Steller
sea lion Food Webs: Who Eats Whom and How Much?
|
| Food web analyses
and estimates of prey consumption are essential for understanding
ecosystem dynamics. A recent review by Andrew Trites synthesized
data from over 100 food webs that have been published for
marine ecosystems. It describes the transfer of food energy
from its source in plants through herbivores to carnivores
and higher order predators. It also examines the length
of food chains, rates of consumption, and efficiencies
of transfer and production by trophic levels. It concludes
by discussing the decline of Steller sea lions in Alaska
to highlight the importance of constructing food webs and
estimating consumption.
|
|
click on image to see food
web details |
 Combining
the individual energy requirements of Steller sea lions with information
about the changes that occurred in the Bering Sea energy food
web suggests that the population decline of Steller sea lions
in Alaska was likely related to the inability of young animals
to acquire sufficient energy from the low quality prey available
to them. There is no indication of shortages of low quality prey.
However, energetic modeling and captive feeding studies suggest
it may not be physically possible for young Steller sea lions
to consume enough low quality prey to meet their daily energetic
needs. Consuming fewer calories can stunt growth and cause reproductive
failure (i.e., abortions) —symptoms that have been observed
in Alaska over the period that Steller sea lions have declined.
A lower nutritional plane may also increase the susceptibility
of sea lions to disease, and increase their risk of being killed
by predators—a factor that may account for the apparent high
mortality of juvenile sea lions. Mathematical modeling suggests
that killer whales could have been a significant contributing
factor in the decline of Steller sea lions, and may now be preventing
the population from recovering.
The case of the Steller sea lion is an example of the importance
of constructing food webs and estimating the energy requirements
of marine organisms to understand ecosystem dynamics. This
can only be achieved through a combination of fieldwork, captive
studies and mathematical models—all of which are essential
tools for the responsible management of fisheries and ecosystems.
|
MAJOR
CONCLUSIONS OF REVIEW
- Consumption
of marine organisms
expressed as a % of
an individual's body weight per day:
·about
1-4% for fish
·4-8%
for marine mammals
·10%
for cephalopods
·15-20%
for sea birds
·20-30%
for zooplankton
- Consumption
varies seasonally with periods of growth and reproduction.
- Most groups
of species consume 3-10 times more than what they
produce, and export or pass up the food web about
70-95% of their production.
- Production efficiencies decline
with increasing trophic levels from about 30-40% in
cephalopods, 15-30% in fish, 11-15% in sea birds and
0.2-0.3% in mammals.
- Only about
10% of what is passed up the food chain is converted
to new biomass at each successive trophic level.
- High fish
catches are generally associated with high primary
production and fishing at lower trophic levels.
- Few marine
organisms feed on more than one trophic level. Humans
are one of the few species that can prey upon almost
any level of the food chain and on any size of prey.
|
Full details can be obtained from:
|
