Positive and negative biotic interactions (e.g., facilitation, competition, predation) structure communities, but interspecies interactions may shift as abiotic stress increases (e.g., stress gradient hypothesis, SGH). The McMurdo Dry Valleys (MCM), Antarctica’s soil ecosystem is simple, making it ideal for studying biotic interactions. This food web includes primary levels of algae, bacteria, archaea, fungi, with protozoan and metazoan consumers (two mite, one springtail, two rotifer, two tardigrade, four nematode species). The microbivore nematode, Scottnema lindsayae dominates dry soil landscapes, while most invertebrate diversity exists in limited wet soil along stream/lake margins. S. lindsayae sometimes co-occurs with the nematode Eudorylaimus antarcticus. Recent research shows E. antarcticus as predaceous, but it also consumes algae. We asked if these species co-occur along salinity gradients in dry soil, whether interactions are positive or negative, and if interactions affect trophic structure. According to SGH, we expected a shift from negative to positive interactions as salinity increased. Using electrical conductivity (EC) as salinity’s proxy, we tested co-occurrence patterns using 23 years of invertebrate community and soil chemistry data (MCM Long Term Ecological Research). Data include S. lindsayae and E. antarcticus abundances and population demographics in single- and two-species communities from <30 to >3800 uS/cm EC. Next, we collected soil along salinity gradients and evaluated nematode trophic structure using stable isotope ratios (13C/12C, 15N/14N).
Results indicated that salinity increased stress: salinity was positively correlated with both species’ mortality and negatively correlated with chlorophyll-a abundance. Upper limits of tolerance were 744 and 2833 uS/cm EC for E. antarcticus and S. lindsayae, respectively. At high salinity (>399 uS/cm EC) E. antarcticus was significantly more abundant in two-species than single-species communities and was only found living at >600 uS/cm EC in two-species communities. At high salinity, S. lindsayae’s abundance and fecundity was lowest for two-species communities. Conversely, at low salinity (<300 uS/cm EC) S. lindsayae was more abundant while E. antarcticus was less abundant in two-species communities. Overall, reduced algae availability at high salinity shifted E. antarcticus' survival to availability of prey, S. lindsayae. Isotope results confirm this: 15N enrichment suggests predation at high salinity. S. lindsayae persistence at salinities >744 uS/cm EC suggest a predation escape strategy. While results were not explained by SGH – interactions in two-species communities under high stress were only positive for E. antarcticus (higher abundance) but were negative for S. lindsayae (lower abundance, lower fecundity) – we show predator-prey interactions not found previously for MCM.