This is a story of success and failure. It is a story where a good thing becomes a bad thing and back again, maybe. This is the story of the tamarisk bush and its spread through the American southwest.
Let us meet the good guy, who ultimately goes bad. The tamarisk, also called salt cedar, from the genus Tamerix is native to drier regions of Eurasia and Africa. Tamarisk is an evergreen or deciduous shrub that grows one to eighteen meters tall. They are gray-green in color, with scale-like leaves and pink flowers in the springtime. This is a wide-spread genus that has been recognized and planted for centuries:
In this case, why he did this, or what meaning this planting has isn’t the point. The point is, he planted a tree that was possibly a tamarisk tree hundreds of years ago.
In the early 19th century, tamarisk was introduced to the US as an ornamental shrub and shade tree. By the later part of the century it had become naturalized in many parts of the country. During the dust bowl, tamarisk was planted on the Great Plains to help fight soil erosion and as wind breaks.
The bad news: tamarisk quickly naturalized, becoming part of established US flora. Tamarisk is fast growing and readily takes over riverbank and flood plain ecosystems. It can tolerate saline soils and crowds out native species that cannot compete with deep taproots. Once established in an area, tamarisk bushes create a dense monoculture that has little value to most wildlife.
Human impact on the land inadvertently helped the tamarisk spread.
Dams were built along the Colorado River. The Hoover Dam, completed in 1936, and the Glen Canyon Dam, finished in 1966, are the two largest and caused dramatic changes to the riverbed below. These dams were built to release water from the bottom of their reservoirs. Controlled water flow in downstream rivers eliminates seasonal flooding, which makes the ecosystem and water table more stable. This stability is perfect for tamarisk plants and their deep taproots, allowing them to exploit this opportunity. An opportunity to invade riparian (riverside) ecosystems.
Less than 2% of the US southwest is considered a riparian ecosystem. While geographically small, over 65% of native wildlife depends on these areas. Many areas that we like to visit are riparian - national parks like the Grand Canyon, Zion and Glen Canyon National Recreation area are all included. Throughout the 1900’s, tamarisk plants spread down the riverbeds in these areas and are now found throughout riverbanks and flood plains.
Riparian ecosystems are slowly becoming more saline. Higher soil salt concentrations are stressful for native species. Tamarisk plants not only tolerate soils with up to 15,000 ppm soluble salts, they also help increase salt concentration at the soil surface. To do this, tamarisk roots take up salts from the soil solution and concentrate it in their leaves. When the leaves fall off, due to changing seasons or old age, the salts are deposited right on the soil surface. Many desert plants have large networks of roots near the soil surface to catch as much water as possible. As areas around tamarisk plants become more saline, native plants can’t compete and slowly die out, allowing dense monocultures of tamarisks to form.
Controlling the invaders
Efforts to control and remove tamarisk started in the 1960s and have been ongoing ever since. Uprooting plants is a short-term option. Established plants have very long taproots that are difficult to fully remove. Flowers produce thousands of seeds, so if there is one seedling, there are probably thousands more. Burning plants encourages flowering and sprouting of new shoots; so that’s not a viable option either. Herbicides are used, but must be applied carefully so surrounding plants are not effected. Frequently, bushes are cut down by hand and an herbicide, like round-up, is applied to the stump.
Cutting, digging, spraying are all labor-intensive methods for removing this beast of a plant. What if there was an easier way?
The deep taproots that are such a problem here are vital erosion control in the native habitat of tamarisk. In it’s natural ecosystem, tamarisk plants have a predator. A pest that decimates the plants and can eventually kill them. Meet Diorhabda carinulata, the tamarisk beetle.
Adult insects overwinter in the leaf litter surrounding a tamarisk plant and lay eggs in the spring. Tamarisk larvae feed on tamarisk leaves for about two and a half weeks before developing into adults. In summer, the tamarisk lifecycle can be as short as four weeks. As tamarisk beetle populations multiply more and more foliage is consumed, this feeding frenzy will eventually kill a tamarisk plant.
Using insects, or any other animal, as a means of biocontrol is risky - the introduced species might not be very effective if introduced to a new ecosystem. Analysis of projects from 1832 to 1997 that released insects as a means of biocontrol found that only 20% of target species were effectively controlled.
Nonetheless researchers in the 90’s thought the species-specific tamarisk beetle would be a good bet and started studies. At first, beetles were released into 10 field cages in the US Southwest to monitor their effect in a contained area. After the first year, the cage doors were opened and scientists began monitoring the dispersal and effectiveness of tamarisk beetles on tamarisk plants. So far, these insects have defoliated over 10,000 acres of tamarisk plants in the US.
Although tamarisks have little value to most wildlife, the southwestern willow flycatcher and other migrating birds use the bushes as nesting sites. If tamarisk bushes were destroyed, native vegetation like cottonwoods, willows, and ash would not grow fast enough to provide habitat for these endangered birds.
Not everyone is convinced that tamarisk beetles are a bad thing. States have their own programs to eradicate tamarisk plants. In Colorado, such programs mix tamarisk removal with willow tree restoration. As tamarisk are destroyed, willows and other natural plant species are planted.
Popular opinion on the best courses of action changes as new information is collected. Four years ago the NYTimes reported on the USDA ending their involvement with using tamarisk beetles as biocontrol, due to the endangered bird. A few weeks ago they released a story on Arizona’s successful use of the beetle, citing water issues in the west as a dominant problem.
So, who really is the bad guy in this story? The introduced tamarisk for becoming an invasive plant species? The tamarisk beetle for threatening the southwestern willow flycatcher? Humans for causing many of these problems?
This story is similar to many stories of invasive species. Once a species is part of an ecosystem, it is impossible to simply get rid of it. As part of an ecosystem, other animals become accustom to the introduced species and begin relying on it for food or shelter. Working back towards a more natural ecosystem takes time - possibly more than species introduction did.
Introduced, invasive, and endangered species are popular topics on Feed the Data Monster. Tamarisk is not the only species with a saga behind it's introduction and invasion of new ecosystems. Check out posts by other FTDM contributors:
Serious reading on tamarisk and tamarisk beetles:
Host specificity of the leaf beetle, Diorhabda elongata deserticola (Coleoptera: Chrysomelidae) from Asia, a biological control agent for saltcedars (Tamarix: Tamaricaceae) in the Western United States