Lateral dispersal of benthic invertebrates from sub-arctic lakes to land

Background/Question/Methods

Dispersal is important both for structuring communities and for determining the strength of cross-ecosystem subsidies. Unlike many freshwater taxa, aquatic insects with a terrestrial, adult life stage have the opportunity to colonize new lakes/streams. The goal of this project is to determine the extent of inland dispersal of a variety of aquatic insect taxa and examine implications both for community structure across the landscape and for aquatic subsidies onto land. We conducted the study during July 2012 in Stordalen myre in Sweden (68°21’ N, 19°03’ E). We set sticky traps (30X30 cm placed 1.3 m above ground) and deposition traps (clear plastic boxes 18X18X11 cm placed on the ground with soapy water) along 4 transects perpendicular to shore from 3 small lakes. Traps were set at 0, 5, 10, 50, 100, and 200 m from shore. On the fourth day, traps were retrieved, and insects were identified and counted. We used ANCOVA to test for declines in insect abundance with increasing distance from shore while also blocking by lake. We also compared the efficiency of deposition vs. sticky traps.

Results/Conclusions

The abundance of Trichoptera declined with increasing distance from the lake shore in both sticky traps and deposition traps. However, the rate of decline was much greater in deposition traps; abundance in the 200 m deposition traps was 3% of that near shore, but was 89% in the 200 m sticky traps. Chironomid abundance significantly declined in deposition traps (3% at 200 m traps relative to shore), but not in sticky traps (97% at 200 m traps relative to shore). In deposition traps, the abundance of Ceratopogonidae and Limoniidae significantly declined with increasing distance. Other taxa were not abundant (<5 individuals/trap) in either trap type and did not significantly change with distance: Plecoptera, Ephemeroptera, Megaloptera, Tipulidae, and Culicidae. Sticky traps were more effective than deposition traps at capturing all taxa except Chironomidae. For these taxa, the different patterns observed in sticky traps vs. deposition traps indicate that fewer individuals land on the ground far from shore, but insects flying in the air are still abundant 200 m from shore. Most lakes in the Stordalen myre are within a 200 m distance of at least one more lake, indicating that populations between lakes may have the ability to mix.