Selective Herbivory by Abert's Squirrel Mediated by Chemical Variability in Ponderosa Pine

Abert's squirrels (Sciurus aberti) are restricted to stands of ponderosa pine (pinus ponderosa) and feed heavily on the inner bark tissues of these trees. Although preferences for inner bark of particular trees (target trees) have been well documented, the basis of the squirrels' selectivity has not been clearly demonstrated. Compared with matched nontarget (uneaten) trees, and with whole populations of ponderosa pine, target trees had significantly lower xylem oleoresin flow rates, and significantly lower levels of β—pinene and β—phellandrene in the xylem oleoresin. In feeding trials with free—ranging animals, food laced with ecologically realistic concentrations of either β—phellandrene were eaten significantly less than control food. The phloem of target trees had significantly higher concentrations of nonstructural carbohydrates and sodium, and significantly lower concentrations of iron and mercury, than the phloem of matched nontarget trees. Oleoresin characteristics associated with selective herbivory wee unaffected by simulated herbivory over 2 yr, supporting existing evidence that these characteristics are under strong genetic control. Because herbivory is associated with these host traits and results in dramatic reductions in host fitness, Abert's squirrels are probably important agents of natural selection in host ponderosa pine populations.