Invasive Plants and Animals of our Region
Main sources: Tellman, Barbara, ed., 2002, Invasive Exotic Species in the Sonoran Region, Tucson: University of Arizona Press and the Arizona-Sonora Desert Museum; Chambers, Nina & Trica Hawkins, n.d., Invasive Plants of the Sonoran Desert; Collins, James, "Where have all the Frogs Gone?", Natural History June 2004, pp. 44-49.
By "invasive species" we mean those exotic species (introduced -- usually by humans -- from outside the reference environment) which have become so well adapted to their new environment that they interfere with the species that are native there (here we follow the definition provided by Tellman et al (2002) pp. xix-xx). As a general rule, the more disturbed a habitat is by human activity, the higher the number and percentage of non-native species will be. Other factors support the invasion of exotics. For example, in the Southwest, most native plants (cactuses, shrubs, and trees) have evolved so they do not form continuous stands, and so most of them do not support wildfires, and consequently such plants have not fire-adapted and are seriously damaged by fire. In contrast, many invasives cluster into continuous stands, support wildfires and thrive on them.
The purpose of the present pages is to identify the invasives appearing on Saguaro Juniper lands so that we may identify relevant locations and be prepared to act if and when these forms appear to be threatening our natives or are otherwise damaging the ecosystem. What follows below is structured mainly by the list provided by Tellman et al. (op.cit.), of "the most invasive exotic species in the Sonoran Desert Region", supplemented by additional exotics specifically present in our area as we identify them. (Click on names printed in blue and underlined below for elaborations on the relevance of these life forms to Saguaro Juniper.)
For further information on the kinds of exotic invasive plant and animal species in Arizona and elsewhere, consult the following:
They are spreading along the San Pedro River, and they are appearing along Saguaro Juniper watercourses (see the "Tamarisk" link above).
This invasive grass is wideswpread in all of our major washes and in all the old earthen tanks of Saguaro Juniper land. See the link for further details.
This invasive grass is found in a number of our washes. See the link for further details.
This invasive grass is found in a number of our washes and water tanks. It is probably spreading as we speak. See the link for further details.
Originally native to the Mediterranean, Johnson Grass now occurs all over the world in warm-temperate regions. It entered the U.S. southeast early in the 19th century and by the 1890s was becoming a problem in Arizona's Salt River Valley. It is now common throughout the state below 6,000 feet.
African sumac (Rhus lancea)
An arid-land tree of South Africa, Rhus lancea was originally imported into the US by the landscape industry and was introduced to Tucson in the 1920s by a UA botanist. Its pollens disperse from November to February and are highly allergenic. Female trees produce large quantities of seeds, which are distributed by the birds who eat them.Once established along a wash or roadside, it spreads quickly, and since its canopy produces very deep shade it tends to crowd out native species. Since its root system is invasive and deep, the plant is hard to remove, and has now become a common weed in Pima County urban areas (though not yet in our SJ area). Click on Rhus and Rhus-2 for detailed images.
Tree of Heaven (Ailanthus altissima)
Tree of Heaven was introduced from China, into the east in the 1780s and into California in the 1850s as a host tree for a silkmoth also brought to the US. It can be confused with native sumacs, having large compound leaves 1-4' long. It reproduces both sexually and through lateral-rooted suckers, so existing plants are hard to remove. It grows rapidly, is a prolific seed-producer, overwhelms native vegetation and forms thickets. These plants are invasive mainly in Santa Cruz County, but have also been found in Cochise County near the Mexican border. For images and details, see Ailanthus.
Buffelgrass (Pennisetum ciliare)
Buffelgrass is a water-efficient C-4 African grass which is actively invading our Sonoran Desert. First introduced into the U.S. in the 1940s for erosion control, it was then planted on a large scale in Mexican drylands in an effort to augment pasturage for cattle, and efforts continue in Sonora to clear large areas (by means of chain and blade bulldozing) for seeding it. It is a fairly large, ragged perennial bunchgrass which produces long, bristly brown seed-heads whose numerous seeds are wind-dispersed, and it forms dense stands (unlike most desert grasses and other plants). Buffelgrass thrives with fire, and has spread into the Tucson area where it has become a significant pest.
The following link will take you to the Learning Center of the American Southwest, where a PDF may be printed out for selected invasive plants, including Buffelgrass: Fact Sheets for Invasives.
Honeybees were introduced into Mexico by the Spaniards, who prized them for both honey and candle wax. Although some Native American peoples had long kept native bees for their wax and honey, by the 19th century the European honeybee became dominant, and the bees spread from there into the Southwest. Honeybees also came here from both California and the eastern U.S. during the 19th century, and of course the Africanized honeybee -- cross-bred from several European subspecies -- has arrived late in the last century.
Asian Tiger Mosquito
Bullfrogs are native east of the Mississippi River, but were introduced into Arizona in the 1920s by what is now the Arizona Game and Fish Department, which observed at the time that "These lusty chaps seem to prosper in our climate," and that the Department sought more opportunities for the public to "enjoy frog hunting as a sport and frog flesh as food...."(Tellman, p. 43). The practice of inserting thousands of tadpoles into Arizona streams continued into the 1980s.
Bullfrogs are voracious predators (butterflies, dragonflies, native frogs, fish, turtles, birds and small mammals), and reproduce prolifically. According to Collins (cited above, pp. 47-48), these invaders are gradually becoming a global displacer of native amphibians, not only through competition and predation, but also by carrying new pathogens (fungi and viruses now known to be contributing to declines of native frogs).
Crayfish (Orcontectes virilis & Procambarus clarkii)
Crayfish are voracious eaters of snails, tadpoles, native fish (our local native fish appear to be especially vulnerable), frogs, and small turtles, and they disturb stream bottoms and thus increase the murkiness of water, inhibiting plant growth. In doing so, they strip streams of aquatic plants. We have not yet encountered them in the Hot Springs Canyon area. For more details and images of crayfish, see this link: Northern Crayfish Control Methods in Arizona Streams.