Sea otters and kelp forests

James A. Estes

University of California Santa Cruz

+ -

Scope of presentation

1. Questions

2. Approaches

3. Findings

4. Management and Conservation

Scope of presentation

1. Question

2. Approaches

3. Findings

4. Management and Conservation

Question?

What are the effects of sea otters on kelp forest ecosystems?

Scope of presentation

1. Question

2. Approaches

3. Findings

4. Management and Conservation

Approaches

I--Food web structure II--History

Players and Processes

CO2

Production & Habitat

Aleutian archipelago

Southeast Alaska

Vancouver Island

Approach:

• With vs. without otters

• Before vs. after otters

Sampling methodology for kelp and sea urchins

Scope of presentation

1. Question

2. Approaches

3. Findings

4. Management and Conservation

Direct effect--sea otters on sea urchins

0

100

200

300

400

500Otters absent

Otters present

Aleutian Islands SE Alaska

Urc

hin

abundance (

g 0

.25m

-2)

In situ grazing

-20

0

20

40

60

80

100

Otters Abundant Otters Absent

Perc

en

t L

oss p

er

24 h

r

Experimental

Control

Indirect effect—sea otters on

kelp

0

1

2

3

4

5

6Otters absent

Otters present

Aleutian Islands SE Alaska

Kelp

abundance (

no. 0.2

5m

-2)

A “trophic cascade”

Transitions between kelp forests and urchin barrens?

Eco

syste

m S

tate

Otter abundance Absent Carrying

Capacity

0

5

10

15

20

25

Ke

lp D

en

sity

0 200 400 600 800

Urchin Biomass

Ecosystem State vs. Otter Density (skiff surveys)

0

0.5

1

0 2 4 6 8 10 12 14 16 18

Otter Density (# per km surveyed)

Ec

os

ys

tem

Sta

te (

0 =

urc

hin

, 1

= k

elp

)

Raw Data

Logistic Function P

Threshold Density =6.3 otters/km

Attu 2000

Influences of sea otter-kelp forest trophic cascade on other species and

ecosystem processes?

CO2

Production & Habitat

Production

Abundant

Absent

Sea otters

0

5

10

15

Mussel growth

mm

yr

1

Duggins et al., Science, 1989

Kelp forest fish abundance

Abundant

Absent

Sea otters

Fish abundance

0

10

20

30

40

50

Catc

heffort

Reisewitz et al., Oecologia, 2006

Glaucous winged gull diet

Abundant

Absent

Sea otters

Irons et al.,Ecology, 1986

0

20

40

60

80

100

% D

iet

Fish Inverts Fish Inverts

Bald eagle diet

Abundant

Absent

Sea otters

Anthony et al., Ecology, 2008

0

20

40

60

80

% D

iet

Bird Fish Mamm Bird Fish Mamm

Sea otters and carbon

-

-

+

CO2

Wilmers et al. Front. Ecol. Env, 2012

Mitigation by sea otters?

Data i. Areal extent of sea otter habitat—A0

ii. Area of rocky reef habitat—Ar

iii. Kelp biomass density—Bw (with sea otters) and density—Bwo (without sea otters)

iv. Percent kelp dry mass--Pd

v. Percent dry mass that is carbon—Pc

From which, we can compute carbon density,

o

rdcd

A

APPBC

Production (flux)

Kelp production to biomass

3.1 - 5.0 yr-1

Reed and Brzezinski 2009

Storage effect in living kelps

Sea otters absent Sea otters present

Kelp biomass (wet weight) 75-133 gm-2 911-1618 gm-2

Kelp carbon 8-14 gCm-2 101-180 gCm-2

NPP 25-70 gCm-2yr-1 313-900 gCm-2yr-1

Difference in scenarios with and without otters

throughout their North American range

Kelp carbon 4.4 to 8.7 TgC

Atmospheric carbon pool 5.6 to 11 %

Atmospheric carbon pool

since pre-industrial times 21 to 42 %

Value of kelp carbon standing

stock $304 to $603 million

Sequestration Effect

Sequestration

scenarios*

Carbon sequestration tCyr-1 Yearly value*

1% (1.3 - 4.5) x 105 $9-31 million

5% (0.6 - 2.3) x 106 $44-157 million

10% (1.3 – 4.5) x 106 $87-314 million

50% (0.6 – 2.3) x 107 $436-1,570 million

* Yearly value is based on Dec 10, 2010 Futures on the European Carbon Exchange.

Atmospheric CO2

Sea otters

Coastal Fishes

Sea urchins

Kelp

Gulls

Bald Eagle

Sea otters

Coastal Fishes Coastal Fishes

Sea urchins

Kelp

Gulls

Bald Eagle

Darwin’s simile “A tangled bank”

Concluding Points

I. Sea otters have a strong direct negative effect on kelp forest invertebrates (shellfish)

II. This direct negative effect on invertebrates results in a strong indirect positive effect on kelp abundance and distribution

III. Kelp forests have myriad effects of other species and ecological processes

IV. Sea otter management should weigh the costs and benefits of all these effects