In late September 2014, I joined a small three-day expedition to collect herbarium specimens and more thoroughly document the botany of far northeastern Imperial County, California. Arranged by Jon Rebman, Curator of Botany at the San Diego Natural History Museum, the purpose of the trip was to fill in some of the scientific gaps in the vouchered flora of the region. There had been significant summer monsoon rains in this normally very arid region flanking the Lower Colorado River, starting in late July, with more in August, and then culminating with even more rainfall in September. (See my posts on Hurricanes Norbert and Odile for more on the September events at my place near Yucca, Arizona.)

These warm-season rains cannot be counted on to reliably occur every summer, and even when they do they are frequently marginal and sparse and cannot necessarily support the emergence and growth of some of the summer annuals that are otherwise difficult to scientifically document. But some years, such as that of 2014, are wetter than average and do bring forth the normally hidden flora of the summer Sonoran Desert. And it was with this rare opportunity in mind that we decided to make the journey to see what was happening.

 

Guided by a botanist’s instincts, Jon Rebman closes in on his quarry, a cottontop barrel cactus, Echinocactus polycephalus.

One of the plants documented on this short botany trip was a lonely specimen of cottontop barrel cactus, Echinocactus polycephalus. Growing on a ridge high in the remote Palo Verde Mountains after a long hike into the heart of the range, I spied this isolated plant across a small valley and alerted Jon, who went over to inspect it. Since there were fruits on the cactus, a good herbarium specimen could be made and so I went over to bring Jon a sturdy paper grocery bag to collect fruit and spine samples.  What follows is a series of pictures demonstrating the simultaneous danger and romance of botany field work, as the scientist bleeds for his craft. Now there’s dedication for you! Working past injuries is all part of the job.

 

Jon pops off some fruits from the Echinocactus. These will be cut in half, dried, and pressed to form part of an herbarium sheet documenting the presence of this species in the Palo Verde Mountains of Imperial County in 2014.

Cottontop barrels are among the slowest growing of all cacti, even when they are happy. “Rapid” growth could be said to be a plant that is putting out 3 to 5 new areoles per head, per year, which is maybe, possibly, 1/4″ of enlargement annually, making specimens this size easily well over 100 years old. This slow growth rate and the low reproductive success of seedlings in such an arid environment makes them pretty vulnerable to ecological disturbances. Despite being tenacious survivors of extremes of heat and drought, cottontops are potentially subject to population crashes from which they are unlikely to easily recover. Care must be taken to not unduly disturb the plants or their habitats. The common name of cottontop comes from the densely woolly apex of each head, often obscured by stout spines, which helps insulate the plant’s meristematic dividing growth cells from intense UV sunlight, heat, and cold temperatures.

 

A cottontop barrel cactus yields a few areoles and fruits to the benefits of scientific inquiry and documentation, via becoming an herbarium specimen. Note the small new head developing on the plant’s side in the lower left corner.

When collecting cactus samples for herbarium sheets, it is usually impossible to collect the entire plant, or even a significant portion of it. So what is done instead is to collect several areoles containing spine clusters, which will show the count and arrangement of the spines. This, along with reproductive materials such as fruits, flowers, or both will serve to make a botanically worthwhile specimen. This does leave a scar on the plant’s ribs, but nothing that it cannot callus over and recover from. And of course we took nothing more than that which was necessary (including photos) for the purposes of science, slightly impacting only this one individual.

Blood sacrifice. Have you ever seen such suffering? Good thing we narrowly survived!

 

A remote ridge in the Palo Verde Mountains supports a single Echinocactus polycephalus. We found no others at this site.

The cactus we made the herbarium specimens from was growing apparently alone, as no others of the same species were observed within the nearby vicinity. There were apparently viable black seeds inside the several fruits, however, which should indicate that at least one other individual is located within flight distance of a pollinating insect. Regardless, this is a very marginal, difficult environment for this species to survive in, and it will be a lucky thing if it manages to reproduce before it dies. Note the yellowish-green caterpillar of a white-lined sphinx moth feeding on Boerhavia sp brought forth by the recent summer rains in front of the barrel cactus.

Caterpillar of white-lined sphinx moth on a canyon wall in the Palo Verde Mountains.

White-lined sphinx moth caterpillars (Hyles lineata) were abundant at this particular place in September 2014. We came across many hundreds of them on every hillside and desert flat, feeding upon various ephemeral plants brought forth by the rains 6 to 8 weeks beforehand. These large insects are also called hawk moths, as well as hummingbird moths, for their ability to hover silently at flowers sipping nectar. They are a common species across the United States, including in these dry desert regions. White-lined sphinx moth larvae feed on a wide variety of plants and can undergo population explosions in good rainfall years. In the lowest and warmest southwestern deserts they can be present year round if the food supply permits, and an army of ravenous caterpillars has been known to cut short an otherwise promising display of spring wildflowers simply by consuming them out of existence.

Jon Rebman photographs his first wild desert tortoise (Gopherus agassizi). Despite the thousands of hours Jon has spent outdoors doing botany, including in desert areas, he swore that this was definitely his first time coming across one. It was also my birthday. You’re welcome.

What on earth is this supposed vegetarian doing, eating meat?

Even more intriguing than finding the desert tortoise was the fact that we caught it doing something unexpected: Eating a caterpillar! Desert tortoises are strict vegetarians and generally show no interest in trying to capture or consume insects, lizards, bird eggs, or anything else that entails being carnivorous. This behavior struck me as extremely unusual.

I took a series of photos of the tortoise with a half-munched caterpillar hanging from its mouth and tried to find out online whether this behavior is documented elsewhere, but my search was inconclusive. It made me wonder whether the tortoise accidentally chewed up the caterpillar in the process of trying to eat a plant they were both feeding upon, but given the lack of plants on the sandy floor of the wash channel we were in that explanation seemed a bit improbable. We did find caterpillars wandering around on the sand and rocks of the same dry wash however, and it appears that the tortoise may easily have come across one and decided for whatever reason to eat it. Might it have been incidental, in that the tortoise mistook the bright yellow caterpillar for a piece of leaf or fruit? Or was it an opportunistic behavior that wild tortoises do occasionally engage in, just mostly unobserved by humanity?

Whatever the case, caterpillars and other bugs are certainly not a primary part of the diets of desert tortoises. However the same cannot be said for many other desert animals. When white-lined sphinx moth larvae are abundant, they can readily become the primary food source of many other desert creatures, if not usually tortoises. Kit foxes, gray foxes, and coyotes all prey upon them when they are able to since the larvae are rather defenseless and nontoxic, unlike the caterpillars of certain other moth and butterfly species which are. During the spring migration season, birds of prey from kestrels to burrowing owls to Cooper’s hawks have all been observed gleaning caterpillars from dense population explosions on the aforementioned wildflower fields. Other birds like roadrunners and ravens will also capitalize upon the fat, juicy, easily-captured larvae. And so goes the desert food chain as it converts rainfall into plant biomass into insects and then into top mammal and bird predator biomass. It’s all part of the cycle of life.

Echinocactus polycephalus on desert pavement flats of remote Imperial County, California. A second smaller plant resides in the middle distance.

Some final thoughts upon the conservation status of cottontop barrels: The future of these wonderful and long-lived cacti is in question since their already-arid habitat is projected to become even hotter and drier with climate changes already in progress. Without enough adult plants to produce enough seeds to germinate when there is enough rare rainfall to enable seedling survival, most Sonoran Desert stands of this plant are probably in long-term decline. Individuals of this species are few and far between in southeastern California, southwestern Arizona, and northwestern Sonora, Mexico. It would not surprise me to see them disappear entirely from all three of those regions within the next 50 to 100 years due to lack of reproduction as aged, ancient adults die out. Mojave Desert populations at higher, cooler, and wetter elevations to the north are also at risk, but the situation is probably not as dire there as it is here, in the hottest and driest ecoregion of the United States.

A very large and ancient example of E. polycephalus subsists on minimal rainfall annually in the Cabeza Prieta National Wildlife Refuge of southwestern Arizona, just north of the international border. I would not be surprised if one this large in a very arid climate is well over 300 years old. Ron Parker, who took this photo, says that while individuals are not very common there, that most of them look happy. Which is good for well-established and extremely drought-tolerant adults, but without seedlings and juveniles coming along to replace them, the species’ future looks worrisome.

Additionally, many accessible populations of Echinocactus polycephalus have already been poached out of existence by careless collectors and clueless desert landowners who seek “landscaping” materials, and who are unaware of their precarious survival odds and the great likelihood that plants removed from their tough habitat are quite likely to simply die within a few years. Even if these barrels do survive in some random trailer yard, without nearby plants to cross-pollinate with and set seeds, not to mention some sort of effort to ensure proper seed distribution and seedling survival, these isolated “desert garden” plants are effectively dead from an ecological standpoint. They may struggle on in some yard somewhere, but when finally they do eventually die, they will not be replaced, because all the wild ones will also be missing.

My friend Larry sits in front of another magnificently large E. polycephalus in Death Valley National Park, California. While not in the Sonoran region under discussion, I included this dry locality to illustrate that the same long-term declining dynamic is possibly at play in much of the Mojave Desert.

Please, do not take individuals of Echinocactus polycephalus of any age from the wild! Not even “just one little one” – every plant matters for the long term viability of the species. This is not a very common plant despite having a fairly large geographical range. And once again, people should understand the important facts that populations are sparse, individuals are thinly scattered across wide areas of landscape, and reproductive rates are very low in the deserts they live in. That is why removal of even a small number of small or middle-sized plants will have a potentially harsh effect upon the long-term status of the species in a given area.

If you want to grow E. polycephalus, seeds are available from vendors online, and so are small plants. And yes, unfortunately they are slow to grow even in captivity, a thing that works against people being willing to propagate them in greenhouses and nurseries. Faster-growing barrels in the similar genus Ferocactus, and even the closely-related golden barrel (Echinocactus grusonii) are more readily propagated by growers since they are quite a bit more vigorous and less sluggish to develop into attractive adults than E. polycephalus is.

It’s not that E. polycephalus is difficult to grow in captivity as a seedling; it’s that it requires patience that many people seeking instant gratification lack. Taking plants from the wild is unethical, and really, you don’t need one so badly that you are willing to participate in hastening the potential extinction of the entire species by poaching youngsters and adults from the wild, right? You’re not that selfish, right?

Right?