Impacts
of Exotic Species
In the United States, exotic species were introduced deliberately (for ornamental, aesthetic, medicinal, or agricultural reasons by humans) or accidentally (through animal vectors, ballast water discharge, movement of people, or by trading of nursery stock). Today, invasive species are posing a major threat to humans and to terrestrial and aquatic ecosystems. According to recent statistics by researchers and conservation experts, it is estimated that invasive species infestations cover nearly 100 million acres in the United States. These species cost the U.S. economy more than $138 billion dollars per year in damages and control efforts.
Some of the impacts of exotic species include:
Loss of Biodiversity
Soil erosion or disturbance
Habitat loss
Alteration of soil moisture and salinity levels
Alteration of nutrient levels in soil
Increase in fire frequency and intensity
Hybridization with native species
Alteration of food chains
How exotics cause problems: Removing a species from its native environment frees it from natural predators, parasites, pathogens and competitors that limit its numbers. Therefore, an exotic species is often capable of explosive population growth. In many instances, exotic species become pests by out-competing or preying upon endemic species. Hawaii, Florida and California have experienced major ecological problems with exotic plants and animals that have been introduced by humans.
Learn more about the impacts of these exotic species:
Cheatgrass
(Downy brome)
Saltcedar (Tamarisk)
Wild boar (Feral pig)
Brown Tree Snake
Fayatree (Fire Tree)
Smooth Cordgrass
Zebra Mussel
- Cheatgrass
(Bromus tectorum)
Cheatgrass, also called downy brome, is a widespread exotic weed. It was introduced to the U.S. from Europe in the late 1800's and now occurs in nearly all 50 states. It is abundant in the Pacific Northwest and Intermountain/ Great Basin region. Cheatgrass is an annual plant. The seeds germinate with the rains and warm weather of early spring, reaching a height of 50 to 60 cm. It grows quickly and out-competes native grasses for water and nutrients. The fibrous root system may reach a depth of 30 cm (more than 12 inches) into the soil and extracts high levels of soil nutrients and ground water. Although cheatgrass provides good forage early in the season, the plant matures and is dried out by early summer. The fine-textured blades and dense growth of this grass allow it to ignite easily ignited and carry fire well, so cheatgrass-infested rangelands are prone to fast-burning wildfires that may cover large expanses. The fires release nitrogen into the soil and air, which enhances the growth of the cheatgrass on the burned land. This leads to a cycle of frequent fires which cannot be tolerated by most native plants, allowing cheatgrass to dominate the landscape almost exclusively. Other impacts of cheatgrass include increased soil erosion, loss of wildlife habitat, reduced stocking rates, and loss of recreational value on the infested land.
- Saltcedar
or Tamarisk (Tamarix sp.)
Saltcedar, also called tamarisk, was introduced to the United States in the early 1800's as an ornamental plant. It is now widespread in the Intermountain Region and also occurs in other states across the country. Saltcedar begins as a small evergreen shrub but can grow rapidly to become more than 20 feet tall at maturity. It grows rapidly and produces thousands of bright flowers and tufted seeds, which easily disperse great distances. Saltcedar naturalizes in flood plains, stream bank areas, waterways, and other habitats where the soil is moist. It extracts large amounts of water and eliminates excess salts by special glands in the leaves; when the plant sheds the leaves they excrete salts and increase the salinity of the soil. Because of tolerance to a wide variety of environmental conditions (including heat, cold, drought, flooding, and high salinity) saltcedar forms dense thickets. They reduce the availability of soil and water resources required for the growth of the competing species, thus providing poor habitat for wildlife and native plants. However, the saltcedar does benefit a federally endangered bird, the Southwestern willow flycatcher, by providing suitable nesting habitat.
- Wild
boar or feral pig (Sus scrofa)
Pigs originated in Europe, Asia and North Africa, but are now established in many other parts of the world. The first individuals of the species were introduced to North America in the 15th century and are now found in much of the United States, with the highest numbers present in the mountains of the South, Southeast, and California, and in the wet forests of Hawaii. They often occur on the steep slopes of mountains, as well as in wetlands and grass plains. Wild pigs breed quickly and produce large populations which displace native species. Their rooting and trampling habits cause extensive damage and create bare areas that are susceptible to erosion and invasion of exotic plants. Because of this activity, they also favor the spread of exotic species and alter the structure of the entire ecosystem. In Hawaii, an area with healthy sedges was turned into bare land within 3 years of the invasion of pigs. Recovery of the same land took 7 years after fencing was added to prevent entry of pigs. Feral pigs also pose a threat to the livestock industry. In the rangelands of eastern Australia, feral pigs prey on newborn lambs. In addition, feral pigs carry many parasites and transmit diseases to native wildlife, livestock, and humans.
- Brown
Tree Snake (Boiga irregularis)
According to recent estimates, nearly 1 million brown tree snakes are prowling the Pacific Island of Guam. The snakes were introduced to the island in the early 1950's through accidental transport on U.S. military ships from Australia, New Guinea and the Solomon Islands. Prior to its introduction, the Island of Guam had no snakes. The snakes prey on birds, lizards, and fruit bats, many of which are unique to the island. The snake is especially devastating because the animals of the island evolved without this predator and are now easy prey; consequently Guam has lost many of its many of its unique species. As the snake removes native species that eat insects, insect populations increase and in turn become serious pests to native and agricultural plant species. Guam has lost many other things because of the snake. The snake poses a human health risk because it has venom that is mildly poisonous to adult humans, but can be dangerous to young children. Economically, It is a serious threat to the tourism industry because many people fear snakes and because the snake climbs power poles and causes frequent power outages by being tangled in the electric wires.
- Fayatree or Fire Tree (Myrica faya)
Fayatree is a noxious, evergreen tree found at elevations of approximately 200 to 2000 meters in the Azores Islands (where is it native) and in the forests and mountain terrains of Hawaii. The Portuguese introduced it to Hawaii in the late1800's. In Hawaii it is highly detrimental, growing to around 12 meters tall and out-competing and displacing native flora and fauna. Fire tree is a successful invader and infests large areas for a variety of reasons. It produces numerous flowers that are pollinated by wind and birds and abundant fruits that are dispersed by birds. The plant has an additional advantage of possessing a root system that can capture atmospheric nitrogen, a compound that limits the growth of many plants. Because it provides itself fertilizer, it is able to re-establish and colonize aggressively in new sites, where it forms dense monocultures. Because of its root system, fayatree is also able to colonize recently deposited volcanic soils, changing island succession.
Smooth cordgrass is a non-native aquatic plant found mainly along the Pacific Coast of the United States. It is adapted to salt marshes and estuaries and it rapidly infests tidal mudflats, which disrupts this unique habitat. Smooth grass converts the natural unvegetated areas to dense marsh meadows, thus altering the hydrology and local ecological processes. There is a significant impact on shorebirds, which migrate across the aquatic waters along the coast. The mudflat habitat provides critical food supplies during low tides to these birds. Smooth cordgrass also clogs and changes the natural flood channels and hampers the flow of water leading to coastal flooding.
The zebra mussel is a freshwater mollusk introduced from the Caspian Sea to the Great lakes in 1988 through ballast water. It has since spread by recreational boating, angling, aquaculture activities, and naturally dispersed through canals. Zebra mussels affect aquatic habitats by attaching to hard surfaces and by their filter feeding activities. Their habit of covering hard surfaces has forced many municipalities along the Great Lakes to periodically clear intake pipes clogged by these small mollusks. In addition to causing problems for water pipes and canals, many native mussel species have suffered severe declines because large numbers of zebra mussels suffocate them. Zebra mussels also alter the aquatic ecosystem because each individual can filter about one liter of water per day despite being less than 4 cm in length. They feed on many forms of plankton, and also remove other particles from the water, thereby clarifying the water. However, plankton serve as the food base for most young sport fish and its removal likely decreases fish production.