presences and unexpected absences. He used it to
identify what he termed holes and outliers. Holes
are where widespread species are absent from
otherwise rich faunas, and outliersare where
uncommon species occur in depauperate faunas.
He applied his index to the data for boreal
mammals and birds of the North American Great
Basin.He found that for mammals most of the
departures from perfect nestedness were due to
holes, whereas for birds most departures were due
to outliers. Cutler speculates that this difference
could be a function of the superior dispersability of
birds, allowing them to generate greater numbers
of outliers by recolonization events. ‘Supertramp’
species (Chapter 5) might also appear as outliers in
analyses of this sort. The preponderance of holes in
the mammal data supports the importance of
extinction as a driving force in this system, with the
holes representing departures from the generally
predictable sequence of species losses that have
occurred during the Holocene.
Such analyses might appear to provide impor-
tant information for conservation. However,
Simberloff and Martin (1991) rightly caution that, to
a large degree, extinction across these mountain-
top habitat islands has been inferred rather than
demonstrated to have happened. The simple statis-
tics can, moreover, mask more complex and impor-
tant underlying details. In illustration, they show
that the same nestedness score can be generated for
species with widely differing underlying distribu-
tions. In the Maddalena archipelago (Sardinia), two
species have a nestedness score of zero (by the
Wilcoxon statistic). One, the rock dove (Columbia
livia), is found across the entire range of island sizes,
and is absent from only 2 of 16 islands. The second,
the little ringed plover (Charadrius dubius), has been
observed only once in the archipelago, breeding
only in one year. As they point out, there may be dif-
ferent explanations for accordance or deviation
from nestedness in particular species. The explana-
tion of such patterns requires basic information on
species’ presence–absence, abundances, minimum
area requirements, habitat use at different sites, and
temporal regularity in occupying differing types of
patch. In their view, given such data, it becomes
possible to offer sensible input into conservation
decision-making—input in which the contribution
of the nestedness statistics is fairly limited.
Moreover, there are many different indices for
assessing nestedness, and much debate over
which is the best to use for particular purposes
(Rodríguez-Gironés and Santamaría 2006). Cook
(1995) undertook a comparative study of 38 insular
systems using 6 different indices of nestedness,
ordered both by area and richness. The analyses
ordered by richness returned higher nestedness
scores than those ordered by area in between 35
and 37 cases (depending on the metric used).
Cook’s analyses show that the broad patterns of
nestedness are robust, but that there is a danger
of over-interpreting the details of a nestedness
analysis.
Formal consideration of compositional structure
was, for too long, sidelined in the SLOSS debate
and related literature. The attention now being
given to species compositional patterns across sys-
tems of habitat islands is thus welcome (Fischer
and Lindenmayer 2005). A nestedness index can
provide one compositional descriptor and can per-
haps aid identification of risk-prone species; but
should not be given primacy in conservation plan-
ning. The discovery of nestedness at a particular
point in time does not necessarily provide clear
insights as to the probability of maintaining the
same sets of species (or any particular species) over
time (Simberloff and Martin 1991). The isolates may
be subject to turnover and/or species attrition in
new ways dictated by the changing biogeographic
circumstances of the landscape in which the frag-
ments occur. As Worthen (1996, p. 419) put it, nest-
edness is not a ‘magic bullet’,‘...no single index
should be expected to distil the informational con-
tent of an entire community, let alone predict how it
will react to habitat reduction or fragmentation’.10.10 Edge effects
The boundary zone, or ecotone, between two habi-
tats, being occupied by a mix of the two sets of
species, is often richer in species per unit area than
either of the abutting core habitat types. In the
reserve context, attention has focused on the
reserve edge, typically where a woodland reserve is276 ISLAND THEORY AND CONSERVATION