Effects of increasing shrubs in arctic tundra on associated arthropods and migratory songbirds

In areas across the Arctic, shrubs are becoming more abundant, altering habitat for consumer species. Since 2010, we have been comparing open and shrub tundra at four sites in the vicinity of the Arctic LTER site to describe arthropod communities associated with these vegetation types. We also monitor migratory songbird populations, nesting locations, and nesting success to determine how changes in vegetation will affect these important species during their breeding season. Shrub vegetation differed across our four sites in terms of shrub height, branching complexity, and canopy greenness, dependent somewhat on landscape position (particularly if the shrubs were in a riparian zone). Open vegetation was relatively similar across sites, comprised of a mixture of sedges and dwarf shrubs. Arthropods tended to be more abundant and more diverse in shrub plots, perhaps reflecting increased opportunities for feeding and reproduction by specialist taxa; this pattern occurred for ground-dwelling, canopy-associated, and flying arthropods. Abundance of the most commonly trapped arthropod orders also differed between vegetation types and among sites. The two bird species of focus both nest on the ground, but white-crowned sparrows prefer to nest at the base of shrubs (birch or willow) on average 40-50 cm tall whereas Lapland longspurs hide their nests inside sedge tussocks that average 10-15 cm tall. The birds can sample across these two environments for food when they first arrive on the tundra, but once they are supporting nestlings, must forage closer to their nests. Because shrub tundra supports more arthropod food resources than open tundra, Lapland longspurs and other open tundra nesters may be at a disadvantage for both food and nest site availability in a shrubbier tundra. Assessing these effects is crucial to understanding how a changing tundra affects these migratory birds that play important roles as consumers and prey in the Arctic and at temperate latitudes during the winter.