Loss of foundation species: consequences for the structure and dynamics of forested ecosystems

ecological change (Reid et al. 2005). Through habitat conversion, over-consumption of resources, and worldwide introductions of pests and pathogens, humans are causing species extinctions at a record rate: the sixth extinction crisis in the billion-year history of eukaryotic life on Earth (Eldridge 1998). The loss of a common or abundant foundation species (sensu Dayton 1972; see Panel 1), which by virtue of its structural or functional attributes creates and defines an entire ecological community or ecosystem, can have dramatic effects on our perception of the landscape and broad consequences for associated biota, ecosystem function, and stability. Foundation species differ from keystone predators (Paine 1966) in that the former usually occupy low trophic levels whereas the latter are usually top predators. They are also distinct from core species (Hanski 1982) in that foundation species are not only locally abundant and regionally common but also create locally stable conditions required by many other species. They also serve to stabilize fundamental ecosystem processes such as productivity and water balance. Trees are most likely to be foundation species in forested ecosystems, as their architecture and functional and physiological characteristics define forest structure and alter microclimates, while their biomass and chemi479