Climate-change effects on the sex ratio of free-living soil nematodes – perspective and prospect

Human-induced global environmental change is predicted to alter the stability and functioning of ecosystems worldwide. Most research in recent decades has focused on studying climate-change effects on aboveground systems, causing a poor understanding of belowground responses. However, gaining knowledge on environmental-change effects on soil biota is of crucial importance, as soil-ecosystem services are indispensable for human well-being and contribute fundamentally to the functioning of terrestrial ecosystems. Nematode communities play a central role in various soil ecosystem processes and are therefore commonly used as biological indicators to assess soil conditions and soil health. While causing overall shifts in community composition patterns, which are most often examined, climate change might also alter nematode population dynamics and the sex ratio (number of males per female). Previous studies on plant-parasitic nematode species suggest that changes to unfavorable environmental conditions trigger reduced development of females and favor sexual rather than parthenogenetic reproduction. Therefore, we are presenting the working hypothesis that predicted climate change causing reduced resource availability and enhanced environmental stress will lead to an increased proportion of males in soil nematode communities. Our systematic literature review revealed that climate- and environmental change effects on the sex ratio of free-living soil nematode populations are inconsistent, but heavily understudied. Data on sex ratios have been treated mostly as additional information, presented without any underlying theory and hypotheses, as well as limited discussion. In this perspective paper, we thus propose that future studies should include clear hypotheses and test if the sex ratio of free-living nematodes increases with climate change due to more stressful environmental conditions and low resource availability. Furthermore, we conclude that experimental studies investigating the specific roles of male and female nematodes are needed to better predict the implications of a changing climate on soil ecosystem functioning.