although larger odonates might prefer higher perches to offset their increased wing loading, species’ mean perch height did not correlate with changes in mean or maximum wind speeds. Rather, perch height selection is best explained by competitive interactions. Mean mass (log10 transformed) of these species are distributed in a significantly non-random manner, consistent with community-wide character displacement. Also, observations of aggressive interactions and the response to decoys of three abundant species revealed a competitive hierarchy based on body size. Libellula luctuosa, the largest species, avoided stations with conspecific decoys but was attracted to sta tions with the decoys of two smaller species. L. incesta avoided stations with larger L. luctuosa decoys, but was attracted to stations with smaller Pachydiplax longipennis decoys. P. longipennis avoided stations with conspecific and L. incesta decoys. L. luctuosa was also more successful in displacing perchers (82.4%) than L. incesta (68.9%) and P. longipennis (46.6%). In pair-wise contrasts, the larger species was always more successful at displacing the smaller species. Finally, P. longipennis was attacked at significantly higher rates when it perched on high perches than when it perched at lower perches. We conclude that interspecific competition causes niche partitioning of perch height in this community.