Going beyond taxonomic diversity: deconstructing biodiversity patterns reveals the true cost of iceplant invasion

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Although invasion has been linked to species losses in native plant communities, it is unclear how invasive species affect other important aspects of native community biodiversity, such as the composition of functional traits or the degree of phylogenetic relatedness. Here, we ask whether declines in taxonomic diversity (TD) associated with the spread of a highly invasive South African species (iceplant) are linked to similar losses in functional (FD) and phylogenetic diversity (PD). Motivated by recent advances in coexistence theory, we aim to infer the mechanisms involved in driving the exclusion of native species following invasion. Coastal dunes of central Italy. We sampled 2 × 2 m vegetation plots characterized by varying degrees of abundance of iceplant (Carpobrotus spp.), and combined species occurrence data with life-history trait information and a dated phylogeny of the native species pool. Rao's quadratic entropy index was used to quantify the TD, FD and PD of each plot, which we then related to iceplant abundance via linear models. Finally, to better understand the mechanisms driving changes in diversity, we characterized both species and communities according to phylogenetic relatedness to iceplant and functional strategy. We found that the negative association between the level of invasion and native community TD is mirrored by quantitatively similar declines in FD, as well as a shift in community phylogenetic structure. These changes appear to result from the selective exclusion of specific functional groups/clades, likely via a combination of niche- and fitness-related processes. We found that iceplant poses a greater threat to biodiversity than previously understood and that on top of causing declines in species richness invasion may also hamper ecosystem functioning and reduce evolutionary potential. Accounting for FD and PD holds promise for gaining a better understanding of how invasive species alter the structure of native communities.

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