Christensen Kerry M, Whitham Thomas G, Keim Paul
Department of Biological Sciences, Northern Arizona University, 86011, Flagstaff, AZ, USA.
Oecologia. 1995 Jan;101(1):29-36. doi: 10.1007/BF00328896.
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. 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 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. Stem-boring moths were significantly more abundant on trees at "hybrid" sites compared to trees at "pure" sites. In addition, within hybrid sites, hybrids supported significantly more moth larvae than pure trees of either species. These two patterns support the hybrid susceptibility hypothesis in which hybrid breakdown results in increased susceptibility to herbivory. In contrast to stem-borers, there were significantly more pitch moth 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 significantly greater than on pure P. edulis. These within-site comparisons support the dominance hypothesis where hybrid resistance differs from one tree species, but not the other. Scale insects exhibited the most restricted distribution; over the 250 km transect they were found only in the hybrid zone. 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). 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 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 =0.646). 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.
我们在美国亚利桑那州中西部一条250公里长的样带上，研究了纯合和杂交矮松上三种常见食草昆虫的数量。利用六个形态特征，我们制定了一个杂交指数，将树木分类为纯加州矮松、杂交种或纯食脂松。这些昆虫（蛀干蛾，白纹迪夜蛾；蚧虫，松球蚧；以及几种在树干和树枝上造成伤口的蛀果蛾）在不同树型间呈现出不同的分布模式。与“纯合”样地的树木相比，“杂交”样地的树木上蛀干蛾的数量显著更多。此外，在杂交样地内，杂交树比任何一个纯合树种的树木支持更多的蛾幼虫。这两种模式支持了杂交易感性假说，即杂交衰退导致对食草动物的易感性增加。与蛀干蛾相反，纯加州矮松样地的树木上蛀果蛾造成的伤口比杂交和纯食脂松样地的树木上显著更多。在杂交区内，纯加州矮松和杂交树上蛀果蛾的数量相等，且两者都显著多于纯食脂松上的数量。这些样地内的比较支持了优势假说，即杂交抗性在一个树种中与另一个树种不同。蚧虫的分布最为受限；在250公里的样带上，它们仅在杂交区内被发现。这支持了杂交易感性和/或胁迫假说（即处于其分布范围边缘的物种遭受更大的胁迫，更容易受到食草动物的侵害）。我们汇总了各地点这三种常见食草动物的平均数量，发现杂交样地支持的食草动物数量分别是纯加州矮松样地和纯食脂松样地的2.1倍和3.9倍。此外，杂交区内的树木死亡率平均比每个物种的纯合区高35倍，且与食草动物的累积数量相关（r = 0.646）。无论这种死亡率是由于昆虫侵害、胁迫还是两者兼而有之，这些结果都表明杂交区是自然选择的重要场所。
