The Cryptic impacts of invasion: functional homogenization of tropical ant communities by invasive fire ants

The diversity and distribution of traits in an ecological community shapes its responses to change and the ecosystem processes it modulates. This 'functional diversity', however, is not necessarily a direct outcome of taxonomic diversity. Invasions by exotic insects occur in ecosystems worldwide, but there is limited understanding of how they impact functional diversity. We present the first comprehensive trait-based investigation of the impacts of an ant invasion, and the first incorporating intraspecific polymorphisms in species-level functional diversity. The fire ant Solenopsis invicta is an invasive species with a global distribution. Focusing on invaded and uninvaded plots in tropical grasslands of Hong Kong, we investigated how the presence of S. invicta affects the diversity and distribution of ant species and traits within and across communities, the functional identities of communities, and functionally unique species. Using trait probability density functions, we built trait spaces for 29 species, and scaled up these components to calculate functional diversity at community and landscape levels. We found that invasion had limited effects on species and functional richness but pronounced effects on functional composition. Specifically, invaded communities had fewer functionally-unique individuals, and were characterized by species with narrower heads and bodies and shorter mandibles. Moreover, invaded communities showed substantially higher levels of functional redundancy (+56%) due to a clustering of trait values. Consequently, across the landscape, invaded communities displayed 23% less functional turnover than uninvaded communities despite showing comparable levels of taxonomic turnover – a result confirming theoretical predictions of the effects of high local functional redundancy. In sum, the presence of S. invicta alters the functional properties of multiple local communities selectively, resulting in functional homogenization across the landscape. The disparities between taxonomic and functional impacts of invasion highlight the need to consider how trait diversity across ecological scales shapes biodiversity and its responses to change.

生态群落内的性状多样性与分布格局，决定了该群落对环境变化的响应方式，以及其所调控的生态系统过程。然而，这种功能多样性（functional diversity）并非分类学多样性的直接产物。全球各类生态系统均有外来昆虫入侵事件发生，但目前学界对其如何影响功能多样性的认知仍较为有限。本研究首次针对蚂蚁入侵的影响开展了全面的基于性状的调查，同时也是首个将种内多态性（intraspecific polymorphisms）纳入物种水平功能多样性分析的研究。红火蚁（Solenopsis invicta）是一种具有全球扩散范围的入侵物种。本研究以香港热带草原的入侵样地与未入侵样地为研究对象，探究红火蚁的定植如何影响群落内部及跨群落的蚂蚁物种与性状的多样性与分布、群落的功能属性，以及功能独特物种（functionally unique species）。本研究通过性状概率密度函数（trait probability density functions），为29个蚂蚁物种构建了性状空间（trait spaces），并将这些组分进行尺度上推，以计算群落与景观尺度的功能多样性。研究结果显示，红火蚁入侵对物种丰富度与功能丰富度（functional richness）的影响较为有限，但对功能组成（functional composition）具有显著作用。具体而言，入侵样地的群落中功能独特性个体数量更少，且群落的优势物种具有更窄的头部与躯体、更短的上颚。此外，由于性状值的聚类分布，入侵样地群落的功能冗余（functional redundancy）显著提升（增幅达56%）。因此，尽管入侵与未入侵样地的分类学周转（taxonomic turnover）水平相当，但在景观尺度上，入侵样地群落的功能周转（functional turnover）较未入侵样地低23%——这一结果验证了高局地功能冗余所带来影响的理论预测。综上，红火蚁的定植会选择性地改变多个局地群落的功能属性，最终导致景观尺度上的功能均质化（functional homogenization）。入侵事件的分类学影响与功能学影响之间存在显著差异，这凸显了我们需要深入探究不同生态尺度下的性状多样性如何塑造生物多样性及其对环境变化的响应。