Biodiversity of Freshwaters - Freshwater Ecology: Concepts and Environmental Applications - page 188

These complex behavioral adaptations can arise only in a diverse animal
assemblage.
Tanganyika also supports many other endemic animals, including 12
of 20 leech species, 37 of 60 gastropods, 33 of 69 copepod species, 74 of
85 ostracods, and 22 of 25 decapod species (Coulter, 1991b). As in Lake
Baikal, the millions of years this lake has existed has allowed for diversifi-
cation of many animal groups.
An obvious question when considering the various groups that have
produced large numbers of endemic species in ancient lakes is the follow-
ing: Why does a particular group diversify (Gillespie et al., 2001)? For ex-
ample, the teleost fishes are very diverse in the African Rift Lakes, but the
amphipods have the greatest number of endemic species in Baikal. Am-
phipods occur in the African lakes, and teleosts occur in Baikal, so why not
extensive speciation in both groups in both lakes? Chance plays a large part
in evolution. If the right group already happens to be present and already
is fairly diverse, it may be predisposed to amplify the existing variation
when conditions occur that are favorable to evolution of new species. Cer-
tain kinds of biological traits may favor rapid speciation in a group. The
gastropods provide an example of some characteristics in a specific group
that can lead to evolution of high numbers of species. Gastropod taxa that
are depth tolerant, brood their young, and have poor dispersal ability can
diverge more rapidly. All these characteristics increase the chances of re-
productive isolation and, consequently, evolution of independent lineages,
as is known from study of evolutionary mechanisms (Michel, 1994).
River basins can also have a continuous existence over geological time,
leading to evolution of high numbers of unique species. The Mississippi
River drainage has been geologically stable, the river has flowed continu-
ously, and little Late Cenazoic extinction has occurred. Much of the di-
versity in North American fishes has radiated from this drainage basin
(Briggs, 1986); there are 300 fish species found in Tennessee alone (Etnier,
1997). The mussel assemblages in the superfamily Unionacea have evolved
227 native species in North America (McMahon, 1991), most of which are
found in the Mississippi River basin. Many of these mussel species are now
endangered by human activities.
Geographic isolation is another factor that leads to high levels of en-
demism. It has long been known that islands have relatively high numbers
of endemic species (but lower total numbers of species). Islands can be
thought of as an extreme case of geographic isolation. Such isolation occurs
in continental habitats as well. Geographic isolation into different habitat
types has been demonstrated to lead to species divergence in three species of
sticklebacks ( Gasterosteus spp.), but divergence did not occur where isolated
habitats were similar (Rundle et al., 2000). Dry landscapes can lead to more
isolation of aquatic habitats and greater divergence of amphipod species
(Thomas et al., 1998) and leeches (Govedich et al., 1999). Nine species of
ostracods in the genus Elpidium evolved in Jamaica (Little and Hebert,
1996). These ostracods inhabit the small pools of water that form in bromeli-
ads (terrestrial epiphytic plants). Thus, segregation of habitats that leads to
evolution of new species can occur over very small spatial scales.
Groundwaters in aquifers with channels large enough to allow move-
ment of animals have given rise to many unique endemic species. The