Layer of protective sand shields desert plants from predators
You wouldn’t cover yourself in sand for no reason. Aside from the comfort issues, it would also take some effort and resources on your part just to get the sand to stick to your skin in the first place. It’s apparently worth the effort to over 200 species of desert-dwelling plants that collect sand either with spines, hairs, sticky coatings or combinations thereof. While being covered in sand doesn’t necessarily serve the same purpose for all those plants, some research in southern California has started going through a list of possibilities, starting with armor.
Scientists started with two species of sand-encrusted plants, the sticky sand verbena (Abronia latifolia) and the prickly honeyscented pincushion plant (Navarretia mellita). While the plants grab their sand-armor in slightly different ways, they both face predatory snails, mice, deer and jackrabbits. With such a clear environmental pressure, the experiment was then to see if the psammophory (or sand armor) was actually helping with that unwanted nibbling. With the sticky verbena, researchers wiped off sand and stickiness on some plants, then returned later to see if those plants had been eaten more than sandy ones. The prickly pincushion plant didn’t want to give up its gritty cover so easily, and so researchers picked a batch to pour extra sand on. If the sand was acting as a defense against herbivores, then the extra armor should work better than what was accumulated on a unaltered plant.
After two months of checking up, it appeared that sand did indeed provide protection against being eaten. Cleaned verbena plants were chomped twice as often as their sandy neighbors, and only one of the 19 super-sandy pincushions were eaten, compared to eight of the 18 natural specimens. It seemed like sand was providing protection, but maybe it was actually due to prevention? What if the sand wasn’t armor, but camouflage?
To test if the appearance sandy plants was key, researchers then applied a fresh batch of normal, tan sand to some A. latifolia, and then stem-green sand to their experimental group. If the benefit of psammophory was actually that the plants blended in with the surrounding, sandy environment better, then the green plants were expected to receive more bites. However, the green color didn’t seem to impress the local predators, who ate both groups equally. Since the presence of sand made a bigger difference, it looks like the sand really is armor, and that foraging animals hate getting sand in their mouth as humans do.
While this seems like a tidy explanation, it may not be the full answer. Sand may provide other benefits, such as acting as a sunscreen for the plant, insulate stems and leaves from drying out, or act as armor from… blowing sand. It’s also quite possible that it’s providing multiple benefits at once, but further research will be needed to further narrow down the possibilities. In the mean time, it’s safe to say that if you see any “dirty” plants, brushing off the sand isn’t in their best interest.
Source: Plants Build Sand Armor to Fight Hungry Animals by Elizabeth Preston, Inkfish