Beautés fatales: Acanthaceae species as invasive alien plants on tropical Indo‐Pacific Islands

Abstract

Most, if not all, alien plant species of the family Acanthaceae (acanths) found in tropical islands were intentionally introduced as garden ornamentals, because of their showy coloured flowers, bracts or leaves. Some have ‘escaped’ gardens and have naturalized in human‐disturbed areas as weeds, adventives, or ruderal species. A few species have successfully invaded secondary and relatively undisturbed native wet forests. This paper reviews the naturalized alien acanths in tropical islands, and focuses on the currently invasive and potentially invasive species. This study is based on recent (1994–2004) botanical surveys and field observations conducted in several tropical oceanic islands of the Pacific and Indian Oceans, and on bibliographical searches of other tropical islands and countries. A total of 52 acanth species are naturalized in the Indo‐Pacific islands; 26 of them are native to Asia, 18 to tropical America, and only 8 to Africa. The number of naturalized acanths in selected tropical oceanic and continental islands varied from 7 to 25 species and the rate of naturalized acanths (number of naturalized species/number of introduced species) from 27% to 62%. We recorded eight major invasive species: the erect herbs or shrubs Justicia carnea, Odontonema strictum, Phlogacanthus turgidus, Sanchezia speciosa and Strobilanthes hamiltonianus form dense monospecific thickets in the understorey of wet forests; the woody vine Thunbergia grandiflora smothers native trees; the creeping herb Hemigraphis alternata forms dense carpets that totally cover the ground; and the herb Ruellia brevifolia colonizes the understorey of closed‐canopy wet forest. We also discuss eight potentially or incipient invasive acanths which are subspontaneous or sparingly naturalized, but which are not yet considered invasive. Most of the currently invasive acanths are sterile, the lack of fruit production being explained by the absence of pollinators and/or to their particular floral structure and reproduction modes. They reproduce vegetatively by stem fragmentation or by root suckers, and their range expansion is thus relatively slow. Their success may be attributed to their long (50–100 years) residence times, and long‐distance dispersal by humans. Many potentially invasive acanths are newly introduced ornamentals that produce seeds, thus constituting potentially greater threats. The Acanthaceae is not yet recognized as an ‘aggressive’ plant family (e.g. compared to the Fabaceae, Melastomataceae, Poaceae, or Rosaceae). It is, however, one of the most popular ornamental families in the Tropics, and should receive more attention because of the increasing number of current and incipient invasive species found in tropical islands.

Residence time and human-mediated propagule pressure at work in the alien flora of Galapagos

“…4 Information for each of the inhabited islands; a percentage of species naturalized (light grey), invasive (mid grey), and transforming (dark grey), labels are % of evaluated alien species; b median MRT of the non-naturalized alien species; c human population; d alien plant species richness. Boxes show interquartile range, and outliers are excluded in other oceanic islands (Meyer and Lavergne 2004).…”

Pour une autre représentation métaphorique des invasions biologiques

“…Les espèces invasives changent donc les règles du jeu et touchent à la morale (Marris, 2005). Sournoisement, elles prennent parfois les traits d'étrangères au charme ravageur : ce sont alors des « beautés fatales » (Meyer et Lavergne, 2004), voire des « peaux de vache » (Dalla Bernardina, 2003). L'usage de telles métaphores nous montre que les espèces invasives assurent également une fonction de miroir en nous renvoyant à nos propres aversions morales.…”

“…Les modes verbaux conspuant les espèces invasives procè-dent eux-mêmes d'une communication fondée sur la menace (Selincourt, 1992 ;Meinesz, 1997 ;Kendle et Rose, 2000 ;Low, 2001 ;Baskin, 2002 ;Meyer et Lavergne, 2004 ;Clergeau et Nunez, 2006 ;Lambertini et al, 2011 ), même si de nombreux auteurs militent pour l'utilisation de termes neutres (Subramaniam, 2001 ;Dalla Bernardina, 2003 ;Colautti et MacIsaac, 2004 ;Brown et Sax, 2005 ;Gobster, 2005 ;Larson, 2005 ;Warren, 2007 ;Rémy et Beck, 2008 ;Tassin, 2010 ;Davis et al, 2011). À tel point que l'invasion biologique est moins, aujourd'hui, un concept écologique qu'un « objet » anxiogène à peu près vidé de son contenu informatif et, à l'inverse, « surgonflé » d'une dimension émotionnelle parfois indécente (Davis et al, 2001 ;Colautti et MacIsaac, 2004 ;Brown et Sax, 2005 ;Sagoff, 2005 ;Warren, 2005 ;Moore, 2006 ;Rémy et Beck, 2008 ;Larson, 2010).…”

Plant introduction, naturalization, and invasion in French Guiana (South America)

“…In French Guiana, about 20% of the alien flora is composed of palms (96 species) which become more and more popular in horticulture. Some species are now reported to be naturalized and even invasive, especially in tropical island ecosystems (Meyer and Lavergne 2004;Meyer et al 2007).…”