The acquisition and utilization of resources is a foundational task for all organisms. Yet, the ability to achieve this task is often compromised by the competing demands of species interactions, such as mounting costly defenses to parasitism. As such, tradeoffs are frequently predicted to exist between energetically demanding activities, such as somatic growth, metabolic processes, and defensive mechanisms. Although studies have shown that parasitized individuals exhibit lower consumption rates, it remains unclear if mounting defenses against parasitism also causes declines in the underlying digestive physiology. Moreover, ecological factors such as host population density, which can mediate resource competition, typically vary among populations, suggesting that such tradeoffs may not be fixed at the species level and should instead vary among populations differing in density. To further understand whether such tradeoffs exist and how population density may affect their intensity, we determined how digestive physiology and immune response of damselfly larvae (Enallagma vesperum) covaried between low- and high-density populations. To do so, we used an experimental approach with artificial parasites to induce an immune response in individuals from low- and high-density lakes. We then conducted growth trials to elucidate potential tradeoffs between immune response and digestive physiology.
Results/Conclusions
Damselflies from the low-density lakes mounted a stronger immune response than individuals from the high-density lake. This suggests that high densities of conspecifics may weaken the ability to mount immune responses, as might occur when organisms compete for limiting food resources necessary for generating immune responses. Consistent with this, individuals from the high-density lake tended to have lower relative growth rates and production efficiency. Furthermore, we found that average daily metabolic rate in the low-density lake did not change when individuals were mounting an immune response, whereas individuals in the high-density lake increased their daily metabolic rate. Despite finding that inducing an immune response did influence some aspects of digestive physiology, we found no evidence of any tradeoffs between immune responses and any measures of digestive physiology in either the high- or low-density lake. We hypothesize that considerable intraspecific variation within populations obscured any potential tradeoffs between digestive physiology and immune response that may be present. Thus, our preliminary results suggest that intraspecific variation, and not tradeoffs between energetically demanding activities, may be a driving factor sustaining damselfly abilities to engage in meeting multiple competing demands that arise within complex communities.