Survival of Freshwater Diversity

What happens when the effects of modernization reach the riverbanks?

Who would think that freshwater zooplankton could ever have any influence on the life of the lumbering grizzly bear, or on the migration patterns of the Bald Eagle. But they do. In great Flathead Lake, Montana, when opossum shrimp were added as a supplemental feed for the kokanee salmon, the shrimp fed on the salmon's favorite meal, the zooplankton. The shrimp then unexpectedly hid from the salmon in deeper waters. The salmon population declined, and the bears went elsewhere in search of a picnic. Because of the reduction in salmon, the eagles dropped the lake from their migration pattern, ecotourism dropped and the local community felt the economic pinch. This scenario, known as secondary extinction, is just one way in which the biodiversity of lakes, rivers, and streams can rapidly go haywire when tampered with.

Conservation within waterways receives little attention. Yet our rivers and streams are critical to the water cycle, and nutrients flux. We rely on them for drinking water, plants and animals, travel and transport, waste removal, and renewable energy.

Waterways are highly resilient to external influences and their potential for recovery is significant. But today factors that offend this resilience are taking their toll and freshwater diversity is imperiled. At present, roughly one in three North American fish is either endangered, threatened or deserving of special concern.

Sinister Sextet

The extinction of a species is rarely attributable to a single cause, rather a combination of six pressures is usually involved. The "sinister sextet" comprises: habitat loss or degradation, the spread of exotic species, over exploitation, secondary extinctions, chemical and organic pollution, and climate change.

In order of destructiveness, habitat loss and species introduction topped the list in a study of freshwater species loss in North America, followed by chemical pollution, hybridization, and over harvesting.

Fortunately waterways are extremely resilient to alteration. By definition they are natural self-cleansing. The continuous supply of biota travelling downstream facilitates rapid recovery. Opportunities for isolation and dispersal further encourage diversity.

Home Wrecking

Damage to an aquatic habitat can be large- scale and blatant, i.e. massive dams or deforestation. Or it can be in the form of smaller, repetitive assaults such as agricultural activity, and human settlements.

Dammed waters can disrupt an ecosystem by breaking migration patterns or changes in water chemistry and nutrient content. Often the most serious (and unexpected) effects become evident years or decades later.

The guiding principle behind habitat restoration is simple: Try to recreate the original environment. What is often overlooked however is the needs of non-target species, such as the non-sport fish, which are of equal importance to the biological system.

Land Transformations & Water

The most widespread impacts on waterways are probably the least dramatic: draining of flooded areas, grazing of livestock, timber harvest, road building, human settlements, and intensification of agriculture.

Floodplains, when they are not modified with dams, are a natural way to enhance biological productivity through biota exchange. Today the majority of untouched floodplains exist only in the tropics. Drain tiles that control runoff export water that would otherwise recharge soils and aquifers.

Deforestation near streams can have far-reaching effects such as changes in water temperature, discharge rate, and nutrient loss. Fortunately, preserving even small amounts of forested land near a stream can minimize this.

Agriculture has contributed greatly to river modification, usually through channel straightening and reduction of streamside vegetation, both increase erosion and sediment exportation. As habitats degrade, populations within change in species number and proportion. Populations fragment and exotic species invade.

Aliens Invade

Exotic species have been very successful in reestablishing themselves. Streams offer a multitude of environments with many opportunities to gain a foothold. Colonists were instrumental in the spread of fish species, as was the spread of pond culture. Exotics are also imported to control weeds or mosquito larvae, for example. Intercatchment water transfers also contribute to the movement of species.

As of 1980, some 35 species have been established in the U.S., another 50 were recorded. Twelve of those established were brought in for sport fishing, 23 were intended for the living room fishbowl. In an analysis of 31 cases, 77% of native species declined after the introduction of exotics. Invading species affect native species through habitat shift, by hybridization, and through the diseases and parasites they introduce. Once exotic species are established they are virtually impossible to eradicate.


Generally overharvesting is less serious a threat than other human disturbances. It is a greater problem in the developing world where fish are exploited for both human consumption and the aquarium trade. Overfishing also results in a "fishing up" effect by which large, slow- growing fish become scarce and are replaced in the market and in the environment by smaller, fast-growing species.

Secondary Extinctions

Secondary extinction occurs when one species is eliminated, and the affected species becomes either undesirably rare or popular. This change reverberates throughout the entire food chain as the zooplankton-shrimp-salmon-bear scenario illustrates.


Water pollution alone is unlikely to eliminate a species, but it has been linked to 38% of the extinctions in North America. Despite everything that the modern world dumps into them, water bodies have a remarkable capacity for recovery.

The Thames is one such instance. In 1955 a 70 km area of the Thames around London was incapable of sustaining fish life- except eels. Within 10 years improvements were made so there was enough oxygen for fish to make a safe passage. Within 20 years, 62 species could be found in a single catch in the river and estuary.

Global Warming

Global Climate Change is perhaps the most unpredictable factor. Biota at the highest risk are those that will not be able to escape to more suitable habitats. The most notable changes are expected at the middle and highest latitudes. It is expected that species will shift toward higher latitudes. Reduction of diversity within watersheds is predicted.

The future of the biodiversity hosted in streams and rivers is in certain danger. Each threat is driven by specific forces that impact aquatic habitats directly and indirectly. These forces must be addressed. For example, tighter control over the aquarium trade, along with education on the effect of introducing exotic species. Conservation policy must include measures to control development along river corridors, and compensation to owners for not using land. Protective measures aimed at streams and rivers protect not only a species, but the whole ecosystem as well.

J.D. Allen, A.S. Flecker. 1993. Biodiversity Conservation in Running Waters. BioScience, Vol. 43, No. 1.

L. Kaufman. 1992. Catastrophic Changes in Species-Rich Freshwater Ecosystems. BioScience Vol. 42, No. 11.