Horizontal and vertical structuring in the dispersal of adult aquatic insects in a fragmented landscape
Didham, Raphael K.; Blakely, Tanya J.; Ewers, Robert M.; Hitchings, Terry R.; Ward, John B.; Winterbourn, Michael J.
published: Feb 1, 2012
ArtNo. ESP141018001003, Price: 29.00 €
Determining the relative importance of longitudinal dispersal of adult aquatic insects along stream corridors versus their lateral dispersal through upland terrestrial habitats is crucial to understanding the impact of land-use change on ecological and evolutionary processes within streams. However, there is a curious mismatch between trap capture studies, which find low lateral movement rates away from streams, and many population genetic studies, which show low levels of genetic divergence among streams, implying high rates of lateral movement. Here, we take advantage of a serendipitous observation of high relative capture frequencies of adult aquatic insects in the forest canopy, to question whether the flight-height preferences of adults might resolve this apparent 'interbasin dispersal paradox'. In a large-scale study of the effects of habitat fragmentation on invertebrate community structure, 347 passive flight interception traps were deployed at ground level (10,211 trap-days sampling effort) and canopy level (7,595 trap-days) to determine not only the horizontal component of land-use impacts on adult aquatic insects, but also the vertical component of adult movement through the forest canopy in a heavily-fragmented landscape. Two-thirds of adult aquatic insects (Ephemeroptera, Plecoptera, Trichoptera, Megaloptera and Odonata) were captured in the forest canopy, rather than at ground level. Multivariate ordination analysis showed that vertical trapping height and surrounding terrestrial land use had the greatest effects on species composition of dispersing adults, whereas distance to the nearest stream had no significant effect. Of the species that were abundant enough to test statistically, the majority of caddisfly and mayfly species were captured significantly more frequently in the canopy than expected by chance alone, whereas stoneflies were more frequently captured at ground level. These results provide a unique insight into the possible reason why long-distance dispersal of aquatic adults has so rarely been observed. The prosaic explanation may simply be that adults of many species disperse through the forest canopy, and well above ground level.