Did you see a plant there? Me neither.

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Plants? Rocks? Both?

At a seminar on sexual selection in frogs I attended recently, an evolutionary ecologist (who studies plants), joked that compared to things like frogs that make noise and move, plants are really pretty dull. I can think of many reasons why plants are in fact the awesomest (being in the plant biology department makes me a little biased), and one particular example that came to mind was from an Am Nat paper I recently read by Strauss and Cacho. In the paper, the authors address crypsis in sparsely vegetated habitats. Crypsis is a really well-documented adaptation in animals as a defense against apparency (snowshoe hares, anyone?) But this paper is about plants!

Sparsely vegetated habitats have little plant life in them for a reason: harsh growing conditions, often due to soils. Soils with low resource availability (nutrients, water, etc) tend to not support many individuals. Those individuals that do grow in harsh soils tend to be specialists that are adapted to these conditions. So once you’re adapted to low resource availability, life should be pretty good, right? Not necessarily. Plants growing in sparsely vegetated habitats might be more apparent to enemies. Like being the last person left on the dodge-ball court or the zebra separated from the rest of the herd, a plant lacking neighbors could be an easier target for herbivores than a plant surrounded by other plants. So plants that are adapted to harsh, bare soils must have defenses against apparency. In their study, Strauss and Cacho examine jewelweed species (Streptanthus sp.) that grow on sparsely vegetated serpentine outcrops in northern California.

Strauss and Cacho set out to see (1) if there is greater enemy pressure on plants and animals inhabiting sparsely vegetated habitats and (2) if there is evidence for defensive coloration in plants as an adaptation to apparency. They found the following, through observations and experiments:

(1)   Enemy pressure is greater for plants and caterpillars in more sparsely vegetated sites, and this is in part due to greater apparency in these sites.

– Observationally, the more bare ground surrounding Streptanthus, the greater the herbivore damage tended to be. This was also shown experimentally—when neighboring plants were removed from around Streptanthus growing on serpentine, plants tended to receive more herbivore damage.

– Life-like polymer caterpillar models were attacked more frequently by birds when placed in sparsely vegetated serpentine areas than in the more highly vegetated surrounding grassland.

(2)   Leaf coloration can serve as a crypsis defense against apparency

– Increased herbivore damage is correlated with a reduction in plant fitness (and as stated above, more apparency = more herbivore damage).

– By measuring reflectance spectra, the authors show that Streptanthus breweri plants more closely match the substrate they grow on than do plants that grow in surrounding grasslands. S. breweri individuals also more closely matched the local serpentine patch they grew in than other serpentine patches.

– Experimentally mismatching S. breweri leaves and serpentine substrate (making plants more apparent by surrounding them with sand or sifted soil) increased herbivore damage.

This study demonstrates, rather elegantly, the role that enemies and apparency can play in plants’ adaptation to harsh, bare soils. The observations and experiments made use of all kinds of techniques, from measuring light reflectance to making polymer clay models of caterpillars (I support any scientific endeavor that involves getting to go to the craft store). The authors also present a lot of details about the natural history of the organisms and the study site—I recommended taking a look at the manuscript to see the pictures of Streptanthus on serpentine. And finally, this study shows that animals aren’t the only ones who use crypsis.

Not so special now, are you, snowshoe hares?

SY Strauss and NI Cacho (2013) Nowhere to run, nowhere to hide: the importance of enemies and apparency in adaptation to harsh soil environments. Am Nat 182: E1-E14

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