easily grasped without reference to such com-
plicating features.
Figure 4.1 illustrates immigration and extinction as
hollow curves, for the following reasons. When the
number of species on an island is small relative to the
mainland source pool, P, a high proportion of
propagules arriving on an island will be of species
not resident on the island immediately before that
point. As the island assemblage gets larger, fewer
arrivals will be of species not already present, and so
the immigration rate (as defined in Box 4.2) has to
decline as Snears that of the total mainland pool. The
hollow form of the extinction curve recognizes that
some species are more likely to die out than others,
and that in accord with our knowledge of SADs, the
more species there are in a sample or isolate, the rarer
on average each will be, and so the more likely each
is to die out. At equilibrium, immigration and extinc-
tion rates should be approximately in balance and
thus equal the rate of species turnover (IET).
Islands of different size or different isolation may
have the same turnover, or the same species num-
ber, but they cannot have both the same turnover
and species number. Figure 4.1 thus shows how the
theory predicts different combinations of SandTas
a function of area and isolation. This reading of the
figure uses it as a representation of space. The dia-
gram may also be understood as a representation of
change through time for a particular island, com-
mencing with initial colonization by the first inhab-
itants, when immigration rate is high, species
number low, and extinction rate low. The curves
may be followed towards the equilibrium condition
when species number is high and stable, immigra-
tion rate has declined, and extinction rate has risen
to meet it. (However, note that the visual appear-
ance of the curves would be different if expressed
in normal units of time: as shown in Fig 4.4 the
increase in species richness per annum slows
greatly as IandEconverge.) If we take the case of a
small, near island, species number at equilibrium
will be Snsand the rate of turnover at equilibrium
will be Tns. The portion of the IandEcurves to
the right-hand side of their intersection can, of
course, be disregarded, as once they have met,
neither rate should vary further to any significant
degree; the island has reached its dynamic
equilibrium point.THE DEVELOPMENT OF THE EQUILIBRIUM THEORY 85Box 4.3 Definitions of terms involved in analyses of species turnover
MacArthur and Wilson (1967, pp. 185–191)
developed the following formulations:
Immigration. The process of arrival of a propag-
ule on an island not occupied by the species. The
fact of an immigration implies nothing concerning
the subsequent duration of the propagule or its
descendants.
Immigration rate. Number of new species arriv-
ing on an island per unit time.
Propagule. The minimal number of individuals of
a species capable of successfully colonizing a hab-
itable island. A single mated female, an adult
female and a male, or a whole social group may
be propagules, provided they are the minimal unit
required.
Colonization. The relatively lengthy persistence
of an immigrant species on an island, especially
where breeding and population increase are
accomplished.Colonization curve. The change through time of
numbers of species found together on an island.
Extinction. The total disappearance of a species
from an island (does not preclude recolonization).
Extinction rate. Number of species on an island
that become extinct per unit time.
Turnover rate. The number of species eliminated
and replaced per unit time.MacArthur and Wilson’s definitions are fine in
theory, but in practice difficult to apply in
empirical work. Very often available survey data
do not allow researchers to distinguish between a
species that has immigrated and one that is
observed in transit or in insufficient numbers or
condition to found a population. Similarly, total
disappearance of a relatively inconspicuous plant
or animal from a large, topographically complex
island may be more apparent than real.