Biology Reference
In-Depth Information
communities. As in reef fish discussed in the previous section, centers of
diversity have the greatest number not only of all species but of endemic
species as well.
Freshwater fishes: diversity is determined by the effects
of latitude, area, and history, but the effect
of productivity is ambiguous
Freshwater systems contain roughly 40% of the Earth's fish species and
almost 20% of all vertebrates (Myers
1997
,p.126).Figure
9.8
shows that
both area and latitude are correlated with species richness of freshwater fish,
but historical events are also important. Richness is markedly greater in
North America than in northern Eurasia, although both regions are at
roughly the same latitude. The difference can be explained by the presence
of refuges during the Ice Ages in North America but not in Eurasia,
permitting the survival of more species in the former than the latter
(Tonn et al.
1990
). Madagascar is tropical and has large freshwater bodies
(600 000 ha), but its diversity is very low (even when marine species
regularly or sporadically invading freshwater are included), probably
because of its long (Pre-Tertiary) isolation from larger continents (Kiener
and Richard-Vindard
1972
).
Concerning fishes in freshwater lakes, the greatest numbers of species
are found in some large African tropical lakes. For freshwater lakes,
productivity is generally higher at low than at high latitudes, although
there is much overlap (net primary productivity of tropical lakes
100-7600mg C/m
2
per day, 30-2500 g C/m
2
per year; temperate lakes
5-3600mg C/m
2
per day, 2-950 g C/m
2
per year) (Likens
1975
, p. 192).
Lakes Victoria, Tanganyika and Malawi (121 500 km
2
) have a total of
about 1450 freshwater fish species (17% of the Earth's total) (Myers
1997
,
p. 127). Of these, Lake Malawi (28 231 km
2
) contains at least 550 fish
species. In contrast, the North American Great Lakes (246 900 km
2
)
contain 173 fish species (Myers
1997
, p. 127), Lake Baikal has 39
(Sheremetyev, personal communication). Lake Baikal and the North
American Great Lakes together contain 31% of the Earth's freshwater,
but have only 212 fish species. Annual productivity (g C/m
2
per year)
compares as follows: North American Great Lakes, c. 100 to 310 (Wetzel
1975
, Table 14.10), 80-90 to 240-250 (Likens
1975
pp. 194-195); Lake
Baikal, 122.5 (Likens
1975
, p. 194). Lake Victoria, in contrast, has an
annual production of 640 (Wetzel
1975
, Table 14.10), which is among
the highest for freshwater lakes on Earth. Hence, at least for these
