Spatial Distribution of Dominant Arboreal Ants in a Malagasy Coastal Rainforest: Gaps and Presence of an Invasive Species

We conducted a survey along three belt transects located at increasing distances from the coast to determine whether a non-random arboreal ant assemblage, such as an ant mosaic, exists in the rainforest on the Masoala Peninsula, Madagascar. In most tropical rainforests, very populous colonies of territorially dominant arboreal ant species defend absolute territories distributed in a mosaic pattern. Among the 29 ant species recorded, only nine had colonies large enough to be considered potentially territorially dominant; the remaining species had smaller colonies and were considered non-dominant. Nevertheless, the null-model analyses used to examine the spatial structure of their assemblages did not reveal the existence of an ant mosaic. Inland, up to 44% of the trees were devoid of dominant arboreal ants, something not reported in other studies. While two Crematogaster species were not associated with one another, Brachymyrmex cordemoyi was positively associated with Technomyrmex albipes, which is considered an invasive species—a non-indigenous species that has an adverse ecological effect on the habitats it invades. The latter two species and Crematogaster ranavalonae were mutually exclusive. On the other hand, all of the trees in the coastal transect and at least 4 km of coast were occupied by T. albipes, and were interconnected by columns of workers. Technomyrmex albipes workers collected from different trees did not attack each other during confrontation tests, indicating that this species has formed a supercolony along the coast. Yet interspecific aggressiveness did occur between T. albipes and Crematogaster ranavalonae, a native species which is likely territorially dominant based on our intraspecific confrontation tests. These results suggest that the Masoala rainforest is threatened by a potential invasion by T. albipes, and that the penetration of this species further inland might be facilitated by the low density of native, territorially dominant arboreal ants normally able to limit its progression.

我们在马达加斯加马索亚拉半岛的热带雨林中，沿三条距海岸距离递增的带状样带开展调查，以探究当地是否存在非随机分布的树栖蚂蚁群落（arboreal ant assemblage）——例如蚂蚁镶嵌（ant mosaic）格局。在多数热带雨林中，具领地优势的树栖蚂蚁种群会形成庞大群落，守卫着以镶嵌格局分布的专属领地。本次调查共记录到29种蚂蚁，其中仅9种拥有足够庞大的群落，可被视为潜在领地优势种；其余物种群落规模较小，划归为非优势种。然而，用于检测群落空间结构的零模型（null-model）分析并未发现蚂蚁镶嵌格局的存在。在内陆区域，最高可达44%的树木未被优势树栖蚂蚁占据，这一现象此前未见其他研究报道。尽管两种举腹蚁属（Crematogaster）物种之间无显著关联，但科氏短尾蚁（Brachymyrmex cordemoyi）与白头长脚蚁（Technomyrmex albipes）呈现正相关关系——后者被认定为入侵物种，即对入侵栖息地造成负面生态影响的非本土物种。后两种蚂蚁与拉瓦诺那举腹蚁（Crematogaster ranavalonae）呈现相互排斥的分布格局。与之相反，海岸样带内以及沿岸至少4公里范围内的所有树木均被白头长脚蚁（T. albipes）占据，且蚁群通过工蚁行进的蚁道相互连通。在对峙试验中，从不同树木上采集的白头长脚蚁工蚁并未互相攻击，这表明该物种在沿岸区域已形成超级群落（supercolony）。但白头长脚蚁与拉瓦诺那举腹蚁之间存在种间攻击性——后者为本土物种，根据我们的种内对峙试验结果，其大概率属于领地优势种。上述结果表明，马索亚拉热带雨林正面临白头长脚蚁（T. albipes）潜在入侵的威胁；而当地本土领地优势树栖蚂蚁种群密度偏低——这类蚂蚁本可限制其扩散，这或许进一步促进了该物种向内陆的渗透。