Can temporal niche separation allow for parasite coexistence? Parasites in their free-living, transmission stage often find themselves in variable external environments. Parasites can adopt different strategies by either expending energy in searching for hosts, or remaining relatively inactive until they receive host cues. The relative success of these strategies will depend on spatio-temporal host availability and survival probability in the transmission stage. Here, we examine how these factors affect interspecifc competition at both the within- and among-host scales using two sympatric nematode species, Steinernema costaricense and S. kraussei (COS and KRA, respectively). These species obligately infect, kill and reproduce in lepidopteron hosts and persist as non-feeding transmission stages in the soil. Previous work has shown that COS are generally more motile than KRA, and that when individual hosts were inoculated simultaneously KRA increases in within-host competitive success relative COS with transmission-stage age. We used single and mixed species soil mesocosms to distinguish among- from within-host competitive interactions at either high or low host density and examined the effect of persistence in the transmission stage by adding hosts in two pluses separated by a two-week absence. Hosts were tracked daily for mortality and removed from the mesocosms to quantify nematode emergence.
Results/Conclusions
We expected that COS would outperform SKA initially, but that this dominance would shift with time, if COS suffered higher mortality or reduced quality due to its increased activity. In contrast to this prediction, in the single-species mesocosms, COS maintained its competitive dominance at both time points and host availabilities. More specifically, COS took significantly lesser time to kill hosts, greater probability of emergence, and more propagules than SKA. This result indicates that in our mesocosms, COS mobility outweighed any negative effects on persistence. However, when we examined the mixed-species mesocosms, we find that SKA did increase in competitive dominance with time. Specifically, in the mixed treatment at high host availability, solely COS emerged from hosts initially, while the two species showed equal probability of emerging from hosts at the later time point. This shift in competitive dominance supports the hypothesis that COS activity does negatively impact its later performance and highlights the importance of examining both within- and among-host competition when examining parasite coexistence.