Insect herbivory as a major factor in the shade distribution of a native crucifer (Cardamine cordifolia A. Gray, bittercress)

1 The relationship between herbivory and plant distribution in shade vs. in sun was investigated for a native crucifer (Cardamine cordifolia, bittercress). The experiments tested the hypothesis that chronic herbivory determines the observed shade habitat distribution of this native plant, 2 In the first experiment, removal of shade above plants in situ resulted in significantly increased herbivory. That increase was associated with decreased plant growth and reproduction and lowered density. Thus, ecotypic differentiation in plant resistance cannot explain lower densities of bittercress in the sun, 3 In the second experiment, protection of plants in the sun with insecticide resulted in growth and reproduction equivalent to plants in the shade. Thus, physiological adaptation to shade cannot explain lower densities of C. cordifolia in the sun. 4 The combined outcomes of these experiments suggest that chronic differential herbivory by adapted insects in the sun is a major factor in the shade habitat restriction of Cardamine cordifolia within its indigenous region, the central Rocky Mountains, USA. 5 Leaf toughness, thickness, and water deficits were greater for plants in the sun, whereas glucosinolate concentrations were lower. Leaf nitrogen concentrations were similar between habitats. Insect abundances were generally higher in the sun. These observations suggest that lower glucosinolates, higher insect abundances, and positive feeding responses by insects to moderately water-stressed plants were responsible for the differentially higher levels of insect herbivory on plants in the sun.