0198566123.pdf

The higher densities of rodent populations indi-
cated by the crowding effect are an important part of
this analysis. As already highlighted, because islands
tend to have fewer species on them than equivalent
continental areas, population densities will differ
between islands and mainlands, in effect as a statis-
tical artefact. Williamson (1981) therefore distin-
guishes the notions of density compensationand
density stasis. Density compensation is where
island communities have the same total population
density but distributed over fewer species, i.e. with
large population sizes per species (above). Density
statis is where the overall population of the commu-
nity on the island is less than that of the reference
mainland system, such that population sizes per
species are the same as the mainland. Distinguishing
between the two conditions is tricky and necessitates
an appropriate experimental design.
According to Adler and Levins (1994) a number of
studies of island rodents have avoided the pitfalls
identified by Williamson, demonstrating that higher
average densities of rodent populations do indeed
occur in some cases on islands. They cite several
examples of experimental studies using fenced
enclosures, which have shown that rodent popula-
tions in such circumstances can reach abnormally
high densities and often destroy the food supply
(the ‘fence effect’ of Table 7.3.), thereby indicating
dispersal to be an important regulator of population
densities under normal circumstances in which
there is no (complete) fence. As rodent populations
on small islands lack an adequate dispersal sink, the

fence effect may be invoked as a force providing

selective influence on the population.

One of the mainstays of island ecologicalanalysis

in recent decades has been the study of the relative

significance of island area and isolation (Chapter 4).

Adler and Levins’ (1994) scheme extends to dealing

with how these factors may work in an island evo-

lutionarycontext (see also Adler et al. 1986). They

hypothesize that average population density

might, in general, increase with increasing isolation

and hence with reduced dispersal, but that this

fence effect might decline with an increase of island

area to the point where it disappears altogether as

islands more closely resemble a mainland. Thus the

island syndrome might be expected to occur only in

islands which are: (1) sufficiently isolated, (2) not

too large so as to resemble mainland, and (3) not so

small that they cannot support persistent popula-

tions. Figure 7.7 illustrates the differing effects that

area and isolation are envisaged to have. Effects

which may be expressed largely as reaction norms

in the short term i.e. within the range of phenotypes

of the founding population, may, under sustained

selection pressure, produce rapid evolutionary

change and locally adapted island populations that

differ from mainland populations. The effect of iso-

lation is a direct one, on dispersal, whereas that of

area is less direct. Larger islands and mainlands

typically have more predators, competitors, and

habitat types, and because of this (and especially

because of predation effects), densities are

depressed compared with those of small islands.

NICHE SHIFTS AND SYNDROMES 193

REDUCED

DISPERSAL

GREATER

NEIGHBOUR

FAMILIARITY

REDUCED

AGGRESSION

MORE

STABLE

SOCIAL

STRUCTURE

GREATER

POPULATION

STABILITY

FOUNDER

POPULATION

BETTER

SURVIVAL

REDUCED

PREDATION

HIGHER

DENSITY

REDUCED

REPRODUCTIVE

EFFORT

GREATER

BODY

SIZE

Isolation

Reduced

area

Figure 7.7Schematic diagram showing the initial effects of island isolation and area on rodent populations. Long-term effects of insularity are
directional selection for increased body size, reduced reproductive output, and reduced aggression (Redrawn from Adler and Levins 1994, Fig. 2).