Biology Reference
In-Depth Information
account. However, for lakes of average size, lakes with relatively low
productivity had greatest diversity.
Tonn et al.(
1990
) have made an extensive and well documented
comparative study of species diversity of fish in small lakes in North
America (northern Wisconsin) and Finland. Environmentally, the lakes
are quite similar, but species composition is very different. Regional fish
diversity in Finland is only about half that in northern Wisconsin, a result
of different glaciation effects. Pleistocene extinctions in northern Europe
were widespread because of the lack of nearby refuges (Banarescu
1975
,
cit. Tonn et al.
1990
), and mainly large, long-lived, and migratory species
survived (Mahon
1984
, cit. Tonn et al.
1990
; Moyle and Herbold
1987
,
cit. Tonn et al.
1990
), whereas in Wisconsin the Mississippi River basin
provided a nearby refuge and center of speciation (Burr and Page
1986
),
favouring species with short life spans and narrow habitat ranges (Mahon
1984
, cit. Tonn et al.
1990
). In other words, North American fish species
are more specialized (references in Tonn et al.
1990
). Tonn et al. demon-
strated the following differences for the fish assemblages studied by
them. In Wisconsin, assemblages were of the presence-absence type,
i.e., small fish prey species were absent from lakes containing piscivores,
and there was habitat specialization. In contrast, in Finland, assemblage
types could be distinguished only on the basis of relative abundance of
species. Plotting local against regional diversity revealed an asymptotic
relationship in Wisconsin; in Finnish lakes the asymptotic part of the
regression line was absent. This may mean that competition may have
limited species numbers in Wisconsin local communities, but the authors
stress that isolation, ecological impoverishment, and habitat severity of
small forest lakes and size-limited predation are probably more important.
''Currently, a general explanation accounting for various local-regional
relations cannot be provided.''
In conclusion, historical events such as glaciations, long isolation of
islands from a large continent, and latitude and area, are important in
determining richness of freshwater fish communities, but evidence for the
role of productivity is ambiguous.
Latitudinal diversity gradients: equilibrium and
nonequilibrium explanations
Anybody who has visited a tropical rainforest and a temperate forest,
anybody who has dived on a coral reef and a cold-water shore, and anybody
who has visited a tropical fishmarket and one in a temperate climate,
