Biology Reference
In-Depth Information
equator. The great complexity of coral reefs, often used as an example of
the importance of heterogeneity for species richness, is the result of, rather
than the reason for, great diversity, as pointed out by Rohde (
1998a
, see
above: productivity).
Narrower niches in the tropics
Several authors have suggested that greater species richness in the tropics
leads to denser species packing (e.g., MacArthur and Wilson
1967
;
MacArthur
1972
). One important aspect of a species' niche is its latitu-
dinal range. There have been extensive discussions in the literature on
Rapoport's rule, which claims that latitudinal ranges of organisms are
generally smaller at low rather than at high latitudes (Stevens
1989
). There
is controversial evidence for the generality of the rule, which will not be
discussed here. The reader is referred to the review in Rohde (
1999
).
However, smaller latitudinal ranges in the tropics (in cases where they
exist) do not give an ''explanation'' of greater numbers of tropical species,
they could also be the result of faster evolution at low latitudes, which has
led to closer species packing and hence smaller niches including latitudinal
ranges. Indeed, Rohde (
1998a
) suggested the existence of ''two opposing
trends: on the one hand, tropical species should have large latitudinal
ranges because temperature conditions in the tropics are uniform over a
much larger latitudinal range than at higher latitudes; on the other hand,
newly evolved taxa (in particular subspecies, most of them in the tropics)
that have little vagility and no dispersal stages, should have narrow ranges
because they did not have the time to spread away from their place of
origin even within the tropics. So, Rapoport's rule should apply to
recently evolved groups with little vagility, it should not apply to older
and more vagile groups'', but in either case they do not give an ''explan-
ation'' of the latitudinal diversity gradient. Moreover, Thorson's rule,
according to which marine benthic invertebrates in the tropics generally
have widely dispersing pelagic larvae, whereas benthic invertebrates at
high latitudes often have larvae produced by viviparity, ovoviviparity,
and brooding, counteracts Rapoport effects at least in the oceans (see
Rohde
1989
,
1992
for a discussion and references).
Latitudinal gradients in niche width of parasites of marine fish were
examined by Rohde (review
1989
, references therein). In the ectopara-
sitic Monogenea infecting the gills of fish, host ranges, the number of host
species used by a parasite species, are more or less the same at all latitudes,
although relative species diversity (the number of parasite species per
