62
Martin V. ANGEL
samples, the more representative becomes the species
list. Even so, the inventory of species for a region
may only approach completion after several years of
thorough and systematic sampling at all depths and at
all seasons. Estimates of species richness are strongly
influenced by the sampling methods and procedures,
and also by chance events that may result in the
exceptional occurrence of species in an area.
The second aspect of biodiversity is described as
dominance or conversely evenness, and is dependent
on the proportional representation of individuals of the
component species in a community or sample. Populations dominated by many individuals of a very few
species are considered to be of low diversity, whereas
a population in which all the species are represented
by similar numbers of individuals are considered to be
of high diversity. A samples containing 100 specimens
of two species divided 50:50 is considered to be richer
than another also containing 100 specimens but with
three species divided up 98:1:1.
Studies of diversity in pelagic communities have
revealed some consistent trends related to depth and
latitude. The number of pelagic species present in
the total water column increases from high to low
latitudes. The number of species present also increases
to a maximum at a depth of around 1000 m (Fig. 3.8)
and slowly declines at greater depths (Angel, 1997).
The relatively species-poor pelagic assemblages that
occur at high latitudes tend to be dominated by a very
few species. As one moves towards lower latitudes
the number of species increases but the dominance
decreases (or alternatively the evenness increases). The
same trend occurs with increasing depth, at least to a
depth of 1000 m. However, these patterns run counter
to the trends on productivity. Where productivity is
higher – for example, where there is strong seasonality,
upwelling or eutrophication – the assemblages become
less species-rich and the dominance by a few species
increases.
The latitudinal trends are exemplified by data from
collections of copepods from 44ºN, 13ºW (Roe, 1984),
from the Western Mediterranean (Scotto di Carlo
et al., 1991) and from the Canary Islands (Roe, 1972)
(Fig. 3.9). The sampling procedures were not the same,
so that the results are not strictly comparable; but the
differences in the results are striking. They also serve
to highlight another general characteristic of pelagic
assemblages – namely, that they contain large numbers
of infrequent and apparently rare species. However,
r
r
r
r
r
r
r
r
r
r
r
r
r
r
n
n
r
n
n
n
n
n
n
n
n
n
n
n
r
r
r
r
r
r
r
r
r
r
s
s
r
r
r
s
Numbers of species
0
10
20
30
40
0
1000
Depth m
q
q
q
q
q
q
q
q
q
q
q
q
n
11ºN
18ºN
30ºN
40ºN
44ºN
53ºN
60ºN
s
s
s
s
s
s
s
s
s
s
s
s
Fig. 3.8. Profiles of the numbers of planktonic ostracod species
throughout the surface 2000 m along 20ºW in the N.E. Atlantic,
showing how at latitudes lower than 40ºN there are more species
at all depths than at higher latitudes. The change in average species
richness appears to occur at the southern boundary of seasonal turnover in the near-surface waters and the regions where there is a
springtime peak in phytoplankton production (Angel, 1997).
rarity in such samples may arise for a number of
reasons:
(1) The sampling range may not have extended over
the normal range of that particular species, so the
specimens caught were outliers.
(2) The mesh size may be too large to retain most
specimens.
(3) The species may normally be active enough to
avoid the sampler in use.
(4) The analysts may have overlooked many of the
specimens, especially those species that are either
very fragile or very similar to other abundant
species.
(5) At the time of sampling most specimens of the
species may have been at a life-history stage that
was either not sampled or unidentifiable.
Martin V. ANGEL
samples, the more representative becomes the species
list. Even so, the inventory of species for a region
may only approach completion after several years of
thorough and systematic sampling at all depths and at
all seasons. Estimates of species richness are strongly
influenced by the sampling methods and procedures,
and also by chance events that may result in the
exceptional occurrence of species in an area.
The second aspect of biodiversity is described as
dominance or conversely evenness, and is dependent
on the proportional representation of individuals of the
component species in a community or sample. Populations dominated by many individuals of a very few
species are considered to be of low diversity, whereas
a population in which all the species are represented
by similar numbers of individuals are considered to be
of high diversity. A samples containing 100 specimens
of two species divided 50:50 is considered to be richer
than another also containing 100 specimens but with
three species divided up 98:1:1.
Studies of diversity in pelagic communities have
revealed some consistent trends related to depth and
latitude. The number of pelagic species present in
the total water column increases from high to low
latitudes. The number of species present also increases
to a maximum at a depth of around 1000 m (Fig. 3.8)
and slowly declines at greater depths (Angel, 1997).
The relatively species-poor pelagic assemblages that
occur at high latitudes tend to be dominated by a very
few species. As one moves towards lower latitudes
the number of species increases but the dominance
decreases (or alternatively the evenness increases). The
same trend occurs with increasing depth, at least to a
depth of 1000 m. However, these patterns run counter
to the trends on productivity. Where productivity is
higher – for example, where there is strong seasonality,
upwelling or eutrophication – the assemblages become
less species-rich and the dominance by a few species
increases.
The latitudinal trends are exemplified by data from
collections of copepods from 44ºN, 13ºW (Roe, 1984),
from the Western Mediterranean (Scotto di Carlo
et al., 1991) and from the Canary Islands (Roe, 1972)
(Fig. 3.9). The sampling procedures were not the same,
so that the results are not strictly comparable; but the
differences in the results are striking. They also serve
to highlight another general characteristic of pelagic
assemblages – namely, that they contain large numbers
of infrequent and apparently rare species. However,
r
r
r
r
r
r
r
r
r
r
r
r
r
r
n
n
r
n
n
n
n
n
n
n
n
n
n
n
r
r
r
r
r
r
r
r
r
r
s
s
r
r
r
s
Numbers of species
0
10
20
30
40
0
1000
Depth m
q
q
q
q
q
q
q
q
q
q
q
q
n
11ºN
18ºN
30ºN
40ºN
44ºN
53ºN
60ºN
s
s
s
s
s
s
s
s
s
s
s
s
Fig. 3.8. Profiles of the numbers of planktonic ostracod species
throughout the surface 2000 m along 20ºW in the N.E. Atlantic,
showing how at latitudes lower than 40ºN there are more species
at all depths than at higher latitudes. The change in average species
richness appears to occur at the southern boundary of seasonal turnover in the near-surface waters and the regions where there is a
springtime peak in phytoplankton production (Angel, 1997).
rarity in such samples may arise for a number of
reasons:
(1) The sampling range may not have extended over
the normal range of that particular species, so the
specimens caught were outliers.
(2) The mesh size may be too large to retain most
specimens.
(3) The species may normally be active enough to
avoid the sampler in use.
(4) The analysts may have overlooked many of the
specimens, especially those species that are either
very fragile or very similar to other abundant
species.
(5) At the time of sampling most specimens of the
species may have been at a life-history stage that
was either not sampled or unidentifiable.