5. Conclusions
In descending order, the most common threats to plants in the continental US include outdoor recreation (especially from ORVs and hiking and related activities), livestock, invasives, then construction and maintenance of roads and railroads plus their berms. Alarmingly, there is some evidence that these threats are either understated or increasing in prevalence. None of these threats receive attention in the peerreviewed scientific literature that is proportionate to their actual prevalence in the United States. Our results paint a formidable conceptual and strategic challenge for conservation. Rare plants in the US face a complex “mega-syndrome” in which threats act coincidentally to affect non-exclusive groups of species. Thus there seems to be no simple, highly-effective strategy that can alleviate threats to the majority of species at once. Rather, integrated coarse-scale strategies like setting aside land for preservation combined with fine-scale strategies like spot control of invasives or educating and diverting recreationists within established parks will be necessary to address the diversity of threats facing plants in the United States. We do note that inspiring work is developing methods for addressing multiple threats at regional scales (e.g., Conlisk et al., 2013; Auerbach et al., 2015; Tulloch et al., 2016), but more work needs to be done to extend these techniques across scales. We hypothesize that mega-syndromes arise as a product of diverse patterns of land use and exploitation associated with complex economies. If this is indeed the case, then novel conceptual tools and plans of action are required to address the mega-syndrome of threats facing rare plants.
