Biology Reference
In-Depth Information
concluded the differences may be fortuitous, which does not exclude the
possibility that they may have an adaptive value, i.e., enabling the species
to extract the blood from different parts of the gills, although this is not
necessarily due to competition for feeding sites. Most importantly, char-
acter displacement may be the result of reinforcement of reproductive
barriers. This is discussed fully in Chapter
5
.
A more general objection to the approach explaining differences
between species by competition for limited resources resulting in char-
acter displacement was made by Andrewartha and Birch (
1984
), who
point out that one should expect to find similar species in similar habitats.
One should rather ask: why can species with different ecological require-
ments live together? Another important point to be established is how
solid the evidence given for ecological displacement really is, a question
discussed next.
Habitat segregation as evidence for interspecific competition
Niches of sympatric species are never strictly separated, as shown by
two of many examples: five abundant tropical terns breeding sympatri-
callyshowedsomedegreeofsegregationintheirdietsandforaging
ranges, but there was nevertheless much overlap (Surman and Wooller
2003
), and two sympatric jackals in northwest Zimbabwe do not have
clear-cut niches and coexistence is probably possible because of extreme
flexibility in diet and behaviour (Loveridge and Macdonald
2003
).
Overall, it is highly unlikely and probably impossible that two species
overlap exactly in their niches including habitat, and a null model
should be based on the assumption that such differences are fortuitous.
Nevertheless, many authors have explained habitat segregation of spe-
cies by interspecific competition, without giving evidence or consider-
ing other explanations. Rohde (
1989
) drew attention to alternative
explanations for habitat segregation in helminths, usually explained by
interspecific competition. For example, Schad (
1962
,
1963
)examined
the distribution of eight species of the nematode genus Tachygonetria
(three subsequently transferred to Mehdiella) in the intestine of the Greek
tortoise, Testudo graeca. Four species had a paramucosal distribution, the
other four were found in the lumen; species in each group were
segregated to various degrees although all showed some overlap.
Species differ in body size, length of the male copulatory sclerites and
male genital papillae, etc. If species occurring in the lumen are arranged in
order of overlap, the series Tachygonetria numidica, T. macrolaimus, T. dentata,
