Climate change threatens endangered key tree cactus (Pilosocereus robinii) in the Florida Keys, USA

Background/Question/Methods Climate change-related sea level rise and increased frequency and intensity of storms threaten rare species and ecosystems along coastlines.  Within the last decade, the precipitous decline of federally endangered Key tree cactus (Pilosocereus robinii) populations alarmed land managers in the Florida Keys, USA.  Canopy closure, a pathogen, and increased soil salinity from storm surges and/or sea level rise were hypothesized as causes for plant mortality.  We surveyed populations at eight sites throughout the Keys and conducted an ecological study on the east and west side of one lower Key, known historically to have the largest population.  Using 57 plants on the lower Key, we evaluated five different habitat variables: soil salinity, canopy cover, elevation, habitat structure, and species composition to determine which variables differed significantly between east and west sides of the key and which variables differed significantly near living or dead cacti.

Results/Conclusions     In eight surveyed populations, Pilosocereus lost approximately 81% of plants and 88% of stems between 1994 and 2007.  Within the lower Key, the west side suffered significantly higher mortality (95% plants/ 99% stems), was 0.3 m lower in elevation, had significantly 223 ppm higher soil salinity, and 17% less canopy cover than the east side, which had moderate mortality (20% plants/ 87% stems).  However, none of the variables tested by the Akaike Information Criterion were found to be strong predictors of plant condition.  In the lower Keys, increased soil salinity coupled with impacts from herbivory and poaching have likely contributed to Pilosocereus demise.  Because Pilosocereus is restricted to rare, fragmented low elevation island habitats, where there is little or no connection to higher ground, it will likely require assisted migration to novel habitats to secure its future as climate change impacts increase.