Plants Coat Themselves in Sand to Prevent Being Eaten

Sand coats the flower of the honey-scented pincushion plant (Navarretia mellita), which helps it avoid being eaten by herbivores. Eric LoPresti

More than 200 species of plants, found around the world, have the curious habit of covering themselves in sand. This is usually done by excreting a glue-like substance that allows sand or soil to stick and hold fast.

Scientists don't fully understand the reasons for this habit, called psammophory. Researchers' best theories are that the sand dissuades herbivores from taking a gritty bite or that it helps camouflage the plant, making it blend in with its sandy environs. There are also theories that the coating protects that plant from being damaged by sandstorms and allows it to hold on to water by preventing evaporation.

To test the first two hypotheses, Eric LoPresti, a graduate student in ecology at the University of California, Davis, and his adviser Richard Karban did a series of experiments on two Californian plants that have this habit: the sand verbena (Abronia latifolia) and the honey-scented pincushion plant (Navarretia mellita), both of which are occasionally eaten by herbivores like rabbits.

As explained by Discover Magazine, the researchers cleaned the sand from a sample of wild verbena, while leaving an equal number covered by the natural process of sand-stickiness. They also added sand to a number of wild pincushion plants, while locating an equal number that were as yet sand-free. In both cases, the interventions had striking results. The clean verbena plants had twice as much herbivore damage as the sandy plants; meanwhile, only one of 19 grit-covered pincushions was eaten, while eight of 18 untreated, clean plants were chomped upon by a rabbit.

A second experiment, using green-colored sand (colored as such to make the plant stand out against its sandy backdrop), showed that the hue of the material didn't matter—thus suggesting that its function isn't primarily to camouflage but to deter predation, according to the study, published February 29 in the journal Ecology.

The finding makes sense—it's hard to imagine biting down on sand wouldn't be a deterrent. Done regularly, it can actually wear down teeth. Bison and elk that live in Yellowstone National Park and which are forced to feed on sandy grass in the winter (due to lack of any other plant food in the region) wear down their teeth much faster than those in other areas, so much so that it can shorten their life spans significantly.