Abstract
Variation in local herbivore pressure along elevation gradients is predicted to drive variation in plant defense traits. Yet, the extent of intraspecific variation in defense investment along elevation gradients, and its effects on both herbivore preference and performance, remain relatively unexplored. Using populations of Arabidopsis halleri (Brassicaceae) occurring at different elevations in the Alps, we tested for associations between elevation, herbivore damage in the field, and constitutive chemical defense traits (glucosinolates) assayed under common-garden conditions. Additionally, we examined the feeding preferences and performance of a specialist herbivore, the butterfly Pieris brassicae, on plants from different elevations in the Alps. Although we found no effect of elevation on the overall levels of constitutive glucosinolates in leaves, relative amounts of indole glucosinolates increased significantly with elevation and were negatively correlated with herbivore damage in the field. In oviposition preference assays, P. brassicae females laid fewer eggs on plants from high-elevation populations, although larval performance was similar on populations from different elevations. Taken together, these results support the prediction that species distributed along elevation gradients exhibit genetic variation in chemical defenses, which can have consequences for interactions with herbivores in the field.
Highlights
In light of the widespread and ongoing impacts of human action on natural systems, there is increasing interest in understanding the adaptive responses of plants and other organisms to environmental change [1]
Our data show that rates of herbivory on field populations of Arabidopsis halleri decline with increasing elevation and that this variation in field damage is associated with an elevational cline in constitutive glucosinolate profiles
Relative to low-elevation populations, plants from high-elevation populations produced a greater proportion of indole glucosinolates when grown under common environmental conditions and suffered lower levels of herbivore damage in the field
Summary
In light of the widespread and ongoing impacts of human action on natural systems, there is increasing interest in understanding the adaptive responses of plants and other organisms to environmental change [1]. Relatively few studies found clear associations between intraspecific levels of particular chemical defenses and rates of herbivory along elevation gradients, perhaps reflecting the complexity of abiotic and biotic factors that shape the evolution of plant defense and stress responses [2,3] or confounding environmental effects that arise when plant defenses are measured for plant tissues collected in the field. The current study explores elevational variation in the chemical defense phenotype and its role in herbivore resistance of Arabidopsis halleri, a Brassicaceous perennial herb which is strictly outcrossing but highly clonal and tends to be found in human-disturbed alpine meadows from around 300 to 2400 m above sea level in the Alps Because this species exhibits a fragmented distribution across a broad elevational range, it provides an attractive study system for exploring the effects of genetic vInatr. To determine whether herbivore preference and performance reflected the observed patterns of field damage and constitutive glucosinolate production across populations, we conducted two experiments using the specialist herbivore Pieris brassicae and A. halleri plants cloned from plants collected from the field and grown under greenhouse conditions for at least one year
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