Biology Reference
In-Depth Information
opportunities provided by species j for the formation of new species i,
(t
T )
¼
time lag in species diversification, (t)
¼
factor indicating vari-
ability due to environmental effects, and n
¼
total number of species.
The main difference between species diversification and population
growth is that, whereas the population size of a species is reduced in the
presence of competing species, species numbers are increased due to the
opportunities created by species, particularly for parasitic and other
''dependent'' species. Rohde (
1980d
) concluded that species saturation
has not been reached at least in some free-living groups and probably in
none of the parasitic and other ''dependent'' groups, and more species
have accumulated in warm environments due to faster evolutionary rates
at high temperatures (for a detailed discussion of temperature effects on
evolutionary speed see Rohde
1992
, and the section on ''Latitudinal
gradients'', pp. 152-165). Each first order species (plants, herbivores,
predators) can support many second order species (parasites, symbionts,
commensals), but there is a reduced number of higher order species
(hyperparasites of various degrees), which may be due to the more
precarious lifestyle with low colonization and high extinction rates,
resulting in an extreme time lag which has not yet permitted colonization.
Evolution is slower in cold environments (e.g., Rohde
1992
). Therefore,
saturation of habitats with species would be reduced at high latitudes,
with greater relative species diversity (number of species per host species)
of some parasite groups in tropical than in cold-temperate seas, and in the
Pacific than in the Atlantic Ocean. There would be diffusion of many
tropical species into cold-temperate regions, and occurrence of many
''progressive'' traits in the tropics. However, exceptions are possible.
Thus, it seems possible that in certain situations, e.g., the great East
African lakes with their large numbers of recently evolved cichlid fish
species, parasites could not keep up with the diversification of their hosts,
which may have led to reduced relative species richness (number of
parasite species per host species) of the parasites. Also, paradoxically, in
spite of the higher saturation with free-living organisms at low latitudes,
the number of extant vacant niches for ''dependent'' species, such as
parasites or symbionts, may be even greater in the tropics, because of
the larger number of potential host species.
Future studies should aim towards getting estimates for numbers of
''secondary'' species such as symbionts, parasites, etc., and assessing their
contribution to the overall diversity. Estimates of local and global species
richness generally ignore them, but they are of overriding importance not
only for estimating current diversity but also for estimating numbers of
