Effects of Wildfire on Interactions Among Nematode Parasites, Mayfly Hosts and Trout Predators

Wildfire activity is increasing globally, highlighting the need to understand how fire disturbance affects species interactions. In particular, few studies have examined how fire influences interactions among parasites, hosts and predators in freshwater streams. We characterised host–parasite and parasite–predator interactions involving nematode parasites (Family Mermithidae), mayfly hosts (Order Ephemeroptera) and trout predators (Salvelinus fontinalis, Salmo trutta and Oncorhynchus clarkii) at 8 burned and 8 unburned stream sites in the southern Rocky Mountains for 2 years following severe wildfires. Mayfly density, infection probability, and density of infected mayflies (infected mayflies/m²) were all lower at burned sites 1 year after fire but returned to levels similar to unburned sites after 2 years. Density of infected mayflies increased with overall mayfly density; however, infection prevalence (%) ranged from 0% to 26% across burned and unburned streams, and there was no relationship between mayfly density and infection prevalence. Based on dissections of > 20,000 mayflies, intermediate-size (4–6 mm) mayflies in the family Baetidae had the highest infection probability and were also the most common mayfly family and size class found in trout stomachs. Wildfire did not affect the number of mermithids consumed per trout, and infection prevalences of mayflies consumed by trout were significantly lower than in the benthos, suggesting predator-avoidance behaviour by infected mayflies. Overall, our results suggest that mermithid nematode responses to fire reflected changes in host density, consistent with the single-host life cycle of mermithids. These results help integrate host–parasite–predator interactions into our understanding of disturbance ecology in freshwater streams, with implications for parasite roles in energy flow through food webs.