The role of enemies and conspicuousness in edaphic adaptation

Background/Question/Methods

Edaphic specialists contribute greatly to overall biodiversity, as they often comprise a large percentage of all endemic species in a region. In California, more than 10% of native plant species are specialized to ultramaphic serpentine outcrops that make up only 1% of the land area in California (Kruckeberg 1984).  Some of the traits involved in these edaphic shifts include the ability to tolerate low or high concentrations of necessary or toxic elements, drought and heat.  The role of biotic interactions may also be central to fully understanding the processes that generate edaphic specialists.  Specifically, harsh soils are often depauperate in the density of plant communities, and thus inhabitants of these outcrops across all trophic levels may be conspicuous to enemies.

We conducted vegetation and damage surveys across three serpentine –nonserpentine gradients to document differences in enemy pressure and plant communities. In addition, we weeded around naturally ocurring plants along a serpentine gradient, and placed plasticene caterpillars on and off serpentine outcrops to examine differences in enemy pressure.  We also conducted spectrophotometric measurements of leaves of a serpentine endemic mustard Streptanthus breweri and of the surrounding soil because we hypothesize that  brown leaf pigments might provide defense from enemies through crypsis.

Results/Conclusions

We show that vegetation is ten times less dense on serpentine outcrops than in adjacent nonserpentine areas less than 20 meters away. Moreover, herbivory levels are also greater on sparse outcrops than in more vegetated areas at the edges. Plasticene caterpillars were attacked by birds five times more often on central serpentine than adjacent nonserpentine areas, suggesting that all trophic levels must deal with conspicuousness in these habitats. Brown pigmented leaves of S. breweri may be one adaptation to increased enemy pressure and conspicuousness– leaf pigments more closely matched the home outcrop than other serpentine outcrops in four populations of S. breweri, suggesting a role for crypsis. We hypothesize that investment in defense required by increased conspicuousness on outcrops may ultimately preclude serpentine specialists from competing effectively off serpentine soil, thus resulting in edaphic specialization.