12.5. Questions and Tasks
243
otters increase their population size in the long run, urchin populations are
significantly decimated and kelp forests become re-established.
12.5. Questions and Tasks
1. Change the carrying capacity of each of the three species, one at a time,
keeping the other values constant.
a) What effects do changes in the carrying capacity have on the species
whose carrying capacity was not changed?
b) How do those changes affect interspecies dynamics?
2. Additional stressors on kelp and urchins are harvesting activities.
a) How can harvesting be incorporated into the model?
b) Are there harvesting rates that would result in roughly the same overall dynamics? If so, what are they?
3. It is well known that otters have a very narrow foraging range , only foraging 1-2 kilometers off the coast and up to 25m deep. Urchins, on the
other hand, have been found in depths of 200-400 meters (California
State University 2000). Additionally kelp can grow up to 61 meters in
height (Catalina Conservancy 2000).
a) How can these depth considerations be included into the model?
b) What effects do these depth variables have on interspecies dynamics?
5. Although not reflected in the model, otters also prey on other shellfish,
as well as finfish. Include these other food sources in the model. Quantitatively, how does the ability of food-switching affect the amount and
distribution of urchins in the model?
6. Despite the model 's assumption, sea otters may not necessarily be the
top-level predator of this ecosystem. In the late 1990s it was noticed that
killer whale (Orcinus orca) populations in Alaska had shifted their diet
to include sea otters . It has been hypothesized that this shift may have
resulted from overfishing in the North Pacific and Bering Sea (Stephens
1998). Expand the model to include killer whales, fish and fishermen .
7. Recalculate the feeding rate of urchins based on the formula given in the
text to a "worst case" or other scenario. Be sure to keep the way you determine what diameter to use constant. How does this change the dynamics between the kelp and urchins?
KELP-URCHIN-OTTER MODEL
Kelp
Drift_Kelp_Biomass(t) = Drift_Kelp_Biomass(t-dt) +
(Kelp_Die_off-Urch_eat_Drift-Drift_Kelp_Exits_System) *
dt
INIT Drift_Kelp_Biomass = 1 {kg}
243
otters increase their population size in the long run, urchin populations are
significantly decimated and kelp forests become re-established.
12.5. Questions and Tasks
1. Change the carrying capacity of each of the three species, one at a time,
keeping the other values constant.
a) What effects do changes in the carrying capacity have on the species
whose carrying capacity was not changed?
b) How do those changes affect interspecies dynamics?
2. Additional stressors on kelp and urchins are harvesting activities.
a) How can harvesting be incorporated into the model?
b) Are there harvesting rates that would result in roughly the same overall dynamics? If so, what are they?
3. It is well known that otters have a very narrow foraging range , only foraging 1-2 kilometers off the coast and up to 25m deep. Urchins, on the
other hand, have been found in depths of 200-400 meters (California
State University 2000). Additionally kelp can grow up to 61 meters in
height (Catalina Conservancy 2000).
a) How can these depth considerations be included into the model?
b) What effects do these depth variables have on interspecies dynamics?
5. Although not reflected in the model, otters also prey on other shellfish,
as well as finfish. Include these other food sources in the model. Quantitatively, how does the ability of food-switching affect the amount and
distribution of urchins in the model?
6. Despite the model 's assumption, sea otters may not necessarily be the
top-level predator of this ecosystem. In the late 1990s it was noticed that
killer whale (Orcinus orca) populations in Alaska had shifted their diet
to include sea otters . It has been hypothesized that this shift may have
resulted from overfishing in the North Pacific and Bering Sea (Stephens
1998). Expand the model to include killer whales, fish and fishermen .
7. Recalculate the feeding rate of urchins based on the formula given in the
text to a "worst case" or other scenario. Be sure to keep the way you determine what diameter to use constant. How does this change the dynamics between the kelp and urchins?
KELP-URCHIN-OTTER MODEL
Kelp
Drift_Kelp_Biomass(t) = Drift_Kelp_Biomass(t-dt) +
(Kelp_Die_off-Urch_eat_Drift-Drift_Kelp_Exits_System) *
dt
INIT Drift_Kelp_Biomass = 1 {kg}