P1 - Strong interactors that trigger cascades: SEA OTTERS...

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Strong interactors that trigger cascades: SEA OTTERS Can forage down to 60m Voracious: eat 20-25% body weight/day Prey on sea urchins voracious grazers that can extirpate kelp o Kelp is potential dominant, structuring immediate subtidal  URCHINS as strong interactors Can graze and detach kelp, bore thru basal stipe, export huge  amounts Collectively can wrestle and weigh down kelp Can persist/subsist on uptake of dissolved organic matter or  grazing microalgae (w/out large kelp) graze all new sporelings and  maintain “urchin barrens” KELP as strong interactor Prodigious growth rates and huge biomass Refuge for fin fish Wave break, decreases erosion in intertidal, causing sediment  deposition Supplies intertidal w/ detritus, allows mudflats to be deposited 2 vs 3 lvl food chains: Aleutian Islands, SE Alaska 2 trophic lvls: urchin barrens   sea urchin 3 trophic lvls: otter state   sea urchin   kelp Evidence  Rat Island (otters) o Complete cover of offshore kelp o Rare urchins o Lots of fin fish, harbor seals, bald eagles Near Island (no otters) o o Dense large mussels o Few fish, harbor seals, or birds Killer Whale Link 27 years of monitoring NE Atlantic revealed precipitous sea otter  decline in 1990s throughout archipelago off W Alaska, down to  10% of 1980s pop. Hypotheses:
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o Toxins (PCB’s) o Disease o Starvation o Climate Stress o Predation
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1991: 1 st  killer whale attack on otter observed; compute if whale  predation rate constant over 27yr census, then P(not observed  before 1990) = 0.006 Ecosystem responses: urchins inc. 8x, kelp dec. 10x where otters  missing Clam Lagoon (otter refuge too shallow for whales to enter): otters  still present, no other food web or ecosystem changes (urchins rare 
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P1 - Strong interactors that trigger cascades: SEA OTTERS...

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