Are community patterns in flight height driven by antagonistic interactions?


Large libellulid dragonflies often fly higher than smaller libellulids. We hypothesized that this size-related pattern in flight height might be caused by aggressive displacement. We tested this hypothesis by using a 30 m “dragonfly zip-line” to reel dead dragonfly decoys of four species of different sizes (Erythemis simplicicollis, Libellula incesta, Pachydiplax longipennis, and Perithemis tenera) along a shoreline at four different heights (20, 60, 100, and 140 cm), counting the number of investigations made by large patrolling Libellula incesta and Libellula luctuosa males. We hypothesized that decoys of smaller species would be investigated and attacked at higher frequency when they were reeled high, in the Libellula flight zone, than when they were reeled at their natural low height. This hypothesis was falsified; both L. incesta and L. luctuosa investigated high-flying decoys significantly less frequently than low-flying decoys. Perithemis tenera decoys were investigated less frequently than other decoys by both species, but L. incesta investigated E. simplicicollis, P. longipennis, and L. incesta decoys with increasing frequency whereas L. luctuosa investigated these three species at equal rates. These patterns correlate with the degree of morphological similarity between patrolling species and decoys, consistent with likely patterns of “mistaken identity” by patrolling L. incesta and L. luctuosa males. We suggest that patrolling males may preferentially investigate other low-flying males in the hopes of finding a mate-guarded female.

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