Biology Department, Stanford University, Stanford, CA 94305, USA.
Oecologia. 2012 Nov;170(3):677-85. doi: 10.1007/s00442-012-2340-x. Epub 2012 May 6.
The effects of herbivory on plant fitness are integrated over a plant's lifetime, mediated by ontogenetic changes in plant defense, tolerance, and herbivore pressure. In symbiotic ant-plant mutualisms, plants provide nesting space and food for ants, and ants defend plants against herbivores. The benefit to the plant of sustaining the growth of symbiotic ant colonies depends on whether defense by the growing ant colony outpaces the plant's growth in defendable area and associated herbivore pressure. These relationships were investigated in the symbiotic mutualism between Cordia alliodora trees and Azteca pittieri ants in a Mexican tropical dry forest. As ant colonies grew, worker production remained constant relative to ant-colony size. As trees grew, leaf production increased relative to tree size. Moreover, larger trees hosted lower densities of ants, suggesting that ant-colony growth did not keep pace with tree growth. On leaves with ants experimentally excluded, herbivory per unit leaf area increased exponentially with tree size, indicating that larger trees experienced higher herbivore pressure per leaf area than smaller trees. Even with ant defense, herbivory increased with tree size. Therefore, although larger trees had larger ant colonies, ant density was lower in larger trees, and the ant colonies did not provide sufficient defense to compensate for the higher herbivore pressure in larger trees. These results suggest that in this system the tree can decrease herbivory by promoting ant-colony growth, i.e., sustaining space and food investment in ants, as long as the tree continues to grow.
食草作用对植物适合度的影响是通过植物防御、耐受性和食草动物压力的个体发育变化来综合的。在共生蚂蚁-植物互惠关系中，植物为蚂蚁提供筑巢空间和食物，而蚂蚁则保护植物免受食草动物的侵害。植物维持共生蚁群生长的好处取决于不断增长的蚁群的防御能力是否超过植物在可防御区域和相关食草动物压力下的生长速度。这些关系在墨西哥热带干旱森林中考察了 Cordia alliodora 树和 Azteca pittieri 蚂蚁之间的共生互惠关系。随着蚁群的生长，工蚁的产量相对于蚁群的大小保持不变。随着树木的生长，叶片的产量相对于树木的大小而增加。此外，较大的树木栖息的蚂蚁密度较低，这表明蚁群的生长速度跟不上树木的生长速度。在实验中排除蚂蚁的叶片上，单位叶片面积的食草量与树木大小呈指数增长，表明较大的树木每单位叶片面积承受的食草动物压力比较小的树木更高。即使有蚂蚁的防御，食草量也会随着树木的大小而增加。因此，尽管较大的树木有更大的蚁群，但较大的树木中的蚂蚁密度较低，而且蚁群提供的防御不足以补偿较大树木中更高的食草动物压力。这些结果表明，在这个系统中，只要树木继续生长，树木就可以通过促进蚁群的生长来减少食草作用，即维持对蚂蚁的空间和食物投资。
