The influence of terrestrial resource subsidies on the top-down effects of multiple predators in a freshwater community

Background/Question/Methods

In many biological systems, multiple predators exert strong top-down effects. Predator species may interact through intraguild predation, competition or facilitation all of which can have important consequences for community structure. While top-down effects are well documented, the dynamics of predation are heterogeneous across landscapes and may be influenced by resource availability and food web structure. In Northern California streams, Juvenile steelhead (Oncorhychus mykiss) and Pacific giant salamanders (Dicamptodon tenebrosus) are the dominant vertebrate predators. Beyond basic diet studies, little is known about the interaction between these two predators and the consequences of these interactions for the stream community. Juvenile steelhead and salamanders have a high degree of diet overlap, and throughout their ontogeny either species may be predator or prey to the other. In headwater streams, inputs of terrestrial carbon are an important source of energy for aquatic communities through prey subsidies as well as leaf litter and other organic material. Using a field based experimental manipulation of predator assemblages and availability of terrestrial in-fall we sought to understand how terrestrial resource subsidies influence predator populations and their effect on the aquatic community.

Results/Conclusions

Experimental covers constructed over individual pools on average reduced terrestrial in-fall 91 fold. Reductions in the availability of terrestrial resources led to significantly lower growth in both one year old and young of the year trout while salamander growth was unaffected.  Predator assemblage had no detectable effect on the growth of either age class of steelhead, however salamander growth was facilitated by the presence of steelhead predators. Differences in predator assemblage and terrestrial in-fall did not change primary productivity as measured by Chlorophyl-a and AFDM but salamander predators did cause changes in the benthic invertebrate community, depressing the biomass of emergent plecoptera in experimental pools. Our work highlights the importance of terrestrial resources for fish growth in stream ecosystems but showed no indication that interspecific competition limits growth for either predator species.