Top Down or Bottom Up? 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 +/- Page 496

Keystone Species 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 Page 471

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.

Sea Otters Key Stone Predator in North Pacific –Once extremely abundant, reduced to near extinction in the early 1900’s 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.

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.