“...everyone knew that all islands were worlds unto themselves, that to come to an island was to come to another world.” ― Guy Gavriel Kay

By DR. CAMERON BARROWS

Looking from the sea toward a desert island, it could be easy to imagine a landscape nearly devoid of life. You couldn’t be more wrong. Desert islands are certainly challenging environments, but those challenges have spawned a myriad of solutions for life to exist and thrive there. Ever since Charles Darwin stepped foot on the Galapagos Islands, naturalists have sought out desert islands to gain insights to understand the patterns of life. Sure, there is greater biodiversity in the Amazon’s tropics, but there the diversity can be overwhelming, seemingly chaotic. On desert islands those patterns make sense.

The Galapagos Islands are about 300 miles off the coast of Ecuador. Each of these islands was formed from volcanoes, created as the oceanic crust shifted over a volcanic “hot spot”, as with the Hawaiian Islands further north and east, spewing lava until it eventually reached to ocean surface and islands grew with each subsequent eruption. That means these islands were never connected to any continental land mass, and that means that the ancestors of everything that now lives there had to cross at least 300 miles of open ocean. Imagine being swept out to sea during a flood, and then imagine the odds of then arriving at a tiny cluster of islands far out into the ocean before you ran out of whatever resources that were swept out to sea with you. Those odds are almost incalculably small, yet tortoises and lizards and cacti and hundreds of other species beat the odds and populated this archipelago.

If our ocean traveler was a lizard, perhaps a green or spiny-tailed iguana, a vegetarian, and abundant in tropical forests from southern Mexico to Peru, it then had to adapt to life on the very arid, non-tropical Galapagos Islands to survive. Some became specialists, eating the algae growing on the boulders in the intertidal ocean edge, with the larger males diving for still deeper algae-encrusted boulders Others found cactus fruits more to their liking. Eventually two very distinct forms evolved, marine iguanas and land iguanas. In the Galapagos Islands today there are three distinct species of land iguanas. One species was only recently discovered high on a volcano – it is bright pink in color (the other two species are shades of yellow, tan, and brown). The marine iguanas also diversified into different species or subspecies, separated by size and color and each restricted to a specific island. Tortoises did the same. Each island has, or had, its own giant tortoise species, big enough for an adult human to ride on its back. Unfortunately for the tortoises, they apparently taste good, and could be stored alive for months within the hulls of circa 1700-1900s era sailing ships. Many of the tortoises that had occupied the smaller islands, and so we easy pickings for those mariners, are now extinct.

For a naturalist accustomed to the North American deserts, at least the lower elevations of the Galapagos Island seem familiar. The difference is in size. Opuntia cactuses are common, but are giants, 20-30 feet tall. The iguanas are huge as well, weighing up to 20-30 pounds or more, and at least 2-5 times larger than their distant green or spiny-tailed iguana cousins. The tortoises can weigh well over 100-200 pounds. There are some familiar birds as well, Vermilion flycatchers and yellow warblers. Some of the most common woody plants are in the same genus as our desert plants Tiquilia, and Croton.  Except the island species are, again, giants in comparison. On some of the larger islands with higher elevations there are less arid, seasonally wet forests. If a tropical tree did beat the odds and arrive on the shore of one of these islands it would have almost certainly quickly died in what for it would have found only inhospitable, desert conditions that dominate the lower elevations of each island. Yet there are trees in those wetter uplands, however, they have no genetic connection to the tropical trees of Central or South America. Rather they are members of the sunflower family. Somehow some aster seeds made it to these islands, and once there evolved into woody trees, sometimes 30-40 feet tall.

Closer to home, at the edge of the Sonoran and Colorado Deserts there is another desert island archipelago found in the Gulf of California between the Mexican mainland and Baja California Peninsula. There are 25 separate, named islands and many smaller islets. While not a rule, like in the Galapagos Islands, gigantism is common. Two islands each have a different species of giant chuckwalla, some 10 times larger than our desert chuckwalla. One island has a giant species of side-blotched lizard, closer to the size of our desert spiny lizard. There is a giant barrel cactus, similar in size to a Saguaro Cactus, found on at least two islands.

The “giant” side-blotched lizard lives and a small island, Isla San Pedro Martir, with little vegetation except a stand of Cardon cacti. For most of the year there is little for these lizards to eat. However, there are a few months when brown and blue-footed boobies descend upon this island to nest and raise their young. Boobies are fish eaters and exclusively feed their chicks regurgitated fish, mainly sardines and anchovies. Feeding booby chicks is a messy process, and lots of fish fragments end up strewn around the nests. Those fish fragments then become a banquet for the very large side-blotched lizards. For those couple months the lizards gorge themselves, and then survive largely on those fat reserves for the rest of the year. Variable, seasonal, feast or famine food availability may explain the gigantism patterns that occur in desert island lizards. Get big, store reserves in fat and muscle, and then survive until the next time food is available. Stay little and starve.

Examining patterns of biodiversity found on desert islands has resulted in a general idea of how these islands get populated. Some of these insights are obvious. Bigger islands have more habitat area and often more diverse habitats, so more species, and so higher levels of biodiversity. The more isolated an island is, the more likely it will have species unique to that island. The less isolated islands, those closer to continental landscapes or to other islands, the more likely they are to share the same species. Near the southern end of the Gulf of California, near the city of La Paz, there are two large islands, Islas Espiritu Santo and Cerralvo. Although similar distances from the Baja California Peninsula, Espiritu Santo lies in shallower water and so was connected to the peninsula during the Pleistocene Epoch when so much of the world water supply was tied up in massive glaciers. The ocean level was much lower. Cerralvo lies in much deeper water and so was still an island throughout the Pleistocene. Espiritu Santo has the same species of lizards as does the adjacent peninsula. Cerralvo has many fewer lizard species, but most are found only on Cerralvo, nowhere else on earth.

These same patterns apply to the habitat “islands” within the North American deserts. Sand dunes are isolated habitats within a “sea” of rocky-gravelly desert landscapes. Species that have evolved adaptations allowing them to thrive on sand dunes typically cannot live anywhere but on a sand dune. Bigger dune systems have more species. More isolated sand dunes have more species restricted only to that dune. They follow the same island “rules”. The same is true for our “sky island” desert mountains.

Nullius in verba

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