Level of environmental threat posed by horticultural trade in Cactaceae

Abstract

Ornamental horticulture has been identified as an important threat to plant biodiversity and is a major pathway for plant invasions worldwide. In this context, the family Cactaceae is particularly challenging because it is considered the fifth most threatened large taxonomic group in the world; several species are among the most widespread and damaging invasive species; and Cactaceae is one of the most popular horticultural plant groups. Based on the Convention on International Trade in Endangered Species of Wild Flora and Fauna and the 11 largest online auction sites selling cacti, we documented the international cactus trade. To provide an in-depth look at the dynamics of the industry, we surveyed the businesses involved in the cactus trade in South Africa (a hotspot of cactus trade and invasions). We purchased seeds of every available species and used DNA barcoding to identify species to the genus level. Although <20% of this trade involved threatened species and <3% involved known invasive species, many species were identified by a common name. However, only 0.02% of the globally traded cacti were collected from wild populations. Despite a large commercial network, all South African imports (of which 15% and 1.5% were of species listed as threatened and invasive, respectively) came from the same source. With DNA barcoding, we identified 24% of the species to genus level. Based on our results, we believe that if trade restrictions are placed on the small proportion of cacti that are invasive and there is no major increase in harvesting of native populations, then the commercial trade in cactus poses a negligible environmental threat. However, there are currently no effective methods for easily identifying which cacti are traded, and both the illicit harvesting of cacti from the wild and the informal trade in invasive taxa pose on-going conservation challenges.

The changing role of ornamental horticulture in alien plant invasions

“…In some taxonomic groups, the most valued species are actually the ones with traits that make them less successful outside of gardens. For example, among cacti, slow‐growing species are usually favoured by gardeners (Novoa et al, ), and they should be less likely to naturalise and become invasive (Novoa et al, ). For orchids, which are strongly underrepresented in the global naturalised flora (Pyšek et al, ), some hobby growers are willing to pay more for species that are rare in trade and most likely difficult to cultivate (Hinsley, Verissimo & Roberts, ).…”

Invasion syndromes: a systematic approach for predicting biological invasions and facilitating effective management

“…Pathways can be characterized in general terms (e.g. ''disseminated as ornamental plants''), or more specifically by identifying vectors involved in the introduction of particular alien taxa from specific donor regions (Hulme 2009;Novoa et al 2017). Whatever the level of pathway specificity, the goal should be to provide insights of direct relevance for management and policy.…”

The Biogeography of South African Terrestrial Plant Invasions

“…By contrast, there have been only two main phases of Cactaceae introductions. Initial introductions of a few species for agriculture in the nineteenth century (for food, cochineal, and as barrier plants); and in the last few decades the introduction of many species for ornamental horticulture (Kaplan et al 2017;Novoa et al 2017). Interestingly, due to correlations between growth forms, life-history traits and usages, most cactus species suitable for agriculture are invasive whereas many of the taxa widely used in horticulture pose minimal risk (Novoa et al 2015).…”

“…Importantly, several of the invasive species are not on Glen's list, and many species on Glen's list could not be found at known planting sites. Walters et al (2011) estimated that around 400 alien species of Cactaceae are present in South Africa, and Novoa et al (2017) presented evidence that about 300 species of cacti are imported to South Africa annually (though the vast majority of these are not new to South Africa). Currently, 35 species of Cactaceae are invasive (Kaplan et al 2017).…”