Data from: Trees as islands: canopy ant species richness increases with the size of liana-free trees in a Neotropical forest

The physical characteristics of habitats shape local community structure; a classic example is the positive relationship between the size of insular habitats and species richness. Despite the high density and proximity of tree crowns in forests, trees are insular habitats for some taxa. Specifically, crown isolation (i.e. crown shyness) prevents the movement of small cursorial animals among trees. Here, we tested the hypothesis that the species richness of ants (Sa) in individual, isolated trees embedded within tropical forest canopies increases with tree size. We predicted that this pattern disappears when trees are connected by lianas (woody vines) or when strong interactions among ant species determine tree occupancy. We surveyed the resident ants of 213 tree crowns in lowland tropical forest of Panama. On average, 9.2 (range ± 2–20) ant species occupied a single tree crown. Average (± SE) Sa was ca. 25% higher in trees with lianas (10.2 ± 0.26) than trees lacking lianas (8.0 ± 0.51). Sa increased with tree size in liana-free trees (Sa = 10.99^0.256), but not in trees with lianas. Ant species composition also differed between trees with and without lianas. Specifically, ant species with solitary foragers occurred more frequently in trees with lianas. The mosaic-like pattern of species co-occurrence observed in other arboreal ant communities was not found in this forest. Collectively, the results of this study indicate that lianas play an important role in shaping the local community structure of arboreal ants by overcoming the insular nature of tree crowns.

生境的物理特征塑造局域群落结构，经典案例便是岛屿化生境（insular habitats）的面积与物种丰富度之间的正相关关系。尽管森林内树冠密度极高且彼此邻近，但对于部分类群而言，树木本身便是岛屿化生境。具体而言，冠层隔离（即冠层害羞（crown shyness））会阻碍小型奔走类动物在树木间移动。本研究检验了如下假说：嵌入热带林冠中的孤立个体树木，其栖居蚂蚁的物种丰富度（Sa）会随树木尺寸增大而提升。我们预测，当树木被藤本植物（木质藤本）连接，或是蚂蚁物种间存在强相互作用以决定树栖占位时，这一规律将不复存在。我们在巴拿马低地热带森林中调查了213个树冠的栖息蚂蚁。单个树冠平均栖居9.2种蚂蚁（范围为2~20种）。有藤本的树木其平均Sa（标准误（standard error, SE）±0.26）约为10.2，较无藤本树木的8.0±0.51高出约25%。在无藤本的树木中，Sa随树木尺寸增大而提升（Sa = 10.99^0.256），但在有藤本的树木中未观察到此规律。有、无藤本的树木其蚂蚁物种组成亦存在差异。具体而言，独居觅食型蚂蚁在有藤本的树木中出现频率更高。此前在其他树栖蚂蚁群落中观察到的镶嵌式物种共存模式，在本研究的森林中并未发现。综上，本研究结果表明，藤本植物通过抵消树冠的岛屿化生境属性，在塑造树栖蚂蚁的局域群落结构中发挥了重要作用。