![]() Can J Bot (in press)įloate KD, Kearsley MJC, Whitham TG (1993) Elevated herbivory in plant hybrid zones: Chrysomela confluens, Populus and phenological sinks. Oecologia 97:215–221įloate K, Whitham TG (1995) Insects as traits in plant systematics: their use in discriminating between hybrid cottonwoods. Am Nat 141:651–662įloate K, Whitham TG (1994) Aphid-ant interaction reduces chrysomelid herbivory in a cottonwood hybrid zone. Oikos 68:393–400įloate K, Whitham TG (1993) The “hybrid-bridge” hypothesis: host shifting via plant hybrid swarms. Aust J Bot 29:37–48Įricson L, Burdon JJ, Wennstrom A (1993) Interspecific host hybrids and phalacrid beetles implicated in the local survival of smut pathogens. Comparison of predispersal seed parameters. Aust J Bot 29:25–35ĭrake DW (1981b) Reproductive success of two Eucalyptus hybrid populations II. Generalized seed output model and comparison of fruit parameters. ![]() PWS-Kent Bostonĭrake DW (1981a) Reproductive success of two Eucalyptus hybrid populations I. Wiley, New Yorkĭaniel WW (1990) Applied nonparametric statistics. ![]() CollinsĬonover WJ (1980) Practical nonparametric statistics. Colorado State University Cooperative Extension, Ft. Am J Bot 81:936–940Ĭranshaw W, Leatherman D, Kondratieff B (1993) Insects that feed on Colorado trees and shrubs (Bulletin 506A). New Mexico State University Cooperative Extension Service, Las CrucesĬobb N, Mitton JB, Whitham TG (1994) Genetic variation associated with chronic water and nutrient stress in pinyon pine. Bot Gaz 140:288–294Ĭain R, Cota J, Ward C (1990) Conifer pests in New Mexico. Annu Rev Ecol Syst 15:25–64īunce JA, Chabot BF, Miller LN (1979) Role of annual leaf carbon balance in the distribution of plant species along an elevational gradient. Oecologia 85:92–100īrussard PF (1984) Geographic patterns and environmental gradients: the central-marginal model in Drosophila revisited. Notes R Bot Gard Edinb 44:275–310īoecklen WJ, Spellenberg R (1990) Structure of herbivote communities in two oak ( Quercus spp.) hybrid zones. ![]() J Kansas Ent Soc 63:638–641īailey DK (1987) A study of Pinus subsection cembroides I: the single-needled pinyons of the Californias and Great Basin. Oikos 64:498–504īarker JF (1990) Sunflower trichome defenses avoided by a sunflower stem weevil, Cylindrocopturus adspersus LeConte (Coleoptera: Curculionidae). Regardless of whether this mortality is due to insect infestation, stress or a combination of both, these results suggest that hybrid zones are important arenas of natural selection.Īguilar JM, Boecklen WJ (1992) Patterns of herbivory in the Quercus grisea X Quercus gambelii species complex. Furthermore, tree mortality was on average, 35 times greater within the hybrid zone compared to pure zones of each species and was associated with the cumulative abundance of herbivores ( r 2=0.646). 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. This supports the hybrid susceptibility and/or the stress hypothesis (i.e., species at the edge of their range suffer greater stress and are more susceptible to herbivory). Scale insects exhibited the most restricted distribution over the 250 km transect they were found only in the hybrid zone. These within-site comparisons support the dominance hypothesis where hybrid resistance differs from one tree species, but not the other. californiarum and hybrids, and both were significantly greater than on pure P. Within the hybrid zone, pitch moth abundance was equal on pure P. californiarum sites than at hybrid and pure P. In contrast to stem-borers, there were significantly more pitch moth wounds on trees at pure P. These two patterns support the hybrid susceptibility hypothesis in which hybrid breakdown results in increased susceptibility to herbivory. ![]() In addition, within hybrid sites, hybrids supported significantly more moth larvae than pure trees of either species. Stem-boring moths were significantly more abundant on trees at “hybrid” sites compared to trees at “pure” sites. The insects (the stem-boring moth, Dioryctria albovittella, the scale insect, Matsucoccus acalyptus, and several species of pitch moths that produce wounds on the trunk and branches) exhibited different distributional patterns across tree types. Using six morphological traits, we developed a hybrid index to classify trees as pure Pinus californiarum, hybrid, or pure Pinus edulis. We examined the abundances of three common insect herbivores on pure and hybrid pinyon pines along a 250-km transect in west-central Arizona, United States. ![]()
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