Botkin Environmental Science Earth as Living Planet 8th txtbk

88 CHAPTER 5 Ecosystems: Concepts and Fundamentals
Harp seals
5th Trophic
Level
Sea
raven
Flatfish
Sculpin
Atlantic
cod
4th Trophic
Level
Cephalopod
mollusks
Silver
hake
Sand
lance
Echinoderms
Arctic
cod
Capelin
Redfish
3rd Trophic
Level
Herring
Myctophid
fish
2nd Trophic
Level
Polychaetes
Pelagic
mysids
Pelagic
amphipods
Pelagic
decapods Copepods Euphausiids
Phytoplankton
1st Trophic
Level
FIGURE 5.7 Food web of the harp seal showing how complex a real food web can be.
5.3 Ecosystems as Systems
Ecosystems are open systems: Energy and matter flow
into and out of them (see Chapter 3). As we have said,
an ecosystem is the minimal entity that has the properties
required to sustain life. This implies that an ecosystem is
real and important and therefore that we should be able to
find one easily. However, ecosystems vary greatly in structural
complexity and in the clarity of their boundaries.
Sometimes their borders are well defined, such as between
a lake and the surrounding countryside (Figure 5.8a). But
sometimes the transition from one ecosystem to another
is gradual—for example, the transition from deciduous to
boreal forest on the slopes of Mt. Washington, N.H. (Figure
5.8b), and in the subtle gradations from grasslands to
savannas in East Africa and from boreal forest to tundra
in the Far North, where the trees thin out gradually and
setting a boundary is difficult, and usually arbitrary.
A commonly used practical delineation of the
boundary of an ecosystem on land is the watershed.
Within a watershed, all rain that reaches the ground
from any source flows out in one stream. Topography
(the lay of the land) determines the watershed. When a
watershed is used to define the boundaries of an ecosystem,
the ecosystem is unified in terms of chemical cycling.
Some classic experimental studies of ecosystems
have been conducted on forested watersheds in U.S. Forest
Service experimental areas, including the Hubbard
Brook experimental forest in New Hampshire (Figure
5.9) and the Andrews experimental forest in Oregon.
In other cases, the choice of an ecosystem’s boundary
may be arbitrary. For the purposes of scientific analysis,
this is okay as long as this boundary is used consistently
for any calculation of the exchange of chemicals and
energy and the migration of organisms. Let us repeat
the primary point: What all ecosystems have in common
is not a particular physical size or shape but the
processes we have mentioned—the flow of energy and
the cycling of chemical elements, which give ecosystems
the ability to sustain life.