Effects of Phylogenetic Diversity and Phylogenetic Identity in a Restoration Ecology Experiment
Our understanding of the effects of plant biodiversity on ecosystem function rests in large part on experiments that have disentangled environmental variables from local diversity. Yet phylogenetic diversity (PD) effects can be confounded by phylogenetic identity effects in such experiments if assemblages with low or high PD tend to be dominated by a single clade. We illustrate this problem in a 127-species experiment designed to test the effects of angiosperm PD and trait diversity on tallgrass prairie restoration outcomes. In this experiment, the taxon pool exhibits a phylogenetic bias: if species were randomly assigned to experimental assemblages, low PD plots would frequently be dominated by a single clade (the sunflower or daisy family, Asteraceae). We present a visualization tool for examining phylogenetic experiments for this bias and propose a taxonomically constrained experimental design to reduce the most egregious causes of bias. We then present the experimental design we developed using the constrained approach and summarize initial findings from this large-scale restoration experiment. Entanglement of phylogenetic diversity and phylogenetic identity is an underappreciated and likely widespread challenge for PD experiments, particularly those that draw upon a large number of candidate species. By recognizing, quantifying, and counteracting this bias, researchers can better differentiate the effects of PD per se from phylogenetic identity effects.
The authors are grateful to staff, volunteers, and colleagues too numerous to mention by name at The Morton Arboretum and Chicago Botanic Gardens who made this work possible. Natural resources and facilities staff at the Arboretum—in particular Spencer Campbell, Kurt Dreisilker, and P.J. Smith—were especially instrumental. Donald Waller and colleagues at University of Wisconsin-Madison generously provided access to prepublication trait data for 74 species, supported by US National Science Foundation Award DEB 1046355 to DW and collaborators. Carri LeRoy, Will Pearse, Grégory Sonnier, Daniel Spalink, Donald Waller, and Lindsey Worcester provided valuable comments on an early draft of this manuscript. Collaborators at Pizzo and Associates and Prairie Moon Nursery—in particular Kyle Banas, Jack Pizzo, and Bill Carter—were closely involved in species selection and production and went above and beyond in making room for us in their facilities, even at the busiest of times. Lane Scher took pains to get excellent drone photos of the experiment. This work was supported by US National Science Foundation Awards to ALH (NSF-DEB 1354551) and DJM (NSF-DEB 1354426).
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