The Edaphic Factor in Ecology

Given the importance of soil features to plants, the edaphic factor's influence on plant ecology and evolution is unsurprising. In particular, soils with unusual features (extreme pH, nutrient imbalances, limited depth, etc.) may be a strong selective force shaping plant evolution. The floras of many unusual soils (serpentine soils, limestone soils, etc.) have at least some taxa that are found only on those soil types, whereas other species may evolve locally adapted populations (ecotypes, races, etc.). In many cases such taxa or populations have evolved in response to particular features of those soils. In other cases, unusual soils may be refugia for taxa that are unable to compete with species that dominate 'normal' soils.

The ability of soils to affect ecology or evolution of organisms other than plants is less well known. It is also less likely for many animals, in part because their mobility and aboveground lifestyle render them less influenced by the various properties of soils. One soil feature that in specific cases has been shown to directly influence animal evolution is soil color. In habitats with little vegetation, such as deserts and beaches, the color of some animals has evolved to match the color of the soil. For example, white gypsum dunes, for which the White Sands National Monument in New Mexico is named, host a number of animals that are notably lighter-colored than those living on darker surrounding soils. These animals include insects, spiders, scorpions, lizards, amphibians, and mammals. The main selective advantage of this color matching is to provide camouflage that makes color-matched animals less likely to fall victim to predators. The evolution of burrowing and soil-dwelling animals is more likely to be influenced by soil properties due to the greater intimacy of their life histories with those soil features.

These direct effects of soils on biota are supplemented by a variety of indirect ways that soils may influence either animals or plants by affecting organism interactions. This is easily imagined when one considers the importance of plant communities in providing habitat for animals and other organisms. There are several intriguing cases of special plant-insect interactions under extreme edaphic conditions. Some plants endemic to heavy metal-rich serpentine soils harbor unique insect herbivores that are specialized to deal with the high metal concentrations found in the plant tissue (Figure 2). It is rare to find cases in which the effects of

Figure 2 The Ni tolerant insect Melanotrichus boydi (Heteroptera: Miridae) on a flower of its host plant, the California Ni hyperaccumulator Streptanthus polygaloides (Brassicaceae). The plant is found only on serpentine soils in California, and the insect is found only on S. polygaloides. The insect is tolerant of the high levels of Ni found in the plant tissues (usually >3000 mg Ni/g dry mass). The insects, about 5 mm long, contain about 800 mg Ni/g dry mass, enough to make them toxic to crab spiders that hunt for prey on flowers of S. polygaloides. Credit: R. S. Boyd.

Figure 2 The Ni tolerant insect Melanotrichus boydi (Heteroptera: Miridae) on a flower of its host plant, the California Ni hyperaccumulator Streptanthus polygaloides (Brassicaceae). The plant is found only on serpentine soils in California, and the insect is found only on S. polygaloides. The insect is tolerant of the high levels of Ni found in the plant tissues (usually >3000 mg Ni/g dry mass). The insects, about 5 mm long, contain about 800 mg Ni/g dry mass, enough to make them toxic to crab spiders that hunt for prey on flowers of S. polygaloides. Credit: R. S. Boyd.

soil on other organisms indirectly affect plants, but this does occur. For example, pocket gophers tunnel through soil and consume aboveground, and especially below-ground, plant parts. In mountain meadows of Arizona, aspen trees suffer significant gopher-caused mortality on deep meadow soils but not on rocky outcrops where pocket gophers do not occur due to the lack of soil deep enough for them to make tunnels.

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