Sex-skewed trophic impacts in ephemeral wetlands

Predation can have marked impacts on ecosystem structure, function, and stability. However, quantifications of biotic interactions frequently overlook demographic variabilities within populations, which can modulate interaction strengths, such as sex and reproductive status. Compositional population ratios between males and females, alongside reproductive status, are highly variable temporally in ephemeral aquatic systems, and may profoundly mediate levels of ecological impact and thus stability of trophic groups. In the present study, we apply functional responses (resource intake as a function of resource density) to quantify predatory impacts of adult males, non-gravid females, and gravid females of the calanoid copepod Lovenula raynerae (Diaptomidae), an abundant ephemeral pond specialist, on larvae of the Culex pipiens (Culicidae) mosquito complex. We then develop a novel metric to forecast population-level impacts across different population sex ratio scenarios. Lovenula raynerae demonstrated prey population destabilising Type II functional responses irrespective of sex and reproductive status, yet variable functional response magnitudes were found. While male and non-gravid female copepods exhibited similar functional response maximum feeding rates, gravid female feeding rates were substantially higher, implying higher resource demands for progeny development. Ecological impacts of L. raynerae on lower trophic groups increased markedly where their abundances increased but, crucially, also as population sex ratios became more biased towards gravid female copepods. We demonstrate that population-level impacts do not only correlate tightly with abundance but may be further modulated by reproductive status variations. Thus, the development of sex-skewed ratios in favour of gravid females during the hydroperiod probably heightens ecological impacts on lower trophic groups. The implications of these results for prey population stability are discussed in the context of freshwater ecosystems.