Department of Ecology and Evolutionary Biology, Rice University, 6100 Main Street, Houston, Texas 77005, USA.
Oecologia. 2010 Aug;163(4):985-96. doi: 10.1007/s00442-010-1584-6. Epub 2010 Mar 3.
Geographic variation in the outcome of interspecific interactions may influence not only the evolutionary trajectories of species but also the structure of local communities. We investigated this community consequence of geographic variation for a facultative mutualism between ants and wild cotton (Gossypium thurberi). Ants consume wild cotton extrafloral nectar and can protect plants from herbivores. We chose three sites that differed in interaction outcome, including a mutualism (ants provided the greatest benefits to plant fitness and responded to manipulations of extrafloral nectar), a potential commensalism (ants increased plant fitness but were unresponsive to extrafloral nectar), and a neutral interaction (ants neither affected plant fitness nor responded to extrafloral nectar). At all sites, we manipulated ants and extrafloral nectar in a factorial design and monitored the abundance, diversity, and composition of other arthropods occurring on wild cotton plants. We predicted that the effects of ants and extrafloral nectar on arthropods would be largest in the location with the mutualism and weakest where the interaction was neutral. A non-metric multidimensional scaling analysis revealed that the presence of ants altered arthropod composition, but only at the two sites in which ants increased plant fitness. At the site with the mutualism, ants also suppressed detritivore/scavenger abundance and increased aphids. The presence of extrafloral nectar increased arthropod abundance where mutual benefits were the strongest, whereas both arthropod abundance and morphospecies richness declined with extrafloral nectar availability at the site with the weakest ant-plant interaction. Some responses were geographically invariable: total arthropod richness and evenness declined by approximately 20% on plants with ants, and extrafloral nectar reduced carnivore abundance when ants were excluded from plants. These results demonstrate that a facultative ant-plant mutualism can alter the composition of arthropod assemblages on plants and that these community-level consequences vary across the landscape.
种间相互作用的结果在地理上的差异可能不仅影响物种的进化轨迹，还会影响当地群落的结构。我们研究了这种地理变异对蚂蚁和野生棉花（Gossypium thurberi）之间兼性互利关系的群落后果。蚂蚁消耗野生棉花的额外花蜜，并能保护植物免受食草动物的侵害。我们选择了三个在相互作用结果上存在差异的地点，包括互利共生（蚂蚁对植物的适应性提供了最大的好处，并对额外花蜜的处理做出反应）、潜在的共生关系（蚂蚁增加了植物的适应性，但对额外花蜜没有反应）和中性相互作用（蚂蚁既不影响植物的适应性，也不对额外花蜜做出反应）。在所有地点，我们采用析因设计来操纵蚂蚁和额外花蜜，并监测野生棉花植株上其他节肢动物的丰度、多样性和组成。我们预测，在具有互利关系的地点，蚂蚁和额外花蜜对节肢动物的影响最大，而在相互作用中性的地点，这种影响最弱。非度量多维标度分析显示，蚂蚁的存在改变了节肢动物的组成，但只在两个蚂蚁增加植物适应性的地点如此。在具有互利关系的地点，蚂蚁还抑制了腐食者/清道夫的数量，增加了蚜虫的数量。额外花蜜的存在增加了互利关系最强的地点的节肢动物丰度，而当与植物的相互作用最弱的地点花蜜供应充足时，节肢动物的丰度和形态种丰富度都下降了。一些反应在地理上是不变的：有蚂蚁的植物上，总节肢动物丰富度和均匀度下降了约 20%，而当蚂蚁被从植物上排除时，额外花蜜会减少肉食动物的数量。这些结果表明，一种兼性的蚂蚁-植物互利关系可以改变植物上节肢动物组合的组成，而这些群落层面的后果在景观上是不同的。
