Dispersal traits may reflect dispersal distances, but dispersers may not connect populations demographically
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Ecological traits that reflect movement potential are often used as proxies for measured dispersal distances. Whether such traits reflect actual dispersal is often untested. Such tests are important because maximum dispersal distances may not be achieved and many dispersal events may be unsuccessful (without reproduction). For insects, many habitat patches harbour ‘resident’ species that are present as larvae (sedentary) and adults (winged and dispersing), and ‘itinerant’ species present only as adults that have dispersed from elsewhere and fail to reproduce. We tested whether itinerancy patterns were temporally consistent, and whether itinerant and resident species differed in wing morphology, a strong correlate of flight capability. Over 3 years and at multiple locations in a 22 km stream length, we sampled larvae and adults of caddisflies in the genus Ecnomus to categorize species as residents or itinerants. Flight capacity was measured using wing size (length and area) and shape parameters (aspect ratio and the second moment of wing area). Three species of Ecnomus were residents and three species were itinerants, and patterns were consistent over 3 years. On average, itinerant species had larger wings, suggesting a greater capacity to fly long distances. Wing shape differed between species, but did not differ systematically between residents and itinerants. Wing morphology was associated with actual but not effective dispersal of some species of Ecnomus. Morphological traits may have weak explanatory power for hypotheses regarding the demographic connectedness of populations, unless accompanied by data demonstrating which dispersers contribute new individuals to populations.
KeywordsAspect ratio Ecnomidae Insect flight Moment of area Trichoptera Wing morphology
We thank the many people who helped with field collections at various times, including Claire Allison, Wim Bovill, Alena Glaister, Steve Horn, Ashley Macqueen, Bobbi Peckarsky, Jared Polkinghorne, Bob Smith and Allyson Yarra. We are deeply indebted to Wim Bovill and Alena Glaister for their stellar assistance with identifications. This project was supported by a Discovery Grant from the Australian Research Council (DP120103145) awarded to JL and BJD. Adult sampling in 2015 was carried out in conjunction with an NSF Postdoctoral Extension awarded to R. Smith and hosted by the University of Melbourne.
Author contribution statement
JL and BJD collected samples; JL measured wings; JL and BJD analysed the data and wrote the manuscript.
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