Data from: Daphnia invest in sexual reproduction when its relative costs are reduced

The timing of sex in facultatively sexual organisms is critical to
fitness, due to the differing demographic consequences of sexual vs.
asexual reproduction. In addition to the costs of sex itself, an
association of sex with the production of dormant life stages also
influences the optimal use of sex, especially in environments where
resting eggs are essential to survive unfavourable conditions. Here we
document population dynamics and the occurrence of sexual reproduction in
natural populations of Daphnia magna across their growing season. The
frequency of sexually reproducing females and males increased with
population density and with decreasing asexual clutch sizes. The frequency
of sexually reproducing females additionally increased as population
growth rates decreased. Consistent with population dynamic models showing
that the opportunity cost of sexual reproduction (foregoing contribution
to current population growth) diminishes as populations approach carrying
capacity, we found that investment in sexual reproduction was highest when
asexual population growth was low or negative. Our results support the
idea that the timing of sex is linked with periods when the relative cost
of sex is reduced due to low potential asexual growth at high population
densities. Thus, a combination of ecological and demographic factors
select on the optimal timing of sexual reproduction, allowing D. magna to
balance the necessity of sex against its costs.