Environmental Engineering Reference
In-Depth Information
small lake trout (
Salvelinus namaycush
).The effects did not simply stop within
the boundaries of the water bodies themselves. Many species of birds and
mammals such as bald eagles (
Haliaeetus leucocephalus
), gulls (
Larus sp.
), griz-
zly bears (
Ursus arctos
), coyotes (
Canis latrans
), mink (
Mustela vison
), and otter
(
Lutra canadensis
) depended on spawning salmon as a key food source.These
species also declined in abundance in many of the tributary watersheds feed-
ing the Flathead lake system in which the salmon spawn.
Ecosystems in Space: Linkages across
Geographic Boundaries
The above treatment of ecosystem complexity shows the temporal legacy
of human impacts on ecosystems. It also alludes to the fact that ecosystems
cannot be viewed as though they were self-contained entities. For example,
lakes—often viewed as being isolated from the surrounding terrestrial sys-
tems by a hard, shoreline boundary—are affected by seasonal runoff as melt-
ing snow in spring flows down hill slopes carrying with it nutrients into
lakes. Such run-off in turn can be important in sustaining the structure and
functioning of lake ecosystems (Pace et al. 2003).
The older conceptualization of an ecosystem being a self-contained en-
tity is now giving way to recognition that ecosystems are connected to each
other by flows across landscapes in which they are juxtaposed.This new way
of thinking has been championed by the late Gary Polis who essentially
asked the question:What if we focus on the consequences to food chain dy-
namics of the flow of externalities to a system rather than concentrate solely
on the components within the system? Polis began asking this question after
studying oceanic island ecosystems off of Baja California that are sharply
separated from each other and the mainland by large distances and a seem-
ingly impermeable salt water barrier.These arid islands provide a largely in-
hospitable environment. They are covered with
Opuntia
cactus, myriad
species of flying insects, and their web-building spider predators (Polis and
Hurd 1995). Curiously, however, the islands supported extraordinarily high
densities of spider predators and this trend was more pronounced on smaller
islands than on larger ones.This oddity ran counter to current ecological
concepts. It is larger island and mainland ecosystems that are supposed to be
better able to support absolutely more top predators.This is because they
have higher in situ plant production to sustain those higher levels of the
food chain. Polis and Hurd's observations instead suggested that the small
islands held the more productive systems.
