Biotic Interactions II
Presiding: A Tomba, Auburn University; R Schultheis, University of Georgia
NB44D-01 15:30h
Effects of Lake Trout (Salvelinus namaychush) on Arctic Char (S. Alpinus) Populations in Arctic Alaska
Lake trout (Salvelinus namaychush) and arctic char (S. Alpinus) typically are not found coexisting in arctic Alaskan lakes. Lake trout and artic char are generally top predators in lakes where they do not coexist. During the summer of 2004, I examined the effects of lake trout on dietary and habitat selection of arctic char near the Toolik Lake Field Station in arctic Alaska. I examined lakes containing either lake trout or arctic char as the top predator, and lakes containing both lake trout and arctic char. Natural abundace of Δ 15N and Δ 13C of seston, zooplankton, macroinvertebrates, and fish were used to construct food webs to examine any dietary and habitat shifts of arctic char and lake trout when they coexist. Lakes differed in 13C signature of basal resources, but fish tracked benthic 13C in all cases. However, lake trout were more enriched in 15N than arctic char, suggesting greater piscivory. These results strongly suggest that interspecific interactions are not very important in determining the trophic position of top predators in arctic lakes.
NB44D-02 15:45h
Emerging aquatic insects affect riparian spider distribution and growth rates in a temperate rainforest
Emerging aquatic insects from streams provide a temporally shifting, alternative source of energy to riparian web-building spiders. The effects of dynamics in aquatic insect emergence on spider distributions are poorly understood. We manipulated the abundance of aquatic insects in riparian forests of British Columbia by excluding aquatic insects using a greenhouse type covering from May through the end of July. In the absence of manipulations, aquatic insect abundance generally peaks in July. The overall density of riparian spiders was reduced when aquatic insects were excluded in May and July but not in June. As in similar studies, tetragnathid spiders in particular showed a strong response to aquatic insect exclusion. The ideal free distribution predicts that organisms at low densities should have equal access to resources for growth to those at high densities. Using comparisons of body size low and high densities of animals we determined that tetragnathid abundance and growth patterns do represent an ideal free distribution.
NB44D-03 16:00h
Distribution of Brook Trout and Their Food Sources in Meadow vs. Wooded Areas of Sierra Nevada Headwater Streams
Stocked eastern brook trout are now well established in historically fishless, small headwater streams in the Sierra Nevada. Although non-native, brook trout have maintained healthy populations since stocking ended 60-80 years ago. Our primary research question was whether brook trout distribution and feeding ecology is influenced by variation in headwater stream habitats and food sources. Stream habitat characteristics and trout demographic data were collected during June and August 2004 from four forested and three meadow sites among five tributaries to Bull Creek in the Sierra Nevada. Both mean fish mass and total fish biomass were greater in forested versus meadow reaches. Macroinvertebrate drift rate did not differ between meadow versus wooded reaches, but were greater in June than August. However, despite higher fish biomass, trout in forests apparently selected prey from drift, whereas trout diets in meadows reflected availability in drift. The results of this research will ultimately be used in a larger, collaborative, whole-ecosystem study conducted by the USDA-Forest Service addressing how current forest management practices affect stream ecosystems.
NB44D-04 16:15h
Interactions of Amphibians, Fish, and Macroinvertebrates in a Southeastern Wetland
In fishless habitats, amphibians often compete with and are predators of macroinvertebrates. Unlike fish, the effects these interactions have on macroinvertebrate communities have been largely unexplored. We conducted an experiment in a semi-permanent oxbow wetland in the Piedmont region of Georgia to explore interactions between amphibians and macroinvertebrates. The predator community was dominated by Ambystoma opacum (Marbled Salamander) and Notophthalmus viridescens (Eastern Newt). Salamanders and newts were excluded from areas of wetland habitat using wire mesh cages (1.5M x 1.5M, 3mm mesh). The macroinvertebrate communities within the cages were then compared to the ambient habitat outside the cages. Fish, mostly Lepomis macrochirus (Bluegill) and Gambusia affinis (Mosquito Fish), colonized the wetland late in the first year of the study, and became common by year two. Also in year two, Rana catesbeiana (Bullfrog) became established. Thus, we were able to explore the variable effects on the macroinvertebrate community of a changing predator complex over a two year period.
NB44D-05 16:30h
Complex Interactions Among Detritivorous Insects Inhabiting Leaf Packs in a North Carolina Stream
Large detritivorous insects may influence the distribution and abundance of other invertebrates that inhabit leaf packs. In a previous study we observed a complementary distribution between the caddisfly Pycnopsyche and ephemerellid and leptophlebiid mayflies in a headwater stream. Initially, we hypothesized that encounter competition between these organisms was the mechanism responsible for this distribution. When presented with a choice of leaf packs in an enclosure, one of which contained Pycnopsyche, neither ephemerellids nor leptophebiids exhibited significant avoidance of leaf packs inhabited by Pycnopsyche. However, a significant number of mayflies from both families were not recovered suggesting that intraguild predation was occurring. A preliminary stable isotope analysis does not support the intraguild predation hypothesis. In a field experiment we examined invertebrate colonization of leaf packs on which we placed 0, 3, and 6 Pycnopsyche. We hypothesized that leaf packs with no Pycnopsyche would have higher numbers of colonizing invertebrates (particularly mayflies) than leaf packs with Pycnopsyche. Results of this study showed that some taxa increased in the presence of Pycnopsyche while other taxa declined. Our results suggest that Pycnopsyche may influence the distribution and abundance of other invertebrate taxa through various mechanisms.