“The value of biodiversity is that it makes our ecosystems more resilient, …. its wanton destruction is akin to setting fire to our lifeboat.” — Johan Rockstrom

By DR. CAMERON BARROWS 

The value of biodiversity, of landscapes filled with a wide variety of species, is a value held across cultural and economic divides. Gazing upon a meadow will instill a sense of serenity. Adding a few deer nibbling on forbs and grasses, and occasionally chasing each other just for the fun of it, brings a sense of joy to anyone. Adding rabbits, prairie dogs, and burrowing owls increases that joy. Add a bear, or a golden eagle, and that joy is amplified to excitement. These responses seem to be coded deep within the human genome. As naturalists we gain that pleasure from all forms of life, not just those charismatic species, but from fungi to plants to arthropods to reptiles, birds, and mammals. The greater the richness of life, the greater the pleasure. Many of us take that pleasure and start to ask questions about why. Not why does biodiversity make us feel good, but why is one landscape more biologically rich than another? The answer is a complicated calculus of topographic diversity, temperature and rainfall, climatic diversity, disturbance patterns and history, a source of species to populate a landscape, and time.

Increasing topographic diversity means adding places that species can inhabit. Take a sandy broad, featureless flat desert plain, perhaps scattered with creosote bushes, and you would likely find resident black-throated sparrows, and maybe Le Conte’s thrashers. Certainly side-blotched lizards, zebra-tailed lizards, desert horned lizards, and desert iguanas will be there as well. With the lizards there will be tortoises, a few snakes, coachwhips, gopher snakes and perhaps a rattlesnake. There will also be beetles, ants, termites, soil microorganisms, and bees, all in numbers of individuals and numbers of species that dwarf those vertebrate species. Then add rock outcrops, boulders, large and small, and now there will be desert spiny lizards, chuckwallas, leopard lizards, antelope ground squirrels, kit foxes, rock wrens, and coyotes. Then add sand dunes and you will have fringe-toed lizards, sidewinders, shovel-nosed snakes, and a whole new array of dune-adapted plants, darkling beetles, crickets, and ants.

Then add mountains, and with those mountains you will get new temperature and rainfall regimes. Having a mountain likely means earthquake faults, and those fault structures can bring water to the surface, that then may create wetlands, palm oases, and riparian habitats. Available water increases vegetation density and structure such as trees, providing food for insects and birds, as well as nesting structures for those birds. With elevation and different slope directions there are more temperature and moisture choices, so add collared lizards, western fence lizards, and if high enough, western skinks and southern sagebrush lizards, more plant species, more trees, more birds, and more arthropods. Mountains and their temperature-moisture gradients also create climate refugia, places where species can escape periods of drought, heat, or conversely, ice ages. That means greater species stability in the face of environmental shifts. Species’ survivorship rates move up or down a mountain depending on food availability and their physiological needs, rather than going extinct when climates change.

Variable climates, swinging between drought and wet years, create different opportunities for annual plants to occupy the same space. Arid-adapted species germinate better during drier periods, while drought averse species bide their time in the soil as seeds waiting for wetter conditions. Some prefer cooler winter rains, others prefer a bit warmer spring rain, and still others prefer warm summer rains. The result is that the number and species composition of annual wildflowers varies between years, so more species can exploit the same space than if the rainfall and temperature was consistently the same year after year. With variable flowers come variable populations of pollinators.

Disturbances, landslides, floods, or wildfire, temporarily alter the landscape character. They can create a blank “canvas” for disturbance-loving plant species, species that may otherwise not be able to compete with the array of plants that lived there prior to the disturbance. With the disturbance-adapted plants can come a new set of pollinators, adding still more to the species richness of a landscape. Even in hyper biodiverse landscapes, such as tropical forests, disturbance plays a key role in creating that biodiversity. Floods can scour riverbanks creating new habitats where the sun can reach the forest floor, stimulating the grow of early successional plant species. When large old trees inevitably die and fall, they too open the canopy where direct sunlight has not been able to reach for a century or more. Then again, the sun-loving, early successional species flourish for a time, until the late successional, climax vegetation is reestablished.

Over the past few million years there has been an oscillating climate swinging between ice ages and ice sheets covering portions of the northern hemisphere, and back to warmer periods when the ice has retreated to the polar arctic region. The ice sheets scoured millions of acres of previously forested landscapes, leaving nothing but bare earth and rock below thousands of feet of ice. Those once ice-covered landscapes may still be recovering their lost biodiversity. In contrast, more southern regions, such as our Southern California deserts, that had stayed ice-free throughout each ice age, accumulated species throughout that period, and so gained biodiversity.

Then sometimes a landscape’s biodiversity depends on who its neighbors are. The Coachella Valley region, broadly defined, extending into Joshua Tree National Park to the north, and into the Anza Borrego Desert State Park to the south, has the highest documented lizard biodiversity in North America (33 species). In part this diversity is due to its topographic and climate diversity, as it includes both some of the lowest and highest elevations found in such a small geographic area. A larger factor contributing to this biodiversity is the happenstance of geography, occurring at the junction of Baja California, the Mojave Desert, and Coastal California. At least eight lizard species are here only because of that connection to Baja California. Four species occur here due to the proximity to coastal Southern California, and three species are aligned with the Mojave Desert. Without those neighbors the species total for the Coachella Valley would be diminished by nearly half. The plant and arthropod species richness of the Coachella Valley region have similar regional alliances.

With such rich biodiversity comes both joy and responsibility. The responsibility comes in ensuring that these landscapes stay as healthy open landscapes for people and the amazing biodiversity that exists here.

Nullius in verba – Go outside, tip your hat to a chuckwalla (and a cactus), and think like a mountain