Hawks and Hawk Moths

Boom and Bust in the Desert

My drawing of the Dune Evening Primrose, Oenothera deltoides.

            Like most deserts, the Anza-Borrego desert is a boom or bust kind of place. Given enough rain, desert plants of all sorts burst out lush and green, flowering and fruiting with abandon, but during the long, hot rainless times, life shrinks back, leaves shrivel and drop, and plant life desiccates and goes into hiding.  Plants have many ways of meeting the challenges of these wild, only vaguely predictable episodes of desert friendliness and hostility.  One is to spend the bad times as durable seeds, then to sprout, grow, flower and seed with blazing speed after the right amount of rain, and then die.  These are the desert annuals, and their need to get the job done fast has produced some of the desert's most spectacular flowers.  They MUST produce seed in the brief favorable period, for they have no perennial presence as a rootstock, stem, or tree. And to produce seed, most must attract pollinators, and to do that requires showy, attractive flowers.  Under the right conditions, the desert turns color as far as the eye can see--- the yellow and white of dune evening primrose, the heliotrope of sand verbena, the yellow of various composites and Oenothera, and many other species--- and people drive many miles to enjoy the spectacle before it fades.

The desert “superbloom” in the Anza-Borrego Springs Desert, 2017

            How do these annuals "know" when to germinate, for germination is a one-way street? Germination after too small a rainfall means certain death, and no second chance.  At least for some annuals, the trick seems to be that they estimate the amount of rain through the calibrated leaching of a slightly water-soluble germination inhibitor in their seed coat.  Enough rain, enough leaching and the seed germinates.  Not enough, and the inhibitor remains inhibitory. Oddly, too much rain removes it all, and once again inhibits germination, but who knows if that ever occurs in nature.

A patch of Dune Evening Primrose

            Hard upon the heels of such bursts of plant life follow bursts of insect life--- pollinators, herbivores, and predators.  A spectacular variety and abundance of aphids, bugs, beetles, caterpillars, moths, and butterflies seem to materialize as if by magic from nothing.  These insects, just like plants, have to break nature's code to determine when the time is right, and break it they do, although how is still largely a mystery.  The species of Oenothera, including the dune evening primrose can reach extreme abundance, and so can a caterpillar as big as a fat boy's finger, a caterpillar that prefers Oenothera, with the result that Oenothera takes a terrific nose-dive.  The caterpillars even eat the seed pods, so it is a bit puzzling how the plant manages to come back with the next rains.  This larva develops into a hawk moth (or sphinx moth), that when hovering over a flower at dusk, is easily mistaken for a hummingbird (with whom it shares the aerodynamics of its flight). 

The caterpillar of a sphingid moth devouring a brown-eyed evening primrose.

            But such a fat, juicy caterpillar cannot go unnoticed.  In another elegant demonstration of the rule of opportunity, the caterpillar bloom often coincides with the migration of thousands of Swainson's hawks through the Borrego Desert.  As the hawks gorge on caterpillars for breakfast, lunch, and dinner, one can almost hear the Oenothera heave a sigh of relief. Perhaps they manage to produce a crop of seeds after all. 

Swainson’s harwk

But then again, no creature that fails to hedge its bets can live in the desert, and for a desert annual, this hedging means that even under the best of circumstances, one does not germinate all of one's seeds at once.  The most auspicious beginning of a season can easily go South.  The lesson of the desert is never put all your seeds in one basket.

Dune Evening Primrose and Sand Verbena. The pollen of the primrose is shed in sticky strings and its eight chromosomes are connected end-to-end in a big ring. Note the cruciform stigma too. An interesting and peculiar flower.