"What happens to other species also happens to us." — Bill Nye

The multitude of species that we share this planet with is challenging to comprehend. We have guesses of how many other species exist, but they are just (educated) guesses. In 2008 there were 5,079 described species of lizards, but by March of 2022, 7,176 species have been described. Lizards are not evolving new species that fast; rather, using rapidly advancing genetic analyses tools, we are getting better and better at identifying unique and isolated genetic lineages (aka species).

For birds, there are some 9,700 described species, but the estimates for what really exists is closer to 10,000. There are at least 50,000 different species of spiders, but that number pales in comparison to plants, for which the current estimates range between 1.6 to 100 million different species. Then there are insects; one guestimate is that there may be as many as a billion to 10 quintillion (10,000,000,000,000,000,000) species of “bugs.” There are no realistic guesses for bacteria and fungi, but the numbers are huge, almost certainly far beyond those for the aforementioned “macro-organisms.”

These species numbers, as big and difficult to visualize as they are, become all the more challenging when we then grasp the reality that there are connections that tie the fates of each of these species to other species; pollinators, parasites, predators, prey, mutualistic relationships that allow each species to thrive. Many of these interrelationships are all but invisible to our eyes, until as the result of a purposeful or inadvertent introduction of a non-native species into a landscape by our species, those long-evolved relationships can be broken, and native species and biodiversity are put at risk.

To simplify these enormous numbers and interconnections, we have created broader categories that become a shorthand for regional biodiversity. Joshua tree woodlands, creosote bush scrub, Sonoran Desert succulent (i.e., cacti) scrub, desert wash woodlands, and desert fan palm oases are natural communities, each comprised of hundreds to thousands of species and more. Within these broader categories we then single out certain species as icons and indicators of that particular landscape. Joshua trees, saguaro cacti, ironwood trees, desert fan palms, desert tortoises, and chuckwallas. As long as those (and other) indicators are sustaining healthy populations, right or wrong, the assumption then is that biodiversity is intact.

To be clear, “healthy” populations means much more than the indicator or iconic species is present. It means that their populations are comprised of the full sequence of age groups that indicate both successful reproduction and recruitment is occurring. The first few years (animals) or decades (woody plants) of life present the greatest challenges for desert species to successfully reach adulthood and begin reproduction themselves. Once becoming an adult, many desert species can live hundreds of years (woody plants) or even thousands of years (creosote bushes), or up to nearly a century (desert tortoises), so presence alone does not necessarily identify a currently healthy ecosystem.

Take the desert tortoise. Tortoises are of course primarily vegetarians, so that their existence is inextricably tied to plants, especially annual plants, and their pollinators, and harvester ants that distribute seeds, and soil microbial relationships that allow those plants to thrive, and to a climate that supports those plants. Tortoises are tied to creosote bushes and other woody shrubs and trees that provide shade and whose roots help bind the soil together so that the tortoise can dig secure burrows beneath the shrubs. Young tortoises, much more so than adults, are vulnerable to becoming prey for ravens, coyotes, and badgers, so the abundance of those predators, and the availability of other species they can prey on rather than young tortoises is also tied to whether tortoise populations are “healthy.”

The desert tortoise, Gopherus agassizii, was fairly recently determined to actually be three distinct species. Gopherus agassizii is now referred to as the Mojave Desert or Agassiz’s tortoise, but now there is a Sonoran Desert tortoise, also called Morafka’s tortoise, G. morafkai, and a Sinaloan thornscrub tortoise or Goode’s tortoise, G. evgoodei. Through the combination of the uplift of the Colorado Plateau and the establishment of the Colorado River about 5 million years ago, along with increased aridity creating distinct desert versus semitropical thornscrub habitats, each of these tortoises became genetically isolated and then on a path of increasingly divergent genetics. In 1990, G. agassizii was listed as a threated species, well before it was found to be three, not just one species. However, only G. agassizii retained protection under the Endangered Species Act. Agassiz’s desert tortoise is largely confined to the Mojave Desert, although there are small populations in the foothills surrounding the Coachella Valley, the eastern, Colorado Desert end of Joshua Tree National Park, as well as a population centered on the Chuckwalla Mountains south of I-10 and Desert Center.

Over the past 30 years since Agassiz’s desert tortoise was listed as threatened, this species is believed to have declined by 90%. Surveys conducted in the 1970s found population densities of 150-350 tortoises per square mile, declining to less than 10 per square mile in the 1990s. Some populations, especially those in the hotter and more arid portions of their range are now even lower. The reasons for this precipitous decline are many. One is the replacement of native desert annual plants by the non-native cheat grass and red brome, Bromus tectorum and B. rubens. You might question how this could be so important, plants are plants to a herbivore, right? While we bemoan the loss of beautiful desert wildflowers, why should tortoises care? But they do. These plants are not substitutable. The non-native grasses were introduced during livestock grazing in the desert a century or more ago. The grasses are okay for sheep and cattle, and while dead and dry, remain available almost year-round, but are much less nutritious than the wildflowers. And the grass seeds can become imbedded in the tortoise throats, resulting in starvation. The grasses have thrived with the addition of fertilizer to the soil via nitrogen-laden smog wafting into the desert from Los Angeles. These same invasive grasses destroy the important soil microbial relationships native species need to thrive in an otherwise harsh desert climate. These grasses fuel wildfires, fires that would otherwise be exceedingly rare in our desert and fires that remove the important tree and shrub cover the tortoises need for thermoregulation. So many biotic connections, connections that when severed or altered result in a cascade of native species declines.

Another cause for the decline in tortoises is ravens. Ravens were rare in our deserts a century ago. Early naturalists rarely saw them and when they did it was a notable event, written into the pages of their field notebooks. Now ravens are abundant. They are smart, having quickly learned how to survive well thanks to humans providing food (garbage and road kills) and nesting platforms (planted trees and power poles) where little or no nest sites previously occurred. Some ravens have learned to become specialists at eating baby tortoises. Many dozens of baby tortoise shells litter the ground below some raven nests. Altered biotic connections with dire outcomes.

And there are the effects of climate change. Hotter more arid deserts mean less frequent wildflowers (even if the grasses do not take over) and temperature regimes that might become lethal for tortoises. Being listed as threatened may catalyze governments to set aside critical habitat, but unless there are cooler summers, wetter springs, fewer ravens, less invasive grasses, and less fires, then just protecting land will not change the tortoise’s current population trajectory. Of course, it is not just tortoises. It is Joshua trees and ground squirrels and lizards and insects, and ultimately people. It is also ultimately people who can change this trajectory.

Nullius in verba

Go outside, tip your hat to a chuckwalla (and a cactus), think like a mountain, and be safe.