0198566123.pdf

ISLAND ASSEMBLY THEORY 123

Box 5.2 How consistent are species incidence functions?

Incidence functions are simple analyses of the
frequency of occurrence of individual species in
relation to one or sometimes two properties of a
set of islands: typically species richness, area,
and isolation. Bierdermann (2003) provides an
interesting analysis of area–incidence
relationships of 50 species of vertebrates and
invertebrates in which he demonstrates that
area requirements increase essentially linearly
with increasing body size on a log–log scale.
Bierdermann rightly cautioned that it should not
be assumed that species incidence functions are
invariant. In illustration, Hinsley et al. (1994)
have shown for birds of English woodlands that
they may vary through time in relation to
density independent mortality (extremes of
weather). It might also be the case for habitat
islands that they vary across the range of a
species or as a function of the properties of the
landscape in which the fragments are
embedded.
Watson et al. (2005) tested this idea for three
fragmented woodland landscapes in south-
eastern Australia, all sufficiently close that climate
and the regional species pool can be assumed
similar. The landscapes were: (1) an agricultural
landscape, used for both pastoral and arable
farming; (2) a peri-urban landscape featuring
pastures, hobby farms, and small urban areas; and
(3) an urban landscape, namely the city of
Canberra. They used logistic regression to
establish sensitivity of species incidence to area,
and to isolation in each landscape separately.
Results demonstrated considerable variability in
the incidence functions between the three
landscapes and among species.
They may be categorized as follows:
8 species showed no sensitivity to area or isolation
in any landscape
4 species showed minimum area thresholds in all
landscapes
4 species showed minimum area thresholds only
in the urban landscape
1 species showed a minimum area threshold but
only in peri-urban and urban landscapes
3 species showed maximum isolation thresholds
only in the agricultural landscape
6 species showed area effects in all landscapes
and isolation effects (according to the

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Isolation (m)

Area (ha, log scale)

Agricultural

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Isolation (m)

Area (ha, log scale)

Peri-urban

Urban

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Isolation (m)

Area (ha, log scale)

(a)

(b)

(c)

Box Figure 1. Area-isolation incidence plots for the eastern yellow

robinEopsaltria australisin three fragmented woodland

landscapes in south-eastern Australia. Open circles are unoccupied

remnants, closed circles are occupied remnants. The species

displays a compensatory pattern in the agricultural landscape (a);

and minimum area patterns in both the peri-urban (b) and urban

(c) landscapes. Dashed lines represent the threshold where the

probability of occurrence reaches 50%, as determined by logistic

regression (From Watson et al. 2005).