3 Invasive Species in the Sonoran Desert Region

3 Invasive Species in the Sonoran Desert Region 11 INVASIVE SPECIES IN THE SONORAN DESERT REGION Invasive species are altering the ecosystems of the Sonoran Desert Region. Native plants have been displaced resulting in radically different habitats and food for wildlife. Species like red brome and buffelgrass have become dense enough in many areas to carry fire in the late spring and early summer. Sonoran Desert plants such as saguaros, palo verdes and many others are not fire- adapted and do not survive these fires. The number of non-native species tends to be lowest in natural areas of the Sonoran Desert and highest in the most disturbed and degraded habitats. However, species that are unusually aggressive and well adapted do invade natural areas. In the mid 1900’s, there were approximately 146 non-native plant species (5.7% of the total flora) in the Sonoran Desert. Now non-natives comprise nearly 10% of the Sonoran Desert flora overall. In highly disturbed areas, the majority of species are frequently non-native invasives. These numbers continue to increase. It is crucial that we monitor, control, and eradicate invasive species that are already here. We must also consider the various vectors of dispersal for invasive species that have not yet arrived in Arizona, but are likely to be here in the near future. Early detection and reporting is vital to prevent the spread of existing invasives and keep other invasives from arriving and establishing. This is the premise of the INVADERS of the Sonoran Desert Region program at the Arizona-Sonora Desert Museum. We have chosen nine invasive species to initiate the INVADERS program. Seven of these species are invasive plants and two are animal species. The two animal species are not yet in Arizona, but their arrival could be devastating to our state. The nine species are: Buffelgrass -Pennisetum ciliare Fountaingrass - Pennisetum setaceum Natalgrass - Melinis repens Sahara mustard - Brassica tournefortii Tamarisk (saltcedar) - Tamarix chinensis Athel tree - Tamarix aphylla Onionweed – Asphodelus fistulosus Red imported fire ant - Solenopsis invicta Argentine cactus moth - Cactoblastis cactorum Information on each of these species including their identification, impact, history, and distribution are included in the following pages of this handbook. This information is also available on the ASDM website at www.desertmuseum.org/invaders. 12 Buffelgrass (Pennisetum ciliare) What is it? Buffelgrass is a shrubby grass to 1.5 feet tall and 3 feet wide. It looks like a bunchgrass when small (either a seedling or recently burned, grazed, or cut). Older plants branch profusely and densely at nodes, giving mature plants a messy appearance. These nodal branches produce new leaves and flower spikes very quickly after light rains, making buffelgrass an extremely prolific seed producer. Some native grasses such as Arizona cottontop (Digitaria californica) branch sparsely and do not appear shrubby. Bush muhly (Muhlenbergia porteri) is a true densely- branched shrub; it differs from buffelgrass in its very delicate texture. An entire mature plant. A single basal stem teased out, showing the profuse branching above ground. Inflorescences are 1.5 to 5 inches long, fat brown to purplish when fresh or occasionally straw-colored. The spikes are crowded with dense bristly fruit which are actually burs without hardened spines. (For this reason buffelgrass is sometimes included in the genus Cenchrus, the burgrasses.) This is primarily a warm-season grass, but below 3000 feet in our region it can grow and flower after almost any rain. 13 The individual units visible in these The most similar native grass is plains buffelgrass inflorescences are bristlegrass (Setaria macrostachya). The basically soft burs. individual seeds are clearly visible. Why is it a Threat? Buffelgrass grows densely and crowds out native plants of similar size. Competition for water can weaken and kill larger desert plants. Dense roots and ground shading prevent germination of seeds. It appears that buffelgrass can kill most native plants by these means alone. Tumamoc Hill in Tucson, home of the University of Arizona's historic Desert Laboratory visible at left, has been overrun by buffelgrass in the last two decades. It has not burned, but native plants are declining and dying from lack of water. Photos: Travis Bean [Red brome (Bromus rubens) is another invasive grass that has covered huge areas of lower bajadas in the upper Sonoran Desert. This annual grass has caused serious damage in the past several decades. It grows densely only in wetter years and produces relatively mild fires, so fires are infrequent and don't completely kill native 14 communities in one episode. Nonetheless conversion is progressing where it has invaded.] Buffelgrass is a very drought-tolerant perennial, so it can remain dense and even spread in dry years. It is present to burn year round and supports hotter fires than those of red brome. The Sonoran Desert evolved without fire as an ecological factor and most of its plants cannot tolerate it. A single buffelgrass fire kills nearly all native plants in its path. The buffelgrass invasion is now destroying steep hillsides compared to red brome's flatter terrain, and is rapidly converting formerly rich biological communities into monocultural wastelands. The shoulders of this road near Saric, A "natural" roadside with native Sonora are almost 100% buffelgrass, a vegetation. AZ Hwy 85 in Organ Pipe common result where it isn't controlled. Cactus National Monument, where Buffelgrass is abundant in the adjacent hills, buffelgrass is controlled. but heavy grazing keeps it from reaching burnable density. Buffelgrass resprouts vigorously after This hillside near Caborca, Sonora recently fires, but most native desert plants are burned, killing nearly all of the native killed. Near Caborca, Sonora. plants. Only charred skeletons of teddy bear cholla (Cylindropuntia bigelovii) and a palo verde (Parkinsonia microphylla) are visible. The rich Arizona Upland vegetation that once grew here can be seen in the distance. There is growing evidence that buffelgrass depletes soil fertility in a decade or so. It then dies and leaves behind a sterile wasteland. No one knows how much time will be needed to restore such ruined land. 15 Distribution Native to the Old World where it is widespread in Africa, the Middle East, Indonesia and nearby islands, and tropical Asia. Introduced to Australia and the New World. In the Sonoran Desert Region buffelgrass is common in southern Arizona and most of Sonora. Habitat There are two seemingly unrelated habitats in the Sonoran Desert. In valleys and lower slopes buffelgrass invades disturbed areas such as roadsides and cleared or grazed fields. It Arizona it is spreading very rapidly along medians and shoulders of major highways and more slowly on smaller roads. In northern Sonora it has been present longer, and it dominates long stretches of smaller highways. From the town of Imuris, Sonora buffelgrass extends in a continuous ribbon along highway 15 all the way to Sinaloa, interrupted only by a few cities. A typical roadside habitat for buffelgrass This cleared field is nearly pure (under the stopsign). The larger grasses buffelgrass. It has not dominated the are the related fountaingrass (Pennisetum surrounding flats which have not been setaceum), which is also a serious invasive cleared of vegetation. threat. Its other habitat is steep rocky hillsides, mostly east- and south-facing slopes. It is most abundant on debris cones near the angle of repose. These steepest slopes may be the best establishment sites from which it will eventually spread. Some less steep hillsides are completely covered with buffelgrass, so the invasion may still be in its early stages. It occurs from near sea level to 4150 feet elevation. 16 Buffelgrass being censused on a hillside near Magdalena, Sonora. Most of the herbaceous plants have disappeared. A steep rocky hillside in the Pan Quemado Mts, Ironwood Forest National Monument. Several patches of buffelgrass are visible. History Buffelgrass was introduced to the United States about in the1930s as livestock forage. It was in planting trials at the Soil Conservation Service nursery in Tucson from 1938 to 1952. Several experimental plantings were done beginning in 1941 at Aguila near Phoenix. Most did not do well. (Another planting in Avra Valley west of Tucson in the early 1980s also died out where it was planted on flat ground.) Records of collections in natural habitat were sparse until about 1980 when it began a rapid expansion. One example is Organ Pipe Cactus National Monument, where buffelgrass was rare before 1984 (Felger 1990). By 1994 it had occupied 20-25 square miles and was expanding rapidly. Few people other than botanists noticed it in Arizona before 1990. Today it is difficult not to see it in the southern half of the state. A survey of roads done by the Desert Museum in 2004 revealed the full extent of the buffelgrass invasion of Arizona and northern Sonora (map below). 17 Buffelgrass distribution along roads in Buffelgrass distribution in the Tucson Arizona and northern Sonora. Red vicinity. The survey was not completed symbols in both maps denote areas within the urbanized zone; buffelgrass is where buffelgrass is the dominant plant present here in nearly every vacant lot and dense enough to burn. and unpaved road shoulder. What can be Done? Buffelgrass can be controlled by manual pulling and herbicides. Organ Pipe Cactus National Monument nearly eradicated patches covering ca. 25 square miles in three years of intensive manual labor. After the initial control only minor efforts have been required to destroy new infestations (though searching the huge park takes a great deal of time). Volunteer groups such as Sonoran Desert Weedwackers are active in controlling it in other areas such as Tucson Mountains Pima County Park. The highest priority should probably be to control buffelgrass on roads outside of urban zones, because these are the seed sources for invasion of natural habitats.

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