Botkin Environmental Science Earth as Living Planet 8th txtbk

82 CHAPTER 5 Ecosystems: Concepts and Fundamentals
(c)
Island
Sea otter
Otter
Urchin
FIGURE 5.1 The effect of sea otters on kelp.
(a) Sea otter eating a crab. Sea otters feed on
shellfish, including sea urchins. Sea urchins feed
on kelp. Where sea otters are abundant, as on
Amchitka Island in the Aleutian Islands, there
are few sea urchins and kelp beds are abundant
(b and c). At nearby Shemya Island, which
lacks sea otters, sea urchins are abundant and
there is little kelp. 3 Experimental removal of sea
urchins has led to an increase in kelp. 2
Sea urchin
Kelp
Other
shellfish
Island
Island
Ocean depth (m) Ocean depth (m)
6
12
18
24
6
12
18
24
Without sea otters
(Shemya Island)
With sea otters
(Amchitka Island)
80 240 400
Sea urchin density (individuals per m 2 )
Without sea otters
(Shemya Island)
With sea otters
(Amchitka Island)
20
40 60 80 100
Percent kelp cover
to be a focus of controversy. On the one hand, fishermen
argue that the sea otter population has recovered—so
much so that they now interfere with commercial fishing
because they take large numbers of abalone. 7 On the
other hand, conservationists argue that community and
ecosystem effects of sea otters make them necessary for the
persistence of many oceanic species, and that there are still
not enough sea otters to maintain this role at a satisfactory
level. Thus, sea otters’ indirect effects on many other species
have practical consequences. They also demonstrate
certain properties of ecosystems and ecological communities
that are important to us in understanding how life
persists, and how we may be able to help solve certain
environmental problems.