“Death from a thousand cuts” – Lingchi

By DR. CAMERON BARROWS

Change is constant, only the rates of change vary. The Grand Canyon and the Colorado River that carved it are a mere six million years old. Just twenty thousand years ago much of the earth’s northern hemisphere was enveloped in an ice age. Mammoths, camels, bison, ground sloths, and rhinos roamed across Southern California. Then suddenly, from a geologic perspective, they were gone, extinct. Extinction is a puzzling process, in part because based on the fossil record it seems inevitable. The vast sum of life forms that have ever occupied our planet are now extinct, although their DNA continues on in the life forms that replaced those that no longer exist. Dinosaur DNA is alive and well in the form of birds. Trilobite DNA lives in today’s crabs, lobsters, spiders, mites, centipedes, millipedes and insects. As the earth changes, conditions for life change. Those that are better adapted to those changing conditions replace those that are not. Or sometimes it is just good or bad luck. When an asteroid slams into the earth being a small lizard in a burrow on the opposite side of the planet trumps being a truck-sized dinosaur anywhere.

To be clear, the inevitability of extinction does not and should not grant license to those who poison the air and water, or plow and pave landscapes, or introduce invasive exotic, non-native species in the name of progress. We depend on life in all its forms for oxygen, food, recycling wastes and nutrients, and our mental health. Extinctions inevitably have unpredictable consequences across food webs and ecosystems.  For that reason, understanding the causes and results of past extinctions has been a topic of scientific research. That Pleistocene extinction that included mammoths, camels, bison, ground sloths, and rhinos has engendered considerable attention and debate. The post-ice age climate warming and drying of North America is a logical candidate, but that extinction event coincided with the arrival and exponential population growth of the ancestors of today’s indigenous Americans. Was it a too warm climate, or was it overhunting by these people, entering a new land with seemingly endless bounty? I’ve always been skeptical of the overhunting hypothesis, largely because these people had their origins in Asia, where elephants, rhinos, and camels still live today. New research from Southern California’s La Brea Tar Pits sheds a new light on this debate.

The tar pits capture and entomb animals ranging from mammoths and bison to lizards and dragonflies. They also capture pollen and any other air-born particles. As a result, the La Brea site provides an unprecedented, continuous snapshot of what life was like in Southern California over the past 50,000 years. Researchers there found a rapid decline in many, but not all, members of that Pleistocene megafauna around 13,000 years ago. Coyotes were among the species that seemed unaffected by that climate transition. There was also a change in pollen. Juniper pollen was replaced by pollen from species that we know today comprise the chaparral habitats of the California coast ranges, species that were unknown in the tar pit layers earlier than that 13,000-year-old transition. The other new occurrence, contemporary with the vegetation shift, was layers upon layers of charcoal dust from wildfires. Charcoal was largely absent prior to that period of warming and drying. From these data, the researchers hypothesized that people, and their extensive use of fire, catalyzed a more rapid change in the character of the surrounding landscape. The warming and drier climate facilitated that introduction of fire, creating a new habitat type comprised of fire-adapted plant species (chaparral), and in doing so rapidly replaced the previous juniper-savanna habitat preferred by the mammoths, camels, bison, ground sloths, and rhinos. In Asia the climate never became so dry as to allow the proliferation of wildfires. The introduction of extensive fire use as habitat management tool in western North America had the consequence of fostering the rapid evolution of a new community of fire adapted plants. It also may have had the unintended consequence of hastening the demise of the Pleistocene megafauna.

In the California deserts more than a handful of species are or were on a trajectory toward extinction. The Coachella Valley fringe-toed lizard, Coachella Valley milkvetch, and an array of endemic dune insects, all lost more than 95% of their original habitat, the only habitat they ever occupied on earth, to development driven by a proliferation of golf course resorts. The listing of the lizard and milkvetch as threatened and endangered catalyzed a conservation plan that has kept that habitat loss from reaching 100%. So far so good. All the endemic species appear to be, if not thriving, at least holding their own on the remaining habitat “islands”. There are still threats from non-native invasive plants such as Sahara mustard, as well as from the challenges of ensuring that natural sand inputs continue to support healthy sand dune habitats.

Soon after the listing of the fringe-toed lizard and milkvetch, the Mojave Desert tortoise was also listed as threatened. That was in 1990. Tragically, unlike the Coachella Valley sand dune species, the tortoise population continues its downward trajectory. The problem is the tortoise faces threats from so many different sources that their population is experiencing a “death from a thousand cuts.” Invasive grasses have displaced their preferred wildflower foods. The grasses are of lesser nutritional value and the grass seeds can choke and kill young tortoises. The introduced grasses also fuel wildfires across a landscape that is not adapted to fire. Native shrubs and trees that create critical food and thermal cover for the tortoises are killed by fire and may not return for decades if not centuries. Highways crisscrossing the Mojave Desert are death traps for tortoises and other wildlife. Highway wildlife mortality, along with human garbage has provided a smorgasbord for ravens, and while previously ravens were once rare across the California deserts, today the raven population has soared. Some of those ravens have specialized on finding young tortoises with their soft shells and eating them. Then there is solar energy development and expanding desert city footprints, both eating away at the tortoises’ habitat. On top of it all climate change is making it hotter and drier, meaning less food, less pools of water for the tortoises to drink, and often making it too hot for the tortoises to climb out of their burrows to search for food and mates. Death from a thousand cuts.

There are no easy or cheap solutions to ensure tortoises do not go extinct. Rather than trying to come up with an answer that will save them throughout their current range, perhaps its time to accept that the only way to ensure tortoises will be here for centuries to come is to create a system of preserves where focused management can be applied. Sites for such a preserve system would need to include adequate climate refugia, areas with sufficient elevation and topography so that the tortoises can find their preferred thermal “happy places.”  Such sites would need to have few if any highways. Then, within each site ravens would need to be discouraged (eliminate possible nesting sites), and invasive grasses would need to be controlled. That might work. It is clear that the current approach is not.

Nullius in verba

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