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Drying up the Desert Wetlands: Aquatic Invasive Species in the Southwest

When you think of the southwestern United States, you probably think of cacti and tumbleweeds. The states that make up U.S. Fish and Wildlife Service’s Region 2 are Arizona, New Mexico, Oklahoma and Texas--none of which are known for their abundance of water. In fact, some of the driest areas in the United States are found in this region, including Arizona’s Sonoran Desert and the Chihuahuan Desert in New Mexico and Texas. However, you may be surprised to know that the Southwest Region is also home to some of the rarest and most important wetlands in the country.

Desert wetlands make up only 3.5 percent of total U.S. lands. But after tropical rainforests, they're the world's second largest supporters of plant and animal life. Desert wetlands also provide homes to 50 percent of all our endangered animals. Some types of desert wetlands you will find in the Southwest include playa lakes, the marshes surrounding these lakes, and riparian (riverside) wetlands.

Playa lakes are a unique type of wetland usually found in very dry areas of the world. They form in flat, shallow depressions that are seasonally filled with rainwater. During the summer, playa lakes are usually very dry, hard, and smooth. But during the winter, when the lakes fill with water, the fine sediments turn to soft mud. The southern high plains of Texas and eastern New Mexico contain nearly 22,000 of these lakes, which is the largest concentration of playa lakes in the world. Playa lakes provide critical habitat for migratory birds in need of food, shelter, and rest during their long seasonal flights. They also harbor a wide variety of amphibians and invertebrates, including the endangered fairy shrimp. Native salt-tolerant vegetation, such as shadscale and saltbrush, thrive on these flats, providing valuable fodder for both native wildlife and livestock.

Another type of desert wetland found in the Southwest is called a riparian wetland. Riparian wetlands are found along the banks of streams and rivers. They provide crucial habitat for native plants and wildlife. In the arid Southwest, 80 percent of all wildlife species depend on riparian areas during at least some part of their life cycle. One of the biggest reasons riparian wetlands in this region are so important is the abundant and diverse plant communities that thrive there. There are two main types of plant communities in the region’s riparian wetlands. Along the rockier streams, you will find a greater variety of tree species, including willows, walnuts, sycamores, cottonwoods, ash, and alders. But on river plains that are mostly sand, gravel, and clay, you will find mostly cottonwoods and willows. Healthy populations of cottonwoods and willows are extremely important to the region’s native bird population. In fact, cottonwoods and willows are homes to more breeding birds than anywhere else in North America. The leaves of these two species are not only rich in nutrients, they also attract lots of insects--an important protein source for birds raising their young. Some migratory birds, such as white pelicans and sandhill cranes, even use these specific trees as landmarks to guide them to their desert wetland homes.

However, these valuable wetlands are being threatened by rapidly spreading communities of invasive plants. Along the Rio Grande, non-native species now make up 40 percent of tree species, and 25 percent of non-woody plants. The most lethal of these alien invaders are the tamarisk (also known as saltcedar), Siberian elm, and Russian olive. Because these species grow so quickly and produce enormous quantities of seeds, they are able to quickly overwhelm native cottonwoods and willows. In many areas across western Texas, New Mexico, and Arizona, tamarisk has become the dominant riparian species. In these areas, wetland quality suffers from reduced biodiversity, degraded habitat, changes in stream flow, lower water tables, and fewer recreational opportunities.

The good news is that in 1999 the very first biological control agent for tamarisk was approved for use in the United States. A tiny leaf beetle that feeds solely on tamarisk leaves shows promise to help curb the growth of this species and improve the odds for many native plants and animals that are being displaced.

Other aquatic invasives that are posing a serious threat to the region’s wetlands are giant salvinia and hydrilla. Both of these plants form dense mats on the surface of ponds and lakes, preventing dissolved oxygen and sunlight from penetrating the water. In Texas, migratory birds can no longer find food and shelter in several lakes that are almost completely covered by giant salvinia. Anglers are also prevented from using these water bodies, since the dense mats make it impossible to cast into the water. Hydrilla is causing similar problems across the Southwest. Both giant salvinia and hydrilla are spread when boaters and anglers inadvertently transport plant fragments attached to their equipment from one water body to another. Arizona is taking a proactive stance against these invasive plants with aggressive eradication campaigns, using both biological and chemical control methods. When giant salvinia was first discovered in the Yuma River in 1988, federal and state agencies quickly moved to control its spread to other areas. After hydrilla was first detected in two lakes in Phoenix during the mid-1980’s, Arizona natural resource agencies moved quickly to successfully eradicate these isolated populations.

But attempts to eradicate invasive species don’t always meet with success. And sometimes well-intentioned eradication campaigns can even cause more problems. In many cases, biological control methods are extremely effective. Yet, they also can pose many risks. Take the grass carp, for example. Originally introduced across the Southwest and other regions to help eliminate aquatic invasive plants like hydrilla, this non-native species is quickly becoming a nuisance. Though introduced grass carp were thought to be sterile, breeding populations have now been found in Oklahoma, and escaped individuals have even made their way up the Mississippi River. Grass carp have been known to clean entire lakes of their aquatic plant populations. Other types of introduced Asian carp species, such as the bighead and silver carp, eat huge quantities of the algae, plankton, and insects that native fish rely on for food. In some areas, Asian carp species have become so prolific that commercial fishermen are abandoning favorite fishing spots because they are unable to lift their nets that always end up overloaded with these big, heavy fish. Despite the negative effects the carp are having on native plants and wildlife, these species are still being stocked in Texas and many other areas of the country. Many concerned anglers, however, are attending public forums to let resource managers know about the negative impacts of these Asian carp species on native game fish.

What can you do as a concerned outdoor enthusiast in the Southwest? You have already taken the first step by becoming more knowledgeable about the nature of the invasive species threat. You can help expand the growing network of informed citizens by sharing this newsletter with your friends, family, and coworkers. Consider joining or volunteering for a conservation group that is working on removing invasive species or preventing their spread in your local area. Many of the wildlife refuges, state parks, and other natural areas that provide so many important conservation and recreation opportunities, rely heavily on volunteers to help eradicate non-native invaders. One nation-wide volunteer program that is really helping to make a difference is called Invaders: Citizen Scientists Combat Invasive Species. This innovative program was developed by the Arizona-Sonoran Desert Museum, and started out as the Invaders of the Sonoran Desert Region volunteer citizen scientist team. Through this program, volunteers are trained to use a specially developed “Invasive Species Early Detection and Reporting Kit,” which empowers ordinary citizens to become the first line of defense against a non-native species invasion. A national database operated by the U.S. Geological Survey allows the data you collect to be used by land managers on the ground for invasive species control and management. Find out more about this exciting program and learn how to get involved by visiting www.desertmuseum.org/invaders.

America's Least Wanted:
Tamarisk

Removing trees is not the type of wetland conservation advice you might expect to hear in the arid states of the southwestern United States. But in the case of the highly invasive tamarisk, it is a management tool that could save thousands of acres of vulnerable desert wetlands from destruction. Tamarisk represents one of the largest threats to southwestern riparian wetlands and the multitude of plants and animals that depend on these unique ecosystems for survival.

The catch-all term “tamarisk” actually represents several different tamarisk species that were originally brought to the United States from Asia in the 1800’s as ornamentals, for use as wind breaks, and to help control erosion. But by the 1960’s, these trees had spread to cover 1.2 million acres of riparian habitat. Though tamarisk has been reported in 34 states, it has caused the most severe damage in the Southwest. Extensive construction of dams and other flood control devices in this region drastically changed the natural flooding patterns of many of the rivers, and provided ideal conditions for the uncontrolled growth, reproduction and spread of tamarisk.

Tamarisk has several adaptations that have allowed it to become especially problematic in the arid Southwest. A very extensive root system allows the tamarisk to draw from deep groundwater reservoirs, which contain a high percentage of mineral salts. This salt is then excreted by its specialized leaves and left in deposits on the soil, which weakens less salt-tolerant native plants. Tamarisk is also a very prolific breeder and grows in stands so thick that native plants are crowded out and water sources become inaccessible to wildlife. Eventually, the diverse native tree and shrub communities that naturally grow in riparian areas are replaced with a community made of just one dominant species--a tamarisk monoculture. These less diverse plant communities support fewer species of insects, birds, and other wildlife, and cause the overall wetland habitat quality to decrease.

Luckily, there is hope for the wetland communities that have become overrun by tamarisk. Just like for many other of the country’s worst invasive species, a biological control agent has become the weapon of choice in the battle to stop this species from spreading. Scientists have discovered an ally among the tamarisk’s natural enemies in Asia--a tiny beetle with a huge appetite. Both the adults and larvae of the Chinese leaf beetle (Diorhabda elongata) eat only the leaves of tamarisk, and can completely defoliate a tree many times during the growing season. This limits the tree from getting the nutrients it needs, which will eventually kill it. In 2004, thousands of beetles were released along the Colorado River and have successfully reduced tamarisk concentrations in many sites. A combined campaign of manual cutting and carefully applied herbicides has also met with success in many tamarisk-infested areas.

Many of these success stories could not have been possible without the hard work and dedication of citizen volunteers. Invasive removal campaigns can be both a fun and fulfilling way to get involved in wetland conservation in your community. So why not take a day during American Wetlands Month this year to help rid a wetland in your area of non-native species? Visit the NBII Invasive Species Information Node at http://invasivespecies.nbii.gov/outreach.html to find out the many different ways you can get involved in an invasive species project near you. You can also order educational materials and get free signs, bumper stickers, and posters to help you educate others in your community about the dangers of invasive species.

VOLUME THREE
Issue No. 3