Dragonfly population densities across three life stages: Oviposition site selection overrides post-recruitment spatial variation

Background/Question/Methods

Many animals have life stages that occupy different habitats.  Although population demographics in one stage can carry over to subsequent – even spatially separated – life stages, most studies of habitat associations have been restricted to a single life stage.  Understanding the relative influence of different life stages on population demographics is critical for predicting how modifications to either habitat could affect animals with spatially separated life stages.  Dragonflies in the Gomphidae family (Odonata: Anisoptera) require both aquatic and terrestrial habitats for different life stages, but the relative importance of each habitat remains unclear.  As with many animals, dispersal occurs during the more mobile adult dragonfly stage, whereas survivorship during the more vulnerable juvenile stage limits population sizes.  Although recruitment via adult oviposition establishes initial population sizes of the aquatic larvae, spatial variability in larval survivorship could override the expected correlation between adult and larval densities.  I used surveys conducted during summers 2005 and 2006 of Gomphidae larval, emergence, and adult stages from 22 lake sites in northern Wisconsin, USA, to investigate (1) whether the density of each life stage correlated with that of the preceding life stage and (2) what habitat factors help explain variation in densities at each life stage.
Results/Conclusions

I found that adult densities from the previous season helped predict densities of early-instar larvae, which suggests that oviposition site selection controlled the local larval distribution more than survivorship or movement during the first year of the larval phase.  Late-instar larval densities helped predict densities of exuviae that emerged later the same season, suggesting that variation in survirorship of final-instar larvae among sites is small relative to the variation in larval recruitment. Emergence densities did not help predict adult densities from the same year.  In addition to the densities of Odonates in previous life stages, water clarity helped predict larval densities, and riparian wetland vegetation helped predict emergent exuvia densities.  Habitat parameters only weakly correlated with adult densities.  This study demonstrates that population dynamics of one life stage carry over to spatially separated life stages and subsequent generations.