Congruency between adult male dragonflies and their larvae in river systems is relative to spatial grain

Abstract

Dragonflies are globally renowned bioindicators, with larvae, exuviae and/or adult life stages used in freshwater quality assessments. However, little is known about the extent to which conspecific adults and larvae occur within close proximity of each other, or how they comparably respond to biotic and abiotic factors. Firstly, we test the extent to which adult male dragonflies are congruent with their larvae at three independent sample unit scales (small 10 m × 3 m, medium 90 m × 3 m, and large 450 m × 3 m) along four rivers, along with a subset of 40 randomly selected small scale sites (small 40) to test for a possible effect of sampling design on the outcomes of the spatial scale analyses. Secondly, we test the extent to which adult males and larvae share similar responses to environmental variables. At medium and large spatial scales, larvae and adults were strongly congruent for abundance, species richness, and Dragonfly Biotic Index (DBI) scores. Despite this, at the small spatial scale, only 15% of observations matched (contained adults and conspecific larvae). This increased to 46% at the medium scale, and 60% at the large scale, neither of which were significantly different from the number of mismatches. Dragonfly species composition differed between larval and adult assemblages at the small, small 40, and medium scales but did not differ at the large scale. Water temperature was the only variable that generally elicited similar responses in both life stages, at all spatial scales. Exuviae here were so under-represented that they provided no extra information. Assessments, where medium or large spatial scales are suitable for sampling, such as measuring the state of a river, can utilize either life stage. However, for comprehensive biodiversity surveys, both larvae and adults should be sampled.