Evaluating the impacts of wildfire on California coastal stream food webs

Background/Question/Methods

The frequency and severity of wildfire in North America has increased substantially in recent years with poorly understood impacts on recipient ecosystems. In particular, the effects of fire on top trophic level organisms in stream ecosystems have not been well studied. Presumably, if wildfire can alter the abundance or distribution of top predators in an ecosystem there will be cascading effects throughout the food web. However, the availability of pre-disturbance data is rare due to the unpredictability of wildfire. As such, it has been relatively difficult to understand the mechanisms by which wildfire impacts stream ecosystems. In this study we examine the incipient impacts of the Lockheed wildfire that burned 40% of the Scott Creek watershed in coastal California, August 2009. With extensive pre-fire data from study sites in burned and unburned regions, across multiple trophic levels, and at various spatial scales we were provided an opportunity to investigate wildfire impacts on stream ecosystem processes. For example, we have attained three full Light Detection and Ranging (LiDAR) scans of the entire watershed from before and after the wildfire. This unprecedented LiDAR dataset has allowed us to measure fire-induced changes in canopy density for the entire watershed. We examine how changes in canopy density impact stream temperatures and subsequently the energetic budget of rainbow trout (Oncorhynchus mykiss), one of the top predators in this system. We ask, does the impact of wildfire on top predators magnify or attenuate as it propagates through the food web?

Results/Conclusions

Our analysis indicated that the wildfire decreased canopy density in burned areas, albeit the impact was highly heterogeneous. Mean daily stream temperatures increased after the wildfire in burned regions and the degree of temperature change was strongly correlated with burn damage. Over the 2010 summer we observed local changes in predator biomass that was strongly correlated with temperature driven energetic demands. In addition, we found a strong negative correlation with invertebrate biomass and increased predator biomass. In total, we’ve determined that the Lockheed wildfire altered the abundance and distribution of top predators with cascading effects to lower trophic levels in this system. Our results indicate that large scale disturbance events can have highly heterogeneous impacts at small spatial scales and across multiple trophic levels. This study has provided a rare and detailed insight into watershed scale spatio-temporal interactions of wildfire and stream food webs and considerably improved our understanding of how large-scale perturbations impact stream ecosystems.