Abstract
Positive interactions enhance biodiversity and ecosystem function, but can also exacerbate biological invasions. Facilitation of exotic invaders by exotic foundation species (invasional meltdown) has been studied extensively, but facilitation of exotic invaders by native foundation species has attracted less attention. Specifically, very few studies have examined the extent that native foundation species facilitate native and exotic competitors. Understanding the processes that mediate interactions between native and exotic species can help explain, predict, and improve management of biological invasions. Here, we examined the effects of native foundation shrubs on the relative abundance of the annual plant community – including native and exotic taxa – from 2015–2018 in a desert ecosystem at Carrizo Plain National Monument, California, USA (elevation: 723 m). Shrub effects varied by year and by the identity of annual species, but shrubs consistently enhanced the abundance of the annual plant community and facilitated both native (n=17 species) and exotic (n=4 species) taxa. However, at the provenance level, exotic annuals were facilitated 2.75 times stronger in abundance than native annuals, and exotic annuals were always more abundant than natives both near and away from shrubs. Our study reaffirms facilitation as an important process in the organisation of plant communities and confirms that both native and exotic species can form positive associations with native foundation species. However, facilitation by native foundation species can exacerbate biological invasions by increasing the local abundance of exotic invaders. Thus, the force of facilitation can have a dark side relevant to ecosystem function and management.
Highlights
Positive interactions among species, or facilitation, can strongly influence the organisation of plant communities (Callaway 1995; Callaway 2007; Brooker et al 2008), in unproductive environments (Bertness and Callaway 1994; Maestre et al 2009)
The biogeographic origins of beneficiary species is an important consideration because exotic species displace native species in drylands globally (Balch et al 2013; Bellard et al 2016; Vitousek et al 2017), and facilitation by native foundation species can influence the outcome of interactions between native and exotic taxa (Reisner et al 2015)
In each year of the study, native and exotic annual species positively associated with E. californica to become beneficiary species (Fig. 1)
Summary
Facilitation, can strongly influence the organisation of plant communities (Callaway 1995; Callaway 2007; Brooker et al 2008), in unproductive environments (Bertness and Callaway 1994; Maestre et al 2009). Invasive plant species pose a pervasive threat to ecosystem function worldwide (Simberloff et al 2013), including strong effects on historic patterns of nutrient cycling (Liao et al 2008), energy flow (Baxter et al 2004; Pearson and Callaway 2008), and abiotic disturbance (D’Antonio and Vitousek 1992; Balch et al 2013). These disruptions are often associated with sharp reductions in local biodiversity (Vila et al 2011; Bellard et al 2016). Strong facilitation of exotic species relative to native competitors could require conservationists to shift their focus from
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
