Biotic Interactions I
Presiding: A Tomba, Auburn University; R Schultheis, University of Georgia
NB32E-01 10:30h
Top-down Control of Stream Food Webs: Indirect Effects by Changed Behaviour and Species Interactions
Predators may directly control stream food webs by consuming invertebrates. But sub lethal effects on prey such as change of activity rhythm or feeding behaviour may lead to indirect effects of predation on other species. Thus, predators may strongly effect invertebrate community structure. The aim of a currently running paired ecosystem experiment is to detect changes of species interaction induced by benthivorous gudgeon (Gobio gobio). For this purpose we link the measurement of physiological fitness parameters to the observation of behavioural changes. Preliminary studies indicated a top-down control of the drift activity of Baetis larvae, while a bottom-up effect could not be observed. The presence of benthivorous gudgeon led to a significantly changed species composition of the invertebrate drift and reduced drift activity of Baetis larvae compared to the fish free control. The diurnal drift pattern of Baetis larvae with a nocturnal peak was observed both in the control and fish reaches. Thus benthivorous gudgeon controls the drift behaviour in a similar way as known for drift-feeding trout. The content of triglycerides and glycogen did not differ between the drifting and not-drifting individuals. Therefore their energetic status does not seem to control drift the activity of Baetis larvae.
NB32E-02 10:45h
Top-Down Control of Stream Food Webs: How to Detect a Predator's Impact on Physiological Fitness of Macroinvertebrates
Before prey is extinguished by its predator physiological stress increases. This is true for a single individual as well as on a population level. We prove this assumption for the first time in a field experiment. It is designed as a paired ecosystem study of two streams with benthivorous fish as predators. So far, top-down manipulation is well established in lentic habitats in order to improve water quality. However, there is hardly any physiological approach to be found within former projects. Behavioral changes to avoid predator encounters are well known concepts, nevertheless every organism is obliged to obtain food and energy for growth and reproduction, they cannot totally avoid their predators. Increased stress during fight or flight reactions will change the energy charge inside the cells (nucleotide composition). Certain metabolites like phosphagens will decrease while others like lactate may accumulate. On a long time scale increased stress will result in lower energy storage, mainly detectable as lower glycogen and triglyceride content compared to individuals without high predation risks. Together with the determination of species biomasses and abundances it should be possible to develop a comprehensive impression of sub lethal effects within the invertebrate community. (supported by German Research Foundation)
NB32E-03 11:00h
The Influence of Paiute Sculpin (Cottus beldingi) on Trophic Dynamics in a Pacific Northwest Stream
Sculpins are abundant and are the most common predator in many Pacific Northwest streams. We tested the hypothesis that Paiute sculpin (Cottus beldingi) have strong effects on trophic dynamics in Taenum Creek, Washington. Sculpin density was manipulated using 1-m2 enclosure/exclosure cages during two 32-day experiments in July and September of 2004. The biomass of periphyton on unglazed ceramic tiles (50-cm2) and benthic macroinvertebrate community composition and abundance were measured at high density, low density, fish exclusion, and control treatments. Controls consisted of cages that were open on the downstream end. Treatments were replicated once each in six separate riffles. Periphyton biomass (chlorophyll b and ash-free-dry-mass) was significantly different between experiments but did not differ among treatments in either experiment. These results suggest that the effects of Paiute sculpins on macroinvertebrates do not cascade to primary producers. Analysis of benthic samples and stomach contents will help to determine whether sculpins affect macroinvertebrates indirectly or directly through predation.
NB32E-04 11:15h
Interactions Between Trout and Sculpin: Consequences of Trout Introductions for a Native Fish
We examined interactions between a native (brook) and a nonnative (brown) trout with native slimy sculpin, to test whether introductions of a nonnative salmonid has an effect on growth and diet of sculpin where a native trout was present. Enclosures (1m2) were stocked with fish (five treatments: juvenile brown trout with sculpin, juvenile brook trout with sculpin, and single species controls) at three densities. Replicates of each treatment were placed in Valley Creek, MN, and fish growth rates were measured over six 38-day experiments, conducted over three years. We examined growth of each species in combined-species treatments versus growth of each species alone. We did not find evidence of competition between brook trout and sculpin, regardless of density or fish size. However, we observed competition between brown trout and sculpin, which depended on initial mass of each species. Sculpin gained greater mass when alone than with brown trout when sculpin size was large (>17g). Likewise, brown trout gained greater mass alone than with sculpin when brown trout size was large (>26g). We suggest that differences in morphology and foraging modes between the two native species allow them to coexist without competition, whereas nonnative brown trout compete with sculpin.
NB32E-05 11:30h
How parasitism, stream substrate, and movement patterns mediate response to disturbance in the snail Elimia flava
Snails in the genus Elimia are abundant in southeastern USA streams, and also serve as intermediate hosts to parasitic trematodes. Previous work indicated that high-flows decrease snail abundance and trematode prevalence, and others have shown substrate type and snail size affect likelihood of snail dislodgement. To investigate how parasitism, size, substrate, and snail behavior influenced dislodgement, we placed Elimia flava in artificial streams containing tile or gravel substrates, and then exposed them to progressively increasing flow velocities ( ~10, 40, 90 cm/s) for 5 minutes each. We recorded snail behavior and time to dislodgement, and then preserved snails to quantify their size and parasite load. Snails on tile dislodged significantly faster than snails on gravel, and snails with high parasite loads also dislodged faster than snails without parasites. Parasitism also appeared to affect movement patterns: snails showing predominantly downstream movement had higher parasite loads than those that did not. Behavior also affected dislodgement probability, as snails moving upstream or to the waterline remained on the substrate longer than snails not showing those behaviors. Parasitism, substrate composition, and snail movement are useful predictors of the likelihood of dislodgement, and parasitism and substrate may both increase snail vulnerability to flow disturbance.