Geoscience Reference
In-Depth Information
CHAPTER 14
Taxonomic Distinctness as a Measure
of Diversity of Stream Insects Exposed
to Varying Salinity Levels in
South-Eastern Australia
Richard Marchant
1
and Ben Kefford
2
1
Department of Entomology, Museum of Victoria, Melbourne, Australia
2
Department of Environmental Sciences, University of Technology, Sydney, Australia
Introduction
species richness and most other biotic indices,
which are cumulative measures that increase with
sample size. Delta
+
is an average measure rather
than a cumulative one. Thus it is potentially of
great value when the aim is to relate a measure
of biological diversity to environmental disturbance
based on data collected over a large spatial scale
where it is likely that sampling effort and perhaps
even sampling techniques vary.
An additional advantage is that it is possible to
test delta
+
for departures from expectation. An
expected value can be calculated from the master
list of all species likely to be encountered in a
region. As delta
Average taxonomic distinctness (or delta
+
)isa
measure of biodiversity that has been applied
successfully to various marine communities (Clarke
and Warwick, 2001) but has not been used with
freshwater communities until recently. Marchant
(2007) applied it to a large dataset of benthic insects
from rivers in Victoria, Australia and others have
used it on more restricted sets of freshwater taxa
(Heino
et al
., 2005; Abellan
et al
., 2006; Bhat and
Magurran, 2006).
Delta
+
is a measure of the taxonomic breadth
of a sample based on the average taxonomic
distance apart of every pair of species present.
Distance is derived from a taxonomic tree (or
hierarchy) incorporating species, genera, families
and so on. Thus it acknowledges that a measure
of diversity should incorporate information not
just on the number of species present, but also
on their degree of relatedness, giving lower scores
to samples consisting of closely related species. It
also has the great advantage of not being sensitive
to variations in sampling effort (as demonstrated
mathematically by Clarke and Warwick, 1998; see
Methods for an empirical demonstration), unlike
does not vary with sampling
effort a sub-set of species (say the species found
at a single site) will be expected to have the same
delta
+
value as that for the master list. If the value
for a site is lower than expected, then a true loss
of diversity has occurred and for aquatic habitats
this is usually the result of habitat disturbance,
pollution or other forms of degradation (Clarke and
Warwick, 2001; Marchant, 2007).
Increasing levels of salinity are a widespread
occurrence in rivers in south-eastern Australia,
largely as a result of clearing of vegetation
by European settlers,
+
and have
long been
