Using Small-Scale Studies to Prioritize Threats and Guide Recovery of a Rare Hemiparasitic Plant: Cordylanthus rigidus ssp. littoralis

Abstract

BackgroundRecovering endangered species would benefit from identifying and ranking of the factors that threaten them. Simply managing for multiple positive influences will often aid in recovery; however, the relative impacts of multiple threats and/or interactions among them are not always predictable. We used a series of experiments and quantitative observational studies to examine the importance of five potential limiting factors to the abundance of a state-listed endangered hemiparasitic annual forb, Cordylanthus rigidus ssp. littoralis (C.r.l., California, USA): host availability, mammalian herbivores, insect seed predators, fire suppression, and exotic species. While this initial assessment is certainly not a complete list, these factors stem from direct observation and can inform provisional recommendations for management and further research.Methodology and Principal FindingsStudies were conducted at five sites and included assessments of the influence of host availability, exotic species, exclusion of mammalian herbivores and insect seed predators on C.r.l. productivity, and simulated effects of fire on seed germination. C.r.l. was limited by multiple threats: individuals with access to host species produced up to three times more inflorescences than those lacking hosts, mammalian herbivory reduced C.r.l. size and fecundity by more than 50% and moth larvae reduced seed production by up to 40%. Litter deposition and competition from exotic plant species also appears to work in conjunction with other factors to limit C.r.l. throughout its life cycle.Conclusions and SignificanceThe work reported here highlights the contribution that a series of small-scale studies can make to conservation and restoration. Taken as a whole, the results can be used immediately to inform current management and species recovery strategies. Recovery of C.r.l. will require management that addresses competition with exotic plant species, herbivore pressure, and availability of preferred host species.

Herbivory mediates direct and indirect interactions in long‐unburned chaparral

“…Deer at FONR clearly affected some plants, such as Ceanothus in this study, oaks ( Q. agrifolia ; L. R. Fox, unpublished), and the endangered annual seaside bird's beak, Cordylanthus rigidus ssp. littoralis (Watts et al, 2010), but there have been no reported extirpations.…”

Suaeda californica in San Francisco Bay: phenology, germination, and high tide refuge

“…Multiple life history stages must be supported to ensure long-term survival (Jackson et al 2009) and an extensive literature points to factors that could limit rare plant recovery at various phenophases. These include restricted geographic range and narrow habitat requirements (Schemske et al 1994), seed predation (Watts et al 2010;Combs et al 2013;Leja et al 2015;Kurkjian et al 2017), herbivory (Joe and Daehler 2008;Watts et al 2010;Hill Bermingham 2010;Grossenbacher et al 2018), fire suppression (Kaye et al 2001;Paniw et al 2015), water stress (Traveset et al 2003), pollen limitation (Knight et al 2005), partial male sterility that reduces self-compatibility and favors outcrossing with sterile male individuals (Fernández-Illescas et al 2010), lack of pollinators (Schurr et al 2019), and competition with invasive species (Grewell et al 2003;Fiedler et al 2007;Watts et al 2010;Reynolds and Boyer 2010;Schneider 2013). Restricted geographic range, narrow habitat requirements, unknown interactions with pollinators, and partial male sterility are important considerations for the survival and growth of S. californica.…”