Nonequilibrium in communities - Nonequilibrium Ecology - page 32

extinctions and the immigration and recolonization abilities of the vari-
ous species within. Disturbances increase heterogeneity by facilitating
local differentiation through random colonization and founder effects,
and through interrupting natural successions. For other important discus-
sions see Rosenzweig ( 1995 ) and the review by Petraitis et al.( 1989 ).
Hengeveld ( 1994 ) stressed the more general significance of variations
in living conditions, ''as living conditions vary in time and space, species
continually have to adapt spatially and genetically, implying that ecolo-
gical optimization theories based on equilibrium assumptions do not
apply.'' Harris ( 1986 ) demonstrated that environmental disturbances
occur with such frequency that competitive exclusion in phytoplankton
species does not occur, leading to nonequilibrium. These conditions may
explain Hutchinson's ''paradox of the plankton,'' i.e., the fact that many
more species using similar resources co-occur than expected in a compe-
titive equilibrial world. Nevertheless, even in phytoplankton there are
repeated seasonal patterns, i.e., there is a certain degree of predictability
imposed by strong environmental pressures.
A considerable body of evidence suggests that predation allows com-
peting species to coexist (e.g., Caswell 1978 and references therein). In a
model by Caswell, a predator initiates new ''cells for nonequilibrium
growth of the prey species,'' permitting their long-term coexistence.
He further suggests that interactions as represented in his model may be
of major importance in the real world.
We use an important paper by DeAngelis and Waterhouse ( 1987 )to
illustrate the factors that may be responsible for equilibrium in communities,
and those that lead to nonequilibrium. According to DeAngelis and
Waterhouse, equilibrium and stability are not sharply defined for real
systems. They distinguish stably interactive communities, unstably inter-
active communities and weakly interactive communities, referring to
May ( 1973 ) whose ''results, in particular, made theoretical ecologists
acutely aware that there are real difficulties with the idea of the ecosystem
as a balance of interacting species.'' There are two potentially disruptive
influences on ecosystems: (a) nonlinear feedbacks and time-lags in
the interactions of biological systems may lead to instability, and (b)
fluctuations of environmental factors may cause stochastic disruptions
(Figure 2.4 ) . Five hypotheses, illustrated in Figure 2.5 , have been pro-
posed to account for stability in spite of these influences. These are for a, (1)
certain types of predation, inter- and intraspecific interactions, and other