Your turn!Make a hypothesis about what is going to happen if the
sea otters continue to decline or even disappear.
Initially, a dramatic decline in sea otter populations was first
noted which indicated that some sort of major shift in predation as
a result of a disturbance had happened. This is the kind of figures
and data that a real biologist would make. How does it compare to
the bar graphs you made? Is it similar? What happened? The number
of sea otters in this population is on the y axis (left) and the
Year is on the x axis.What is causing this major decline!?Killer
whales had begun to prey upon the sea otters very heavily. But Why?
Both organisms had always been in this ecosystem and this trend had
not happened before.Killer whales prey was at one point, mainly
northern sea lions, whales, and some sea otters but theres been a
shift away from sea lions to sea otters. Why?Make a hypothesis. Why
did killer whales start eating sea otters at such a high rate? What
might have happened in their ecosystem to cause this?
Heres another graph from a Biologist. What do you see? Do the
different animal populations increase or decrease?Decline in
whales, followed by a decline in harbor seals, followed by a
decline in sea lions, and finally by a decline in sea otters. The
decline in whales was due to commercial whaling that became very
heavy following WW 2 and ended at the end of the 1970s. At this
same time, there was also heavy commercial fishing. Because of the
decreasing numbers of fish in the oceans, the harbor seals and sea
lions did not have as much food as they needed. The killer whales
were slowly shifting between prey as the prey became less available
until they ended up eating sea otters. Sea otters have small bodies
so these are not the best food for a large, killer whale. Theyd
much rather use their energy to kill something large bodied that
would provide lots of food and energy.That decline in animals was
happening mainly out in the ocean. Change then began to happen near
shore when the killer whales began eating sea otters. IN this
picture, you can see a sea otter eating a sea urchin in a kelp
bed.When sea otters are present, they eat a huge amount of sea
urchins. This is important because the sea urchins eat the kelp.
Youve seen this image before. You can see a kelp bed with otters
present.This photo was taken at the exact same place as the photo
above but this photo was taken AFTER the killer whales had eaten
all of the sea otters that were present in this area. This image
shows how the sea urchin populations have gotten out of control and
have eaten all of the kelp. We call this sort of an area an urchin
barrens.Here is what happened after the killer whales began
focusing predation on the sea otters.Fig. 1. (A) Changes in sea
otter abundance over time at several islands in the Aleutian
archipelagoand concurrent changes in Sea otters declined on four
different islands.(B) sea urchin biomass, The number of sea urchins
increased. The number of sea urchins in 1997 was MUCH larger than
in the 1980s.
(C) grazing intensity, The amount of kelp the sea urchins ate
also increased dramatically in 1997 from 1990 because there were
MORE sea urchins.
and (D) kelp densityThe amount of kelp decreased because the
otters were gone or were fewer in number and werent eating as many
sea urchins and the increasing number of sea urchins were eating
many, many more kelp beds.
measured from kelp forests at Adak Island. Error bars in (B) and
(C) indicate 1 SE. The proposedmechanisms of change are portrayed
in the marginal cartoons, the one on the left shows how thekelp
forest ecosystem was organized before the sea otters decline and
the one on the right showshow this ecosystem changed with the
addition of killer whales as an apex predator. Heavy
arrowsrepresent strong trophic interactions; light arrows represent
weak interactions.
Population trends and survival rates ofsea otters in Clam Lagoon
(solid squares) andadjacent Kuluk Bay (open circles), Adak
Island,Alaska. Clam Lagoon did not have killer whales.You can see
that the number of sea otters in the area without killer whales had
more sea otters than the area that had killer whales that were
eating sea otters.