temperate estuaries (Deason and Smayda 1982;
Myerset al. 2007). These examples involve a wide
range of organisms and environments (Fig. 8.4),
and suggest that systematic depletion of predators
from the oceans is producing far-reaching indirect
impacts on the structure and functioning of many
marine ecosystems. I divide these examples into
rocky bottoms, continental shelves and pelagic
systems.
8.3.2.1 Rocky bottoms
The most famous and dramatic marine trophic
cascade involves the four-level food chain from
predatory orcas (killer whales) to sea otters to
grazing sea urchins to dominant kelps in the
northeast Pacific Ocean (Fig. 8.4a). After fur tra-
ders exterminated sea otters on several islands in
the 18th century, their sea urchin prey exploded
and in turn eliminated kelp forests, whereas kelp
beds remained vigorous on islands too remote for
otter harvesting (Estes and Palmisano 1974; Estes
and Duggins 1995). Loss of kelps on the otter-free
islands in turn led to pervasive ecosystem-level
changes, including local extinction of several ma-
rine species associated with the kelp habitat, pos-
sibly reduced abundances of coastal raptors that
depend on fishes, and increased coastal storm
damage stemming from the loss of buffering by
kelp forests (Mork 1996). In recent decades, killer
whales (orcas) began attacking sea otters, evident-
ly as the pelagic food chains supporting them
withered, and the effects of killer whale predation
cascaded down through sea otters and sea urchins
to reduce kelp again, demonstrating a four-level
trophic cascade (Esteset al. 1998). Similarly strong
cascades involving vertebrate predators, sea urch-
ins and macroalgae have since been documented
on rocky bottoms throughout the world. In kelp
beds of the western North Atlantic, archaeological
data and time series suggest that overfishing of
cod and other groundfish released grazing sea
urchins from predatory control, and cascaded
down to decimate kelps (Stenecket al.2004).In
the warmer Mediterranean Sea, experiments and
comparisons of marine protected areas with near-
by fished areas also showed that harvesting of
predatory fishes allowed urchins to proliferate
and overgraze macroalgae, converting large areas
to ‘barrens’ of structure-free coralline algal pave-
ments (Salaet al. 1998; Guidetti 2006). Finally, on
rocky reefs of Tasmania, predation by spiny lob-
sters and fishes on urchins cascades to macroalgae
(Shears and Babcock 2003; Pederson and Johnson
2006). In all these systems, loss of top predators
shifts a structurally complex, diverse community
dominated by macroalgae to a depauperate ‘ur-
chin barren’ dominated by crustose coralline
algae and maintained by intense grazing.
But cascades are not limited to urchin-domi-
nated communities. An intriguingly similar exam-
ple involves small herbivorous crustaceans as the
intermediate link. There have been several reports
of perennial seaweeds such as rockweeds (Fucus)
and giant kelp (Macrocystis) being decimated by
anomalous outbreaks of grazing amphipod and
isopod crustaceans (e.g. Kangaset al. 1982; Haah-
tela 1984; Tegner and Dayon 1987). For example,
after an El Nin ̃o warm-water event destroyed kelp
communities in California during the early 1980s,
recovering kelp beds were left without their nor-
mal assemblage of fishes, and populations of the
kelp-curler amphipod (Peramphithoe humeralis)ex-
ploded, devastating the kelps again, probably as a
result of relaxed predator control (Tegner and
Dayon 1987). Mesocosm experiments suggested
that particular species of grazing amphipods
might mediate these shifts in dominance by large
brown algae (Duffy and Hay 2000). Recent field
experiments in California support this hypothesis
(Davenport and Anderson 2007), showing that in-
vertebrate-feeding fishes reduced amphipod
abundances and grazing impact, and that these
effects in turn cascaded to increase kelp blade
growth by 100–300%, with a trend toward also
reducing kelp mortality. A survey of unmanipu-
lated kelp reefs similarly showed that amphipod
abundance was negatively related to that of fishes.
Intriguingly, many of these communities resemble
the ‘wasp-waist’ architecture found in some pel-
agic communities, in which the intermediate
trophic level is dominated by one or a few strongly
interacting species. This hints that low diversity
is an important mediator of these strong trophic
dynamics, as suggested by verbal theory
(Strong 1992; Duffy 2002), a point to which I return
below.102 APPLICATIONS