Foraging patterns of an invasive predator, Eleutherodactylus coqui, indicate potential direct and indirect effects on invertebrate prey food webs

Background/Question/Methods

Invasive predators can profoundly impact naïve communities, especially where functionally equivalent predators were historically absent. Beyond direct consumption, predators can indirectly affect entire communities by altering food web linkage. Where invasive predators consume prey from multiple distinct sources, novel links may couple dynamics and create indirect relationships between previously disparate prey communities. Thus, evaluating the spatial and temporal foraging patterns of an invasive predator should provide insight into avenues of influence on native communities and overall food web complexity. This study focuses on Eleutherodactylus coqui where they have invaded Hawaii, reaching unprecedented densities with vast potential to alter native prey communities. Eleutherodactylus coqui is a generalist sit-and-wait predator that inhabits daily underground refuges and nightly understory perches.  Beyond direct consumption, E. coqui’s daily migrations may create or strengthen linkage between the highly invaded invertebrate litter food web and predominantly native foliage web. Here I used lab trials and field manipulations on the Big Island of Hawaii to evaluate temporal and spatial feeding patterns of invasive E. coqui. I characterized temporal foraging patterns by offering individual frogs multiple mobility-matched prey species for 12h (night or day) in lab mesocosms with vertical structure and natural refuge. In the field, I established plots where natural C3 leaf litter was replaced by locally occurring C4 litter to determine vertical (litter versus understory) spatial partitioning of naturally foraging E. coqui.

Results/Conclusions

Lab trials revealed that frogs consumed invertebrates during both day and night periods; however, they ate more consistently and consumed twice as many prey at night. Carbon isotopes indicate that E. coqui are connected to both foliage and litter food webs, with a clear litter contribution but a stronger dependence on foliage prey. These results suggest the need for further assessment and consideration of foraging habits, since they verify a recent report of litter invertebrates in invasive E. coqui gut contents, which differs from native range diets. Since my study indicates that primary foraging is occurring at night in the understory, E. coqui may have a stronger impact on the native foliage invertebrates than previously thought.  Furthermore, consumption from both food webs, likely the result of daily vertical migration, suggests the potential for novel or strengthened coupling of litter and foliage food webs. By consuming prey from spatially distinct communities, E. coqui’s direct effects on native Hawaiian foliage invertebrates may be indirectly amplified by abundant invasive prey in the litter community.