Abstract
Understanding which species are introduced and become invasive, and why, are central questions in invasion science. Comparative studies on model taxa have provided important insights, but much more needs to be done to unravel the context dependencies of these findings. The cactus family (Cactaceae), one of the most popular horticultural plant groups, is an interesting case study. Hundreds of cactus species have been introduced outside their native ranges; a few of them are among the most damaging invasive plant species in the world. We reviewed the drivers of introductions and invasions in the family and seek insights that can be used to minimize future risks. We compiled a list of species in the family and determined which have been recorded as invasive. We also mapped current global distributions and modelled the potential global distributions based on distribution data of known invasive taxa. Finally, we identified whether invasiveness is phylogenetically clustered for cacti and whether particular traits are correlated with invasiveness. Only 57 of the 1922 cactus species recognized in this treatment have been recorded as invasive. There are three invasion hotspots: South Africa (35 invasive species recorded), Australia (26 species) and Spain (24 species). However, there are large areas of the world with climates suitable for cacti that are at risk of future invasion-in particular, parts of China, eastern Asia and central Africa. The invasive taxa represent an interesting subset of the total species pool. There is a significant phylogenetic signal: invasive species occur in 2 of the 3 major phylogenetic clades and in 13 of the 130 genera. This phylogenetic signal is not driven by human preference, i.e. horticultural trade, but all invasive species are from 5 of the 12 cactus growth forms. Finally, invasive species tend to have significantly larger native ranges than non-invasive species, and none of the invasive species are of conservation concern in their native range. These results suggest fairly robust correlates of invasiveness that can be used for proactive management and risk assessments.
Highlights
The increased movement of humans around the world has facilitated the intentional and accidental transportation of species far from their native ranges, often in a manner that can facilitate invasions (Wilson et al.2009)
We found no significant differences between the projected species richness maps for the two modelling approaches, i.e. using native range occurrence records only or native and invasive occurrence records (Fig. 3)
Only a seventh of the cactus species used in their native ranges for food or fodder have been introduced elsewhere, and we found no official records of species having been introduced for medicinal or other purposes
Summary
The increased movement of humans around the world has facilitated the intentional and accidental transportation of species far from their native ranges, often in a manner that can facilitate invasions (Wilson et al.2009). Much work has recently focussed on reviewing the invasive performance of particular genera or closely related groups of organisms in different situations around the world (e.g. Richardson et al 2011; Moodley et al 2013; Potgieter et al 2013; Shackleton et al 2014). Such studies aim to update knowledge on the global occurrence and potential range of these taxa and to understand the complex drivers of human-mediated introductions and invasions. It is important to consider whether such comparisons yield broad generalities or whether generalizations apply only to a subset of taxa
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