A systematic assessment of threats affecting the rare plants of the United States Haydée Hernández-Yáñez a,b , Justin T. Kos a , Matthew D. Bast a,c , Janeisha L. Griggs a , Paul A. Hage a , Alex Killian a , M. Isabel Loza a,b , Matthew B. Whitmore a , Adam B. Smith a,b, ⁎ a Department of Biology, University of Missouri-St. Louis, 223 Research Building, One University Boulevard, St. Louis, MO 63121-4499, USA b Center for Conservation and Sustainable Development, Missouri Botanical Garden, 4344 Shaw Boulevard, St. Louis, MO 63110, USA c St. Louis Community College, Meramec, 11333 Big Bend Boulevard, St. Louis, MO 63122-5799, USA abstract article info Article history: Received 28 April 2016 Received in revised form 19 September 2016 Accepted 4 October 2016 Available online xxxx Characterizing the distribution of threats facing species is a crucial, first step toward designing effective conser- vation strategy. The last comprehensive analysis of threats facing rare plants in the United States was conducted nearly 20 years ago. Here we systematically analyze the threats facing 2733 rare and vulnerable plants in the US using textual analysis of the most comprehensive database available. In the continental US plants are most com- monly threatened by outdoor recreation (affecting 35% of species), especially from off-road vehicles (19%) and hiking and related activities (13%). The next-most common threats are from livestock (33%), residential develop- ment (28%), non-native invasives (27%), and roads (21%). In Hawaii invasives threaten 95% of species followed by increases in fire intensity/frequency (26%) then livestock (19%). Multivariate analyses indicate threats do not form distinct “syndromes” (clusters of threats) but rather a single “mega-syndrome” with high degrees of overlap between most threats. We also compared the prevalence of threats to the distribution of research effort. Nearly 75% of threats are understudied relative to their prevalence, including five of the six most common threats while a few rare threats (missing species like pollinators; pathogens; logging; climate-induced ecosystem movement; and crop-based agriculture) receive most of the attention. In comparison to a benchmark assessment from 1998 (Wilcove et al. BioScience 48:607-615) we find little difference in threat prevalence, though temporal trends sug- gest increasing frequency of nearly all threats. Overall rare plants in the US are affected by a dense network of threats across which research attention is disproportionately directed. © 2016 Published by Elsevier Ltd. Keywords: Agriculture Urbanization Plants Invasive species Recreation Threats 1. Introduction Five major threats endanger biodiversity: habitat alteration, over- harvest, invasive species, pollution, and disease (Millennium Assessment, 2005) with climate change expected to become yet anoth- er driver of biodiversity loss (Thomas et al., 2004). Each of these broad- ly-defined threats can be further divided into specific threats from diverse factors like urbanization, agriculture, native versus non-native invasive species, and so on. Detailed characterization of threats facing species is crucial for effective recovery planning (Lawler et al., 2002; Hayward, 2009), directing conservation strategy (Murray et al., 2014), allocating resources across conservation actions (Wilson et al., 2007), and estimating the political feasibility of abating threats (Prugh et al., 2010). Hence, there is a pressing need to describe the distribution of threats across species as specifically as possible. The last such analysis for plants in the United States was performed nearly 20 years ago (Wilcove et al., 1998). Threats can act in concert to affect groups of species (Burgman et al., 2007; Budiharta et al., 2011). For example, agriculture, overexploitation, and urbanization each threaten generally distinct groups of carnivorous plants (Jennings and Rohr, 2011). These threat “syndromes” (sensu Burgman et al., 2007) can be related to geographic co-location of species (Jono and Pavoine, 2012), range size (Burgman et al., 2007; González-Suárez et al., 2013), habitat type (Burgman et al., 2007), tax- onomy (Budiharta et al., 2011; McCune et al., 2013), or the fact that some kinds of human activities engender multiple threats to species (e.g., road construction can facilitate spread of invasives). Syndromes offer both opportunities and challenges for managers and researchers. On one hand, addressing sets of co-occurring threats increases efficien- cy and knowledge transfer because they may have a common origin (Burgman et al., 2007). On the other hand, addressing groups of threats can be difficult if they are diverse in nature and require very different strategies to ameliorate (Auerbach et al., 2015; Tulloch et al., 2016). For science to adequately inform threat abatement, research effort should be apportioned in rough accordance to the actual incidence Biological Conservation 203 (2016) 260–267 ⁎ Corresponding author at: Center for Conservation and Sustainable Development, Missouri Botanical Garden, 4344 Shaw Boulevard, St. Louis, MO 63110, USA. E-mail address: adam.smith@mobot.org (A.B. Smith). http://dx.doi.org/10.1016/j.biocon.2016.10.009 0006-3207/© 2016 Published by Elsevier Ltd. Contents lists available at ScienceDirect Biological Conservation journal homepage: www.elsevier.com/locate/bioc