“Life finds a way.” – Jeff Goldblum, Jurassic Park

Our planet is about 4.5 billion years old. Sometime around 3.7 billion years ago, based on well accepted fossil evidence, we find the first signs of life. It could have been even earlier, but earlier fossil traces are less clear and rocks that old are hard to find. What this means is that when the earth had barely cooled enough from an initial orb of molten rock, enough so that liquid water could exist on its surface without boiling away, at that point there was already life in those primordial lakes and seas. At least some, if not all that original water is thought to have an extraterrestrial source, from comets and ice asteroids slamming into our early planet.

To be clear, life then was comprised of extremely primitive, barely single-celled, organisms that existed in those mineral-rich waters. There was little or no free oxygen in that atmosphere, fostering the idea that the bacteria associated with deep-sea thermal vents spewing hydrogen sulfide mimic those early earth conditions. Then about 2.4 to 2.0 billion years ago, some of those primitive creatures evolved the ability to use the sun and carbon dioxide to make energy to fuel their internal physiological processes. Their metabolic effluent was oxygen. Oxygen was toxic to most, but not all life on that early earth, and so that accumulation of free oxygen resulted in a mass extinction called the Great Oxidation Event. Only those creatures that could tolerate, or even thrive in an oxygen-rich atmosphere and ocean survived and became our very distant ancestors. It would take another 1-1.5 billion years (540 million years ago) before complex macroscopic multi-cellular creatures emerged in the fossil record; trilobites dominated the earth for the next 250 million years or more.

The common denominator for life then and now was not oxygen, it was water. When astronomers and exobiologists search for life elsewhere in the universe, they are searching for planets that are just the right distance from their stars so that they are not too hot and not too cold so that liquid water can exist — the “Goldilocks” zone. The greatest abundance of life on earth exists in very wet places, in the ocean and the warm humid tropics. But deserts are species-rich as well, even though they are water-starved much of the year. That is the enigma of deserts. Apparently, it is relatively easy for life to evolve and thrive in wet places. But deserts present a challenge, making life here even more interesting.

Biodiversity starts with primary producers, plants – the food that sustains all other life. Some desert plants are almost certainly relicts of a water-rich temperate or even tropical time before the rise of the rain shadow, the Peninsular, Transverse, Sierra Nevada, and Cascade Mountain ranges that created the California deserts. Rather than adapting to increasing aridity, those plants became increasingly confined to the few desert areas where water was still plentiful at or near the surface. Phreatophytes, plants that require having their roots in contact with water year-round, are rare in deserts, but they do exist. Desert fan palms are a classic example. Rather than reduced or ephemeral leaves, the palms sport huge water-wasting leaves. They can be so extravagant because they only exist where their relatively shallow roots are bathed in year-round water – typically associated with earthquake faults. They have not adapted to desert aridity because, since they can find abundant near-surface water, they do not have to. Honey mesquite is another example, although unlike the palms, mesquite have deep roots to keep them in contact with deeper groundwater sources. Most other desert plants do not have that luxury.

Then there is another group of desert plants — those that simply avoid arid conditions. Annual plants (annuals) are especially diverse in deserts; there are a multitude of desert niches to exploit, based on soil characteristics, temperatures, elevation, shading, and when and how much rain falls. That multitude of niches has resulted in more annual plant species in deserts compared to any other California biome. Annuals have adopted a particular avoidance strategy to exist in deserts — they only appear when deserts receive enough rain at the right time. Usually, although not every year, deserts have a wet season, albeit a very short one. When the soil is wet enough, annuals can germinate and then race to grow, flower, set fruit, and disperse their seeds before being overwhelmed by aridity and then shriveling into dry stems blowing in the wind. Those seeds can stay ensconced in desert sands for many decades before enough water is again available and presents an opportunity to grow. Their challenge is “deciding” if it is wet enough to complete that entire cycle. Once the seeds germinate there is no turning back; choosing wrong could mean they dry out before they can flower and fruit; poor choices mean they do not pass their genes onto future generations. To hedge their bets, not all seeds germinate in any given year; even in an extraordinarily wet year perhaps only 20-30% of the available seeds might germinate. The rest of the seeds wait in the sand for their chance in future years — collectively they are referred to as a “seed bank” — an appropriate term in that they are an “investment” for the future.

Plant leaves are where pores, called stomata, are concentrated. These pores open during the day and close at night to allow gasses such as carbon dioxide to enter the plant to the become the building blocks of more complex carbon-based molecules used to conduct all levels of the plant’s metabolism, growth, and reproduction. Those same pores are where other gasses, the effluents of those metabolic processes, oxygen and water vapor are emitted. Plants can adapt to arid conditions by reducing their number of stomata by reducing their leaf size or dropping their leaves all together during dry conditions. Ocotillos are “leaf droppers." When there are sufficient rains, the ocotillo stalks are covered in verdant green leaves. However, when their roots no longer have access to water, they drop their leaves and hold on to what water they have left in their thick-skinned stalks. But if it rains again in a month or two, ocotillos sprout new leaves and flowers. They can repeat this leaf drop and leaf growth cycle up to three times in a year if the rains are sufficient and spaced out sufficiently. Paloverdes are also leaf droppers. Other plants have gone “all-in” and eschewed having any leaves at all. Smoke trees are the best example of that strategy. Then there are various ways leaves can reduce leaf temperature and so retard water loss. Waxy reflective surfaces with leaf edges pointing toward the sun (creosote bush) or gray fuzzy leaf surfaces to reflect and disperse the sun’s energy (desert lavender, salt bushes, brittle bush).

Cacti are iconic examples of adaptation to deserts. No leaves, thick, succulent stems to store water. Yet cacti first evolved as epiphytes attached to tree branches high in the canopies of tropical forests exposed to the unfiltered rays of the hot tropical sun. Yes, it rains a lot in the tropics, but without soil on those tree limbs usable water that can be absorbed by plant roots is still scarce; cacti that evolved sponge-like tissue to store water were able to survive better than those that did not. They also evolved a new metabolism, Crassulacean Acid Metabolism (CAM). Even without leaves they have stomata imbedded in their thick skins that could lose that precious water if opened during the day. Cacti and some other tropical epiphytes evolved a strategy of closing those stomata during the heat of the day to avoid water loss. During the day they can use stored carbon dioxide to continue on-going physiological activities, then at night when it is cooler their stomata open, releasing oxygen and minimizing water vapor loss and absorbing carbon dioxide to be used during the next day. As the deserts formed in North America, cacti moved into those newly available niches, diversifying and already able to deal with the hot arid conditions these relatively new deserts provided.

The diversity and adaptability of desert plants then create niches for similar levels of abundance and diversity of insects and birds to pollinate those plants, with an expanding food web that helps explain the overall species richness of deserts, despite the seasonal lack of water.

Nullius in verba

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