Noncompetitive mechanisms responsible for niche restriction and segregation - Nonequilibrium Ecology - page 85

similarity index (Rohde and Hobbs 1986 , 1999 ), that congeners overlap
less than non-congeners, and that those congeners that showed consider-
able overlap had markedly different copulatory organs, in contrast to
those that were spatially segregated. The only limiting factor for all of
these parasites is space for attachment, since food (blood, epithelial cells
and mucus) are in unlimited supply as long as the host is alive. But use of
space does not differ between congeners and non-congeners. The authors
concluded that not competition for space, but reinforcement of repro-
ductive barriers is responsible for niche segregation in those cases in
which it occurs. As phrased by Rohde ( 1991 ) ''if competition to reduce
resource exploitation was responsible for segregation, it should affect
species with similar or dissimilar copulatory organs in the same way''
(Figure 5.2 ). For a recent, very well documented study of niche segrega-
tion in 52 monogeneans of the genus Dactylogyrus infecting 17 species of
freshwater cyprinid fish see Jarkovsky et al.( 2004 , further references
therein), who showed that not only differences in the size and shape of
copulatory organs, but also of attachment organs are important for repro-
ductive segregation.
One of the most convincing studies of character displacement which
has led to reinforcement of reproductive barriers is by Kawano ( 2002 ),
who examined male morphology of two closely related rhinoceros
beetles, Chalcosoma caucasus and C. atlas in Laos, Thailand, Malaysia,
Indonesia and Mindanao. The beetles occur in 7 sympatric and 12
allopatric locations. The species has major and minor morph males.
Major morph males have large horns and fight with other major morph
males, whereas the smaller minor morph males, which have rudimentary
horns and relatively large wings, secure mating by emerging earlier in the
year and the day and by flying over longer distances, thus avoiding contact
with major morph males. The qualitative features and the variation in
each species is the same in allopatric and sympatric locations, and there is
almost complete overlap in dimorphism, body size, horn size, and size of
genitalia between the two species in the allopatric locations. In all sym-
patric locations, differences between species in all characters are highly
significant (Figure 5.3 ). In particular, differences in the size of genitalia are
much greater than would be expected if due to general body size dis-
placement (Figure 5.4 ) . The author concludes that the differences have
evolved to avoid interspecific competition and bring about reproductive
isolation. However, evidence for interspecific competition is fairly weak
if there is evidence at all, but there can be no doubt (in view of the much
greater size differences of genitalia than of body length in sympatric