Alaska Sea Grant

Teacher Background for Investigation 3

Results

Type of Data or Observation
(Intertidal Zone)

Rat Islands
(Amchitka)

Near Islands
(Shemya)

Sea Otter Presence/Absence

Abundant (10-30/km2 of habitat)

Scarce or absent

Kelp Presence/Absence

Complete mat of kelp

Kelp sparse or absent

Average Intensity of Grazing

<1% of kelp plots grazed

50-75% of kelp plots grazed

Average Urchin Density (per m2.)

8

78

Urchin Size (Length in Inches)

<12 mm

Maximum of 21 mm

(Adapted from Estes and Palmisano. 1974.)

Conclusions:

1. The differences between the two benthic (bottom) communities on the Near and Rat islands are probably related to the presence or absence of sea otters.

2. The otters control the sea urchin populations, and the absence of grazing pressure allows the kelp beds to flourish.

3. Reducing the population of sea otters makes it possible for sea urchin populations to increase, and this leads to a significant reduction in the size of kelp beds and associated animals.

Evaluation:

James Estes and his colleagues did repeat the study in 1986-87 (when otters had reached the Near Islands and were thought to be at maximum density), and during the 1997-2000 period. In the ten-year period between 1987 and 1997, they found that sea urchin size and density increased to produce 8 times the amount of biomass, while kelp density decreased to one-twelfth. The percentage of kelp grazed increased from 1.1% per day in 1991 to 47.5% in 1997—all indicative of an “urchin barrens” in the absence of a high density of otters.

The data they collected at Amchitka in 1997 were similar to those for Adak (high urchin numbers, small urchin size, sparse kelp beds, high grazing rates on kelp).

Sample Data Table:

1990s Observations

Type of Data or Observation
(Intertidal Zone)

Rat Islands
(Amchitka)

Near Islands
(Shemya)

Sea Otters

 

Few

(Teacher fills in)

Few

(Teacher fills in)

Kelp

 

(Student Prediction)

"little" or "not much"

(Student Prediction)

"little" or "not much"

Urchins

(Student Prediction)

"lots" or "many"

(Student Prediction)

"lots" or "many"

 

seagrant UAF logo Alaska Department of Education and Early Development noaa
© 2007 - 2017 Alaska Sea Grant