Biodiversity of Freshwaters - Freshwater Ecology: Concepts and Environmental Applications - page 191

even those as simple as rotifers (Pejler, 1995). Scale also can be an impor-
tant determinant of diversity patterns, with microbes being associated with
the smallest particles and animals such as beavers actually creating and re-
sponding to changes at the watershed level. Figure 5.19 illustrates how
scale can affect biodiversity in streams and rivers.
How habitats are linked to each other is also a central determinant of
biodiversity. It has long been understood that interfaces among habitat
types are regions with high numbers of species. This occurs because species
that are adapted to both habitat types can occur there (species ranges over-
lap), in addition to species that specifically exploit the transitional zone.
The relationships among biodiversity and riparian zones of streams are a
prime example of high diversity associated with an ecosystem interface
(Tockner and Ward, 1999). Maintenance of riparian zones can preserve
both diversity and ecosystem function in a watershed (Naiman et al., 1993;
Spackman and Hughes, 1995; Patten, 1998) and in the stream (Vuori and
Joensuu, 1996). Likewise, hyporheic zones should be managed when bio-
diversity of river systems is of concern (Stanford and Ward, 1993).
Dispersal ability is also important in determining distributions of
species and biodiversity. Widely distributed freshwater species (cosmopoli-
tan species) often have the following characteristics: (i) life history stages
that can survive transport (often desiccation resistance); (ii) production
of large numbers of individuals in the transportable life stage form; and
(iii) the ability to survive, including competing successfully and avoiding
predation, in new habitats (Cairns, 1993).
Some species are distributed very broadly. For example, the protozoa
have cosmopolitan distributions of many species; about 8% of all species
ever described on Earth can often be found in a single sample (Fenchel
et al., 1997). They have all the characteristics listed in the prior paragraph
and thus are very successful at colonizing new habitats. Consequently,
identification keys for protozoa that have been written anywhere in the
world are useful in most other locations (Cairns, 1993).
Some of the natural modes of species transport include swimming or
being moved by currents through connected habitats; transport on the feet
or in the guts of animals (including insects, birds, and mammals); and
windborne dispersal. The possibility of transport of algae by waterfowl in
their guts has been documented (Proctor, 1959, 1966). A few larger zoo-
plankton species can be transported by wind, but are not as likely to be
transported by waterfowl (Jenkins and Underwood, 1998)
Resting stages of organisms are particularly important in some cases—
not only in dispersal but also in the ability to respond to long-term changes
in the environment. For example, copepods produce eggs that can remain vi-
able but physiologically inactive in sediments for long periods of time (dia-
pause). This long-term survival is an adaptation to changing environmental
conditions and allows the copepods to become established in environments
that only sporadically support a reproductive population (Hairston, 1996).
Hairston et al. (1995) studied sediments of two small freshwater lakes in
Rhode Island. They dated the sediments of the lakes with isotopes and
hatched eggs taken from sediments of different ages. In one lake, the mean
age of eggs that hatched was 49 years, and the maximum was 120 years. In
the second lake, the mean was 70 years and the maximum was 332 years.