|
Introduction of exotic plant and animal species
The intentional or accidental introduction of non-native species can have devastating impacts on the delicate balance of a natural system. When introduced species have great impacts, multiplying rapidly and to the exclusion of the native flora and fauna, they are said to be invasive species. They are typically most problematic when no natural predators exist in the newly invaded territory. Invasive species can have a variety of negative impacts. By crowding out native species, invasive plants can reduce the quality of habitat and food resources available for native wildlife. Often they also reduce overall species diversity in the natural community, degrading the functioning of the system.
 In riparian habitats, plants which artificially alter the hydrology are especially problematic. Tamarisk, or salt cedar, (Tamarix spp.), compete aggressively with native riparian plants for soil moisture. Salts accumulate in the leaf litter of these plants, reducing establishment of other species. When established in large stands, thirsty tamarisk plants can lower the water table and reduce stream flows, especially problematic in an environment where surface water is limited and very precious.
Invasive species also can disrupt natural disturbance regimes. Buffelgrass (Pennisetum ciliare) and red brome (Bromus tectorum) are two grasses that have been introduced to desert scrub communities across Arizona. These species burn readily, threatening native vegetation by bringing fire to a community not adapted to surviving periodic fires.
 
Left: Red brome fills in spaces between native shrubs and creates ample fuel to carry a wildfire. Right: Following a wildfire, much of the native vegetation is dead. Shelter and food resources for native wildlife are significantly reduced. Images courtesy of www.birdandhike.com.
 Bullfrogs (Bufo americanus) are a troublesome invasive animal of riparian areas across the United States. Adult frogs eat all kinds of native fauna including fish, snakes, and native frogs and toads. They can survive very adverse conditions and reproduce in staggering numbers - one female can lay 50,000 or more eggs a year. Their rapidly increasing numbers coupled with a healthy appetite and ability to survive drought have had significant impacts on native wildlife in recent years.
 Crayfish (Orconectes virilis) are another introduced species with devastating effects on the native fauna of Arizona's wetlands and streams. Brought to Arizona as bait for stocked game fish, crayfish have spread across the state. With no predators, these animals have multiplied and spread rapidly. They eat all types of aquatic plants and animals and have been implicated with the decline in Mexican garter snakes, Chiricahua leopard frogs and other amphibians.
Many introduced plant species are troublesome for riparian and aquatic systems as well. Much information regarding invasive aquatic plants is available on this Aquatic Plants page offered by the National Invasive Species Information Center (NISIC) and the University of Arizona.
For more information...
| 
