Landscapes do not exist in isolation. Landscapes are nested within larger landscapes, that are nested
within larger landscapes, and so on. In other words, each landscape has a context or regional setting,
regardless of scale and how the landscape is defined. The landscape context may constrain processes
operating within the landscape. Landscapes are "open" systems; energy, materials, and organisms
move into and out of the landscape. This is especially true in practice, where landscapes are often
somewhat arbitrarily delineated. That broad-scale processes act to constrain or influence finer-scale
phenomena is one of the key principles of hierarchy theory (Allen and Star 1982) and 'supply-side'
ecology (Roughgarden et al. 1987). The importance of the landscape context is dependent on the
phenomenon of interest, but typically varies as a function of the "openness" of the landscape. The
"openness" of the landscape depends not only on the phenomenon under consideration, but on the
basis used for delineating the landscape boundary. For example, from a geomorphological or
hydrological perspective, the watershed forms a natural landscape, and a landscape defined in this
manner might be considered relatively "closed". Of course, energy and materials flow out of this
landscape and the landscape context influences the input of energy and materials by affecting
climate and so forth, but the system is nevertheless relatively closed. Conversely, from the
perspective of a bird population, topographic boundaries may have little ecological relevance, and
the landscape defined on the basis of watershed boundaries might be considered a relatively "open"
system. Local bird abundance patterns may be produced not only by local processes or events
operating within the designated landscape, but also by the dynamics of regional populations or
events elsewhere in the species' range (Wiens 1981, 1989b, Vaisanen et al. 1986, Haila et al. 1987,
Ricklefs 1987).