Once distributed across the Pacific Rim, sea otters were hunted to near extinction in the 18th and 19th centuries. Less than 1000 remained by the early 1900's in the Aleutian Archipelago. A predator-prey population model is used to describe the top-down control that sea otters have in kelp forests.
1. High density of sea otters,
Low density of macroinvertebrates (e.g. sea urchins, limpets, chitons)
High density of macrophytes, fish and harbour seals
2. No sea otters
High density of macroinvertebrates
Low density of kelp, fish and harbour seals
Sea otters feed on macroinvertebrates, especially sea urchins, keeping their population in check. When some populations of sea otters were harvested to extinction in the Aleutian Archipelago, the macroinvertebrate populations grew unchecked. The macroinvertebrates grazed heavily on kelp leaving little food or shelter for fish, which declined in number. In turn, the harbour seals, which feed predominantly on fish, also declined in number. The threshold density of sea otters required to shift the system back to State 1 is unknown. Also, the threshold density of macroinvertebrates that can be sustained before the numbers of macrophytes, fish and harbour seals declines is unknown.
State 1: No control on otter harvesting by Aboriginal Aleuts.
State 2: Sea otters were protected in the early 1900's. The Alaska Department of Fish and Game reintroduced animals to southern Alaska in 1968-1971.
Contact
Jacqui Meyers
Email
jacqui.meyers@csiro.au
CSIRO Sustainable Ecosystems,
PO Box 284,
Canberra ACT 2601
Keywords
empirical data, model, population harvesting, sea otters, seals, predator-prey interactions, Aleutian Islands, kelp, sea-urchins, colonization, bottom-up forces, top-down forces, recruitment, predation, Alaska, herbivores, oceans, populations
