Geoscience Reference
In-Depth Information
Table 5.1
Typical simple
subjective abundance classes and
their numerical equivalents.
photograph the area and use the photograph(s). This allows both
a more visual comparison between different areas for qualitative
measures (Section 5.5.1) of abundance, and image analysis of
the area to obtain a quantitative measure (Section 5.5.2).
Descriptor
Numerical
scheme
5.5.1 Presence/absence and qualitative
abundance estimates
Very abundant
5
5
The advantage of presence/absence scores is that they are easy
to obtain, but this ease has to be weighed against a substantial
information loss incurred because the amount of a particular
taxon is not recorded.
Abundant
4
Frequent
3
Occasional
2
Qualitative visual estimates of abundance are useful because
they are simple and rapid to use (Figure A1.1). Their popularity
has spawned a number of schemes designed to improve their
accuracy and comparability. The simplest scheme consists merely
of ranking the taxa within a sample into a series of abundance
classes with a quasi-logarithmic numerical scheme (Table 5.1).
Rare
1
The main problems with such a scheme are that small taxa
(fragments) tend to be underrated, consistency between different
workers is lower in the frequent, occasional and rare categories,
and clumping of taxa biases visual assessment. However, fully
quantitative abundance measures have not been shown to be
totally necessary or reliable, and may actually prove less
effective in revealing an assemblage pattern than the consistent
and reasonable objective use of visually assessed scales.
5.5.2 Quantitative measures of abundance
Frequency
The frequency of a taxon is a measure of the chance of fi nding
it within any one sample within a given set of samples. It is
usually expressed as a percentage. Sample size greatly affects
the observed frequency and should be stated. Frequency is also
particularly sensitive to specimen (fragment) size and
distribution in relation to sample size.
Density
Density is a measure of the number of individual fossils
(whether representing whole or fragmented organs or
organisms) of individual taxa that occur within a sample. As
such it is a direct, potentially accurate and relatively easy-to-
measure expression of abundance. Density measures also
contain a strong taphonomic component in that easily
fragmented material will tend to be overrepresented. Therefore
whole-body fossils should be counted separately from fragments.
Cover
Plant cover has been defi ned as 'the proportion of a planar unit
sample area, aligned parallel to the bedding, that is occupied
