Richard Corlett on “Invasive aliens on tropical East Asian islands”

Corlett, R. T., 2009. Invasive aliens on tropical East Asian islands. Journal of Biodiversity and Conservation. DOI 10.1007/s10531-009-9624-4 [Published online: 31 March 2009]

“Tropical East Asia (TEA) has numerous islands, both continental and oceanic. This study uses information on invasive aliens in terrestrial habitats on these islands to test the generality of the continental-oceanic contrast in invasibility, assess the conservation impacts of invasive species, and suggest ways to mitigate these. The continental islands of Hong Kong and Singapore are worst-case scenarios for continental invasibility and alien species often dominate in chronically disturbed sites, but very few have successfully invaded closed forests, with the exception of birds in Hong Kong. On other, less densely populated, continental islands, closed-canopy forests appear to resist invasions by all taxa, with few known exceptions.

Excerpt – “Tropical East Asia has numerous continental islands, ranging in size from tiny rocky outcrops to the giant islands of Borneo (750,000 km2) and Sumatra (440,000 km2). The densely populated islands of Hong Kong (particularly the 80-km2 Hong Kong Island) and Singapore (the 600-km2 main island) can be considered as examples of the ‘worst case scenario’ for continental invasibility, with pervasive human impacts, numerous accidental and deliberate alien introductions through their ports and botanic gardens, and a massive propagule pressure arising from the close proximity of urban areas and native vegetation. They difer in that Hong Kong (22° N) has a subtropical monsoon climate and was largely deforested centuries ago (Dudgeon and Corlett 2004), while Singapore (1° N) has an aseasonal climate and was largely covered in primary forest until less than 200 years ago (Tan et al. 2007). On both islands, alien plants dominate only in disturbed urban and agricultural areas, although this dominance can apparently persist on some sites after disturbance is halted (e.g. for Hong Kong—Corlett 1992b; Dudgeon and Corlett 2004; Leven and Corlett 2004; Chung and Corlett 2006; for Singapore—Corlett 1988, 1992a; Tan et al. 2007). In contrast, only single alien plant species, Syzygium jambos in Hong Kong (Leung et al. 2009) and Clidemia hirta in Singapore (Teo et al. 2003), have signiWcantly invaded native, closed-canopy forests. Additional aliens are established in the margins of some forest areas and along forest paths on both islands. Several species have invaded fire-maintained native grassland and shrubland in Hong Kong along streamsides (particularly Mikania micrantha) and in areas disturbed by feral cattle, which are not present on Hong Kong Island, but occur on nearby Lantau Island and the adjacent mainland (Leung et al. 2009).

The information available suggests that alien invertebrates in both Hong Kong and Singapore are also largely confined to areas that experience chronic human disturbance (Dudgeon and Corlett 2004; Fellowes 1999; Tan et al. 2007). There have, however, been two dramatic exceptions to this in Hong Kong, where the pinewood nematode (Bursaphelenchus
xylophilus
) and pine needle scale (Hemiberlesia pitysophila) have largely eliminated the native Pinus massoniana since the 1980s (Dudgeon and Corlett 2004). It is very likely that other, less conspicuous, alien invertebrates have been overlooked in the forests, since the native faunas are also poorly known.

Alien amphibians and reptiles have also been largely excluded from forest and other semi-natural vegetation in both areas, but a number of mammal and bird species have been successful invaders, particularly in Hong Kong. Indeed, more than half the >20 alien bird species established in Hong Kong (including the islands and adjacent mainland) are found in forests and both their diversity and abundance is increasing (Leven and Corlett 2004; Kwok 2007; Leven pers. com.). This pattern of invasion reXects the origin of most alien bird populations in Hong Kong from Buddhist bird releases. Wild-caught birds from similar forests in southwest China are imported into Hong Kong as cage-birds and have been released in large numbers over recent decades, resulting in optimum conditions for establishment (Chan 2007). Buddhist releases have been less common in Singapore, but may be the source of the only widespread avian forest invader there, the white-crested laughingthrush (Garrulax leucolophus) (Wang and Hails 2007). The exotic squirrels in both Hong
Kong (Callosciurus flavimanus; Chung and Corlett 2006) and Singapore (C. finlaysonii; Baker and Lim 2008) were probably also established from deliberate releases. In Hong Kong, where no native squirrel species survived deforestation, C. xavimanus has successfully invaded native forests, but in Singapore, which has high densities of native tree squirrels, the exotic species is currently confined to suburban areas, although this may simply reflect its relatively recent introduction. Both areas have free-ranging dogs, but these appear to include truly wild populations only in Hong Kong, where feral cats are also established on Lantau Island, which lacks the native leopard cat, Prionailurus bengalensis (Dudgeon and Corlett 2004). Feral cattle (Bos taurus) are also established in fire-maintained grasslands on Lantau Island.”

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