Chemicals A number of chemical contaminants are hard to detect and study because they rarely outright kill their victims. In some fish and reptiles these contaminants interfere with reproduction and can even cause sterility. Toxicologists continue to try and understand this complex subject, but meanwhile we continue to emit more and new kinds of these contaminants into our environment. Introductions of Exotic Species Humans are constantly introducing animals and plants from their native habitats into new areas. Sometimes this is done intentionally, for example stocking a lake with game fish, and sometimes accidentally, such as the release of Zebra Mussells into the Great Lakes.
Many amphibians can only survive in fishless ponds.
When these are stocked with fish the amphibians are eventually eliminated. Ironically, amphibians themselves are sometimes the introduced species. Introduced species may also out- compete and eventually displace native species. However, the most insidious effect of introducing exotic species comes with the diseases they may harbour. The tragic story of how European diseases killed thousands of native Canadians is well known.