Effects of cattle, an exotic species, on an Aleutian island

Exotic species may threaten biodiversity worldwide, but their effects on biological communities are often poorly understood.  Humans introduced cattle to Sanak Island, located in the Aleutian Archipelago, in the 1890s.  Sanak’s inhabitants ranched cattle until the 1960s, when they abandoned the island.  Cattle are currently feral, and are occasionally hunted.  We describe direct and indirect effects of introduced cattle on the biological communities and food-web of Sanak.  Areas preferentially grazed by cattle have lower plant species diversity, lower plant biomass, and higher abundances of exotic Kentucky bluegrass and dandelions than neighboring tundra and cattle-free sites.  Kentucky bluegrass and dandelion abundances are positively correlated with each other, the location of ancient Aleut village sites, and cattle activity.  Cattle maintain areas of altered vegetation, or ‘grazing lawns’, in ancient Aleut village sites.  Cattle dung, carcasses, and blood supply nutrients to various insects, and may increase their abundances.  Native vole abundances decrease in relation to cattle.  Birds also may be affected by the encroachment of exotics: insects and plants are consumed by as many as half of the birds found on Sanak; thus fluxes in insect and plant communities likely influence bird populations.  This evidence supports the ‘invasional meltdown hypothesis’: the presence of one exotic invader (cattle) facilitates the establishment of others, creating positive feed-back loops that affect various trophic levels.  These combined studies illustrate the transformation of a landscape and food-web by cattle, and, more generally, the alterations that introduced species can provoke in ecosystems.