PORCUPINE MARINE NATURAL HISTORY SOCIETY NEWSLETTER

44
species records by major regional blocks, from the
relatively species rich SW, through NW and NE to the
relatively species poor SE (Figure 5).
Fig. 5. Number of species recorded from each major
regional block.
A similar pattern was seen in the numbers of species
with different distribution permutations (Figure 6).
The largest group (to the left of the chart) represents
those species (47%) that have been recorded from all
four regional blocks. The second largest group (10%)
was for those species found in all areas except the
SE (XSE in Figure 6). The third largest group (8%)
included those species recorded only from the SW
(NB: in Figure 6 ‘SW’ represents SW only).
Fig. 6. Numbers of species recorded from different
permutations of distribution between regional blocks.
The key lists permutations in sequence, as they appear
clockwise in the chart, beginning with the ‘12:00’
position.
Improved detail could be seen on Sea Area maps
produced for individual species chosen to represent
each permutation. One example is shown below
(Figure 7) to represent ‘distributed in all regions
but the SE’, the most common regional distribution
permutation (other than ubiquitous).
PMNHS Newsletter No.24 Summer 2008
Fig. 7. Sea area distributions and photograph for
Lepedepecreum longicorne.
Discussion
While it was not possible to confidently identify
regional faunule boundaries with the data collated
to date, this analysis provides quantitative evidence
for a commonly observed pattern in the marine
biogeography of the British Isles: that is, species
richness is highest in the southwest and lowest in
the southeast. These patterns could be explained in
terms of sensitivity to low winter temperatures or
high summer temperatures, respectively for warm or
cold water species:
(i) highest species richness in the southwest, as
a result of warm water species, followed by the
northwest and northeast,
(ii) moderately increased species richness in the
northwest and northeast, as a result of cold water
species,
(iii) low species richness in the southeast, by
default.
The concepts are illustrated by the following
figure.
Fig. 8. Stylised warm and cold water faunal influences
around the British Isles; numbers of species involved
are indicated by thickness of arrows, which do not
necessarily imply short-term movement of animals.
There are other potential explanations for the
observed patterns. There may be low larval dispersal
to the southeast, due to the greater movement of
water over deeper areas. Another explanation might
be that habitat diversity is low in the southeast. The
fact that the area is deficient in deep water habitats
was factored out as far as possible, through the
elimination of deep water species and offshore Sea
Areas from the analyses. The low diversity of some
other habitats, such as hard substratum communities
remains a potential explanation for the pattern,
although the absence of certain biogenic habitats
could be considered a function of low species
richness in itself.