Generalist parasites can strongly influence interactions between native and invasive species. Host competence can be used to predict how an invasive species will affect community disease dynamics; the addition of a highly competent, invasive host is predicted to increase disease. However, densities of invasive and native species can also influence the impacts of invasive species on community disease dynamics. We examined whether information on host competence alone could be used to accurately predict the effects of an invasive host on disease in native hosts. We first characterized the relative competence of an invasive species and a native host species to a native parasite. Next, we manipulated species composition in mesocosms and found that host competence results did not accurately predict community dynamics. While the invasive host was more competent than the native, the presence of the native (lower competence) host increased disease in the invasive (higher competence) host. To identify potential mechanisms driving these patterns, we analyzed a two-host, one-parasite model parameterized for our system. Our results demonstrate that patterns of disease were primarily driven by relative population densities, mediated by asymmetry in intra- and interspecific competition. Thus, information on host competence alone may not accurately predict how an invasive species will influence disease in native species.
Many organisms can reproduce both asexually and sexually. For cyclical
parthenogens, periods of asexual reproduction are punctuated by bouts of
sexual reproduction, and the shift from asexual to sexual reproduction
has large impacts on fitness and population dynamics. We studied
populations of Daphnia dentifera to determine the amount of
investment in sexual reproduction as well as the factors associated with
variation in investment in sex. To do so, we tracked host density,
infections by nine different parasites, and sexual reproduction in 15
lake populations of D. dentifera for three years. Sexual
reproduction was seasonal, with male and ephippial female production
beginning as early as late September and generally increasing through
November. However, there was substantial variation in the prevalence of
sexual individuals across populations, with some populations remaining
entirely asexual throughout the study period and others shifting almost
entirely to sexual females and males. We found strong relationships
between density, prevalence of infection, parasite species richness, and
sexual reproduction in these populations. However, strong collinearity
between density, parasitism, and sexual reproduction means that further
work will be required to disentangle the causal mechanisms underlying
these relationships.
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