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Page 496 Top Down or Bottom Up? Bottom Up Control resources control community NVHP Top Down Control Predators control the community NVHP 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 471 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 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.