HERBIVORY AND TREE MORTALITY ACROSS A PINYON PINE HYBRID ZONE

We examined the abundances of three common insect herbivores on pure a nd hybrid pinyon pines along a 250-km transect in west-central Arizona , United States. Using six morphological traits, we developed a hybrid index to classify trees as pure Pinus californiarum, hybrid, or pure Pinus edulis. The insects (the stem-boring moth, Dioryctria albovittel la, the scale insect, Matsucoccus acalyptus, and several species of pi tch moths that produce wounds on the trunk and branches) exhibited dif ferent distributional patterns across tree types. Stem-boring moths we re significantly more abundant on trees at ''hybrid'' sites compared t o trees at ''pure'' sites. In addition, within hybrid sites, hybrids s upported significantly more moth larvae than pure trees of either spec ies. These two patterns support the hybrid susceptibility hypothesis i n which hybrid breakdown results in increased susceptibility to herbiv ory. In contrast to stem-borers, there were significantly more pitch m oth wounds on trees at pure P. californiarum sites than at hybrid and pure P. edulis sites. Within the hybrid zone, pitch moth abundance was equal on pure P. californiarum and hybrids, and both were significant ly greater than on pure P. edulis. These within-site comparisons suppo rt the dominance hypothesis where hybrid resistance differs from one t ree species, but not the other. Scale insects exhibited the most restr icted distribution; over the 250 km transect they were found only in t he ii hybrid zone. This supports the hybrid susceptibility and/or the stress hypothesis (i.e., species at the edge of their range suffer gre ater stress and are more susceptible to herbivory). We summed the mean numbers of these three common herbivores across sites and found that hybrid sites supported 2.1 and 3.9 times more herbivores than pure P. californiarum and P. edulis sites, respectively. Furthermore, tree mor tality was on average, 35 times greater within the hybrid zone compare d to pure zones of each species and was associated with the cumulative abundance of herbivores (r(2) = 0.646). Regardless of whether this mo rtality is due to insect infestation, stress or a combination of both, these results suggest that hybrid zones are important arenas of natur al selection.