|
In a world in which it seems that nearly every natural ecosystem
is under stress, freshwater ecosystems—the diverse communities
of species found in lakes, rivers, and wetlands—may be the
most endangered of all. Freshwater ecosystems have lost a greater
proportion of their species and habitat than ecosystems on land
or in the oceans, and they face increasing threats from dams, overextraction,
pollution, and overfishing.
How Crucial Are Freshwater Ecosystems?
Freshwater systems occupy only 0.8 percent of Earth's surface (McAllister
et al. 1997:5), but they are rich in species and vital as habitat.
An estimated 12 percent of all animal species live in fresh water
(Abramovitz 1996:7). Many others, including humans, depend on fresh
water for their survival. In Europe, for example, 25 percent of
birds and 11 percent of mammals use freshwater wetlands as their
main breeding and feeding areas (Kristensen and Hansen 1994:90).
Due to their limited area, freshwater ecosystems only contain about
2.4 percent of all Earth's plant and animal species (Reaka-Kudla
1997:90). On a hectare-for-hectare basis, however, they are richer
in species than the more extensive terrestrial and marine ecosystems.
(See Figure 1.) To date, scientists have discovered some 44,000
aquatic species in freshwater ecosystems, but this probably only
represents a portion of all freshwater species (Reaka-Kudla 1997:90).
In the last 18 years, scientists have described about 309 new freshwater
species each year (Nelson 1976, 1984, 1994).
In addition to being biologically rich, freshwater ecosystems play
a vital role in the lives of many people, providing a source of
drinking and irrigation water, food, recreation, and employment.
Indeed, the majority of the world's population lives near and depends
on freshwater environments, with most inland cities located next
to a waterway (Moyle and Leidy 1992:130). The world's fishers harvested
some 8.2 million metric tons of fish from lakes, rivers, and wetlands
in 1999—about 9 percent of the world's total fish catch (not
including aquaculture) (FAO 2000:6). Rivers and lakes are also crucial
as transportation and shipping routes, as power sources, and, unfortunately,
as waste sinks. All of these human uses take their toll on freshwater
ecosystems.
Why is Freshwater Biodiversity Endangered?
Threats to species in freshwater ecosystems are widespread. Habitat
degradation, physical alteration from dams and canals, water withdrawals,
overharvesting of fish and shellfish, pollution, and the introduction
of nonnative species have all increased in scale and impact in the
last century. (See Figure 2: Alteration of Freshwater Systems Worldwide.)
As a consequence, the capacity of freshwater ecosystems to support
biodiversity—the natural variety, abundance, and distribution
of species across the aquatic environment—is highly degraded
at a global level.
In a recent study of freshwater fish, Harrison and
Stiassny (1999) found that while many factors can simultaneously
contribute to extinctions, habitat alteration and the introduction
of nonnative species were the major causes driving the extinction
of fish species. Building dams and water diversions, channelizing
riverbeds, and draining wetlands are typical habitat alterations.
The study reported that habitat alteration contributed to 71 percent
of extinctions; nonnative species (which can compete with or feed
on native species) contributed to 54 percent; overfishing contributed
to 29 percent; and pollution to 26 percent of extinctions (Harrison
and Stiassny 1999:298-299).
Of the many ways in which humans alter freshwater
ecosystems, dams are probably the most widespread and significant
in their impact. Today, the world's rivers are dotted with more
than 45,000 large dams—dams higher than 15 meters or impounding
more than three million cubic meters of water (WCD 2000:8,11). Most
of these dams were built in the last 50 years (ICOLD 1998:13).
Dam building has slowed in many countries, particularly in the developed
world. In the United States, for example, since 1998, more dams
have been decommissioned than built (WCD 2000:10). However, dam
construction is still robust in other countries. As of 1998, there
were 349 dams over 60 meters high under construction around the
world, mostly in developing countries (IJHD 1998:12-14).
Dams provide unquestionable benefits—from water supply to power
generation—but they disrupt the hydrological cycle profoundly,
suppressing natural flood cycles, disconnecting rivers from their
wetlands and floodplains, disrupting fish migrations, and altering
the deposition of sediments downstream. For example, the World Commission
on Dams found that more than 60 percent of the large dams it surveyed
report significant problems with disrupted fish migrations (WCD
2000:82).
Rivers with multiple dams can become little more than chains of
connected reservoirs, with consequent changes not only in the temperature
and chemistry of river water, but in the living functions of the
riverine ecosystem. Waterfalls, rapids, riparian vegetation, and
wetlands are some of the habitats that disappear when dams impound
rivers (Dynesius and Nilsson 1994:759). These habitats are essential
feeding and breeding areas for many aquatic and terrestrial species,
and also help to remove pollutants and maintain water quality.
How Threatened Are Freshwater Species?
One measure of the actual condition of freshwater biodiversity is
the extent to which species are threatened with extinction. Globally,
scientists estimate that more than 20 percent of the world's 10,000
recorded freshwater fish species have become extinct, threatened,
or endangered in recent decades (Moyle and Leidy 1992:140). This
number, however, may well be an underestimate (Bräutigam 1999:4).
According to the 1996 IUCN Red List of Threatened Animals, 734 species
of fish are classified as threatened, of which 84 percent are freshwater
species (IUCN 1996:intro p. 37; McAllister et al. 1997:38). For
some countries and regions more detailed information is available.
In South Africa, 63 percent of freshwater fish are threatened or
endangered; in Europe, 42 percent; and in Iran, 22 percent (Moyle
and Leidy 1992:138).
Unfortunately, global data on the status of the range of different
freshwater species—plant and animal—is sparse, making
it very difficult to quantify the overall condition of the world's
freshwater biodiversity. Where we do have data, however, trends
look bleak. In the United States, which has comparatively detailed
data on freshwater species, 37 percent of freshwater fish species,
67 percent of mussels, 51 percent of crayfish, and 40 percent of
amphibians are threatened or have become extinct (Master et al.1998:6).
Indeed, studies indicate that freshwater species are being lost
at an "ever-accelerating rate" (Moyle and Leidy 1992:163). Based
on recent extinction rates, an estimated 3.7 percent of freshwater
animal species will be lost in North America each decade, a rate
nearly five times that of terrestrial animals (Ricciardi and Rasmussen
1999:1221).
Responses to Freshwater Threats
Concern for freshwater systems and their biodiversity is growing
in many parts of the world, particularly among scientists. There
is also recognition of the need to maintain functioning ecosystems
that continue to provide the goods and services humans depend on
and value, like clean water, fish, and recreation. In some cases,
this has resulted in a change in practices for the better. For example,
when Argentina recently sought World Bank funding to address flooding
along the Parana River near one of its richest agricultural regions,
it agreed to adopt a "living river" approach to the US$400 million
flood control project. This means the river's floodplain and wetlands
will be left intact as much as possible to preserve their functions.
Consequently, traditional flood control structures like levees and
concrete channels will be minimized—a significant departure
from past practices (Castro 2001).
More positive still is a nascent trend toward restoring some damaged
freshwater ecosystems. In the United States, Congress has approved
a $7.8 billion project in southern Florida to help restore some
of the natural water flow in the Everglades, an immense system of
freshwater wetlands disrupted in the 1950s with an extensive system
of canals and levees for flood control.
There have also been some positive changes in dam operations to
try to minimize ecological damage. For example, at least 29 countries
have adopted policies aimed at insuring that dams release enough
water to maintain a minimum flow that supports the aquatic organisms
downstream. During the dry season or in drought times, these "environmental
flow releases" can mean the difference between survival and death
for the large range of species that depend on the river's riparian
zone (WCD 2000:81).
Unfortunately, these signs of greater awareness of freshwater ecosystems
are still the exception rather than the rule (Duda 2001). With rising
demand for water and food, as well as the increasing number of water
development projects worldwide, experts stress that the need to
manage freshwater ecosystems as the critical resource they are—not
an afterthought or a luxury—is urgent. |
