conditions change. Second, and in contrast, func-
tional redundancy can enhance stability against
perturbations that cause extinction by reducing
the probability that extinction removes a function-
ally unique species. The last two phenomena are
sometimes combined as the insurance hypothesis
(Yachi and Loreau 1999).
In the context of applied marine ecology, a ques-
tion of particular interest is how diversity influ-
ences resistance to human predation, i.e. fishing.
Are more diverse marine food webs more stable in
response to such anthropogenic perturbations? The
theory most relevant to the case of fishing effects in
marine systems involves the role of prey richness in
buffering the community from predator impacts.
Leibold (1989, 1996) argued that a resource base
with more species is more likely to contain at least
one species that is resistant to consumption and can
dominate in the presence of a consumer, such that a
more diverse prey community will maintain higher
aggregate biomass under predation. Similarly, if
different prey species are resistant to different pre-
dators, or gear types in the case of fishing, then a
diverse prey community will maintain higher bio-
mass under fishing pressure. This general argu-
ment has been extended to suggest that trophic
cascades also should be most prevalent in low-
diversity systems, with one or a few important
species at each trophic level (Strong 1992; Duffy
2002).
8.4.2 Evidence linking diversity and stability in marine systems
Growing empirical evidence suggests that chang-
ing horizontal (i.e. within-trophic level) diversity
can have several important consequences for ma-
rine food web interactions and ecosystem processes
(Emmerson and Huxham 2002; Duffy and Sta-
chowicz 2006; Stachowiczet al. 2007). Experimental
research has supported a stabilizing effect of diver-
sity on community biomass in some competitive
plant assemblages, aquatic microbial food webs
and soil microfaunal communities (reviewed by
McCann 2000; Cottinghamet al. 2001; Loreauet al.
2002). There is also some experimental evidence
that marine species diversity can enhance trophic
level resistance to top-down control (Hillebrand
and Cardinale 2004; Duffyet al. 2005) and to inva-
sion by other native (France and Duffy 2006) and
non-native species (Stachowiczet al. 1999, 2002a).8.4.2.1 Comparisons through time
Observational evidence from fishery science corro-
borates predictions (Tilmanet al. 1997; Doaket al.
1998) that diversity can enhance stability of aggre-
gate biomass or production (i.e. overall catch), both
in the general sense of reducing long-term fluctua-
tions and in the specific sense of providing resis-
tance to perturbations of fishing and environmental
forcing. For example, time series of fish biomass
from the North Sea show that aggregate biomass
is less variable than that of individual fish species
(Fig. 8.5; Jennings and Kaiser 1998), supporting the
suggestion that diversity enhances general stability.
Diversity can also provide resistance to specific
perturbations. Among the most intriguing cases is
the link between stock diversity and productivity of
Alaskan salmon under decadal-scale climate varia-
tion (Hilbornet al. 2003b). Because salmon return to
the streams or lakes of their birth to spawn, popula-
tions are genetically highly structured into distinct
genetic populations or ‘runs’ that differ substantial-
ly in life history, phenology and ecology. Hilborn
et al. (2003b) used historical catch records for Bristol
Bay sockeye salmon (Onchorhyncus nerka) dating
back to the 1890s to show that the relative contribu-
tions of different populations to total salmon catch
differed greatly through time as individual popula-
tions responded differently to long-term variation
in climate forcing by the El Ninˇo Southern Oscilla-
tion (ENSO) and the Pacific Decadal Oscillation
(PDO). The population-specific variability in re-
sponse to changing environmental conditions re-
sulted in an aggregate salmon catch that was more
stable through time than was that of any individual
population (Hilbornet al. 2003b). This link between
stock diversity and stability in response to human
predation is consistent with the proposed impor-
tance of niche differentiation as a mechanism by
which biodiversity can stabilize biomass and pro-
duction (Loreauet al. 2002).
Similarly, there is considerable evidence that func-
tional diversity of herbivores is important to main-
taining coral dominance over algae on tropical reefs.
In the Caribbean, overharvesting of herbivorousSTRUCTURE AND FUNCTIONING OF EMERGING MARINE COMMUNITIES 105