Fish Ecology and Fisheries - Freshwater Ecology: Concepts and Environmental Applications - page 431

diversity and distribution of fish. Patterns will be described from the most
general to the smallest scale.
The numbers of species are approximately the same for freshwater and
marine fish. On a continental scale, fish communities are more diverse on
large continents than on smaller continents or islands. Also, consistent with
many other animal and plant groups, fish species are richer in the tropics
than in temperate zones. Data compiled by Matthews (1998) demonstrated
that approximately 7% of samples from both tropical and temperate
streams had no species in them (completely unsuitable for fishes), but the
maximum possible biodiversity is considerably higher in tropical streams
and rivers. This is partially related to glaciation of higher latitudes that led
to local extirpation of fish species. Also, the time for evolution of fish
species has been greater in the tropics because more generations per year
can be produced. Thus, the number of species per sample peaks at 10-20
in temperate streams, but many samples in tropical streams have 50 or
more species (Fig. 21.2). A striking example of tropical adaptive radiation
is the diverse and highly coevolved assemblage of fishes associated with the
riparian zone in the Amazon basin (Sidebar 21.1). The spectacular diver-
sity of fishes in some African Rift lakes was described in Chapter 10.
Within continents, different ecoregions have varied diversity. For ex-
ample, in North America exclusive of Mexico, there are about 740 fish
species. About 300 of these can be found in the Mississippi basin. This
basin has an area about equal to that of the Hudson Bay drainage, which
has about 100 species. Thus, lack of glaciation and a long evolutionary his-
tory have led to evolution of more species in the Mississippi basin.
Within a basin, fish diversity in rivers and streams increases at greater
distances from the headwater streams (Fig. 21.3) because larger rivers are
less likely to dry, small streams and rivers are more subject to debris torrents
and catastrophic flooding, and the floodplain of a large river contains a

0.12

0.10

Tropical streams
Temperate streams

0.08

0.06

0.04

0.02

0.00

0 0 0 0 0 0 0 0 0 0 0
Number of fish species

FIGURE 21.2 The proportion of total samples containing various numbers of fish species
from tropical and temperate streams. The total numbers of samples were 204 in the tropical
streams and 815 in the temperate streams (after Matthews, 1998).