Dining on disease: how interactions between infection and environment affect predation risk

Despite growing interest in ecological interactions between predators and pathogens, few studies have experimentally examined the consequences of infection for host predation risk or how environmental conditions affect this relationship. Here we combined mesocosm experiments, in situ foraging data, and broad-scale lake surveys to evaluate (1) the effects of chytrid infection (Polycaryum laeve) on susceptibility of Daphnia to fish predators and (2) how environmental characteristics moderate the strength of this interaction. In mesocosms, bluegill preferred infected Daphnia 2-5 times over uninfected individuals. Among infected Daphnia, infection intensity was a positive predictor of predation risk, whereas carapace size and fecundity increased predation on uninfected individuals. Wild-caught yellow perch and bluegill from in situ foraging trials exhibited strong selectivity for infected Daphnia (3-10 times over uninfected individuals). In mesocosms containing water high in dissolved organic carbon (DOC), however, selective predation on infected Daphnia was eliminated. Correspondingly, lakes that supported chytrid infections had higher DOC levels and lower light penetration. Our results emphasize the strength of interactions between parasitism and predation while highlighting the moderating influence of water color. P. laeve increases the conspicuousness and predation risk of Daphnia; as a result, infected Daphnia occur predominantly in environments with characteristics that conceal their elevated visibility.     

Festering food: chytridiomycete pathogen reduces quality of Daphnia host as a food resource

When parasitic infections are severe or highly prevalent among prey, a significant component of the predator's diet may consist of parasitized hosts. However, despite the ubiquity of parasites in most food webs, comparisons of the nutritional quality of prey as a function of infection status are largely absent. We measured the nutritional consequences of chytridiomycete infections in Daphnia, which achieve high prevalence in lake ecosystems (>80%), and tested the hypothesis that Daphnia pulicaria infected with Polycaryum laeve are diminished in food quality relative to uninfected hosts. Compared with uninfected adults, infected individuals were smaller, contained less nitrogen and phosphorus, and were lower in several important fatty acids. Infected zooplankton had significantly shorter carapace lengths (8%) and lower mass (8-20%) than uninfected individuals. Parasitized animals contained significantly less phosphorus (16-18% less by dry mass) and nitrogen (4-6% less) than did healthy individuals. Infected individuals also contained 26-34% less saturated fatty acid and 31-42% less docosahexaenoic acid, an essential fatty acid that is typically low in cladocera, but critical to fish growth. Our results suggest that naturally occurring levels of chytrid infections in D. pulicaria populations reduce the quality of food available to secondary consumers, including planktivorous fishes, with potentially important effects for lake food webs.     

Ecological Engineering Practices for the Reduction of Excess Nitrogen in Human-Influenced Landscapes: A Guide for Watershed Managers

Excess nitrogen (N) in freshwater systems, estuaries, and coastal areas has well-documented deleterious effects on ecosystems. Ecological engineering practices (EEPs) may be effective at decreasing nonpoint source N leaching to surface and groundwater. However, few studies have synthesized current knowledge about the functioning principles, performance, and cost of common EEPs used to mitigate N pollution at the watershed scale. Our review describes seven EEPs known to decrease N to help watershed managers select the most effective techniques from among the following approaches: advanced-treatment septic systems, low-impact development (LID) structures, permeable reactive barriers, treatment wetlands, riparian buffers, artificial lakes and reservoirs, and stream restoration. Our results show a broad range of N-removal effectiveness but suggest that all techniques could be optimized for N removal by promoting and sustaining conditions conducive to biological transformations (e.g., denitrification). Generally, N-removal efficiency is particularly affected by hydraulic residence time, organic carbon availability, and establishment of anaerobic conditions. There remains a critical need for systematic empirical studies documenting N-removal efficiency among EEPs and potential environmental and economic tradeoffs associated with the widespread use of these techniques. Under current trajectories of N inputs, land use, and climate change, ecological engineering alone may be insufficient to manage N in many watersheds, suggesting that N-pollution source prevention remains a critical need. Improved understanding of N-removal effectiveness and modeling efforts will be critical in building decision support tools to help guide the selection and application of best EEPs for N management.     

Instream Large Wood: Denitrification Hotspots with Low N2O Production

We examined the effect of instream large wood on denitrification capacity in two contrasting, lower order streams — one that drains an agricultural watershed with no riparian forest and minimal stores of instream large wood and another that drains a forested watershed with an extensive riparian forest and abundant instream large wood. We incubated two types of wood substrates (fresh wood blocks and extant streambed wood) and an artificial stone substrate for nine weeks in each stream. After in situ incubation, we collected the substrates and their attached biofilms and established laboratory‐based mesocosm assays with stream water amended with ¹⁵N‐labeled nitrate‐N. Wood substrates at the forested site had significantly higher denitrification than wood substrates from the agricultural site and artificial stone substrates from either site. Nitrate‐N removal rates were markedly higher on woody substrates compared to artificial stones at both sites. Nitrate‐N removal rates were significantly correlated with biofilm biomass. Denitrification capacity accounted for only a portion of nitrate‐N removal observed within the mesocosms in both the wood controls and instream substrates. N₂ accounted for 99.7% of total denitrification. Restoration practices that generate large wood in streams should be encouraged for N removal and do not appear to generate high risks of instream N₂O generation.     

Effects of species diversity on community biomass production change over the course of succession

Over the past decade an increasing number of studies have experimentally manipulated the number of species in a community and examined how this alters the aggregate production of species biomass. Many of these studies have shown that the effects of richness on biomass change through time, but we have limited understanding of the mechanisms that produce these dynamic trends. Here we report the results of an experiment in which we manipulated the richness of freshwater algae in laboratory microcosms. We used two experimental designs (additive and substitutive) that make different assumptions about how patches are initially colonized, and then tracked the development of community biomass from the point of initial colonization through a period of 6-12 generations of the focal species. We found that the effect of initial species richness on biomass production qualitatively shifted twice over the course of the experiment. The first shift occurred as species transitioned from density-independent to dependent phases of population growth. At this time, intraspecific competition caused monocultures to approach their respective carrying capacities more slowly than polycultures. As a consequence, species tended to over-yield for a brief time, generating a positive, but transient effect of diversity on community biomass. The second shift occurred as communities approached carrying capacity. At this time, strong interspecific interactions caused biomass to be dominated by the competitively superior species in polycultures. As this species had the lowest carrying capacity, a negative effect of diversity on biomass resulted in late succession. Although these two shifts produced dynamics that appeared complex, we show that the patterns can be fit to a simple Lotka-Volterra model of competition. Our results suggest that the effects of algal diversity on primary production change in a predictable sequence through successional time.     

Landowner Satisfaction with the Wetlands Reserve Program in Wisconsin

We evaluated ecological monitoring data and landowner perceptions to the federally funded Wetlands Reserve Program (WRP) in a three-county region in Wisconsin. We surveyed landowner satisfaction, involvement, participation, and use of the WRP restoration sites. We found that landowners are satisfied with the overall program (mean, 3.6 ± 0.2 [SE], on a scale of 1–5, with 5 being completely satisfied). WRP restorations significantly increased the area of wetland within the sites surveyed, the increase was primarily of fresh meadow (736.32 ha after restoration). Satisfaction is related to landowner participation during restoration and to the economic incentives provided by the WRP, Landowner satisfaction and the number of plant communities after restoration are unrelated to each other or to restoration and easement costs per hectare. Survey participants recommended some changes to the WRP, including a reduction in the tax rate of land enrolled in the WRP, approval for permanent deer stands, and increased communication with WRP officials during the restoration. Monitoring information collected for WRP restoration sites does not allow assessment of whether WRP sites are functionally equivalent to natural sites. We suggest that the WRP require a more rigorous monitoring program, including guidelines for invasive species control. Managers should also encourage collaborations with external researchers and consider restorations within an experimental framework.