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

Abstract

Parasites frequently reduce the fecundity, growth, and survival of individual hosts. How often do these virulent effects reduce the density of host populations? Spectacular examples show that recently invaded parasites can severely impact host populations--but what about parasites persisting long-term in host populations? We have addressed this issue using a zooplankton host (Daphnia dentifera) that becomes infected with a fungal microparasite (Metschnikowia bicuspidata). We combined observations of epidemics in nine lakes over 6 years, fine-scale sampling of three epidemics, and a mesocosm experiment. Most epidemics remained small (<10% maximum prevalence) and exerted little influence on host densities. However, larger epidemics more severely depressed the populations of their hosts. These large/severe epidemics started and peaked earlier than smaller/benign ones. The larger epidemics also exerted particularly negative effects on host densities at certain lags, reflecting the delayed consequences of infection on fecundity reduction and host mortality. Notably, negative effects on the juvenile stage class manifested later than those on the adult stage class. The results of the experiment further emphasized depression of host density by the fungus, especially on the density of the juvenile stage class. Consequently, this common parasite reduces the density of host populations when conditions foster larger outbreaks characterized by an earlier start and earlier peak. Given these considerable effects on host density seen in a number of large epidemics, parasitism may sometimes rank highly among other factors (predation, resource availability) driving the population dynamics of these hosts.

Parasitism by water mites in native and exotic Corixidae: Are mites limiting the invasion of the water boatman Trichocorixa verticalis (Fieber, 1851)?

“…Smith (1977) previously showed that the spatial distribution of two sympatric water boatmen was determined by the presence of water mites, which exclude one of them at lower salinity. In many host-parasite systems, values of prevalence exceeding 10 %, as in our study for T. v. verticalis, are enough to exert a negative influence on host density (Hall et al 2011). Moreover, total prevalence and intensity of mites recorded in our study were probably underestimates.…”

Chronic contamination decreases disease spread: aDaphnia–fungus–copper case study

“…Contamination reduced the total size of epidemics in Daphnia populations by 50 per cent relative to epidemics occurring in uncontaminated conditions. This reduction in epidemic size matters because larger epidemics typically reduce densities of Daphnia more severely [35]. Indeed, Daphnia populations experienced a smaller parasite-induced decline in densities in copper contaminated tanks with smaller epidemics.…”

Potassium stimulates fungal epidemics in Daphnia by increasing host and parasite reproduction

“…Virulent parasites can harm host populations (Frick et al 2010, Vredenberg et al 2010, Hall et al 2011, alter competitive interactions (e.g., Tompkins et al 2003), and exert strong selective pressure on hosts . The emergence and resurgence of infectious diseases among wildlife populations also poses a major challenge for species conservation (Frick et al 2010, Vredenberg et al 2010, Fisher et al 2012.…”