Biodiversity of Freshwaters - Freshwater Ecology: Concepts and Environmental Applications - page 185

other indices of diversity throughout the years. Some mathematical bene-
fits and drawbacks of the various indices are discussed by Pielou (1977)
and Magurran (1988) but are beyond the scope of this text. Once diver-
sity is estimated, the next issue to be considered is the major patterns of

diversity.
One approach to classification of habitats is to use the organisms (

diversity) and abiotic characteristics, termed the ecoregion concept. Ecore-
gions are relatively large areas of land or water that contain a geographi-
cally distinct assemblage of natural communities (Abell et al., 2000). For
example, the freshwater of North America has been divided into 76 ecore-
gions from eight major regions based on biological distinctiveness with re-
gard to fish, amphibians, crayfish, unionid mussels, and aquatic reptiles
(Abell et al., 2000). It is not clear how well these ecoregions extend to in-
sects, plants, and algae. Given that freshwaters can be biologically distinct,
the next issue I will consider is the processes that lead to observed distri-
butions of diversity.

TEMPORAL AND SPATIAL FACTORS INFLUENCING EVOLUTION OF
FRESHWATER ORGANISMS

The ultimate source of biological diversity is evolution. Two main fac-
tors influence the evolution of new species: time available for evolution and
reproductive isolation. These factors, coupled with what Hutchinson (1959)
called the mosaic nature of the environment (what we now call spatial and
temporal aspects of habitat heterogeneity) have resulted in millions of
species. I discuss these factors with regard to lakes, streams, groundwaters,
and wetlands. Consideration of spatial and temporal scale is important in
describing the biological processes. Specific examples of situations in which
evolution has resulted in high numbers of species will be described. Some
short-term determinants of biodiversity will be explored in the next section.
One of the most striking demonstrations of the importance of evolu-
tion to biological diversity is the high number of species found in geolog-
ically ancient habitats that have had ample time for new species to arise.
Typically, such habitats contain many endemic species, those with a re-
stricted distribution. Additionally, such habitats provide some of the clear-
est examples of adaptive radiation (evolution of many species from a sin-
gle or few founder species).
Ancient lakes contain a large proportion of freshwater biodiversity
(Cohen, 1995). These tectonic lakes have a unique assemblage of verte-
brates and invertebrates compared to the Great Lakes of North America
(Fig. 10.3). This high degree of endemism occurs even though the surface
area (i.e., potential habitat) of the Great Lakes is almost 10 times greater
than that of the largest of the ancient lakes.
Lakes that have existed continuously for millions of years generally
have been subjected to changes that lead to geographic isolation. Varia-
tions in water level, formation and isolation of various subbasins, and flu-
vial processes can allow for divergence and evolution of new species
(Brooks, 1950). The evolution can occur because of reproductive isolation
of populations, leading to divergence and speciation.