Download Comparative ecosystem dynamics

Comparative ecosystem dynamics
Kenneth T. Frank, Department of Fisheries and Oceans, Bedford Institute of
Oceanography, Dartmouth, Nova Scotia, Canada
• Outline:
• Structure of marine ecosystems (collaboration with B.
Petrie, N. L. Shackell, W.C. Leggett, J. Choi, J.A.D. Fisher)
• Focus on the eastern Scotian Shelf (NW
Atlantic)
– Cascading trophic interactions
• Comparative analyses: Is there a general
pattern of response to over-exploitation?
• Summary
• Structure of marine ecosystems
#
Marine food chains normally operate this way – bottom-up
TL 4
#
Positive
correlation
between
any
trophic
level in
food chain
#
TL 3
#
TL 2
Trophic Level 1
Time
Southeast Scotland
Frederiksen et al. 2006
Faroe Shelf
Steingrund and Gaard 2005
Tuna
Squid
Sardine
Shrimp
Catch of four main fisheries in Gulf of California
Under bottom-up control assume that:
• Variability at base of food chain determines
productivity at higher trophic levels
• Exploitation of top predators has no impact on lower
trophic levels in food chain
• Over-fishing effects are reversible
• So, in terms of EBM, why worry about anything else
but the target species
• In early 1990s groundfish stocks in the
Northwest Atlantic, particularly cod, were
over-fished and on the verge of collapse
NW Atlantic Cod stocks that collapsed in the 1990s
Location
% Decline
in Stock
Size
Year of
Moratorium
on Fishing
Labrador and
Northeast
Newfoundland
99.8
1992
Southern Grand
Bank
96.4
1993
95.9
1993
Southern Gulf of
St. Lawrence
85.4
1993
Eastern Scotian
Shelf
95.8
1993
Northern Gulf of
St. Lawrence
Despite long-term fishing moratoria nearly all have failed to recover
#
If top predators depleted then marine food chains may operate differently
TL 4
#
Negative
correlation
between
adjacent
trophic
levels in
food chain
#
TL 3
#
TL 2
Trophic Level 1
Time
• Large, complex systems considered immune to
cascading trophic interactions
• An exception to the rule - the eastern Scotian
Shelf
~108,000 km2
Data rich area:
Scientific surveys – 30+ years
Four trophic levels
Loss of entire functional group
161 cm
112 cm
120 cm
60 cm
76 cm
121 cm
73 cm
142 cm
108 cm
Response of major forage fish
species (planktivores)
- Release from predation
- Similar increases in macroinvertebrates
Other geographic areas responded the same way
TL4
TL3
TL1
TL2
• Large, complex
systems not immune
to cascading trophic
interactions
• Growing number of
examples in marine
ecosystems
Under bottom-up control assume that:
Exploitation of top predators has no
impact on lower trophic levels in
food chain
Over-fishing effects are reversible
Why worry about anything else but
the target species
Not true
Not so simple
Lots to worry about
Lack of recovery despite no fishing, revealed weakness of
knowing how ecosystems respond to exploitation
Not all fisheries collapse when heavily
exploited nor does the structure of all
systems undergo transformation
Comparative approach – 47 areas in the
Northern Hemisphere (literature review)
Published studies during past 10 years (a few earlier)
Time series of single (dominant) species or functional groups
TL1
TL2
TL3
TL 1
TL2
6
TL3
2
1
TL4
7
6
25
Surveys (local and large scale), landings data
-- Positive correlations among TLs
-- Negative correlations among TLs
Contemporary diagnosis of 47 systems
45
42
47
44
42
40
43
41
- Strong latitudinal gradient in structure
46
Water temperatures exhibit strong latitudinal gradients
< 5o C
> 6o C
Fish diversity exhibits strong latitudinal gradients
Western Atlantic
Number of fish species
Eastern
Coastal
Shelf
Abyssal
Latitude (deg)
Macpherson 2002
• In North Atlantic,
pattern of trophic
forcing related to
temperature and
species richness
western areas
eastern areas
All areas heavily exploited yet
species rich and warmer water
areas remain bottom-up
Potential mechanisms -from Frank et al 2006 TREE
• Temperature influences growth rate, age at maturity,
etc.; targeted species inhabiting warmer waters more
resilient to exploitation effects (Myers et al. 1997)
• Species richness acts through compensatory effects
where the probability that a depleted species will be
replaced by another is higher (e.g.Shackell and Frank
2007)
• Trophic cascades can be eliminated or reduced by
compensatory mechanism preventing the effects to propagate
down the food web
• Suggests warmer water, species rich systems can withstand
higher levels of exploitation without changing trophic structure
• Current research attempting to define exploitation thresholds
to avoid undesirable states
– linking exploitation rates, species richness, and temperature
across many different systems
Can withstand increasingly high
exploitation until top-down
Rapid change in system with
modest increase in exploitation
2oC increase in temperature
corresponds to 10% higher
exploitation threshold
Summary
• Trophic structure of warmer water, species rich
systems appears resilient to exploitation
effects; conversely, structure of colder water
systems much less resilient
– Expect fishing effects to be reversible on target
species in warmer water systems, but concern for
rapid re-direction of fishing effort on compensating
species
– Fishing effects not easily reversed in colder water
systems and large marine ecosystems are not
immune to trophic cascades
• Whether or not cod will recover remains
unknown – going on 15 years since exploitation
eliminated/greatly reduced
• Could be very difficult to return to former state simply by
wait and see approach
• May be in permanently altered state
• Should other types of intervention be
considered?
• Reintroductions as in terrestrial systems
• Culling/removal of forage fishes (since no interest in a
commercial harvest) or competitors (such as seals)
• Some elements of fishing industry satisfied with current
situation
• Thank you for your attention