Nowhere to run, nowhere to hide: the importance of enemies and apparency in adaptation to harsh soil environments.

Bare, simplified searching environments, often associated with sparsely vegetated harsh soils, may cause both plant and animal inhabitants to be apparent and conspicuous. "Apparency" has been a key concept to explain the diversity of plant defensive chemistry but has been difficult to test. In animals, there is extensive work on camouflage and crypsis, adaptations to apparency that reduce detection by predators. Here, we explore apparency as a challenge in bare soil habitats characterized by sparse vegetative cover for both plants and animals. Using experiment and observation, we show that attack rates from enemies on vulnerable plants and undefended caterpillar models are greater in barer serpentine habitats than in adjacent more vegetated ones. Palatable Streptanthus species (Brassicaceae) may have adapted to apparency with a crypsis defense, typically considered the purview of animals. In Streptanthus breweri, leaf color is locally matched to soil outcrop color, and experimental mismatching of leaf and substrate color increases damage to plants, suggesting adaptation to apparency per se. Herbivore coloration may, too, have been influenced by greater enemy pressure and apparency in these sites. Adaptation to increased enemy pressure and apparency, with concomitant trade-offs in competitive ability, may be an underappreciated aspect of specialization to harsh soils, especially in plants. Apparency may be a useful framework for understanding trade-offs driving soil specialization and global biodiversity patterns.