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Biodiversity Debate Discussion Questions

Background Information
Discussion Questions

Background:

When people build a dam on a river it changes the ecosystem in many ways. Dams may impact, both positively and negatively, the biodiversity of terrestrial plant communities, fish and other aquatic life, mammals and birds. See the Further Discussion questions following the activity for more information about these impacts.

Note that there are many other arguments for and against a new reservoir that do not relate specifically to biodiversity. These include economic opportunity (irrigation or drinking water) versus economic losses (flooded communities or cost of construction), recreational opportunities (boating and fishing), and opportunities for developers (water front housing), aesthetic considerations and more.

Other Stream Side Science activities such as, That’s Predictable and Missing Macroinvertebrates, may be helpful for the students to review how ecosystems change due to various factors.

More information about dams can be found on the internet sites listed on the Resource pages.

Further Discussion:

1. What impacts could a dam have on the biodiversity of terrestrial plant communities?

Increased agriculture may replace native plant communities with monocultures (e.g., fields of corn).
Reduced or eliminated flooding events downstream of reservoir may harm native riparian communities. These communities depend on floods for seed germination, replenishing soils with nutrients and to increase shallow groundwater.
Diversion of irrigation water to canals may create new riparian areas along these new waterways.

2. What impacts could a dam have on the biodiversity of fish populations?

The reservoir provides new habitat for fish and invertebrates and often results in a new sport fishery.
Large reservoirs in big rivers such as the Columbia and Snake disrupt migration patterns of salmon. Salmon have evolved to migrate during the high flows of spring runoff from the high mountain areas to the ocean. When a dam is introduced into the system, the salmon start down the stream and are stopped by a reservoir. The reservoir slows the fish, and delays their arrival at the ocean. As salmon travel to the ocean, their bodies change to allow them to live in a salt water environment. When the migration is slowed, these changes occur before they reach the ocean, which can be fatal to the fish.
Changes in downstream fish communities will depend on the temperature of the water released from the reservoir.
• If cold, deep water is released from the reservoir, downstream temperatures may be too cool for native warm-water fish, because the cool temperatures slow the development of eggs and young.
BUT, cold water releases from reservoir may create a trout fishery below the dam where none existed before.

3. How might a dam affect the physical structure of a river system?
Reservoirs are natural sediment traps, so downstream of the dam, the water may be much clearer. This may cause loss of beaches and gravel bars, but may allow aquatic plants and trout fisheries to thrive. When a dam causes a stretch of river to dry up completely, all invertebrates and fish are lost from that area. Canals do not typically have water year round, so they do not create similar habitat.

4. What impacts could a new dam have on diversity of birds and mammals?

Reservoirs would provide new habitat for ducks and other waterfowl.
New wetland areas near their inflows may also be created, providing new habitat for mammals and shorebirds.
A new reservoir would flood existing habitat.