Community Ecology Processes, Models, and Applications

more profitable for fishers to target other species,
which provides the overfished species with a refuge
and allows them to recover. This explanation is
consistent with the negative relationship between
fished taxa richness and interannual variation in
catch (Wormet al. 2006, see also Fig. 8.5).

8.4.2.3 Mechanisms

What is the mechanism for these putative effects
of biodiversity on ecosystem resilience? One likely
candidate is functional compensation (or functional
‘redundancy’) among species within a guild or
functional group, such that decline of one species
is compensated by increase in another species with
similar functional characteristics, e.g. through
relaxed competition. Evidence potentially consis-
tent with this mechanism comes from the tropical

Atlantic, where longline fisheries for billfish show a

pattern of sequential depletion of species (Myers

and Worm 2003), with decline of blue marlin in

the 1960s accompanied by a rise in catch of sailfish,

which then declined in turn as swordfish catches

increased through the late 1970s and 1980s. The

result was that total billfish catch remained relative-

ly stable through time despite boom and bust pat-

terns in the catch of individual species. Similar

patterns have been observed in demersal ecosys-

tems, using both catch data (Myers and Worm

2003) and fisheries-independent data (Shackell

and Frank 2007), in which declines of targeted fish

species were accompanied by compensatory

increases in other groups. These patterns of species

turnover are very similar to that predicted by

theory when a consumer imposes mortality on

100 80

80

60

40

20

0

40

30

20

Per cent recovery Fished taxa richness

Average catch (Gg / yr)

Productivity in catch (%)

Coefficient of variation

Collapsed taxa (%)

10

12 34

0

–10

1234

70

60

50

40

30

20

10

0

150

120

90

60

30

0

12

0.9^3500

0.8

0.7

0.6

0.5

0.4

0.3

0 50 100 150

Fished taxa richness Fished taxa richness

(log) Species richness

r= 0.48

p= 0.0001

r= –0.57

p< 0.0001

r= 0.57

p< 0.0001

r= 0.57

p< 0.0001

r= 0.73

pr= 0.0291= –0.30 p< 0.0001

(log) Species richness

(a) (b)

(c) (d)

(e) (f)

(log) Species richness

200

3000

2500

2000

1500

1000

500

0

0 50 100 150 200

34

123

(log) Species richness

4

Figure 8.6Correlations between biodiversity (fish species richness) and the productivity and resilience of fishery catches
across 64 large marine ecosystems. Reproduced with permission from Wormet al. (2006).

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