Sea otters and kelp forests - University of Alaska Southeast

Sea otters and kelp forests
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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
Production & Habitat
CO2
Southeast
Alaska
Aleutian
archipelago
Approach:
• With vs. without otters
• Before vs. after otters
Vancouver
Island
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
Urchin abundance (g 0.25m-2)
500
Otters absent
Otters present
400
300
200
100
0
Aleutian Islands
SE Alaska
In situ grazing
100
80
Percent Loss per 24 hr
Experimental
60
Control
40
20
0
Otters Abundant
-20
Otters Absent
Kelp abundance (no. 0.25m-2)
Indirect effect—sea otters on
kelp
6
Otters absent
5
Otters present
4
3
2
1
0
Aleutian Islands
SE Alaska
A “trophic cascade”
Ecosystem State
Transitions between kelp forests
and urchin barrens?
Absent
Otter abundance
Carrying
Capacity
25
Kelp Density
20
15
10
5
0
0
200
400
Urchin Biomass
600
800
Ecosystem State vs. Otter Density (skiff surveys)
Ecosystem State (0 = urchin, 1 = kelp)
1
Attu 2000
Raw Data
Logistic Function P
0.5
Threshold Density =6.3 otters/km
0
0
2
4
6
8
10
12
Otter Density (# per km surveyed)
14
16
18
Influences of sea otter-kelp forest
trophic cascade on other species and
ecosystem processes?
Production & Habitat
CO2
Production
Sea otters
Mussel growth
Abundant
mm yr
1
Absent
15
10
5
0
Duggins et al., Science, 1989
Kelp forest fish abundance
Sea otters
Fish abundance
Abundant
Absent
Catch effort
50
Reisewitz et al., Oecologia, 2006
40
30
20
10
0
Glaucous winged gull diet
Sea otters
100
Abundant
% Diet
Absent
80
60
40
20
0
Irons et al.,Ecology, 1986
Fish Inverts
Fish Inverts
Bald eagle diet
Sea otters
80
Absent
% Diet
Abundant
60
40
20
0
Anthony et al., Ecology, 2008
Bird Fish Mamm
Bird Fish Mamm
Sea otters and carbon
Mitigation by sea otters?
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+
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CO2
Wilmers et al. Front. Ecol. Env, 2012
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,
B  Pc  Pd  Ar
Cd 
Ao
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
25-70 gCm-2yr-1
313-900 gCm-2yr-1
NPP
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.
Darwin’s simile
“A tangled bank”
Bald Eagle
Sea otters
Gulls
Sea urchins
Coastal
Fishes
Kelp
Atmospheric
CO2
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