Unusual or Extreme Habitats - Freshwater Ecology: Concepts and Environmental Applications - page 305

Ephemeral pools or wetlands form an important habitat for other organ-
isms, including waterfowl. Prairie potholes, many of which only hold water
for the wet part of the year, provide a key habitat for many of the ducks that
live in the central midwestern United States (Batt et al., 1989). The waterfowl
are mainly migratory and utilize the pools during the wet times of year.
The vernal pools of California are unique systems in that they contain
a highly endemic plant community. Georgia, Texas, Mexico, Chile, South
Africa, and Australia also have examples of temporary pools with endemic
plant communities (Thorne, 1984). Likewise, the fairy shrimps (Anostraca)
have adapted to vernal pools throughout the world, with some genera that
are distributed broadly and others that are endemic to local regions (Belk,
1984). In a good example of endemism in temporary pools, when 58 ver-
nal pools in California were sampled, 67 species of crustaceans were
recorded and 30 were probably new species (King et al., 1996).
Impermanent streams have received only a moderate amount of atten-
tion despite their importance in the many arid regions of the world (Davies
et al., 1994). The level of permanence has been clearly related to inverte-
brate community structure (Miller and Golloday, 1996), and drying prob-
ably has stronger effects than flooding (Boulton et al., 1992). Primary pro-
duction by periphyton in streams is resilient to desiccation (Dodds et al.,
1996a) and recovers in days to weeks.
Another specialized aquatic habitat that has received attention from
ecologists is found in the small pools formed in pitcher plants, tree holes,
the leaves of bromeliads, and abandoned car tires. Many different insect lar-
vae can be found in these small pools, and the larvae partition the envi-
ronment so they will not compete for the same resources. Tadpoles of some
amphibians also inhabit these small pools. The pools are attractive study
systems because they form a well-defined ecosystem in which all members
of the community can be identified and easily replicated and sampled.
Pitcher plants (Fig. 15.8) form small, deep wells with slippery sides,
and the pool of water that collects at the bottom serves as an insect trap
as well as a habitat for aquatic organisms, including bacteria, protozoa,
and aquatic invertebrates. More than 17 invertebrate species are obligate
associates of pitcher plants in the southeastern United States (Folkerts,
1999). Pitcher plants probably use the trapped insects as a nutrient source.
Such carnivorous plants typically grow in nutrient-poor wetlands.
A chironomid larva (Metriocnemus knabi) and a culicid (Wyeomyia
smithii) that inhabit the pitcher plant Sarracenia purpurea accelerate break-
down of trapped prey and make nutrients and CO 2 more available than
they are in the absence of the two invertebrates (Bradshaw and Creelman,
1984). These two invertebrates partition the habitat spatially with a third
species (Blaesoxipha fletcheri) and this allows for their coexistence (Giber-
son and Hardwick, 1999).
Small pools with several hundred milliliters of water form in the bracts
of the tropical monocot Heliconia, supporting a complex community. Stud-
ies of insect community interactions have demonstrated that positive and
negative interactions in the pools occur among the residents (i.e., competi-
tion is not the only structuring force in the community). These studies also
provided some of the early direct measurements of interspecific interaction
strengths (Seifert and Seifert, 1976).