Changes in natural processes and ecological drivers influence all wildlife habitats in Arizona and the Southwest. While some habitats are more resilient or resistant to these modifications than others, the increased pressures from multiple interacting threats can take a toll on even the most resilient systems.
Hydrologic alteration of flow regimes, stream channelization, land management practices, and fire suppression are a few of the natural system modifications that have adverse effects on Arizona’s natural communities. These modifications can fragment, convert, or destroy habitats, and greatly alter disturbance regimes (McIntosh et al. 1994, Hessburg and Agee 2003). These human-caused changes have created conditions that are outside of the evolutionary and ecological tolerance limits of native species (Beschta et al. 2004). As a result, the integrity of many terrestrial and aquatic habitats have been severely degraded at the population, community, and species levels of biological organization (Frissell 1993).
Fire and Fire Suppression
Wildfires were once an integral process in many forest, woodland, shrub, and grassland ecosystems in Arizona, and many of these habitats are considered fire-adapted. Fires stimulate grass and forb growth, cycled nutrients, and reduce fuel loads that could contribute to more severe fires in the future (Johnathan et al. 2020). A century of suppression along with a combination of climatic factors and human influence has created optimal conditions for devastating fires that are difficult, if not impossible, to control in forested landscapes. Unnaturally dense canopies, increase in small trees and ladder fuels, lack of understory grasses and other herbaceous vegetation, increased fuel loads, and invasive species are a few of the characteristics of these altered landscapes. The tendency of fires in these areas to burn extremely hot and fast reduces many of the ecological benefits and puts humans, property, and wildlife at risk. Population growth and development within and adjacent to fire-adapted landscapes is a contributing factor because fire suppression is often implemented for protection, thereby altering the natural fire regime that these ecosystems rely on.
Grasslands
Fire suppression and intense grazing has led to shrub encroachment, proliferation of non-native species, and soil erosion in areas that once were dominated by native perennial grasses and forbs. Additional factors such as climate variability (especially rainfall patterns) can have significant influence on community composition and extent, while fragmentation can contribute to altered fire regimes by limiting fire spread (Geiger and McPherson 2005).
Some grassland-dependent birds, including the Arizona grasshopper sparrow, eastern meadowlark, short-eared owl, chestnut-collared longspur, Baird’s sparrow, and Sprague’s pipit, prefer more extensive, open native grasslands with limited areas of woody shrub for breeding or for wintering.
Both intense grazing and fire suppression promotes the expansion of shrub species that, in turn, out-compete native forbs and grasses. Pronghorn, and other grassland-obligate species are highly dependent on these open grasslands in order to meet their daily and seasonal nutritional requirements. The major limiting factors for pronghorn are poor fawn recruitment rates, a result of combined poor habitat conditions, lack of nutritional forage, and predation.
Several rangeland conservation programs exist to assist ranchers in increasing productivity and resilience in Arizona’s grasslands. See AZGFD’s Landowner Incentive Programs for information about these programs.
Forests and Woodlands
Historically, naturally occurring fires within Arizona’s ponderosa pine and lower mixed-conifer forests and woodlands were frequently low-intensity fires. These fires helped maintain stands of older trees with an open, park-like structure (Moir and Dieterich 1988). At higher elevations, mixed conifer and spruce-fir forests (wetter forest types) exhibited less frequent fire return intervals, and fires were generally stand-replacing fires of higher intensity (Pyne 1984, Walstad et al. 1990, Agee 1993). Improper grazing practices and systematic fire suppression over the past century combined with prolonged drought, has altered herbaceous vegetation composition, increased fuel loading, and stimulated development of ladder fuels, leading to increases in fire size, frequency, and intensity. As disturbance regimes shift outside the ranges to which dominant species are adapted, potential for post-fire forest regeneration to prior composition and structure may be compromised (Coop et al. 2020).
In the southwest, evidence of ecosystem conversion has been cited in the Jemez Mountains of New Mexico where a series of three large, severe fires burned through the forest over a 25 year period (SW CASC 2020). The conifer forests in many areas have been replaced by shrublands as shrubby and herbaceous species quickly resprout after fires. Related studies have only recently been initiated, so it is unknown when, or if, the conifer forests will regenerate in these severely-burned areas. Smaller-scale community type conversions have occurred post-fire in the Santa Catalinas of southeastern Arizona where shrubland has replaced mixed-conifer forest. Likewise, on Mount Elden just outside Flagstaff, areas once dominated by ponderosa pine forests are now open grassland more than 40 years post-fire (Falk et al. 2019).
The Four Forest Restoration Initiative (4FRI)
Since 2009, AZGFD has played an integral role in the Four Forest Restoration Initiative (www.4FRI.org), which was created in order to accelerate an ambitious, landscape-scale restoration program to improve and sustain watershed health, enhance wildlife habitat, promote the reintroduction of natural fire, and improve overall forest resilience. This initiative includes Prescott, Tonto, Apache Sitgreaves, Kaibab and Coconino national forest lands, and spans 2.4-million-acres across Arizona.
The 4FRI stakeholders include members of local, county, and state governments, NGOs, institutions, and industry representatives, many of whom are involved in work groups that focus on different initiative components and special projects. A key strategy with this initiative is the utilization of appropriately-scaled business and industry to harvest, process, and sell wood products. This will reduce treatment costs and provide restoration-based work opportunities that will create jobs.
As the southwest continues to experience extreme wildfire events coupled with increased temperatures and long-term drought, researchers are exploring management options and predicted outcomes based on three categories:
Resistance: Actions that attempt to preserve existing ecosystems (i.e. forest thinning)
Recovery: Actions intended to reestablish vegetation types that existed previously (i.e. replanting)
Reorganization: Actions that acknowledge the inevitable conversion of an ecosystem and work within the context of climate change to allow a novel community to establish
Many of our grasslands have experienced increases in woody encroachment which reduces the available habitat for grassland-dependent species such as black-tailed prairie dog and black-footed ferret. Increased fuel loads and climate change have resulted in unpredictable fire patterns in many mountainous habitats which impact New Mexico meadow jumping mouse by both removal of vegetation and degradation of the riparian habitat with post-fire flooding. Fire suppression, livestock grazing, and logging on Mt. Graham in the Pinaleño Mountains has resulted in larger, high-intensity wildfires which directly removes habitat for endangered Mt. Graham red squirrel that can take decades, if not centuries, to replace.
Desertscrub
Desertscrub landscapes historically had very low wildfire frequencies. Although lightning occasionally ignites desert fires, low fuel volumes and sparsely distributed vegetation would ordinarily prevent the significant spread of fire (McLaughlin and Bowers 1982; Brooks 1999). However, inappropriate grazing regimes, rangeland modifications, and climate variability have contributed to large-scale invasion by a variety of invasive grasses and forbs. This increase in fine fuels has resulted in an increase in catastrophic wildfires in the Sonoran and Mohave deserts (Brooks 1999; Esque and Schwalbe 2002; Brooks and Matchett 2006). Larger fires and shortened fire return intervals further alter desertscrub by favoring non-native vegetation over native perennial species (Brooks and Chambers 2011). In the Sonoran Desert, long-lived species such as saguaros and palo verdes that evolved in a fire-free setting, are particularly vulnerable. In some instances, native species thresholds are exceeded such that native desert shrublands are being converted to non-native annual grasslands (Brooks and Pyke 2001; Esque and Schwalbe 2002).
Desertscrub communities are home to several SGCN reptiles and amphibians that may lack the adaptive capacity to respond favorably to altered fire regimes. Although studies of the effects of wildfires on reptile species occupying desertscrub habitat are limited, direct mortalities have been documented during post-fire surveys (Esque et al. 2003; Simons, 1989). Indirect effects of wildfires including vegetation-type conversion, reduced structural complexity, altered thermoregulatory conditions, and reduction in food resources also pose a threat to the abundance and diversity of reptile and amphibian species in desert systems.
Riparian Areas
Stochastic events such as wildfire and post-fire flooding pose serious threats to riparian habitats and native fish populations, particularly in systems already impacted by drought and invasive species. Altered timing of fires, higher fuel loads, broader geographic extent, and increased fire intensity can produce substantially greater quantities of sediment/ash and cause greater loss of native vegetation (Coop et al. 2020). Meanwhile the resulting soil instability reduces infiltration and increases run-off which can have an immediate and detrimental effect on fish and amphibians (Ketcham and Koprowski 2013). Accumulation of the increased sediment may also alter habitat, and reduce water quality, especially dissolved oxygen.
Many native fish populations are now isolated into headwater habitats upstream of barriers, and this can protect them from downstream threats. However, large-scale wildfires can completely eliminate riparian habitat zones and post-fire floods can eliminate entire fish populations. For example, the Wallow Fire of 2011, Arizona’s largest wildfire in history, eliminated or severely impacted several native trout populations and the loss of riparian habitat resulted in high water temperatures above the optimal suitability for trout. These factors make wildfire and post-fire floods one of the greatest threats to native fish in the state.
Dams and Water Management and Use
The combination of an arid climate, highly-populated metropolitan areas, and a substantial agricultural industry makes Arizona’s water resources both highly valuable and highly vulnerable. The chart below from the Arizona Department of Water Resources summarizes Arizona’s water supply sources (Figure 3).
Agricultural and urban areas have always depended on groundwater pumping, dams, reservoirs, channelization projects, and water diversion structures to meet their water demands (Cheney et al. 1990). These water diversion tools divert water out of natural channels which changes the natural variability of stream flow quantity, timing, and frequency across both time and space. The changes to stream flow affect several aspects of the river and riparian habitat, such as physical structure, energy flow, sediment transport, water temperature, and water quality. In turn, these changes impact aquatic and riparian plant and animal species (Ffoliott et al. 2004).
In addition to altering the quantity, timing and duration of surface flow, dams and impoundments also create barriers that fragment species ranges, preventing upstream and downstream movement of fishes and other aquatic species, including riparian plant dispersal. The manner in which a dam regulates water flow can also have significant impacts on the floodplain both upstream and downstream. The altered disturbance regime places additional stress on native species while creating opportunities for non-native species to take hold and invade the vulnerable areas, to the point of becoming the dominant vegetation.
Groundwater drawdown for agricultural, municipal, and industrial uses has and will likely continue to cause aquifer reductions, resulting in loss of stream connectivity and riparian habitat (Zaines 2006). The resulting loss of groundwater discharged as surface flow reduces connectivity and is a major concern that may intensify as a result of extended drought conditions related to climate change. While all riparian habitat is dynamic and subject to natural cycles of loss and regeneration, fragmentation and loss related to groundwater depletion can have considerable effects on small ciénegas, springs, seeps, marshes, alluvial valley aquatic and riparian areas and their associated species (Tiller, Hughes, and Bodner 2012). For example, spring “improvements,” in which spring output is captured in collection structures and exported to make it available to human-determined uses, limits the extent of the wetted zone in the ciėnega or around the spring, changing the biological structure of these highly diverse, increasingly rare habitats (Minckley 2013). Wildlife are affected either through diminishing water availability or, in the case of many native amphibian and invertebrate species, degradation or total loss of habitat.
While Arizona’s intense water management techniques have substantial anthropogenic benefit, the modifications and disturbances lead to degraded and fragmented habitat that is unable to support the historically rich suite of native species that would otherwise thrive in our aquatic and riparian systems. For example, the creation of dams and reservoirs in the Colorado River, including Glen Canyon Dam in 1963, and the construction of the dams and reservoirs on the Salt River from 1912-1958, altered the flow regime and temperatures, impacting several endemic species including Colorado pikeminnow, humpback chub, razorback sucker, and bonytail. The four species were subsequently listed as endangered and recovery efforts have been underway for decades. The creation of the Central Arizona Project (CAP) canal, which delivers Arizona’s allocation of Colorado River water, was predicted to result in the introduction of undesirable non-native species to central Arizona watersheds and which the USFWS determined was likely to jeopardize the continued existence of spikedace, loach minnow, Gila chub, and the Gila topminnow, all of which are listed as endangered.
According to the ADWR, the overall water use in Arizona has decreased over the last 30-40 years even with a considerable increase in population as shown in Figure 4. The reduction is attributed to investments in water conservation and infrastructure and the reuse of water (ADWR 2021).