Hubbard J Andrew, McPherson Guy R
Department of Rangeland Ecology and Management, Texas A&M University, College Station, TX 77843-2126, USA fax: (409)845-6430; e-mail:
School of Renewable Natural Resources, University of Arizona, Tucson, AZ 85721, USA, , , , , , US.
Oecologia. 1997 Mar;110(1):143-146. doi: 10.1007/s004420050142.
By caching acorns, jays serve as important dispersal agents for oak (Quercus) species. Yet little is known about which acorn characteristics affect selection by jays. In the traditional model of jay/oak symbiosis, large, brown, ripe acorns free of invertebrate parasites (e.g., Curculio acorn weevils) are selected by jays. Recently, it has been suggested that a tri-trophic relationship between oaks, jays, and weevils may have evolved to counter the negative dietary effects of acorn tannins. Under the tri-trophic model, jays would preferentially select acorns containing weevil larvae. We tested the assumptions that (1) acorns containing curculionid larvae exist in sufficient quantities to support jay populations and (2) jays can detect, and preferentially select, acorns containing weevil larvae, and investigated the cues by which jays select acorns. Captive Mexican jays (Aphelocomaultramarina) were presented Emory oak (Quercusemoryi) acorns in aviary feeding trials. Large, dense, viable acorns free of curculionid larvae were preferentially selected. Contrary to results of previous research, color did not affect selection. Acorn viability increased and curculionid larval occupancy decreased in adjacent savannas and isolated stands relative to existing oak woodland, perhaps favoring oak recruitment into adjacent lower-elevation grasslands. Our results compel us to reject the tri-trophic model for this system, and are consistent with the traditional jay/oak symbiosis model. Relatively long-distance dispersal of viable acorns favors Emory oak replacement, and spatial patterns of acorn viability and curculionid parasitism suggest expansion of Emory oak into adjacent low-elevation semi-arid grasslands.
通过贮藏橡子，松鸦成为橡树物种重要的传播媒介。然而，对于哪些橡子特征会影响松鸦的选择，我们却知之甚少。在传统的松鸦/橡树共生模型中，松鸦会选择那些个头大、呈褐色、成熟且无无脊椎动物寄生虫（如橡实象鼻虫）的橡子。最近，有人提出橡树、松鸦和象鼻虫之间的三营养级关系可能已经进化，以抵消橡子单宁对饮食的负面影响。在三营养级模型下，松鸦会优先选择含有象鼻虫幼虫的橡子。我们检验了以下假设：（1）含有象鼻虫幼虫的橡子数量足以维持松鸦种群；（2）松鸦能够检测并优先选择含有象鼻虫幼虫的橡子，并研究了松鸦选择橡子的线索。在圈养条件下，对墨西哥松鸦（Aphelocomaultramarina）进行了室外饲养试验，给它们提供埃默里橡树（Quercusemoryi）的橡子。它们优先选择个头大、饱满、有活力且无象鼻虫幼虫的橡子。与之前的研究结果相反，颜色并不影响选择。相对于现有的橡树林地，相邻稀树草原和孤立林分中的橡子活力增加，象鼻虫幼虫的占有率降低，这可能有利于橡树向相邻低海拔草原的补充。我们的研究结果迫使我们拒绝该系统的三营养级模型，这与传统的松鸦/橡树共生模型一致。有活力的橡子相对长距离的传播有利于埃默里橡树的更新，橡子活力和象鼻虫寄生的空间格局表明埃默里橡树正在向相邻的低海拔半干旱草原扩张。
