Out of Place Living Organisms:
Every body of water represents a unique ecological system. While lakes close together may appear alike, they are actually quite different. So, in one body of water, a particular fish, aquatic plant or other aquatic life form may be an integral part of the system's unique balance. However, when put into different waters, that same species may upset the delicate balance of the new system. When an introduced organism upsets this balance, it causes ecological or economic harm and is considered to be a nuisance species.
The most devastating species introduced to the U.S. have been from other countries. Zebra mussels, Hydrilla and sea lampreys are examples of these species. However, even species native to the U.S. can become a nuisance when put into the wrong environment. For example, crappies are excellent sport fish. In many lakes, they fit in well with the balance of the lake. However, in a wrong setting, the crappie could populate faster and out-compete other species for food and space. This could result in having a small lake with a lot of little crappie and little else.
In new surroundings, the out of place organisms are freed from predators, parasites, pathogens and competitors that have kept them in check. Once established these non-native species can create negative impacts.
Not all non-natives are nuisances. Just because a species is not native does not automatically make it a nuisance. For example, fisheries biologists have introduced muskellunge to many inland lakes across the country. This popular sport fish has provided anglers with added recreational opportunities and in most cases has not disrupted the balance of native species. As trained professionals, it is the responsibility of fishery biologists to study an entire ecosystem of a water body before making intentional introductions. However, just because a type of fish is present in one water body does not necessarily make it okay for it to be in another one. Therefore, individuals should not take it upon themselves to take fish from one lake and introduce them into another. In Alabama, stocking fish or other aquatic organism is illegal without a permit.
Upsetting the Balance:
Nuisance species can upset the balance of an ecosystem in a number of ways.
Predation - Species such as the round goby can eat the eggs of native species and destroy future generations of native species.
Food Chain Impacts - Mollusks such as the zebra mussels can filter important nutrients out of the water, many of which are a necessary part of the food chain that supports other native species. Without those nutrients native species perish. Zebra mussels also compete with native mussels for food and destroy native mussels by attaching to them and preventing them from opening and closing.
Competition - Plants such as purple loosestrife and Hydrilla displace valuable native plants, particularly in wetlands. By overtaking native plants that wildlife depend upon, this nuisance plant has caused the decline of many bird species. In addition, unlike native plants, they can grow throughout the entire lake or pond to the point that boaters and swimmers can't even use the water resource.
Diseases - Parasites such as whirling disease directly destroy important fisheries.
Human Health Problems - Bacteria such as Cholera can impact oysters and eventually can cause human health problems.