From: http://cbc.amnh.org/crisis/foodweb.html
Real Food Web
Sea Otter Food Web
Major feeding or "trophic" interactions within communities are depicted as "food webs"
by community ecologists. Depending on the community, these webs may be relatively
simple or incredibly complex, as in coral reefs and tropical rainforests.
Because community interactions can be quite complex, we sometimes simplify a food
web by focusing on only the major interactions and ignoring some of the minor ones. As
a result, the food webs rarely show EVERY organism that lives in a community.
Also, since real food webs are dynamic structures, specific details of a general food web
will vary from place to place, from season to season, and as individual organisms grow
and pass through their life cycles, changing their own food preferences and their risks of
being consumed by other species.
Sea Otter Food Web
We will examine two versions of the "Sea Otter Food Web" to see what the effect the
Sea otters have on the rest of the populations. Start with the “Normal” web that
demonstrates what happens when the Sea otters are present. This example focuses
on the larger organisms that live or feed along the bottom of a shallow, rocky
habitat.
How to set up a food web - Basics
Members of food webs are connected by arrows that point from the consumed to the
consumer. These arrows can be interpreted as the direction in which energy flows
during predation (or grazing).
Remember that not all organisms are consumers. Therefore organisms that don’t
look like they are consuming anything are probably producers in the ecosystem!
Additionally some organisms are scavengers or decomposers. Arrows pointing to
these organisms will be coming from dead things.
Arrow thickness indicates the relative importance of a particular food link to the
overall structure of that food web. Faded images and dashed arrows represent
species and interactions which have become rare or absent from the food web.
In the specific food web you’ve been given, the sea otter, at the top of the diagram,
acts as a "keystone species" in the community. Keystone species are ones that have,
for various reasons, a substantial effect - disproportionate to their numbers - on the
rest of the community. Because they lack the blubber of other marine mammals,
individual sea otters need to use a huge amount of energy each day to stay warm
and healthy. As we all know, using a lot of energy means eating a lot of food.
1
While a population of otters may eat many things, sea urchins are their favorite prey.
As the sea otters eat the Urchins, they prevent the Urchin population from growing
out of control and having a negative impact on the ecosystem. Compare the effect
that Urchins have on the web with and without sea otters.
The impact of sea urchins is relatively well understood. Sea urchins have strong jaws
and very hard teeth and are tireless grazers (herbivores), capable of consuming
tough, woody kelps and invertebrate skeletons. Studies have shown that, in the
absence of otters, some urchin populations can grow so dense that they consume
nearly all the bottom cover of edible algae and sessile invertebrates.
At this extreme, these communities are known as "urchin barrens" so many of the
organisms are depleted because of the overconsumption by the urchins. (Ecosystem
is “barren” of certain species because of the “urchins”. As represented in Part B,
impact of too many urchins is fairly dramatic. Remember Sea otters suppress this
over population of Urchins. However hunting of these sea otters significantly
reduced their numbers increasing the number Urchin Barrens. We are only
beginning to see the otter populations increase since hunting was made illegal and
the otters became protected under the Endangered Species Act.
Humans aren’t the only ones to impact the population of otters. A new study noted
a decline in the otter population in Alaska. They attribute this decline to increased
predation by Orcas (killer whales) who have been forced to change their diet and
seek out otters because of a decline in their primary food source: sea lions. As you
can see, the tiniest change of a distant population can trigger a chain reaction that
can have a dramatic effect somewhere else!
2
3
Discussion Questions:
1. Give a brief explanation of what a food web is. How is a food web different from a
food chain? No, I do not just want a definition of each.
2. Identify which organisms in the “normal” food web (with sea otters) fall into the
following categories:
a. Primary producer
b. Primary consumer
c. Secondary consumer
d. Top predator
3. Which group of organisms would you expect to have the greatest biomass and
why?
4. In you own words, what is a “keystone species” in an ecosystem? Would we fit
this classification? Justify your answer!
5. Suppose are large amount of fertilizer (lots of nitrogen and phosphorus) was
allowed to wash into this ecosystem resulting in an “algal bloom” (an
4
overabundance of primary production). What do you think would happen to
the population of otters? (hint, consider the lake scenarios). If the nutrient levels
return to normal levels the following year, what might be the effect on the otters?
Would this occur in the first generation or the second? Justify.
6. In our own area, the deer population has dramatically increased as our ecosystem
has become more “suburban”. While the natural predator of the deer (mountain
lion) has long been removed from this location, hunting has declined as housing
density increases providing deer with more habitat which is non-threatening.
Historically, the forests in this area were predominantly composed of large oak
trees with some maple. Today, we are seeing that there are much more maple
saplings (young trees) than there are oak saplings. Explain what has happened and
describe how the term “deer barren” applies (you are generalizing the
phenomenon your read about in this handout about “urchin barren”.
5
7. We have discussed that only about 10% of available energy gets passed up from
one trophic level another. Looking at the first (normal) food web in this packet,
trace the most efficient (least energy lost) pathway from primary producer to sea
otter. Why did you choose this as the most efficient?
8. ANSWER THIS QUESTION COMPLETELY AND WITH THOUGHT. Use an
additional page if required. Be sure this is legible!. In northeastern India, there
is a bamboo plant that produces flowers once every 50 years. Rats feast on these
flowers and subsequently swell in population. Once the flowers are no longer
available, the rats then turn to rice for a food source. At times, up to 70% of the
population experiences a food shortage due to this phenomenon. Suggest an
ecological approach that can be taken to stabilize this food web. There are a
couple different routes to take here but make sure you justify it AND hypothesize
any possible negative consequences of your action.
6