Biology Reference
In-Depth Information
are not effective. A further problem addressed is the significance of spatio-
temporal heterogeneity in maintaining diversity.
It is important to understand details of the line of argument used by
Connell. Connell (
1979
) discusses the theoretical background for his
studies. Following Caswell (
1978
), he distinguishes between closed and
open systems. In a closed system, a population is found in a single, fairly
homogeneous habitat without immigration. In an open system, popula-
tions in different ''cells'' are linked by migration of individuals between
them, although populations are still more or less independent, since
exchange of individuals is relatively limited. This corresponds to what is
usually defined as a metapopulation, a term not used by Connell. The
''open'' system seen as a whole may well be a closed system, in spite of the
open nature of its components. Local extinction in open but not in closed
systems can be overcome by recolonization.
The definition of equilibrium is scale-dependent. Connell follows the
usage of Caswell, according to whom equilibrium applies to what is
happening within a habitat cell: a system is equilibrial when, for example,
competitors within a cell can coexist as a result of some underlying
mechanism. However, a whole system can also be considered to be
equilibrial, even when one competitor always out-competes another in
a particular cell, as long as competitors coexist indefinitely in the system.
Connell defines ''gaps'' as empty cells created by disturbances. A gap
will first be invaded by species that are adapted to early invasion. Once a
gap is filled, these early colonizers cannot successfully immigrate any
longer, because their adaptations to early immigration (such as their
ability to germinate or attach in exposed habitats, and high metabolic
rates) are not suited to late invasion under changed conditions. However,
if, for example, major disturbances are repeated, early invaders will have
the chance to colonize newly created gaps and the whole system remains
open and in nonequilibrium. In contrast, lack of further major disturb-
ances will lead to the dying out of early colonizers, i.e., for these species,
the habitat is now a closed nonequilibrium one. Repeated minor dis-
turbances will permit intermediate and late colonizers to fill the gaps, i.e.,
for them the system is open and nonequilibrial. If disturbances are very
rare, only late-succession species can survive, and the system is either
open nonequilibrial or closed equilibrial. Nonequilibrium in this case
depends on the existence of ''compensatory mechanisms'' that permit
coexistence of species. If such mechanisms do not operate, one species
will eliminate all others, leading to a closed and equilibrial system. Connell
mentions as a possible compensatory mechanism frequency-dependent
