The Atascosa Highlands are an area of incredible biological diversity located within one of the most ecologically rich regions on the planet. Taking up less than 1% of Arizona's overall landmass, the Atascosas host at least half of the state’s total bird species and approximately one-quarter of its flora, including several species which are found nowhere else in the United States. While these mountains have been inhabited for thousands of years, the last few centuries of human activity have had a profound effect on this stretch of the US-Mexico border, threatening the natural balance and driving many species to the brink of local extinction from this biological paradise. Though humans have always left their mark on the Highlands, the capability to massively alter landscapes has come to the borderlands only very recently on an ecological timescale. The dramatic effects of large-scale human intervention have irrevocably changed the course of natural history in the Atascosa Highlands, making it essential to document the ecology of the region now. This will provide a baseline for future generations who can use the data gathered today to chart the consequences of this unprecedented era of industrialization and climate change on the ecology of the sky islands.

            In 1943 writer Natt Dodge referred to the Chiricahua mountains of Southern Arizona as a “mountain island in a desert sea.” This became the origin of a new understanding of the isolated ranges of Southern Arizona and Northern Sonora as “sky islands.” This term refers to a series of mountain ranges which connect the Sierra Madre Occidental of Mexico with the Rocky Mountains of the U.S. and Canada, sitting at the confluence of the Sonoran and Chihuahuan deserts. These mountains bridge political boundaries and act as essential migratory routes for species unaware of the arbitrarily defined borders dividing their habitat. Sky islands of the region can rise thousands of feet from their scorching desert bases to cool, wet forests of Douglas Fir (Pseudotsuga menziesii) and Pine (Pinus spp.). The radical geologic, climactic, and topographical variations make the sky island region one of the most biodiverse places on the planet.

            The Atascosa Highlands are a series of four small ranges which form the 203,799 acre Tumacacori Ecosystem Management Area of the Coronado National Forest. The northernmost range is the high bluffed Tumacacoris, which blend into the craggy Atascosas to the South. Running along the border is the Pajaritos, and in the west the San Luis rise sharply out of grassy plains. The Highlands are separated from the Santa Ritas to the East by the Santa Cruz river, and isolated from the Baboquivaris to the west by the broad Altar Valley. The Atascosa Highlands are not the highest of the sky island ranges, reaching their ultimate height of 6,422’ on the rough cut ridgeline of Atascosa Peak. But what they lack in elevation, these mountains make up for in sheer ruggedness. Precipitous cliffs rising hundreds of feet from rolling valleys, create a mosaic of varied microclimates that allow for species with wildly different habitat preferences to grow within a few yards of each other. This topographical variation defies traditional elevational life zone delineations by bringing habitat types together to form diverse ecotones where the lines between ecosystems blur.

            While attempts to rigidly define habitats will always be imperfect, the most prominent ecosystem of the Highlands is referred to as scrub grassland. This biome is found throughout the region and consists of rolling grasslands interspersed with spiny cacti, woody shrubs and stunted trees. These grasslands have been among the habitats most affected by human activity in the Atascosas. Old photos and written accounts provide a picture of wide open plains full of native grama grass (Bouteloua spp.) with few trees or shrubs. But habitual overgrazing and a series of devastating droughts have caused an alarming decrease in total grass cover and encouraged destructive erosion and channel cutting in the shallow washes which cut through the sky islands. Historically, summer storms and indigenous landscape management practices would have facilitated regular fires in the grassland that torched the seedlings of woody plants, while leaving the fire adapted native grasses and annuals largely unaffected. As soon as Europeans began to run cattle on the range, fire suppression became a central tenet of land ethic. Fire came to be viewed as a threat due to a poor understanding of its essential role in maintaining the health of grassland biomes. Over time this has led to a gradual increase in the number of woody plants and succulents, slowly transforming grasslands into shrubby savannah. A history of managing rangeland for the benefit of cattle, rather than native species has caused a severe shift in the species demographics of this incredibly diverse biome.

I. Juniperus arizonica (Arizona Juniper)
II. Noccaea fendleri (Alpine Pennycress)
III. Quercus hypoleucoides (Silverleaf Oak)
IV. Lobelia laxiflora (Sierra Madre Lobelia)
V. Agave palmeri (Palmer's Agave)
VI. Carex chihuahuensis (Chihuahuan Sedge)
VII. Choisya dumosa var. Mollis (Mexican Orange)
VIII. Mimosa dysocarpa (Velvetpod Mimosa)
IX. Asclepias linaria (Pineleaf Milkweed)

           As you move up in elevation and reach the shady sides of canyons, the grassland blends into oak woodland. A half dozen species of oaks (Quercus spp.) grow in the Atascosas, and numerous other species of plants and animals thrive in the dappled shadows they cast. The acorns of these oaks also serve as one of the major food sources for wildlife in the area. These oak woodlands are typically quite high in avian diversity and a walk through this habitat is usually accompanied by the harsh squawk of Mexican Jays (Aphelocoma wollweberi), and the sudden eruption of Montezuma Quail (Cyrtonyx montezumae) from cover. In and around these oak groves manzanitas (Arctostaphylos pungens), sumacs (Rhus virens v. choriophylla), and snakewood (Condalia correllii) provide berries which feed foraging mammals and birds. Many of these stands of oak were cut down for firewood when mining in the Atascosas was in full swing, but they never suffered the intensive and systematic logging that occurred in the higher pine forests of the nearby Chiricahuas and Pinalenos. Scrub grassland and oak woodland are often the most biodiverse ecosystems in sky island ranges; the prevalence of both in the Atascosa Highlands helps explain the extreme species richness of these mountains.

            Beyond the oak woodlands, at the top of some of the peaks in the Atascosas, are patchy forests of Border Pinyon Pine (Pinus discolor) and Juniper (Juniperus deppeana). These areas generally consist of slow growing coniferous trees and an understory of Pinyon Ricegrass (Piptochaetium fimbriatum), Yucca (Yucca “schotti”) , and Sotol (Dasylirion wheeleri). In 2011, a fire swept across Atascosa peak and scorched many of the pines and junipers near the old fire lookout. Periodic fires would have occurred historically, but a warming climate discourages recolonization by these more drought sensitive species and leaves this species assemblage particularly at risk from climate change and the threat of hotter fires carried by introduced species and woody shrubs, which have built up at lower elevations. Pinyon/juniper woodlands are alive with pine siskins (Spinus pinus) that prefer these colder reaches, and squirrels which dutifully gather pine nuts to stash away for leaner months. In these arboreal patches the sweet smell of pine needles wafts on cool breezes and rewards travellers willing to brave the steep climb up the loose, rocky sides of Atascosa Peak or Bartolo Mountain.

I. Cirsium arizonicum (Arizona Thistle)
II. Melia azedarach (Chinaberry)
III. Pseudognaphalium leucocephalum (White cudweed)
IV. Elionurus barbiculmis (Wooly balsamscale)
V. Delphinium scaposum (Barestem larkspur)
VI. Opuntia engelmannii (Engleman’s Prickly Pear)
VII. Stachys coccinea (Scarlet Betony)
VIII. Pinus discolor (Border Pinyon)
IX. Tillandsia recurvata (Ball Moss)

            Where these peaks drop off, time has carved deep cut ravines with semi-perennial water that create riparian corridors. In these lush canyons the Arizona heat is alleviated by the cool water running around the bases of Sycamore (Platanus wrightii) and Ash (Fraxinus velutina and F. gooddingii) trees while the whooping calls of Elegant Trogons (Trogon elegans) reverberate off rhyolitic cliff sides. A citrusy aroma rises from the foliage of Mexican Orange Bush (Choisya dumosa var. mollis) as hummingbirds dart between patches of Sierra Madre Lobelia (Lobelia laxiflora) and dragonflies flit between shallow pools. Sonoran Mud Turtles (Kinosternon sonoriense) wade in the lazily flowing streams and vine snakes blend in to the bright foliage. These riparian corridors form essential travel routes for migratory animals and are where many of the rarest species in the Atascosa Highlands occur. The most prominent riparian corridor in the Atascosas is called Sycamore Canyon and contained within its steep sided walls are species found in no other locality in the United States. The record setting heat of the last half-decade has disproportionately affected riparian ecosystems. 2020 was the hottest year ever recorded for the Southwest, and the effects of extreme heat and drought were clear in Sycamore Canyon, as an absence of monsoon rains sent willows (Salix spp.) and ashes (Fraxinus spp.) that should have been flourishing in late summer humidity into enforced dormancy and early death.

I. Prunus serotina (Chokecherry)
II. Baccharis sarothroides (Desert Broom)
III. Ipomopsis thurberi (El Paso Skyrocket)
IV. Philadelphus microphyllus (Little Leaf Mock Orange)
V. Quercus arizonica (Arizona White Oak)
VI. Passiflora arizonica (Arizona Passion Vine)
VII. Berberis haematocarpa (Algerita)
VIII. Packera neomexicana (New Mexico Groundsel)
IX. Mentzelia montana (Blazing Star)

            There is however another habitat often overlooked in ecological studies, the anthropogenic biome. Places where the dominant factor driving the ecosystem is human disturbance. In the Highlands these range from small cattle ponds to massive man made lakes formed by dammed canyons. From old mine sites to strips of land blasted and cleared for new border wall construction. These “anthromes” are ground zero for the spread of disturbance loving species like mesquite (Prosopis velutina), desert broom (Baccharis sarothroides), bermudagrass (Cynodon dactylon), and puncturevine (Tribulus terrestris). Sometimes, the greatest change wrought by intensive human disturbance is not the complete eradication of life but rather the introduction of radically different species demographics into more delicate native habitats. In a day, a bulldozer can undo the progress of centuries of natural selection, precipitating radical shifts in the flora which ripple out to all the plants and animals of the Atascosa Highlands.