Sea Otters and the Trophic Cascade Hypothesis

20
Sea Otters and the Trophic Cascade Hypothesis indirect effects in complex food webs

description

Sea Otters and the Trophic Cascade Hypothesis. indirect effects in complex food webs. The Sea Otter, Enhydra lutris. Sea Otter Taxonomy. Sea Otter Anatomy. Sea Otter Diets. Altogether more than 75 species are eaten by otters. Prey range from urchins to sea birds. - PowerPoint PPT Presentation

Transcript of Sea Otters and the Trophic Cascade Hypothesis

Page 1: Sea Otters  and the Trophic Cascade Hypothesis

Sea Otters and the Trophic Cascade

Hypothesis indirect effects in complex food

webs

Page 2: Sea Otters  and the Trophic Cascade Hypothesis

The Sea Otter, Enhydra lutris

Page 3: Sea Otters  and the Trophic Cascade Hypothesis

Sea Otter Taxonomy

Page 4: Sea Otters  and the Trophic Cascade Hypothesis

Sea Otter Anatomy

Page 5: Sea Otters  and the Trophic Cascade Hypothesis

Sea Otter Diets• Altogether more than 75 species are

eaten by otters. Prey range from urchins to sea birds.

• Most prey are taken shallower than 20 fathoms (120 ft.).

• The most preferred species are in order of importance urchins, abalone, cancer crabs, and large top shells.

• Three of the four top prey types are herbivores.

Page 6: Sea Otters  and the Trophic Cascade Hypothesis

Sea Otter Physiology• Sea otters use fine fur rather than blubber for

insulation. Fur shows highest hair shaft density in animal kingdom: 10,000/sq.cm..

• As small mustellids, they have high surface to volume ratios resulting in high heat loss.

• High metabolism and high heat loss generates demand for high caloric intake. Otters consume up to a third of their body weight in shellfish each day.

• Energetic demands and territorial habit means that herbivorous invertebrates often limited to very low densities in presence of sea otters.

Page 7: Sea Otters  and the Trophic Cascade Hypothesis

History of Otter Extirpation: Distributions Before and After Hunting by Russian

Furriers.

Page 8: Sea Otters  and the Trophic Cascade Hypothesis

Population Growth of Southern Sea Otter.

Page 9: Sea Otters  and the Trophic Cascade Hypothesis

Population growth thru

range expansion

Page 10: Sea Otters  and the Trophic Cascade Hypothesis

• Extirpation of the sea otter apparently allowed sea urchin populations to explode, causing “urchins barrens” on some coasts.

• This is a example of an indirect effect of otters on kelp.

Page 11: Sea Otters  and the Trophic Cascade Hypothesis

When urchins become more numerous than their algal forage can support, they

roam in pack, causing sea urchin barrens

Page 12: Sea Otters  and the Trophic Cascade Hypothesis

Intertidal brown algal beds and subtidal kelp beds on infrared

film

Page 13: Sea Otters  and the Trophic Cascade Hypothesis

Urchins, Strongylocentrotus spp.

Page 14: Sea Otters  and the Trophic Cascade Hypothesis

Sea Otter Abundances and Trophic Cascades

• Decimation of otters, sheephead, and lobsters in California, may have caused increases in sea urchin numbers, which in turn resulted in the decline in kelp bed distribution. The successive changes in abundances down thru successive trophic levels is termed a trophic cascade. Trophic cascades are characteristic of systems with keystone species.

• Presence or absence of top level consumers in Alaska provides a more certain example of trophic cascades. The top level consumers are humans or killer whales preying on otters and other members of the food web.

Page 15: Sea Otters  and the Trophic Cascade Hypothesis

Sea Otter food webs in Alaska

Page 16: Sea Otters  and the Trophic Cascade Hypothesis

Evidence from middens for “alternative stable states” mediated by sea otters.

Page 17: Sea Otters  and the Trophic Cascade Hypothesis

Midden data indicates Aleuts ate their way down the

food chain causing a kind of trophic

cascade.

Page 18: Sea Otters  and the Trophic Cascade Hypothesis

Kelp beds in Alaska, where sea otters are again plentiful, have few grazers and several stories of kelp

harbor fish

Page 19: Sea Otters  and the Trophic Cascade Hypothesis

Killer Whales occasionally move into a local region to prey on marine mammals. This

reduces otter populations. Open points are data from killer whale areas, solid from areas

without killer whales.

Page 20: Sea Otters  and the Trophic Cascade Hypothesis

When killer whales

move in, “trophic

cascades”.