Arthropod communities and passerine diet: effects of shrub expansion in Western Alaska

Abstract

Across the Arctic, taller woody shrubs, particularly willow (Salix spp.), birch (Betula spp.), and alder (Alnus spp.), have been expanding rapidly onto tundra. Changes in vegetation structure can alter the physical habitat structure, thermal environment, and food available to arthropods, which play an important role in the structure and functioning of Arctic ecosystems. Not only do they provide key ecosystem services such as pollination and nutrient cycling, they are an essential food source for migratory birds. In this study I examined the relationships between the abundance, diversity, and community composition of arthropods and the height and cover of several shrub species across a tundra-shrub gradient in northwestern Alaska. To characterize nestling diet of common passerines that occupy this gradient, I used next-generation sequencing of fecal matter. Willow cover was strongly and consistently associated with abundance and biomass of arthropods and significant shifts in arthropod community composition and diversity. Key nestling prey items were positively associated with both willow and ericaceous shrubs. Diet composition varied significantly among bird species and spatially within species, however, I found that temporal variability in prey abundance did not have a strong relationship to the probability of consumption. I predict that the wide temporal window of prey availability and high diet diversity may protect these birds against negative impacts from climate-driven shifts in prey phenology and abundance. Taken together, my results suggest that shrub expansion could result in a significant shift in Arctic food-web structure and an increase in food availability for insectivores, although future ecosystem change in the Arctic is likely to be heterogeneous as shrub types are expanding at different rates and in different places across the Arctic.