Impact of exotic macroalga on shorebirds varies with foraging specialization and spatial scale.

Exotic species may increase or decrease native biodiversity. However, effects of exotic species are often mixed; and indirect pathways and compensatory changes can mask effects. Context-specific assessments of the indirect impacts of exotic species are also needed across multiple spatial scales. Agarophyton vermiculophyllum (previously Gracilaria vermiculophylla), an exotic, invasive macroalga, has established throughout the western hemisphere with reported positive or neutral impacts on biodiversity. Shorebirds are an important group for conservation in areas invaded by A. vermiculophyllum. We assess the impacts of this invader on shorebirds by measuring behavior and habitat selection at spatial scales ranging from algal patches to the entire study region. Birds were considered either flexible-foragers that used diverse foraging techniques, or specialized-foragers that employed fewer, more specialized foraging techniques. Responses were scale dependent, with patterns varying between spatial scales, and between behavior and habitat selection. However, a general pattern of habitat selection emerged wherein flexible-foraging shorebirds preferred A. vermiculophyllum habitat, and for specialized-foragers, habitat selection of A. vermiculophyllum was mixed. Meanwhile, flexible-foraging birds tended to neutrally use or avoid uninvaded habitat, and specialized-foraging birds mostly preferred uninvaded habitat. Shorebird behavioral response was less clear; with flexible-foragers spending less time on bare sediment than expected, the only significant response. Shorebird response to A. vermiculophyllum differed by foraging mode; likely because flexible, opportunistic species more readily use invaded habitat. Increases in A. vermiculophyllum could result in functional homogenization if the bare habitat preferred by specialized-foragers is reduced too greatly. We hypothesize the effect of scale is driven by differences among tidal flats. Thus, tidal flat properties such as sediment grain size and microtopography would determine whether foraging from A. vermiculophyllum was optimal for a shorebird. Specialization and spatial scale are important when assessing the biodiversity conservation impacts of invasive A. vermiculophyllum.