Hunting for Black Gold
Fellow, Climate Assessment for the Southwest
Stephanie Doerries is a 2018 recipient of the Climate Assessment for the Southwest (CLIMAS) Climate & Society Graduate Fellows Program.
Project Title: Estimating survival and abundance of the endangered Sonoran pronghorn
Abstract: Large herbivores are in decline around the globe. Understanding the impacts of climate change, human activity, and habitat fragmentation on these populations is essential to facilitate recovery of endangered species such as the Sonoran pronghorn. A subspecies of American antelope unique to the Sonoran desert, the Sonoran pronghorn persists at relatively low densities in southwest Arizona and northwest Sonora, Mexico. Because wildlife managers are faced with the problem of accurately estimating population parameters with limited resources in a logistically challenging landscape, I am working with the Arizona Game and Fish Department to improve estimates of survival and abundance for Sonoran pronghorn. For survival, I will develop mark-recapture models for four types of data: location data from telemetry and GPS collars, opportunistic observations, genetic identification of fecal samples, and photographs taken by motion-activated cameras at sites of highly-localized pronghorn activity. For abundance, I will develop a novel sighting probability model to account for imperfect detection of pronghorn during range-wide aerial surveys. With more accurate estimates of survival and abundance, I will analyze the relationship among these parameters and climatic variables and vegetation indices. Results will allow managers to more accurately assess the population viability of Sonoran pronghorn for different recovery efforts.
With the aid of my headlamp, I carefully check the contents of my backpack in the pre-dawn darkness. Food, water, vials, coin envelopes...check. I strap a shovel to the outside of my pack and swing it across my shoulders with a huff, shrugging to adjust the weight. Two and a half gallons of water is not light, but I’ll drink most of it over the course of the next 12 hours. And I always carry a little extra when traipsing around the Sonoran desert in summer. I enjoy the June morning, about 70 degrees Fahrenheit, knowing the temperature will rise at least 30 degrees by midafternoon. A warm breeze blowing across the Pinta Sands, a remote area on the Cabeza Prieta National Wildlife Refuge, hints at the heat to come. I sling the strap of my binoculars over my shoulder and start walking at a brisk pace so I can cover the three plus miles to the first wildlife water before sunrise. If I’m lucky, I’ll see a pronghorn at the edge of the playa—a dried lakebed—like I did last year.
Figure 1: The Sonoran desert of the Cabeza Prieta National Wildlife Refuge
By pronghorn I mean Sonoran pronghorn. Most of us are familiar with the American “antelope” roaming the Great Plains; we’ve sung about them since childhood in the classic tune “Home on the Range”. Few are aware of the distinct subspecies specially adapted to living in the Sonoran desert. Sonoran pronghorn are not easy to find, and they have been listed as endangered since 1967. The U.S. range-wide survey in 2016 estimated that 228 individuals occupy 1.6 million acres—an area the size of Delaware—extending about 60 miles along the U.S./Mexico border. As low as this number is, it is a significant increase from the count of 21 observed after a year of severe drought in 2002. Over the next century, climate scientists predict that such droughts will occur more frequently in the Sonoran desert. The Sonoran pronghorn’s current range lacks any naturally-occurring, perennial sources of water they are likely to visit. Pronghorn are the fastest land mammal in North America, reaching and sustaining speeds over 55 mph, but they cannot outrun drought. To help the Sonoran pronghorn population recover, wildlife managers have strategically built water sources and provided supplemental forage during the hot, dry summer. Combined with captive breeding efforts, management has succeeded in reversing the downward trend in Sonoran pronghorn numbers. These recovery efforts, however, have not been implemented without controversy. Wilderness advocates balk at constructing wildlife waters in the designated Wilderness of the Cabeza Prieta National Wildlife Refuge and Organ Pipe Cactus National Monument, the majority of the Sonoran pronghorn’s range in the U.S. Given the legal difficulties of conducting experiments with endangered species, wildlife managers struggle to quantify the degree to which costly recovery efforts have helped bolster the Sonoran pronghorn population. Doing so requires accurate estimates of survival, a challenging feat given the remoteness of the desert landscape and the pronghorn’s low densities. Fortunately, researcher partners at have devised a way to estimate survival by systematically collecting what I’ve come to know as black gold, more commonly called pronghorn scat. Why do I call scat black gold? Because you can learn a lot about an animal from its poop. You can determine its unique genetic profile, diet quality, stress level,…even pregnancy status for females!
Figure 2: Two male Sonoran pronghorn, called bucks, stand vigilant towards the author
As I approach the wildlife water, I carefully scan for movement and the white and tan pattern unique to pronghorn. The only thing standing out from the background vegetation of chain fruit cholla, creosote, and bursage is the 6-inch tall plywood rim around three-quarters of the below-ground trough, minimizing the amount of sand blown into the water source. Without the rim, the trough would be impossible to see from a distance. I spend an hour carefully searching the surrounding area for pronghorn scat and tracks without any luck. The sun rises higher in the sky and sweat starts to bead on my forehead. While I shovel muck out of the trough, a couple hundred bees maneuver around me to access the valuable water. I try to make my movements slow and smooth to avoid being stung. When finished with the mundane maintenance, I gather my gear and head towards a second wildlife water to continue my search for pronghorn scat.
This year, 2018, is our sixth year collecting scat for genetic identification, which will provide us with five estimates of survival. How does one estimate survival by collecting scat and determining each sample’s genetic profile? By using mark-recapture models. Mark-recapture is a way to estimate survival and population size by capturing individuals, marking them, releasing them, and then performing additional captures after a given time period. The proportion of marked individuals recaptured can be used to estimate the size of the population being sampled. When this process is repeated for two or more years, researchers can estimate survival of individuals. In this case, unique genetic profiles are “marks” so pronghorn can be “captured” without ever being touched. Their scat is “captured” instead! This helps improve the accuracy of our survival estimates while minimizing our impact on an endangered species.
The two-and-a-half-mile hike to the second wildlife water is devoid of fresh pronghorn sign. I repeat my search efforts and maintenance work, measuring the depth of water in the trough as sweat drips down my chest and back. The temperature must be in the 90’s by now. The wind from a nearby dust devil provides a brief reprieve from the heat as I enjoy walking without my heavy backpack. With no pronghorn scat in sight, however, I guzzle some water, shrug into my pack again, and start hiking towards the third and final wildlife water along the bajada.
Figure 3: The author mucking out the trough of a below-ground wildlife water
Why I am visiting wildlife waters to find pronghorn scat? Given that Sonoran pronghorn occur at such low densities, finding fresh scat in randomly located areas would be extremely difficult. Instead, I target areas where pronghorn are known to congregate during the hot, dry season leading up to the monsoon: wildlife waters in the bajadas. I have already collected more than five hundred samples this June and anticipate collecting another two hundred or so before the sampling season ends.
I dart behind the nearest creosote bush when I hear the pronghorn’s warning call—a distinct “snort-wheeze”. Two bucks are visible between the branches about seventy-five yards ahead of me, a quarter mile from the third wildlife water. I crouch as still as possible, balancing my weighty pack with the awkward positioning of the shovel. As I watch the pronghorn through my binoculars, one buck squats to urinate and defecate. I carefully memorize his exact position in relation to the surrounding vegetation and rejoice that my scat collection efforts have not been in vain! After snorting a few more times, the two pronghorn trot away. In this sweltering 100+ degree heat, I feel sorry to have disturbed them during the hottest part of the day. I stiffly rise to my feet and head towards the spot where I first saw the pronghorn. Following a couple minutes of searching, I find the small pile of pellets. I carefully put a dozen into a #1 coin envelope for genetic analysis and another dozen pellets into a vial for hormone analysis. An hour of following pronghorn tracks and searching beneath ironwood and palo verde trees provides me with scat samples for both pronghorn in the group. After completing the routine maintenance on the wildlife water, I spend a few minutes resting beneath the broken shade of a large ironwood.
Figure 4: The author collecting pronghorn scat for hormone analysis
With sufficiently accurate estimates of annual survival for five consecutive years, I hope to analyze the relationship between survival and climatic variables such as rainfall. These results will allow managers to more accurately assess the extinction risk of Sonoran pronghorn for different recovery actions. This is particularly important in light of climate change and predictions that droughts will become more severe and frequent in the southwest and that summers, the most physiologically stressful season for Sonoran pronghorn, will become hotter and longer.
Content with successfully finding scat samples that will ultimately contribute to our understanding of Sonoran pronghorn, I lace up my boots and reorganize my gear. Now for the long, seven-and-a-half-mile slog through the heat back to camp.