science for a changing world

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2005-3006

January 2005

The diversity of environments that characterizes the West 

is responsible for the region’s rich biological heritage. This 

ecological diversity also means that opportunities for invasive 

species are many, varied, and complex. Island ecosystems are 

notoriously vulnerable to invaders as demonstrated in Hawaii 

and West Coast offshore islands. Aquatic invaders impose 

high economic and environmental costs in systems as varied 

as San Francisco Bay and desert springs in the Great Basin. 

Although the West’s arid and montane ecosystems may seem 

resistant to plant and animal invaders, we now know that exotic species have altered physical processes related to fire and 

hydrology in a manner favoring further expansion and persistence of invaders. Natural resource managers value analytical, 

mapping, and genetics tools developed by USGS scientists 

to monitor invasive species and help conserve biological 

systems. USGS biologists conduct research to assist land and 

water managers’ efforts to control invasive species and restore 

natural systems. Throughout the West, the USGS carries out 

studies for early detection and rapid assessment of invaders. 

The following are some examples of how the USGS is making 

a difference in the western United States.

Islands in the Incoming Tide 

Introduction of invasive plant and animal species threatens to damage Pacific island ecosystems and inflict high 

economic cost. Because native island species often lack resistance, the natural resources 

of Hawaii and other Pacific 

islands require protection 

from pests and parasites not 

recognized as problematic 

elsewhere. These isolated 

oceanic archipelagoes are 

highly susceptible to invasions, and extensive loss and 

endangerment of species have 

already occurred. The future 

of Hawaii’s native biodiversity depends on successful 

control of established invasive 

species and prevention of 

establishment of new ones. 

Biologically rich national 

parks and wildlife refuges, as 

well as over 400 listed endangered species, are at risk. 

USGS scientists work with 

Biological Science Capabilities 

◆ Development of tools to analyze and map data 

◆ Development of genetic probes for rapid detection 

◆ Inventory and monitoring system development

◆ Biology of species ranging from microbes to large 

vertebrates

◆ Ecosystem science and restoration science

partners to develop tools for reducing species introductions, to 

develop collaborative survey methods for detecting invasions, 

and to establish mechanisms for evaluating the effectiveness 

of prevention measures. 

Invaders Find Their Way to Alaska (With Help)

Historically, Alaska’s National Parks have been nearly 

free of exotic species, but the rate of alien plant invasion is 

increasing rapidly. As in other places in the West, the USGS 

is conducting comprehensive studies in Alaska National Park 

Service units for early detection and rapid assessment of 

invaders. Results are available in online databases for use by 

resource managers. Scientists in Alaska have found that remote 

units with few visitors and limited access have near-pristine 

conditions. In contrast, parks with heavier visitation and easier 

access demonstrate 

a widespread 

occurrence of 

non-native plants 

in disturbed areas. 

USGS scientists 

are testing methods 

for early detection 

and rapid eradication in Denali 

National Park, as 

well as methods 

to identify factors 

that predict the potential for further spread.

An Invasive Plant Provides Habitat for an Endangered 

Species

Tamarisk (saltcedar), an invasive, exotic shrub first introduced into the Southwest over 100 years ago, is now dominant 

Biology and Invasive Species in the Western U.S.

Photo Credit: John & Karen Hollingsworth, USFWS
in riparian systems throughout 

the arid West. In many places, 

tamarisk has replaced native 

willow and cottonwood, negatively affecting both wildlife 

habitat and water consumption by riparian vegetation. 

Yet tamarisk also provides 

important habitat for migratory 

and resident birds, including 

approximately 25% of the endangered southwestern willow 

flycatcher’s breeding population. Use of tamarisk by avian 

species presents a management 

paradox for state and federal 

agencies that are working to reduce or eradicate invasive 

plants while also conserving sensitive and endangered species. The USGS is conducting research to understand how 

birds such as the flycatcher use tamarisk, how the ecology 

of flycatchers nesting in tamarisk compares with those in 

native habitats, and how managers can balance tamarisk 

control and wildlife conservation.

Introduced Species 

without Parasites 

– One’s Loss Is the 

Other’s Gain

The impact of invasive 

species is a function of 

how efficiently they establish dominant populations. 

Research points to parasites, specifically a lack thereof, as being highly important 

in biotic invasions. USGS biologists have found that exotic 

animals are less affected by parasites where they have 

invaded than where they are native. Having about half as 

many parasites, introduced European green crabs may have 

an advantage over native competitors that are restricted by 

their full complement of parasites. This “super-pest” decimates shellfish in Pacific Coast bays and estuaries. Where 

introduced, green crabs become large and abundant in the 

absence of a parasitic barnacle found in the crab’s natural 

range. Effective biological control could depend on testing 

green crab parasites that are also safe for native species. 

Invasion of Annual 

Grasses – It’s About 

More Than Just 

Plants

Extensive areas of 

western rangeland were 

once dominated by 

native grasses, herbaceous plants, and shrubs such as sagebrush. Eurasian annuals 

such as cheatgrass have invaded millions of acres throughout 

the West, perpetuating fires and threatening human communities, wildlife, and ranching. Rapidly-recurring fires have a 

significant impact on the native plant and animal communities that have few defenses against frequent successive burns. 

USGS researchers are investigating how increased fire size 

and frequency affect arid ecosystems, how fire changes nutrients in the soil, how invasive plants and fire are related, and 

how to protect native plants and animals from further impacts 

of invasive plants. Research is also underway to understand 

how rangeland systems may be restored following exotic plant 

invasion and fire. 

Alien Hitchhikers Affect our Waterways

Ocean-going ships transport non-native nuisance species 

in ballast water and on their hulls. Activities like recreational 

boating and intentional introductions also spread aquatic 

nuisance species in western rivers, streams, lakes, and estuaries. Invasive aquatic plants and animals pose risks in systems 

ranging from salmon-bearing streams in the Columbia Basin 

to isolated desert springs hosting rare invertebrates. Many of 

the native species in these systems are listed under the Endangered Species Act. USGS scientists are testing methods to inventory, control, and remove non-native species from aquatic 

habitats, working with regulators and the shipping community 

to develop methods for early detection and eradication.

Contact:

USGS Biological Resources Discipline

Western Regional Biologist

909 1st Ave., 8th floor, Seattle, WA 98104

Phone: (206) 220-4600

Photo Credit: Erik Ackerson 

Photo Credit: Lawrence Abeita, BIA