Document: KAT-3-71-8

Stable isotopes provide evidence of dispersal by an acid-tolerant stonefly (Plecoptera) .

MACNEALE, K.H.* ^1,2, B.L.PECKARSKY ¹ and G.E.LIKENS ²

Cornell University, Ithaca, NY 14853 ¹
Institute of Ecosystem Studies, Millbrook, NY 12545 ²

Abstract:
Quantifying the mean and extreme distances traveled and the direction of dispersal by organisms is critical for estimating effective population size and for predicting potential for colonization events. We used an isotopic tracer, ¹⁵N, to study the dispersal of individual adult stoneflies, Leuctra ferruginea (Leuctridae), whose larvae live in streams. ¹⁵N was added continuously for four to six weeks to two streams within the Hubbard Brook Experimental Forest, NH, in 1997, 1998, and 1999, and collections were made within 600 m upstream and downstream of the addition site and in adjacent watersheds. Comparison of the initial distribution of labeled stoneflies caught emerging from the stream reaches enriched with ¹⁵N to that of stoneflies caught subsequently in flight showed that prior to feeding as adults, males and females flew less than ~30 m from their site of emergence. Females that had fed extensively and returned to the stream to oviposit flew on average ~109 m upstream, with the maximum dispersal distance being 330 m upstream. These data also suggest that colonization by adults of nearby streams in other watersheds may be possible, as we found adults to fly as far as 250 m laterally from the stream into the hardwood forest. The colonization cycle hypothesis proposes that upstream flight of adult stream insects may be necessary to maintain population densities if larvae drift downstream. Leuctra ferruginea is most abundant throughout the Northeast in small, acidic headwater streams, but drifts infrequently (0.02 individuals/1000L discharge, or 0.04 individuals/hr). Potential selection pressures for upstream flight in species that do not drift in high densities may include greater competitor and/or predator densities, or unfavorable abiotic conditions in downstream habitats.

Keywords: dispersal, stable isotopes, 15N, aquatic insects

This abstract is being presented at: 11:00 AM in session:
Oral Session #55: Invertebrates in Streams: Foodwebs.