The cryptic impacts of an ant invasionSubmitted by editor on 11 February 2020.Get the paper!
Photo by: Francois Brassard
Written by: Mark Wong
Ecologists are increasingly appreciating the multifaceted nature of biodiversity, which comprises not only taxonomic diversity, but also genetic, phylogenetic and functional components. Each captures a unique and important spectrum of the differences among living things at any scale.
Functional diversity, emphasising the differences among organisms' phenotypic traits, is meant to summarise the diversity of ecological interactions in a system without measuring each interaction per se. In this regard, functional diversity is an efficient means for assessing the responses and effects of species in ecosystems (insofar as those interactions affect species' fitness and ecosystem processes). Many fields in ecology need this vital information, which explains why an enormous number of studies are using trait-based approaches to investigate functional diversity across the tree of life. Studies on terrestrial arthropods – our planet's most diverse Eukaryotic organisms – are no exception, and we reviewed this literature in a recent paper (Wong, Guénard & Lewis, 2019). One thing that caught our attention was a general lack of research on how arthropod functional diversity responds to invasions by exotic species, which are pervasive as well as ecologically and economically relevant at a global scale. As is common to trait-based studies in general, we also noted that few investigations of arthropod functional diversity attempted to account for intraspecific trait variation, which may very well influence community dynamics and ecosystem functions.
Our latest paper in Oikos aims to address the above knowledge gaps and limitations with a case study of an invasion by the Red Imported Fire Ant (Solenopsis invicta) in Hong Kong. Concerns about the spread of this invasive species are increasingly echoed in many Asian nations, but ecological assessments remain scarce. We investigated how invasion by S. invicta affected the functional diversity of ant communities, as this probably modulates a wide array of ecosystem functions (herbivore control, nutrient cycling, bioturbation etc.).
We measured multiple traits of individual ants in uninvaded and invaded communities, and built species-level functional spaces incorporating intraspecific trait variation. This was made possible through the recently developed "trait probability density" framework (see Carmona et al., 2016). With this powerful approach, one neat thing we could do for the first time was incorporate trait variation from intraspecific polymorphisms into functional diversity at higher scales. By this we mean the trait variation arising from the different worker subcastes of ant colonies (e.g. small worker ants vs. large and big-headed 'soldier' ants), which may contribute to the ecological success of particular species, or perhaps even the diversity of their ecological interactions. Solenopsis invicta for instance, has polymorphic workers (see gif above).
With our functional spaces for individual ant species, we could then scale up to assess the functional alpha-diversity of ant communities, and relationships across multiple communities (functional beta-diversity). We also dug into other interesting questions, such as how the relative 'functional uniqueness' of particular species changed due to invasion, and whether functional richness was indeed higher in polymorphic species.
One striking result we found: although the species richness of ant communities changed only marginally with invasion, their functional structure at both alpha and beta scales was altered significantly. This implies that assessments using only taxonomic measures (e.g. species richness) to investigate community responses to disturbances (e.g. invasions) may fail to detect more cryptic impacts in other facets of biodiversity such as functional diversity.
If you're curious to know why these changes occurred, and their potential implications for ecosystem functions, please read our paper.
Moving forward, we are applying trait-based approaches to rigorously test different hypotheses concerning the underlying ecological drivers of the invasion.
Carmona, C. P., de Bello, F., Mason, N. W., & Lepš, J. (2016). Traits without borders: integrating functional diversity across scales. Trends in Ecology and Evolution 31: 382-394.
Wong, M. K. L., Guénard, B., & Lewis, O. T. (2019). Trait‐based ecology of terrestrial arthropods. Biological Reviews 94: 999-1022.