Top Down or Bottom Up

1
Top Down or Bottom Up?
Page 496
Bottom Up Control ? resources control community N
? V ? H ? P Top Down Control ? Predators control
the community N ? V ? H ? P Top down control
Trophic Cascade Model Freshwater Pond For
Example Phytoplankton ? Zooplankton ? Small
Fish ? Large Fish Remove large fish then small
fish increase, zooplankton decreases and
phytoplankton increases. Effects will be
propagated up and down food chain as a /-
2
Keystone Species
Page 471

• A species that occupies a specific niche that is
  extremely important in determining community
  structure.
• When that species is removed, the community
  dramatically changes
• Not typically the most common species in a
  community

3
Pisaster ochraceous (a starfish)

• Keystone species in the rocky intertidal
  communities of western North America.
• Is a strong predator for a mussel (Mytilus
  californianus)
• The starfish can not eat large mussels, so the
  mussels have a size-related refuge from predation
• This mussel can out-compete other invertebrates
  for space, but the starfish takes away that
  competitive edge.
• When the starfish were removed, mussel numbers
  increased and excluded other invertebrates and
  algae from attachment sites.

4
Sea Otters

• Key Stone Predator in North Pacific
• Once extremely abundant, reduced to near
  extinction in the early 1900s by the fur trade
• Feed heavily on sea urchins and thus can control
  their populations
• Sea urchins feed heavily on macroalgae (kelp) and
  where sea urchin abundance is high, kelp is
  basically nonexistent
• Where sea urchin abundance is low, kelp is common
  along with all of the other species associated
  with it.

5
Case Study

• Sea otters have declined (sometimes 25 per year)
  in Alaska since about 1990, and the kelp beds
  have begun to disappear as sea urchins increased.
• Killer whales are suspected because their prey
  base (seals, sea-lions) has declined, and their
  predation on sea otters has increased.
• Seals and Sea-lion population declines have been
  attributed to a decline in their food base
  (fish).
• Fish declines have been attributed to
  overharvesting in the North Pacific.
• So, overharvesting of fish may have led to a
  cascade of events that were unexpected.