Just when you think you are beginning to understand things a little, along comes an event that puts you in your place! The rainfall here on the west coast of the United States in 2017 has been a bit odd – very welcome, to be sure, but a bit odd. We have exceeded expected rainfall through this winter and spring by so much that in the northern Pacific states from Washington through Oregon and down into northern California, experts are declaring an official end to a drought that has lasted about a decade. Reservoirs and lakes in that region are now refilled to capacity. Here in southern California, the boon of rain hasn’t erased that long drought as completely, but it still had staggering effects locally. One of the factors that makes the rain so odd is that it occurred completely outside the Pacific oscillation cycle, governed by the El Nino and La Nina oscillations. These rains at Anza-Borrego State Park produced a huge wildflower super-bloom, to the benefit of White-lined Sphinx moths and their predators.
Borrego – a.k.a. Desert Bighorn Sheep
Whenever southern California gets strong winter rains, it’s always great to get out to Anza-Borrego State Park and the area around the town of Borrego Springs to observe the wildflower bloom. This explosion of wildflowers occurs, to some extent, annually, and it is always beautiful to see, but this year’s display was one of the best in decades – there hasn’t been a bloom like this one since the rains of 1997, which was an El Nino year, by the way. The rains are a boon to all kinds of wildlife in the desert around Borrego Springs. Even creatures who are superbly adapted to an existence without major rainfall welcome plentiful precipitation. On our way out to see the wildflowers, we ran across one of the iconic creatures of the region: Desert Bighorn Sheep (Ovis canadensis nelsoni). In fact, “borrego” is a Spanish word for these sheep. Hence, the name of the park and town. We found a group of five borrego rams picking their way among the boulders and cacti of a steep slope on our way to the valley floor.
Sphinx Moth Caterpillars
Another annual phenomenon, tied to the winter rains and the wildflower bloom, is the mass emergence of caterpillars of the White-lined Sphinx Moth, Hyles lineata. The caterpillars always emerge en masse, as a way of avoiding predation, using the sheer weight of numbers to ensure that at least some survive to pupate, emerge as moths and restart the cycle. This same strategy is used by a number of other invertebrates like mayflies and cicadas (“locusts” with 13- and 17-year emergence cycles in the East and Midwest). The caterpillars begin devouring the flowers with gusto, and in years of less rainfall when there are fewer flowers, they can polish off a wildflower bloom in a matter of weeks.
The other survival tactic of the caterpillars is tremendous variability in color pattern. A typical White-lined Sphinx Moth caterpillar is lime green, with a series of black and yellow stripes running the length of their bodies. The black lines are often punctuated with bright red spots. But we also saw some caterpillars that had greatly reduced black so that they appeared more strongly green. Some had bright green heads and tails; others had orange or reddish heads and tails. Even more extreme were the caterpillars that were almost completely black, with just a few longitudinal dashed lines of yellow, green or orange. What is the point of all this variability? The answer is that it helps ensure that some of the caterpillars will escape detection and survive. Who is doing the detection? Well that’s the next piece of this biological mosaic: there are probably multiple predators of the caterpillars, but one of the most prominent and magnificent is the Swainson’s Hawk.
Swainson’s Hawk (Buteo swainsoni) is a migratory hawk of the western and central states with a large breeding range that extends up into eastern Alaska and western Canada and includes parts of 21 states, 6 Canadian provinces and even 4 Mexican states. Virtually the entire population of Swainson’s Hawk winters in north central Argentina, thousands of miles south. As fits a long-distance migrant like that, Swainson’s Hawks have narrower, more-pointed wings than other buteos, for more efficient flight. One population of Swainson’s Hawks migrates through the area of Anza-Borrego and Borrego Springs, where they take up residence for a week or two on their way north. And their arrival and departure is delicately timed to coincide with the sphinx moth caterpillar emergence. Swainson’s Hawks arrive, gorge on caterpillars, regain their strength and then take off for more northern breeding grounds. It is highly probable, though difficult to prove, that predation by the hawks has driven the emergence and camouflage strategy of the sphinx moth caterpillars. One is reminded of the fact that the “selection” in “natural selection” is always a negative event. Things are not so much selected for as they are selected against. And getting eaten would certainly be a negative event, at least from a caterpillar’s point of view!
On the way home, we stopped at another unit of Anza-Borrego State Park, the Tamarisk Campground, so named because of a large grove of these trees that the campsites are nestled into. This site is well known among birders because historically, it has been a roosting site of Long-eared Owls (Asio otus). Normally, one wouldn’t give away such a location because photographers, birders and others may rush to the site and disturb the roosting owls, which are very sensitive to such things. But in this case, the site is already broadly public knowledge, which means that perhaps it may be of benefit as a place to learn about these beautiful creatures with first-hand experience of them. Sure enough, when we arrived, there were already people clustering around a single owl perched fortuitously in the open. We stopped and quietly took photos. When the crowd got larger and noisier, we asked them to be quieter and explained why it was important. We were gratified at how well people responded to this instruction and complied with our requests, becoming much more quiet, backing off a distance, and speaking only in hushed tones. All in all, it was a wonderful end to a very beautiful day.