Epidemic size determines population-level effects of fungal parasites on Daphnia hosts |
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Authors: | Hall Spencer R Becker Claes R Duffy Meghan A Cáceres Carla E |
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Institution: | (1) Department of Biology, Indiana University, 1001 E. 3rd St., Bloomington, IN 47405-3700, USA;(2) School of Biology, Georgia Institute of Technology, Atlanta, GA 30332-0230, USA;(3) School of Integrative Biology and Program in Ecology, Evolution and Conservation Biology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA;(4) Present address: Sweco Environment, Stockholm, Sweden |
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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. |
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