Animals - Freshwater Ecology: Concepts and Environmental Applications - page 178

habitat disturbance. Evidence suggests that
a worldwide decline in amphibian diversity
is occurring (Sidebar 9.2).
Aquatic reptiles, birds, and mammals
are familiar to most people. A list of some
of the important or charismatic species
(Table 9.4) includes some that can be abun-
dant in wetlands, rivers, and lakes. Some of
these species can alter hydrology or nutrient
cycles. Others are of interest because of their
rarity or unusual nature. Most of these
species are associated with shallow habitats,
such as the littoral zone of lakes, ponds,
rivers, streams, and wetlands.

Sidebar 9.2.
Worldwide Decline in Amphibians

Many herpetologists have reported that popula-
tions of frogs, toads, and salamanders are de-
clining. These declines are occurring over a va-
riety of habitats and in many places around the
world. Gauging the extent of the problem and
what is causing it is difficult. The first problem
is that populations of amphibians are naturally
variable (Pechman et al., 1991). However, data
from 936 amphibian populations from around
the world were analyzed and exhibited rapid
declines in the 1960's, and continued declines to
the present (Houlahan et al., 2000). Studies have
documented that at least 5% of all salamander
species already are endangered. Given that 80%
of all amphibian species occur in the tropics,
and habitat destruction has been immense in
these regions, it is likely that much more than
5% are endangered (Mittermeier et al., 1992).
What is the cause of the decline in amphibian
populations? Clearly, habitat destruction is a ma-
jor problem given the number of wetlands and
other aquatic habitats that have been impacted
(see Chapters 4 and 5). Research suggests that
increases in ultraviolet radiation related to ozone
depletion have had negative impacts on repro-
duction and growth of amphibians (Blaustein et
al., 1995, Langhelle et al. 1999). Ultraviolet radia-
tion may also interfere with predator avoidance
(Kats et al., 2000). Disease also may be partially
responsible for the declines, including infection
by chitrid fungi (Berger et al., 1998). It is possible
that environmental stresses combine to lower
immune capacity (Carey et al., 1999). Water pol-
lution, including airborne pesticides and other
toxic organic compounds, may also be a factor
that contributes to declines (Duellman and Trueb
1986). Non-native fishes can also decimate am-
phibian populations (Knapp and Matthews 2000).
Assigning a single cause to the decline or ex-
tinction of an amphibian often can be difficult
(Sarkar, 1996; Alford and Richards, 1999). If am-
phibians can be viewed as a "canary in the mine,"
the possible declines and the causes certainly
require further study because they indicate a ba-
sic problem in the health of our freshwater
ecosystems. Now there are indications of similar
decline in global reptile populations, a further in-
dication of global effects of humans on other
species (Gibbons et al., 2000).

SUMMARY

1. The major groups of animals in
freshwaters are the Porifera, Cnidaria,
Turbellaria, Nemertea, Gastrotricha,
Rotifera, Nematoda, Mollusca,
Annelida, Bryozoa, Arthropoda, and
Chordata.
2. The taxonomy of the smaller organisms
is less completely known than that of
the larger organisms.
3. Sexual reproduction is often sporadic or
nonexistent in more primitive
organisms. It is usually required in
larger organisms such as the vertebrates.
4. Body form can vary with season or
exposure to predation in several groups,
including the rotifers and cladocerans.
A seasonal change in body form is
called cyclomorphosis.
5. The Crustacea and Insecta are the most
diverse animal groups in freshwater
systems and have adapted to all major
aquatic habitats.
6. Aquatic insect larvae are particularly
diverse in rivers and streams.
7. Fish assume an important role in
aquatic food webs. Their body shape is
related to their place in the food web
and their habits.
8. Many of the mammalian, amphibian,
reptilian, and avian species that utilize
freshwater are endangered; some have
become extinct.