Community Ecology Processes, Models, and Applications

stronger top-down control generally and trophic
cascades in particular (Shurinet al. 2002), and an
altered distribution of biomass among trophic levels.
For example, pristine coral reefs support (or did
historically) large populations of apex predators
(Friedlander and DeMartini 2002) but often little
visible plant life, in stark contrast to the rain forests
found above the tide line. In short, top-down control
and trophic transfer are more efficient in the sea, and
marine biomass pyramids tend to be (or were,
primevally) less bottom-heavy than those on land
(Odum 1971; Del Giorgio and Gasol 1995). These
strong trophic interactions should enhance the
ability to detect predicted responses to food web
alteration in marine systems relative to terrestrial
ones.
The second major consequence of water’s density
and buoyancy of biomaterials is the greater impor-
tance in the sea of advection of materials (inorganic
nutrients, detritus) and organisms. In certain terres-
trial systems, migrating birds, mammals and even
insects can transport large quantities of materials
over long distances (Poliset al. 1997). Nevertheless,
constantly moving currents make marine ecosys-
tems more open on average than terrestrial sys-
tems. Although pelagic marine communities and
populations are more highly structured than
might be expected from the superficially featureless
appearance of their habitat, many large predators
nevertheless can swim between ocean basins, and
larvae of many species can drift for hundreds of
kilometres before settling. These features mean
that between-habitat subsidies, source-sink dynam-
ics and gene flow tend to be considerably higher, on
average, in marine communities than on land or
freshwater. They also suggest that simple models
of community structure and dynamics that implic-
itly assume closed systems may be less likely to
apply in the sea, where metacommunity ap-
proaches will probably prove fruitful (see Leibold
et al. 2004; Chapter 5).
The third important consequence of water’s den-
sity, stemming from both the microscopic size of
most primary producers and system openness, is
the much closer and more rapid coupling between
physical drivers and biological processes in marine
(pelagic) systems than those on land (Steele 1985).
For example, nutrient loading can produce re-

sponses of primary producers within days or even

hours in the sea. Thus, many marine communities,

especially pelagic communities, tend both to be

more sensitive to disturbances and to rebound

more rapidly after disturbance than terrestrial

ones. Together with strong trophic interactions,

this sensitivity should enhance the ability to detect

predicted processes and patterns of community

regulation on large spatial and temporal scales.

8.2 The changing shape of marine food webs

8.2.1 Conceptual background

Like other optimal foragers, humans generally tar-

get large and abundant prey preferentially, all else

being equal. Fishing thus represents not only a

strong, but also a selective press perturbation on

marine communities, which has been sustained

for decades and even millennia in some areas

(Wing and Wing 2001; Barrettet al. 2004; Lotze

et al. 2005). The responses of marine communities

to this strong top-down influence depend on both

‘vertical’ components (food chain length, omniv-

ory) and ‘horizontal’ components of biodiversity

(species or functional group richness and composi-

tion within trophic levels), and their interactions

(Duffy 2002; Duffyet al. 2007; Fig. 8.1). These in

turn are mediated by organismal traits. Specifically,

focusing on the key related traits of body size

(Woodwardet al. 2005), feeding traits that deter-

mine trophic level (Paulyet al. 1998) and life history

(Jenningset al. 1998) reveals several apparently

consistent patterns in the changing structure and

functioning of marine ecosystems, and clarifies the

mechanisms involved.

An important consequence of the complex pelag-

ic life histories of many marine animals is that they

pass through a large range of body size, and multi-

ple trophic levels, during their lifetimes. Most new-

born marine animals receive no parental care – in

stark contrast to the birds and mammals that domi-

nate upper trophic levels on land – and are there-

fore highly vulnerable to predation, starvation and

abiotic stress. Importantly, the larvae of apex pre-

dators frequently serve as prey of fishes that the

apex predators hunt as adults. Thus, compared

STRUCTURE AND FUNCTIONING OF EMERGING MARINE COMMUNITIES 97