prevent extinction at low population density. Alter-
native stable states of a population can also result
from size dependence of vital rates such as recruit-
ment, growth and fecundity (Botsford 1981). For
simple predator–prey systems, the range in types
of regime shifts can be generated from the same
model simply by changing parameters, particularly
the ratio between prey carrying capacity and
predator half-saturation constant (K/D), the rela-
tionships between maximal predation rate and
prey growth rate, and the minimum timescale
for shifts (Collieet al. 2004). Alternate attractors
may also arise from idiosyncrasies of species beha-
viour. For example, pelagic ecosystems often show
rapid shifts between decadal-scale states domi-
nated by different planktivorous fish species, such
as sardine and anchovy. At high population density
such fishes are usually found in ‘pure’, monospecif-
ic schools, but, when reduced to low density by
fishing or other processes, their strong schooling
inclination causes them to join schools of other
species, which may place them into conditions
that are poor for feeding and reproduction, driving
their population further toward decline (Curyet al.
2000).
Regime shifts in predator–prey interactions may
also be mediated by the ontogenetic shifts in trophic
level (Fig. 8.2) common in both benthic (Barkai and
McQuaid 1988) and pelagic (Swain and Sinclair
2000; Bakun 2006) ecosystems. In many such sys-
tems, the dominant species in one of the alternative
states feeds on early life history stages of the alter-
native dominant, generating an unstable feedback
loop that prevents the alternative dominant from
gaining abundance. A potential example of this
phenomenon involves the collapse of cod in the
Baltic Sea, which was accompanied by a shift to
dominance by the cod’s ‘wasp-waist’ prey, plankti-
vorous herring and sprat (Bakun 2006). Cod have
failed to rebound from their initial collapse
throughout much of the north Atlantic, despite se-
vere fishing restrictions, probably in part because
abundant herring and sprat feed heavily on cod
eggs and larvae. Thus, for pelagic marine food
webs, the ontogenetic size structuring of trophic
interactions may be a key factor in mediating com-
monly reported regime shifts between alternate sta-
ble states (Bakun 2006).
Regime shifts may also result from effects of
organisms on the environment. In benthic systems
in particular, ecosystem engineers or other species
may modify the environment such that it becomes
less hospitable to species characteristics of the al-
ternative regime. Certain taxa of infaunal inverte-
brates, for example, are both more tolerant of
mobile sediment resuspension and more active in
resuspending it; these may prevent establishment
of species that would otherwise dominate in stable
sedimentary environments (Peterson 1984; van
Neset al. 2007). Similarly, in lakes and probably
also in estuaries, clear-water phases are main-
tained in part by growth of benthic macrophytes,
which bind sediment, preventing its resuspension;
whenmacrophytesarelostforwhateverreason,
the mobility of both sediments and sediment-
bound nutrients foster turbidity in the water col-
umn, which resists re-establishment of benthic
macrophytes.
Finally, a link between changing biodiversity and
regime shifts, though not rigorously studied, is sug-
gested by several lines of evidence. First, rapid
transitions in ecosystem state appear to be better
documented in relatively low-diversity systems, in-
cluding temperate lakes, the Black Sea (Daskalov
et al. 2007) and the North Atlantic (Franket al. 2007).
In particular, wasp-waist ecosystems appear espe-
cially prone to rapid, pronounced ‘regime shifts’
between alternate semi-stable community states,
and this vulnerability has been attributed in part
to the low diversity and low resilience of this inter-
mediate trophic level. Second, experiments show
that invasion of marine communities by exotic spe-
cies, which can trigger irreversible shifts in ecosys-
tem structure and function, is generally more
frequent in communities of low diversity (Stacho-
wiczet al. 1999, 2002a).8.7 Emerging questions in emerging marine ecosystems
The ocean of the 21st century is changing at rates
and in directions never before seen in human histo-
ry. The causes involve both abiotic changes – in-
cluding eutrophication, habitat alteration, and,
increasingly, climate warming and acidification –
and direct alteration of community structureSTRUCTURE AND FUNCTIONING OF EMERGING MARINE COMMUNITIES 113