Songbirds elicit a productivity mediated trophic cascade in temperate montane grasslands
Grassland songbirds are a key predator in temperate grassland ecosystems. But despite dramatic declines in the abundance of these species over the last 40 years, we have little information on how these species influence the diversity and function of the ecosystems they inhabit. Recent evidence suggests that insectivorous birds control diversity in tropical ecosystems through predation on arthropods ultimately influencing both biodiversity and net primary production. These effects are likely mediated by abiotic resource availability. We present the results of a 4-year project that excluded songbirds and small mammals at sites along a gradient of water and temperature in temperate grassland ecosystems. We measured the effects of bird and mammal removal on arthropod and plant abundance and diversity. We addressed the questions: 1) what is the structure of temperate grassland food webs along a gradient of abiotic resources; and 2) how would the loss of grassland songbirds influence the structure and function of these ecosystems.
Our data suggest that the indirect effects of grassland songbirds and small mammals on plants is mediated by water and temperature. The loss of grassland songbirds produces significant changes in the abundance of plants and distribution of arthropod functional groups. Cooler areas with more rainfall had higher plant growth, more plant species, and broad reticulate food webs. Dryer hotter locations had longer linear food webs that responded differently to the removal of songbirds than wetter cooler locations. When song birds were excluded from wetter cooler locations plant biomass decreased only slightly. Alternatively, when songbirds were removed from dry hot locations biomass increased. The indirect effects of grassland songbirds on ecosystem functions such as plant primary productivity occurs through bird predation on arthropods. Human activities that impact species occupying upper trophic levels, whether these be insects, songbirds, or small mammals, may have negative and unanticipated cascading effects on grassland ecosystem function.