Density, parasitism, and sexual reproduction are strongly correlated in lake Daphnia populations

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.