Abstract
Nutrient pollution has the potential to alter many ecological interactions, including host–parasite relationships. One of the largest sources of nutrient pollution comes from anthropogenic alteration of the nitrogen (N) cycle, specifically the increased rate of nitrate (NO3-N) deposition to aquatic environments, potentially altering host–parasite relationships. This study aimed to assess the mechanisms through which nitrate may impact host–pathogen relationships using a fungal pathogen (Metschnikowia bicuspidata) parasitic to crustacean zooplankton (Daphnia dentifera) as a tractable model system. First, the influence of nitrate on host population dynamics was assessed along a gradient of nitrate concentrations. Nitrate decreased host population size and increased infection prevalence. Second, the influence of nitrate on host reproduction, mortality, and infection intensity was assessed at the individual host level by examining the relationship between pathogen dose and infection prevalence at ambient (0.4 mg NO3-N*L−1) and intermediate (12 mg NO3-N*L−1) levels of nitrate. Host fecundity and infection intensity both decreased with increasing pathogen dose, but increased nitrate levels corresponded to greater infection intensities. Nitrate had no effect on host growth rate, suggesting that hosts do not alter feeding behavior in nitrate-treated media compared with ambient conditions. This study suggests that nutrient enrichment may enhance disease through increased transmission and infection intensity, but that high levels of nitrate may result in smaller epidemics through reduced transmission caused by smaller population sizes and increased pathogen mortality.
Highlights
Parasites are a structuring force to host populations (Anderson and May 1978; Townsend et al 2009) and communities (Wood et al 2007), with the potential to influence ecosystem level processes (Hatcher et al 2012) and ecological interactions (Refardt and Dieter 2012)
Ecology and Evolution published by John Wiley & Sons Ltd
Nitrate reduced final host population size (Fig. 2) from an average of 11 individuals at ambient conditions to approximately 5 individuals when exposed to increased nitrate concentrations
Summary
Parasites are a structuring force to host populations (Anderson and May 1978; Townsend et al 2009) and communities (Wood et al 2007), with the potential to influence ecosystem level processes (Hatcher et al 2012) and ecological interactions (Refardt and Dieter 2012). Nitrate pollution is responsible for changes in animal community composition (Smith 2003), plant productivity (Rabalais et al 2002), and pH of streams and lakes (Fenn et al 2003; Sutton et al 2011). While many of these effects are a result of increased phytoplankton growth, nitrate toxicity directly influences animal populations and communities (Johnson and Carpenter 2008; Johnson et al 2010).
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