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