Community Ecology Processes, Models, and Applications

food webs (Fig. 8.3). On a global basis, 70% of
documented extinctions were of predators, while
most invasions were at lower trophic levels, trun-
cating the typical trophic pyramid into a vertically
compressed food web dominated by filter-feeders
and scavengers. Thus, invasion by exotic species
did not counteract the trophic skew caused by
predator depletion but in fact exacerbated it.
One important exception to the general pattern
of declining species richness with trophic height,
with important implications for community dynam-
ics, involves so-called ‘wasp-waist’ ecosystems, in
which one or a few species dominate intermediate
trophic levels, such that their particular biological
characteristics control trophic transfer from primary
production to upper levels of the food web (Cury
et al. 2000; Hunt and McKinnell 2006). The classic
example involves the herbivorous sardines and
anchovies that dominate upwelling ecosystems
throughout the world. Typically, wasp-waist has
been used to refer to pelagic ecosystems. But these
pelagic systems also bear some functional similari-
ties to benthic systems in which the herbivore tro-
phic level is dominated functionally by a single
species of sea urchin. In both cases, factors influen-
cing abundance of these key intermediate species

have a major influence on overall ecosystem struc-

ture and functioning, and can shift the system

between alternate semi-stable states (Salaet al. 1998;

Bakun 2006).

In summary, many marine food webs historically

had a characteristic ‘shape’, with diversity and

abundance generally declining, and body size in-

creasing, with height in the food chain. Human

impacts change the shape of marine food webs

predictably, tending to reduce average food chain

length and skew communities toward dominance

by small-bodied, fast-maturing omnivores, detriti-

vores and suspension-feeders. Trophic skew thus

appears characteristic of human-influenced marine

systems, and altered top-down control should be a

central consequence in the sea.

8.3 Trophic cascades in the sea

8.3.1 Conceptual background

What are the broader consequences of the system-

atic shortening of marine food chains? Hairston,

Smith and Slobodkin (HSS; Hairstonet al. 1960)

initiated one of the longest running controversies

in ecology with their assertion that trophic-level

–14.0%

–26%

–29%

Recorded

extinctions

Original trophic

distribution

Recorded

invasions

–65.1%

Projected distribution

(after 25% turnover)

Current distribution

(after 5.1% turnover)

–24.6%

+50.0%

–3.3%

–5.4%

+8.6%

–0.1%

–24%

–21% +67%

Figure 8.3The changing shape of a coastal marine food web, the Wadden Sea, The Netherlands, based on Byrnes
et al. (2007). Bars represent successive trophic levels: primary producers (dark grey); herbivores, deposit feeders,
detritivores and zooplankton (hatched); omnivorous consumers (light grey); and carnivores and parasites (open).

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