Several contributing factors are increasing the introduction and spread of invasive or undesired plant and animal species throughout Arizona. These include intentional and non-intentional introduction of non-native species, as well as human-related disturbance and alteration of natural systems.
Climate change, and the resulting shifts in plant and animal communities, adds an additional layer of complexity as conditions change and native species are forced to respond through resistance, adaptation, or movement to more suitable areas. At the same time, changing conditions may create new habitat for non-native species. Invasive or undesired species may prey upon, outcompete, or transmit diseases to vulnerable native species. These stressors further impair native species’ ability to cope with changes to their environment. As non-native species become established, impacts to agricultural productivity, public utility operations, native aquatic species, tourism, and outdoor recreation, ecosystem services, and human well-being can all be negatively affected (USFWS 2012).
Once established, invasive or undesired species can be extremely difficult to control. Management is both time consuming and expensive. One study by Cornell University researchers estimated that invasive species damage and control costs the U.S. more than $120 billion annually (Pimental et al. 2000). Early detection and rapid response to invasive species is critical. Likewise, collaborating with public and private land managers at a landscape scale increases the likelihood of successful control while reducing the chances of re-establishment. These coordinated and dedicated eradication programs are possible and may have profound, positive effects on native wildlife populations.
Invasive animal species are a major concern in Arizona. Whether introduced accidentally through imported goods or purposefully as a source of food or for sport hunting or fishing, when not managed properly, undesirable non-native species can cause a wide range of negative impacts to the environment as well as the economy. Several factors may contribute to their tendency to take over in new environments, including a lack of natural predators and diseases, highly competitive nature, and unique reproductive strategies.
Aquatic invasive species in Arizona are having a devastating effect on native aquatic wildlife through disease introduction, predation, and competition (Dolan and Mannan 2009). Aquatic invasive plants and animals are spread via interconnected waterways (e.g. Colorado River reservoirs), inter- and intrastate movement of watercraft, and the discard of live aquatic wildlife and plants (e.g. pets, bait fish, decorative aquatic plants) into local waters (AZGFD 2016). While AZGFD plays a leading role in control efforts as well as public information campaigns, ultimately it is everyone’s responsibility to help control the spread and prevent the introduction of new aquatic invasive species through proper cleaning and decontamination of watercraft as well as appropriately discarding bait.
Quagga Mussels have spread from the Great Lakes to the western U.S. and have infested several important waters in Arizona including lakes Powell, Mead, Mohave, Havasu, Apache, Canyon, Saguaro, as well as the Lower Colorado River drainage, including Lake Pleasant. They pollute the shoreline, damage equipment and infrastructure, alter the aquatic food web, and cost millions of dollars to control. Quagga mussels filter incredible amounts of phytoplankton, reducing the amount available to other organisms, and altering the ecological balance of the lake which can promote the growth of nuisance algae (Invasive Mussel Collaborative 2018).
Invasive Aquatic Animal Species:
Quagga mussels • Zebra mussels • Crayfish • Bullfrogs • Red-eared sliders • barred tiger salamanders • Green sunfish • Applesnails • New Zealand mudsnail
Invasive Terrestrial Animal Species:
Feral burros and horses • Starlings • Cowbirds • Feral hogs
The Don’t Move a Mussel campaign provides the boating public with guidelines for cleaning, draining, and drying their boats to help stop the spread of this highly invasive mussel. This is currently our best defense as cost-effective eradication methods have not yet been developed, although research is underway (Invasive Mussel Collaborative 2018).
Bullfrogs are not native to the western U.S. They were first introduced in the early 1900s for sport, food, and inadvertently during fish stockings. Bullfrogs reproduce prolifically, laying up to 20,000 eggs in a single clutch and are capable of spreading without human intervention. Bullfrogs compete with and often prey upon many native aquatic species including fish, turtles, snakes, and a variety of invertebrates. Of particular concern are the detrimental effects that bullfrogs have on the federally-listed Chiricahua leopard frog and Mexican gartersnake. In addition to preying on or competing with these species, bullfrogs may harbor deadly diseases such as chytridiomycosis that can infect native amphibians (Garner et al. 2007). Efforts to remove bullfrogs from habitats where Chiricahua leopard frogs occur or will be introduced is an AZGFD management strategy that has been effective in several large landscapes in southeastern Arizona.
Crayfish are not native to Arizona and were originally introduced as fishing bait and as a biological control for aquatic vegetation in irrigation canals and ditches. Populations have since exploded in our lakes, ponds, rivers, and streams statewide. Crayfish have few natural predators and feed on aquatic plants, insects and other invertebrates, larval fish, snakes, and turtles. They may have played a role in population declines of several of our SGCN including the Chiricahua leopard frog, narrow-headed gartersnake, Sonoran mud turtle, Three Forks springsnail, Gila topminnow, loach minnow, and Little Colorado spinedace. Though anglers can still use crayfish as bait in waters where they are caught, they can no longer purchase, import, or transport live crayfish as bait.
Along with our federal and university partners, AZGFD has explored potential control measures for crayfish including trapping, pesticides, and pathogens (viral, fungal, and bacterial). Nearly all of these control measures have limitations or concerns on effectiveness or incidental impact on non-target wildlife. From a management standpoint, preventing their spread is the best approach because they are difficult to control once established. Continued outreach to the public is one of our best practices to help limit illegal stocking and movement of crayfish by people.
Although aquatic invasive species have caused great concern in Arizona, native species and habitats are also threatened by terrestrial invasive species:
Starlings, which were brought to the U.S. from Europe in the late 1800s, are a highly aggressive species that can take over nests of native birds, destroying eggs and killing nestlings (Dolin and Mannan 2009). They find saguaro cactus cavity nests particularly appealing to the detriment of native species like the Gila woodpecker.
Cowbirds expanded their range into Arizona from the Plains states as farming operations increased here in the early 1900s. They feed on seeds from harvested fields and grain from feedlots. Cowbirds are brood parasites and will lay their eggs in the nests of other species, replacing one or more of the existing eggs so their young are incubated and raised by the host bird. In this way, the cowbird contributes to population declines of some native species, including orioles, thrashers, towhees, and warblers (Audubon 2020).
Cowbirds and starlings are both common in urban areas where homeowners keep bird feeders full of mixes that include millet and cracked corn. One strategy for deterring these species is to use a mix of black oil sunflower seeds and safflower seeds which these invasive birds seem to find less palatable.
Feral burros and horses have become a concern throughout many parts of Arizona. Feral burros and horses compete with native wildlife for forage and water, and have been observed to displace wildlife from water. Reduced access to water is particularly detrimental during drought. Feral burros and horses also alter plant composition, reduce plant diversity and cover, and increase soil compaction and erosion, thereby putting native habitats at risk. Because these non-native species have few natural predators, and effective management options are currently limited, their numbers have grown beyond those determined to be appropriate for federally-designated management areas in most parts of the state. This has resulted in growing concern about their long-term effects on the health of natural habitats and native wildlife. In addition, they have become a safety hazard for motorists in some parts of Arizona.
Resources and Collaborative Management Efforts for Invasive Animal Species
AZGFD State of Arizona Aquatic Invasive Species Management Plan is a statewide strategy to halt the spread and eliminate, where possible, aquatic invasive species from becoming further established in Arizona waters.
BLM’s Wild Horse and Burro Program provides information on herd management, ways to get involved, and research.
Invasive plant species are causing serious ecological and economic impacts by outcompeting native species, and in some cases, taking over entire landscapes. Native habitats, agricultural lands, and parks that are overrun by invasive species require aggressive and costly invasive species management to control the spread and improve the quality of impaired land. Attributes that contribute to a species “invasiveness” include altered phenology, prolific seed production, seed dormancy, resistance to or dependency on fire, and moderate to high rates of dispersal and establishment.
Problematic Aquatic Plant Species:
Giant salvinia • Elephant grass • Hydrilla • Eurasian watermilfoil • Parrotfeather •Brazilian elodea
Problematic Terrestrial Plant Species:
Buffelgrass • Red brome • Cheatgrass • Fountain grass • Sahara mustard • Globe chamomile • Tamarisk • Sweet resin bush • Russian thistle • Tree of heaven • Diffuse knapweed
Many aquatic invasive plant species were first unintentionally introduced from the aquarium trade and aquatic nurseries where they were sold as ornamentals. Following introduction into an aquatic habitat, many of these species can easily spread to new bodies of water by watercraft and equipment. Once established, they can alter the water flow, quality, and recreational value of our lakes and rivers. As these species take over water bodies, they can form thick mats, blocking sunlight and depleting oxygen levels. Hydrilla, parrotfeather, Eurasian watermilfoil, and Brazilian elodea can all spread through vegetative fragmentation, where pieces of the plant break off and take root in other areas. These species are incredibly difficult to eradicate in infested waters, and the best method of controlling the spread is by thoroughly cleaning and draining watercraft and equipment and properly disposing of unwanted bait.
In low-elevation riparian areas, tamarisk (salt cedar) is posing the biggest threat to native systems. An invasive tree, tamarisk has benefited from variability in precipitation and temperature, because it adapts well to warmer and drier conditions associated with climate change. Dense stands along waterways alter surface flow, lower the water table, increase soil salinity, and displace native species. Native cottonwood, willow, and seep willow species suffer, as do wildlife such as threatened western yellow-billed cuckoos that rely on these native habitats. Tamarisk is fire-adapted and once established it is difficult to control due to deep tap roots and the ability to resprout readily when damaged.
In desert areas where fire is not a common ecological process, invasive species including red brome, cheatgrass, buffelgrass, Sahara mustard, and stinknet (a.k.a. globe chamomile) have invaded natural areas, threatened native species, and provide the fuel that allows fires to spread through desert areas. Once the fire burns through an area, these invasive plants are the first to recolonize, replacing non- fire-adapted native species like blue palo verde and the giant saguaro. The frequency and intensity of these fires decreases biodiversity and alters vegetation structure. Meanwhile, forested areas are seeing cheatgrass, dalmatian toadflax, and diffuse knapweed colonize and spread rapidly in severely burned areas (Coop et al, 2020). Even fire-adapted native species cannot compete with the rapid germination and growth of these invasives.
Resources and Collaborative Management Efforts for Invasive Plant Species
Verde River Cooperative Invasive Plant Management Plan, led by Friends of the Verde River, provides a strategic approach for controlling invasive plants in the Verde River Watershed, while increasing community and stakeholder engagement.
Arizona Department of Forestry and Fire Management Invasive Plant Grant Program provides funding by the State of Arizona and USFS to treat invasive plant infestations that threaten forested, woodland, or rangeland areas.
Arizona Department of Agriculture Noxious Weed List is the official State list of invasive species that are known to aggressively spread and cause ecological and/or economic impacts. The list includes both species known to exist in Arizona and those that are problematic elsewhere.
AZGFD State of Arizona Aquatic Invasive Species Management Plan is a statewide strategy to halt the spread and eliminate aquatic invasive species from becoming further established in Arizona waters.
Additional problematic species in Arizona include insects such as bark beetles that can alter and degrade forest conditions, algae that can be highly damaging to native aquatic species and overall water quality, and native aquatic plants that form monocultures and lead to pond succession. These species are difficult to nearly impossible to control once introduced to an area. In addition, native wildlife and plants, in some situations, can be problematic to the maintenance of SGCN species.
Insect outbreaks cause tree mortality and reduced growth in Arizona’s forests and woodlands (Negrón et al. 2009). Bark beetles and inner bark borers are native invertebrates and perform functions that are instrumental in sustaining forest health. However, under certain conditions the magnitude of impact can cause shifts in vegetative species composition and structure (Haack and Byler 1993). Drought, altered forest conditions, environmental stresses, late spring frosts, wind throw, and air pollution can encourage insect outbreaks (Haack and Byler 1993). For example, closely spaced, single-species stands are more likely to trigger outbreaks than are compositionally and structurally diverse forests, especially during vulnerable periods such as drought (Mattson and Haack 1987, Waring and Pitman 1983).
Overgrowth of some algae in water bodies can be harmful to aquatic wildlife, humans, and the economy. Conditions that promote these algal blooms include ample sunlight, slow moving water, and excess nutrients, especially nitrogen and phosphorus (EPA 2019). Aquatic species are impacted by reduced sunlight, depleted oxygen, and release of toxins to the point where mortality can occur. Some types of algae, including golden algae, a species introduced to Arizona in the past two decades, produce potent toxins leading to extensive fish kills. Algal blooms can have negative impacts to the economy when popular recreational water bodies are overcome with a scum-like surface of algae. Humans may be at risk if toxins are ingested, for example those toxins produced by microcystin. Not all algae are toxic, however, and the Arizona Department of Environmental Quality monitors water quality and reports of algal blooms to protect our water bodies and keep Arizonans informed of potential safety concerns. In order to stop the spread of algae and other invasive aquatic species, it is critically important to thoroughly clean, drain, and dry watercraft and equipment and never move water, live animals, or plants from one body of water to another.
Some native aquatic plants can form dense monocultures and negatively-impact desired wildlife species in an area. For example, there are many ponds in Arizona that house refuge populations of Gila topminnow, desert pupfish, and Chiricahua leopard frogs. Native cattails, bullrushes, and various types of submersed vegetation can colonize these ponds, increase in abundance, leading to pond succession, where the lentic habitats gradually turn into a wetland, thus eliminating the fish habitat. It is therefore necessary to periodically control these types of vegetation to maintain open water habitats.
Occasionally, native wildlife can be problematic to the detriment of SGCN species. For example, American beavers are great modifiers of aquatic and riparian habitat and contribute to the overall health of these systems. Beaver activities can transform a stream from a series of pool-run-riffle sequences, to one more dominated by pools, thus affecting the fish assemblage. For riffle-occupying species like loach minnow, the disappearance of riffles will lead to the local disappearance of the species. Therefore, to maintain a recovery population of the endangered loach minnow, it may at times be necessary to control beaver.