Whitham T G, Morrow P A, Potts B M
Department of Biological Sciences, Northern Arizona University, 86011, Flagstaff, AZ, USA.
Department of Ecology, Evolution and Behavior, University of Minnesota, 55108, St. Paul, MN, USA.
Oecologia. 1994 May;97(4):481-490. doi: 10.1007/BF00325886.
We found the hybrid zone between Eucalyptus amygdalina and Eucalyptus risdonii to be a center of insect and fungal species richness and abundance. Of 40 taxa examined, 73% were significantly more abundant in the hybrid zone than in pure zones, 25% showed on significant differences, and 2% were most abundant on a pure host species. The average hybrid tree supported 53% more insect and fungal species, and relative abundances were, on average, 4 times greater on hybrids than on either eucalypt species growing in pure stands. Hybrids may act as refugia for rare species: 5 of 40 species were largely restricted to the hybrid zone. Also, 50% of the species coexisted only in the hybrid zone, making for mique species assemblages. Although hybrids support more species and greater abundances, all hybrids are not equal: 68% of the 40 taxa examined were significantly more abundant on one hybrid phenotype than another. While herbivore concentrations on F1 type intermediates were rare, concentrations were common on phenotypes resembling backcrosses either to E. amygdalina or E. risdonii. For specialist herbivores, the hybrid phenotype most heavily utilized appears to be determined by its phenotypic affinity to its host species. Generalists exhibit an overall greater abundance on hybrids, but are less likely to utilize one hybrid phenotype over another. Mechanistic explanations for these distributions are numerous and probably species specific, but are likely to include: increased genetic susceptibility of hybrids due to hybrid breakdown; increased stress in the hybrid zone resulting in greater plant susceptibility; and a greater diversity of resources in the hybrid zone which could support more species. Seed capsule production by hybrids and their parental species is negatively correlated with herbivory. However, it is difficult to determine whether herbivores cause this pattern as hybrids may have inherently lower sexual reproduction. Laws enacted to protect rare and endangered species do not include hybrids. We argue that a re-examination of our current "hybrid policy" is warranted. Plant hybrid zones are centers of plant evolution and speciation, sources of economically important plants and potential biocontrol agents, and, as our study suggests, also provide essential habitats for phytophagous communities.
我们发现杏仁桉和里斯登桉之间的杂交区域是昆虫和真菌物种丰富度及丰度的中心。在所研究的40个分类单元中，73%在杂交区域的丰度显著高于纯种植区域，25%无显著差异，2%在纯宿主物种上丰度最高。平均而言，杂交树支持的昆虫和真菌物种多53%，且杂交树上物种的相对丰度平均是纯林中生长的两种桉树种中任一种的4倍。杂交种可能是珍稀物种的避难所：40个物种中有5个主要局限于杂交区域。此外，50%的物种仅在杂交区域共存，形成独特的物种组合。尽管杂交种支持更多物种且丰度更高，但并非所有杂交种都一样：在所研究的40个分类单元中，68%在一种杂交表型上的丰度显著高于另一种。虽然食草动物在F1型中间杂交种上的聚集很少见，但在类似于与杏仁桉或里斯登桉回交的表型上很常见。对于专性食草动物来说，被大量利用最严重的杂交表型似乎由其与宿主物种的表型亲和力决定。广食性动物在杂交种上总体丰度更高，但不太可能偏好某一种杂交表型。对这些分布的机理解释有很多且可能因物种而异，但可能包括：杂交衰退导致杂交种的遗传易感性增加；杂交区域压力增加导致植物易感性增强；杂交区域资源多样性更高从而能支持更多物种。杂交种及其亲本物种的种子荚产量与食草作用呈负相关。然而，很难确定食草动物是否导致了这种模式，因为杂交种可能本身有较低的有性繁殖能力。为保护珍稀濒危物种而制定的法律并不包括杂交种。我们认为有必要重新审视我们当前的“杂交政策”。植物杂交区域是植物进化和物种形成的中心、经济重要植物和潜在生物防治剂的来源，而且正如我们的研究所表明的，还为植食性群落提供了重要栖息地。
