PS 39-6 - Measuring possible trophic cascades from the loss of native birds.

Background/Question/Methods

Biological invasions are one of the greatest threats to biodiversity today. Species diversity determines interactions within food-webs and ecosystem processes and is therefore a key factor for the stability of ecosystems. Islands are powerful study systems for species additions and removals as they often support unique biodiversity, and are susceptible to invasions. The island of Guam was invaded by the brown tree snake (Boiga irregularis) after WWII and destroyed the native bird community. Native birds were the main predators of insects, and without them, spider abundance has increased. Predators play an important role in exerting top-down control on communities, and it is crucial to understand the consequences of their removal. If not replaced by other predators, their loss leads to increases in herbivores and reductions of plant biomass, in what is called a trophic cascade. A few places on Guam and nearby islands still have insectivorous forest birds, allowing comparisons. In this study, we ask whether the loss of birds have caused trophic cascades on Guam. We surveyed insects across areas with and without birds and quantified herbivory on leaves. We looked at how insect abundance, ant presence, and herbivory on leaves differed between different habitat types and bird presence.

Results/Conclusions

Due to COVID restrictions, we were only able to sample on Guam, but will sample the neighboring islands in 2022. Insect collections were conducted along transects that were placed from the edge of the forest (less trees) to inside the forest (more trees). Preliminary results from Guam suggests that the presence of birds had an effect on the insect distribution in the forest. When birds were present, insects were evenly distributed throughout the forest. When birds were not present, there were a higher abundances of insects closer to the edge of the forest. When birds were present, they were often found closer to the edge of the forest by clearings where they can stay on powerlines to avoid the snakes in the trees. Our results suggests that despite the small presence of mostly non-native birds, they may predate enough on insects to effect their distribution. The next set of analyses will focus on ant presence, herbivory and data from neighboring islands. Understanding the food-web shifts on islands due to the brown tree snake invasion is crucial in restoring the native bird community, preserving native insects, preventing invasive arthropods from establishing, and preventing other islands from experiencing irreversible biological losses.