Professor of Botany University of Wisconsin - Madison, United States
Abstract: Identifying drivers of species diversity has long been a central goal of community ecology and can provide insights into processes influencing community assembly. While patterns of functional and phylogenetic diversity have arguably been over-interpreted as proxies for assembly processes, these approaches can still provide valuable information to help focus conservation priorities. This is particularly the case in Hawaii, where many endemics have been pushed to the brink of extinction. On Kohala mountain, the joint threat of invasive plants, feral pigs, and Sphagnum encroachment presents a novel challenge for native understory plants. We use this system as a case study to investigate the influence of climate and novel drivers on biodiversity in an imperiled habitat.
We conducted a vegetation survey of 120 sites across four study zones varying along a gradient of pig population density ranging from 17.5 to 0 pigs km-2. We measured leaf and reproductive traits (SLA, leaf dry matter content [LDMC], %C, %N, leaf thickness, propagule mass) to characterize ecological function, and used a pruned vascular plant phylogeny to represent phylogenetic relationships. We calculated the standardized effect size of mean pairwise distance (ses.MPD) and mean nearest taxon distance (ses.MNTD) for functional and phylogenetic š¯›¼ and š¯›½ diversity by comparing site MPD and MNTD from trait and phylogenetic distance matrices to simulated null assemblages (n = 1000). We used AIC to select best models for š¯›¼ diversity and Mantel tests to understand how relevant drivers influenced š¯›½ diversity.
Sphagnum coverage decreased both functional and phylogenetic š¯›¼ diversity, suggesting communities were homogenizing in response to Sphagnum encroachment. Pig density and disturbance increased local functional diversity but decreased phylogenetic diversity. Pig soil disturbance favored non-native species with resource acquisitive traits, increasing functional diversity with resulting habitat heterogeneity. However, with these increases we see declines in phylogenetic š¯›¼ diversity, representing a significant challenge to native biodiversity. Functional š¯›½ diversity was influenced most strongly by pig density, soil disturbance, and spatial distance, while phylogenetic š¯›½ diversity was little influenced by these factors. Divergence between functional and phylogenetic diversity patterns indicates weak phylogenetic signal in traits; indeed, we found no evidence of phylogenetic signal using Blombergā€™s K and Pagelā€™s lambda (p-values obtained by permutation). Stronger responses of functional š¯›¼ and š¯›½ diversity to these drivers suggest using phylogenetic diversity as a proxy may obscure patterns when traits are not phylogenetically constrained.
