“It is that range of biodiversity that we must care for – the whole thing – rather than just one or two stars.” — David Attenborough

By DR. CAMERON BARROWS

Biodiversity, the sum of different species occurring within a specified geographic area, is often a metric for prioritizing conservation efforts. Biodiversity “hotspots” are typically targets for high profile conservation efforts. Higher levels of biodiversity mean more species can be protected for each acre set aside and conversely more species would be lost if such areas are not protected. 

While conceptually sound, one significant problem is we do not have comprehensive inventories for how many species occur in most places around our planet. And there are deeply entrenched biases that favor, for instance, lush, green landscapes over arid, sandy habitats. We have a pretty good idea how many birds and butterflies and trees and shrubs exist within a landscape, but we’re short on data for most other insects and certainly not for cryptic, nocturnal, seasonally active, or fossorial (living beneath the soil surface) species. 

In lieu of our imperfect knowledge of world-wide patterns of biodiversity, are there surrogates that can inform conservation prioritization? The answer is a qualified yes. Those surrogates include the age, isolation, topographic diversity, and climate of landscapes. All else being equal, older, more isolated, more mountainous, wetter and warmer landscapes tend to be more species-rich.

When looking through a global lens, old landscapes — landscapes that have been in place for many tens of thousands if not millions of years — have higher biodiversity than younger landscapes. With enough time, species adapt and evolve into new varieties, as they encounter unique conditions, food resources, and/or competitors.

The Namib Desert of southwestern Africa is the oldest desert on Earth, at least 55 million years old and perhaps as old as 80 million years. There you can find magnificent red sand dunes, red due to the tiny garnets that make up the grains of sand. There you can also find fringe-toed lizards, except they have no genetic relationship to our fringe-toed lizards here in the California deserts. On the Namib sand dunes, one species lives only on or very near the unvegetated dunes’ steep avalanche faces. Another species lives just a bit farther away on the open, sparsely vegetated flat expanses of dune sand. Other species live on the stabilized, more densely vegetated dune flats. All are within sight of each other, but having evolved from an original single species, over time they have specialized on different portions of the sand dunes. 

The California desert began to take shape roughly 10 million years ago, and its sand dunes formed with flooding associated with the newly created Colorado River about 5 million years ago, and so are comparatively new habitats. The Coachella Valley sand dunes formed separately, due to flooding and sand deposition from the Whitewater River about the same time. Although there are five species of fringe-toed lizards in the Mojave and Colorado Deserts, they each occupy separate, isolated dune systems. Unlike the Namib lizards, our fringe-toed lizards occupy and use all portions of the dunes, the avalanche faces, the unvegetated sand flats, and the vegetated sand hummocks.

Isolation, habitat islands, such as a mountain climbing above a flat desert plain, sand dunes separated by miles and miles of desert scrub and mountains, or classic oceanic islands, limit genetic connections between species’ populations. Given enough time, that lack of genetic connection leads to what geneticists refer to as genetic drift, populations drifting apart from each other until they eventually are different species. Examples include the aforementioned fringe-toed lizards. Another example is the woodyasters (Xylorhiza spp). The one we know best is the Mecca Hills species, X. cognata. But there is another one in the Anza Borrego Desert State Park, X. orcuttii, and another in the Mojave Desert, X. tortifolia. Each isolated from the other by inhospitable terrain.

Mountainous terrains combine isolation with varied climate gradients and slope faces that provide more niches for species to exploit. Desert mountains are often referred to as sky islands because they act much like isolated oceanic islands, except they exist in a “sea” of flat desert plains. The biodiversity of these sky islands reflects shifting species distributions due to changing climates of the Pleistocene ice ages. During those repeated ice ages, plants and animals were pushed south or perished. When warmer conditions returned those species either shifted back north or climbed into the sky islands to thrive in microclimates that match their original habitats. Lodgepole and limber pines, and Rocky Mountain maples are all a few of the many examples of species stranded in our local mountains after being pushed south during the last ice age. 

Another is the southern sagebrush lizard. Pushed south during the ice age, with the return of a warmer climate, some climbed to elevations at and above 6,000’ up to and approaching 9,000’ in our local sky island mountains, while others moved back north into what are now the Great Basin and Colorado Plateau Deserts.  Again, due to isolation and genetic drift, they are now considered separate species with the northern population called the common sagebrush lizard.

Warmer-wetter landscapes harbor richer biodiversity than cold landscapes. Warmer-wetter landscapes have higher levels of plant growth, and so more food to provide sustenance for more species. Tropical forests are more species rich than tundra, boreal, or subalpine forests. Wet or moist landscapes have more species than arid regions. Tropical forests and coral reefs generally have more species than deserts. So, again at a global scale, warm, tropical montane forests that have existed for millions of years will have considerably more species, far richer biodiversity, than flat, arctic tundra that only emerged from beneath a glacier within the past few thousand years.

Those ecologists aiming to find new species focus their attention on tropical climates. One of the more recently surveyed biodiversity hotspots are the montane jungles of Vietnam, Laos, and Cambodia. These areas were off-limits due to raging wars and political strife, but now are being explored. Listing all the species being discovered there could fill library shelves. At the same time, arctic tundra provides habitat for caribou, snowy owls, nesting snow geese, wolves, probably at least a dozen species of nesting shorebirds, and lots of mosquitos. None of which live and or nest anywhere else on Earth. 

Our California deserts have higher numbers of reptile and bee and shrub species than anywhere else in North America. Even in our dry, hot deserts, life has found a way to fill those spaces with species found nowhere else. 

Therein lies the conundrum. If one relies only on surrogates and long accepted biases as to where to find and then protect biodiversity, then the arctic tundra and North American deserts would be ignored by conservationists, and so would be an open door for oil extraction (the arctic) and solar energy development (our deserts).  This is the fallacy of focusing on just numbers, just the hot spots. Conservation efforts must ensure nature, all the pieces, all the species, are protected at least somewhere.

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