Biology Reference
In-Depth Information
host species) is much greater in tropical waters. In the endoparasitic
Digenea, host ranges are larger at high latitudes. However, if the intensity
and frequency of infection with particular parasite species is considered
(using Rohde's
1980e
host specificity index), host specificity is very great
and similar at all latitudes, both for the Digenea and Monogenea (Rohde
1978c
,
1993
). The reason is that, in cold seas, even digenean species with
broad host ranges infect very few host species heavily. All this means that
niche width, at least along the niche dimension host specificity, is not
narrower in high diversity habitats. Microhabitat width of ectoparasites of
marine fishes is also not wider in the tropics than in cold-temperate waters.
Alternative explanations
Many authors have used positive correlations to find ''explanations'' of
patterns, ignoring alternative explanations and ignoring historical events.
For example, correlations between tree diversity and contemporary cli-
mate and, in particular, energy, have been used to claim that explanations
of tree diversity based on extant factors are sufficient and that historical
processes or events are superfluous towards explaining the patterns (refer-
ences in McGlone
1996
). However, it is important to point out that
correlations cannot give an explanation of a pattern, but, at best, suggest
an explanation. This point was well made by McGlone (
1996
) who stated
that diversity-energy correlations of tree diversity are strong only at
regional scales, and cannot predict diversity at small plots within latitu-
dinal bands, or between continents. Also, ''tree diversity cannot have
responded to global glacial-interglacial energy fluctuations because plant
species cannot evolve that rapidly nor, in most areas of the world, can
migration plausibly adjust floral diversity. Thus, contemporary climate or
energy, while yielding excellent correlations with plant diversity, has no
explanatory power.'' Contemporary climate merely acts as a surrogate for
past climatic changes (see also Latham and Ricklefs
1993
; Francis and
Currie
1998
). More generally, positive correlations between energy input
(temperature) and diversity do not explain diversity patterns in the sense
that temperature somehow limits diversity (see for example, the recent
study by Hawkins et al.
2003
, who reviewed the empirical literature and
concluded that contemporary climate constraints and especially measures
of energy, water, or water-energy balance are not the only, but they
are the most important factors explaining spatial variation in species
richness). Such correlations may also mean that temperature is involved
in determining diversity via acceleration of evolutionary processes.
