Why Are Daphnia in Some Lakes Sicker? Disease Ecology, Habitat Structure, and the Plankton

Abstract

Some aspects of habitat seem to enhance the spread of disease whereas others inhibit it. Here, we illustrate and identify mechanisms that connect habitat to epidemiology using a case study of disease in plankton. We see a pronounced relationship between the basin shapes of lakes and fungal (Metschnikowia bicuspidata) disease in the zooplankton grazer Daphnia dentifera. As we work through seven mechanisms that could explain why Daphnia in some lakes are sicker, we can eliminate some hypotheses (i.e., those relating an index of lake productivity to disease through host density, links between resource quality and transmission rate, and variation in host susceptibility) and find support for others involving food-web actors (e.g., selective predation on infected hosts by fishes, "sloppy predation" by an invertebrate, a possible dilution effect in V-shaped lakes). Furthermore, we identify physical mechanisms (gravity currents, turbulence) that could lead to greater transport of fungal spores to habitat occupied by Daphnia hosts in U-shaped lakes. These results highlight how habitat structure, through its effects on food-web structure and physical processes, can shape wildlife disease.

Epidemic size determines population-level effects of fungal parasites on Daphnia hosts

“…Finally, our results illustrate that endemic parasites can be an important but variable factor shaping host populations Lafferty et al 2006Lafferty et al , 2008Hall et al 2010). In our case, this endemic fungal parasite altered host densities during some lake-years-once epidemics became large enough.…”

“…We hypothesize that variation in the start date of epidemics involves interplay between physical processes and species interactions. An increased intensity of mixing of lake waters, induced by storms and convection driven by cooling, can overcome physical barriers imposed by stratification to connect fungal spores to hosts (Bittner et al 2002;Johnson et al 2009;Hall et al 2010;Smyth 2010). Once potentially initiated by physical processes, food web structure then influences how epidemics proceed.…”

“…To determine the frequency and magnitude of disease, we monitored epidemics of Metschnikowia in nine populations of D. dentifera (Barry and Kalamazoo Counties, MI) from 2002 to 2008 Hall et al 2010;Duffy et al 2010). Every 2 weeks during August through October, we collected six bottom-to-surface zooplankton tows with a 153-lm Wisconsin bucket net.…”

“…In laboratory and mesocosm experiments, these parasites have been observed to depress Daphnia densities (Ebert et al 2000;Bittner et al 2002;Duffy 2007). Yet, in natural populations, the severity of microparasite epidemics in Daphnia varies considerably in space and time (Bittner et al 2002;Decaestecker et al 2005;Ebert 2005;Johnson et al 2006Johnson et al , 2009Hall et al 2010). For example, epidemics of a chytrid fungus range interannually from 1 to 34% peak prevalence (Johnson et al 2009).…”

Habitat, predators, and hosts regulate disease in Daphnia through direct and indirect pathways

“…First, selective predators (bluegill sunfish [Lepomis macrochirus]) selectively target and cull infected hosts, reducing prevalence and density of infections (Packer et al 2003, Hall et al 2005; the "healthy herds" hypothesis). Thus, high fish predation lowers infection prevalence of focal hosts (Hall et al 2005(Hall et al , 2010. Fish then consume parasites along with infected hosts ("concomitant predation"; see , resulting in a net loss of fungal spores.…”