Trait-mediated competition drives an ant invasion and alters functional diversity

The assumption that differences in species’ traits reflect their different niches has long influenced how ecologists infer processes from assemblage patterns. For instance, many assess the importance of environmental filtering versus classical limiting-similarity competition in driving biological invasions by examining whether invaders’ traits are similar or dissimilar to those of residents, respectively. However, mounting evidence suggests that hierarchical differences between species’ trait values can distinguish their competitive abilities (e.g., for the same resource) instead of their niches. Whether such trait-mediated hierarchical competition explains invasions and structures assemblages is less explored. We integrate morphological, dietary, physiological and behavioural trait analyses to test whether environmental filtering, limiting-similarity competition, or hierarchical competition explain invasions by fire ants on ant assemblages. We detect both competition mechanisms; invasion success is not only explained by limiting similarity in body size and thermal tolerance (presumably allowing the invader to exploit different niches from residents), but also by the invader’s superior position in trait hierarchies reflecting competition for common trophic resources. We find that the two mechanisms generate complex assemblage-level functional diversity patterns (overdispersion in some traits, clustering in others) suggesting their effects are likely missed by analyses restricted to a few traits and composite trait diversity measures.

长期以来，物种性状差异反映其不同生态位（niche）的假设，一直影响着生态学家通过群落格局推断生态过程的研究范式。例如，诸多研究通过比较入侵物种与本地物种的性状相似性与否，分别评估环境过滤（environmental filtering）与经典限制相似性竞争（limiting-similarity competition）在驱动生物入侵（biological invasions）中的相对重要性。然而，越来越多的证据表明，物种性状值的等级差异可用于区分物种的竞争能力（例如针对同一资源的竞争），而非其生态位分化。这类由性状介导的等级竞争（trait-mediated hierarchical competition）是否能够解释生物入侵并塑造群落结构，目前相关研究仍较为匮乏。本研究整合形态学、食性、生理学及行为学性状分析，旨在检验环境过滤、限制相似性竞争与等级竞争（hierarchical competition）这三类机制，能否解释红火蚁（fire ants）入侵蚂蚁群落的过程。本研究同时检测到了两类竞争机制：入侵成功不仅可通过体型与热耐受性（thermal tolerance）的限制相似性得到解释（该机制可使入侵物种利用与本地物种相异的生态位），同时也可通过入侵物种在反映共同营养资源（trophic resources）竞争的性状等级体系中的优势地位得到解释。本研究发现，这两类竞争机制会产生复杂的群落水平功能多样性（assemblage-level functional diversity）格局：部分性状呈现性状过分散（overdispersion），部分则呈现性状聚集（clustering），这表明若仅针对少数性状或复合性状多样性指数（composite trait diversity measures）开展分析，很可能会遗漏这两类机制的作用效应。