Consumer Influence on Ecosystem Structure and Function II
Presiding: K Bertrand, Kansas State University; B W Taylor, University of Wyoming
NB22A-01 INVITED 10:30h
Structural and Functional Effects of Southern Redbelly Dace (Phoxinus erythrogaster)
We used a combination of field and mesocosm experiments to measure the effects of southern redbelly dace (Phoxinus erythrogaster) on stream ecosystem structure and function. Ecosystem structure was quantified by algal filament length, algal biomass, size distribution of benthic organic matter, algal community composition, and invertebrate community composition. Ecosystem function was based on gross primary productivity (GPP). Throughout the experiments, dace changed ecosystem structure but not ecosystem function. The presence of Phoxinus resulted in reduced algal filament length and reduction in the mean size of benthic particulate organic matter (BPOM). Studies of other grazing minnows (e.g., central stoneroller Campostoma anomalum) also reported structural effects on stream ecosystems (i.e., reduced algal biomass, filament length, and mean BPOM size fraction) as well as effects on ecosystem function (i.e., net primary productivity). Although interspecific variation among grazing minnows may cause differences in ecosystem effects, the experimental context, including methodology, also can strongly affect the magnitude and nature of observed responses.
NB22A-02 INVITED 10:45h
Effects of Stream Fishes on Benthic Primary Productivity: A Mechanistic Test
I simultaneously tested three alternative hypotheses (the trophic cascade, nutrient enhancement via terrestrial nutrient translocation, and nutrient enhancement via bioturbation) for consumer regulation of primary productivity (PPR) by three widely distributed stream fish species (Orangethroat Darter, Western Mosquitofish, and Bullhead Minnow). I used stream mesocosms fitted with fish and terrestrial input barriers to address relative importance of localized fish predation versus access to terrestrial inputs for fish consumer effects. Orangethroat Darter, a benthic invertivore, increased PPR through an apparent trophic cascade, by localized reduction of benthic grazing invertebrate densities. Western mosquitofish, a surface feeding insectivore, increased PPR by enhancing nutrients through terrestrial nutrient translocation, and had no effect on benthic grazer invertebrate density. Bullhead Minnow, a benthic omnivore that disturbed sediments during foraging, increased PPR through nutrient enhancement via bioturbation, but within specific stream mesocosm areas two which the fish was restricted it also reduced benthic grazing invertebrates. Thus, suggesting this species may have affected PPR through a combination bioturbation and trophic cascade mechanisms. These mechanistic pathways are likely common processes by which fish affect food web structure and ecosystem function in many stream ecosystems.
NB22A-03 11:00h
Top-Down and Bottom-Up Forces in an Appalachian Stream: Can Consumers Differentially Mediate Effects of Nutrient Enrichment on Algae?
The effect of nutrient loading on the biotic integrity of streams is of growing concern, given increased anthropogenic inputs from agriculture and urbanization. However, few studies have addressed the potential of top-down forces, both direct and indirect, to regulate increases in primary production that often result from nutrient enrichment. In this study, we examined the effect of nutrients on algal standing crop and community composition in response to both snails (Elimia proxima) and macroconsumers (fish and crayfish), in a 23 factorial design. Insectivorous sculpin (Cottus bairdi), algivorous stonerollers (Campostoma anomalum) and omnivorous crayfish (Cambarus spp.) dominate the macroconsumer community in Betty's Creek, an Appalachian stream of northern Georgia. Snails had the strongest regulatory impact on nutrient-stimulated algae, both with and without macroconsumer predators. In contrast, stonerollers and crayfish were not able to directly suppress increased algal standing crop. However, reductions of invertebrate biomass by both sculpin and crayfish indirectly increased algae, regardless of nutrient addition. These results provide evidence for the existence of weak trophic cascades in lotic systems, even in complex food webs where consumers feed at multiple trophic levels. This work also documents the importance of grazer identity in the mediation of nutrient effects on algae.
NB22A-04 11:15h
Impacts of Invasive Rusty Crayfish on Stream Ecosystems of the Upper Midwestern U.S.
Invasive species can have detrimental effects on structural characteristics of freshwater ecosystems, but relatively few studies have assessed ecosystem-level impacts of invasive species in streams. We studied the effects of invasive rusty crayfish (Orconectes rusticus) on detritus processing and invertebrate and fish abundance in northern Wisconsin and Michigan, USA, streams. We hypothesized that rusty crayfish would increase the rate of detritus processing and reduce fish and invertebrate abundance due to their aggressiveness and competitive superiority for food and habitat. We measured sugar maple (Acer saccharum) decomposition rates in three reaches of a stream with differing densities of rusty crayfish, high (5.05/m2), intermediate (2.27/m2), and none (0/m2) using leaf bags excluding crayfish and open bags allowing crayfish access. We found that open bags decayed significantly faster (k=0.143) than crayfish excluded bags at all sites (k=0.079) (p=0.0005). The reach lacking crayfish had significantly higher densities of invertebrates (p=0.005). We also surveyed an additional 7 streams that contained or lacked rusty crayfish and found significantly higher fish abundance (p=0.019) and biomass (p=0.001) in streams lacking rusty crayfish. Rusty crayfish appear to indirectly affect detritus processing via negative effects on benthic invertebrates, and may have larger-scale impacts on fishes across streams.
NB22A-05 11:30h
Impact of Invasive Fish on Nitrogen Transport to Adjacent Stream Habitats
Consumers can alter in-situ ecosystem processes, but less is known about their impacts on adjacent habitats. Animals may influence ecosystem dynamics in adjacent areas, especially if energy is exchanged between systems. Lake trout (Salvelinus namaycush) were introduced to Yellowstone Lake and are reducing the native Yellowstone cutthroat trout (Oncorhynchus clarki bouvieri) through predation. Both trout live in Yellowstone Lake, but only cutthroat spawn in and transport nutrients to streams. To estimate the indirect impact of lake trout on nutrient cycling in adjacent streams, we estimated how much nitrogen cutthroat transport. Spawners supplied 100 mg NH4-N m-2d-1 prior to lake trout when 50,000 cutthroat spawned, but only 1.8 mg NH4-N m-2d-1 to tributary streams in 2004 when 1,400 fish spawned. In contrast, cutthroat excreted 5.9 mg NH4-N m-2d-1 in Yellowstone Lake prior to lake trout and only 1.9 mg NH4-N m-2d-1 currently. Given a 90 day spawning period, tributary streams are more impacted (8,838 mg NH4-N m-2yr-1) than Yellowstone Lake (1,460 mg NH4-N m-2yr-1). Consumers can have large impacts not only in their primary habitat, but also in neighboring areas. Less transported nitrogen could reduce stream primary and secondary production, and cutthroat recruitment through time.
NB22A-06 11:45h
Does functional redundancy exist?
Human activities are triggering rates of species extinction not seen since prior mass extinctions, and loss of biological diversity now ranks among the most prominent changes to the global environment. It has been argued that reductions in diversity could alter how efficiently biologically limiting resources are captured by communities and converted into new biomass. Although this prediction is derived from one of the most fundamental tenets of species coexistence (niche theory), empirical evidence has been mixed. In fact, many studies suggest that a large fraction of co-occurring species are functionally redundant, and their absence from experimentally assembled communities does not lead to any notable change in the fluxes of energy or matter that underlie ecosystem processes. Can these contrasting theoretical and empirical perspectives be resolved? Here I discuss the mismatch in scale that has led to differing conclusions of theory and empiricism, as well as the opposing forces of coexistence that do verses do not allow species to be functionally redundant. I conclude with a call for research among aquatic ecologists who study systems that may very well hold the key insights needed to overcome the major hurdles currently faced in this area of research.