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

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Presentation on theme: "Sea Otters and the Trophic Cascade Hypothesis indirect effects in complex food webs."— Presentation transcript:

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

2 The Sea Otter, Enhydra lutris

3 Sea Otter Taxonomy

4 Sea Otter Anatomy

5 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.

6 Sea Otter Physiology Sea otters use fine fur rather than blubber for insulation. Fur shows highest hair shaft density in animal kingdom: 10,000/ 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.

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

8 Population Growth of Southern Sea Otter.

9 Population growth thru range expansion

10 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.

11 When urchins become more numerous than their algal forage can support, they roam in pack, causing sea urchin barrens

12 Intertidal brown algal beds and subtidal kelp beds on infrared film

13 Urchins, Strongylocentrotus spp.

14 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.

15 Sea Otter food webs in Alaska

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

17 Midden data indicates Aleuts ate their way down the food chain causing a kind of trophic cascade.

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

19 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.

20 When killer whales move in, “trophic cascades”.

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