Introduced and invasive cactus species: a global review

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.

Keywords: Biological invasions; cactus invasions; climate suitability; introduction pathways; invasion debt; invasive species; phylogenetic signal..

Figure 1.

The 12 growth-form categories of the family Cactaceae considered in this paper.

Figure 2.

Cactus species richness across the native (A) and invasive range (B) as well as the native distribution of invasive cacti (C). Shading indicates the number of taxa per country. Lighter colors correspond to less taxa.

Figure 3.

Potential species richness based on all available records (native and invasive records) of 39 invasive cactus species.

Figure 4.

Bayesian phylogeny based on matK DNA sequence data illustrating phylogenetic relationships among genera within Cactaceae. The aligned matK matrix contained 1354 characters and required 65 gaps (indels), ranging from 1 to 74 characters in size. Overall, the phylogeny yielded well-resolved relationships among all genera included. High nodal support (posterior probabilities >0.9) is indicated at nodes by yellow boxes. Genera with invasive taxa are indicated as red branches where red circles are proportional to the percentage of invasive taxa within each genus. Scale bar = number of substitutions/site. The three main clades are indicated under the numbers 1, 2 and 3. CB: Subfamily Cactoideae, Tribe Browningieae; CC: Subfamily Cactoideae, Tribe Cacteae; CCE: Subfamily Cactoideae, Tribe Cereeae; CH: Subfamily Cactoideae, Tribe Hylocereeae; CN: Subfamily Cactoideae, Tribe Notocacteae; CP: Subfamily Cactoideae, Tribe Pachycereeae; CR: Subfamily Cactoideae, Tribe Rhipsalideae; CT: Subfamily Cactoideae, Tribe Trichocereeae; OA: Subfamily Opuntioideae, Tribe Austrocylindropuntieae; OC: Subfamily Opuntioideae, Tribe Cylindropuntieae; OO: Subfamily Opuntioideae, Tribe Opuntieae; OP: Subfamily Opuntioideae, Tribe Pterocacteae; OT: Subfamily Opuntioideae, Tribe Tephrocacteae; M: Subfamily Maihuenioideae. PE: Subfamily Pereskioideae. The bars in the left graph indicate the percentage of non-invasive species (black) against percentage of invasive species (red) per tribe or subfamily.

Figure 5.

The distribution of invasive cacti within (A) genera, (B) growth forms and (C) IUCN Red List categories. Significance levels were determined by comparing the number of invasive vs. number of non-invasive for any group to all other taxa using Fisher's exact test. Confidence intervals were determined for the per cent invasive or introduced based on an assumption of binomial errors.