Alien plant invasions in tropical and subtropical

Authors: Llewellyn C Foxcroft David M Richardson Marcel Rejmánek Petr Pyšek

Publish Date: 2010/07/11

Volume: 12, Issue: 12, Pages: 3913-3933

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Abstract

Biological invasions affect virtually all ecosystems on earth but the degree to which different regions and biomes are invaded and the quality of information from different regions varies greatly A large body of literature exists on the invasion of savannas in the Neotropics and northern Australia where invasive plants especially African grasses have had major impacts Less has been published on plant invasions in African savannas except for those in South Africa Negative impacts due to plant invasions in African savannas appear to be less severe than in other regions at present As savannas cover about 60 of the continent with tens of millions of people relying on the services they provide it is timely to assess the current status of invasions as a threat to these ecosystems We reviewed the literature contrasting the African situation with that of Neotropical and Australian savannas A number of drivers and explanatory factors of plant invasions in savannas have been described mostly from the Neotropics and Australia These include herbivore presence residence time intentional introductions for pasture improvements fire regimes the physiology of the introduced species and anthropogenic disturbance After comparing these drivers across the three regions we suggest that the lower extent of alien plant invasions in African savannas is largely attributable to 1 significantly lower rates of intentional plant introductions and widespread plantings until recently 2 the role of large mammalian herbivores in these ecosystems 3 historical and biogeographical issues relating to the regions of origin of introduced species and 4 the adaptation of African systems to fire We discuss how changing conditions in the three regions are likely to affect plant invasions in the futureSavannas form one of the world’s largest biomes and are the dominant vegetation type in Africa Scholes 1997 occupying about 65 of the continent Huntley and Walker 1982 About a fifth of the global human population and a large proportion of the world’s ungulates wild and livestock are found in savannas Lehmann et al 2009 A wealth of knowledge now exists on the composition and function of savannas particularly in Africa see Sinclair and NortonGriffiths 1979 Huntley and Walker 1982 Bourlière and Hadley 1983 Cole 1986 Werner 1991 Scholes and Walker 1993 Sinclair and Arcese 1995 Solbrig et al 1996 Cowling et al 1997 Coe et al 1999 du Toit et al 2003 Sankaran et al 2005 2008 Furley 2006 Shorrocks 2007 Sinclair et al 2008 This includes a growing understanding of the inherent complexity of savanna systems and the extent that savannas depend on complex interactions of climatic and edaphic factors and disturbance from both fire and herbivory Sankaran et al 2005 2008 Globally the tropical savannas are the second largest biome extending over 15 × 106 km2 Long et al 1989 Melillo et al 1993Savannas are defined as a tropical vegetation type codominated by a largely continuous layer of grasses generally below 2 m and a discontinuous woody tree layer Bourlière and Hadley 1983 Scholes 1997 Scholes and Archer 1997 A common feature of all savannas is the hot wet season and warm dry season Scholes 1997 an usually high fire frequency Huntley 1982 Scholes 1997 and high habitat heterogeneity Pickett et al 2003 Rogers 2003 Tongway and Ludwig 2005 Savannas vary spatially in composition and function across a number of scales Bourlière and Hadley 1983 For example there is high variation along soil gradients and associated vegetation at the scale of catenas or across a variety of subsavanna type habitats in the subSaharan region see Fig 19 in Shorrocks 2007 Within this spatial arrangement savannas vary structurally from a short grass layer with tall trees to savannas with a range of shrub and tree sizes in between eg Pivello et al 1999a Temporally savannas and grasslands have varied in extent and distribution from glacial/interglacial cycles Dupont et al 2000 to the last few hundred years Gillson 2004Many millions of people depend on savannas for their livelihoods leading to a wide range of land uses such as agriculture grazing and agroforestry including fuel wood harvesting Huntley 1982 Scholes 1997 van Wilgen et al 2001 Therefore savannas are subject to many kinds of anthropogenic disturbances as well as periodic natural disturbances fire drought floods megaherbivores Walker and NoyMeir 1982 Scholes 1997 Anthropogenic disturbances create habitats and conditions suitable for invasions by alien plants thus forming multiple sources for further invasions into savanna systems van Wilgen et al 2001 The Millennium Ecosystem Assessment 2005 assessed the past and current impact of five drivers of ecosystem change for a range of biomes For the tropical grassland and savanna biome invasive species were regarded to have had a relatively low impact over the last century but a trend of very rapid increase of the impact for this biome was noted The current low incidence and influence of invasive plants in savannas relative to some other terrestrial biomes may be related to the fact that disturbance which generally favours invasions is fundamental to savanna functioning Savannas are generally resilient to changes in disturbance regimes Walker and NoyMeir 1982 Harrison and Shackleton 1999 possibly making them relatively resistant to biological invasions Habitat modification has had a high impact and is increasing in importance climate change has had a moderate but very rapidly increasing impact overexploitation a very high and continuing impact and pollution a moderate but very rapidly increasing impact in these biomes Millennium Ecosystem Assessment 2005 Although invasive species are not currently the main threat to the conservation of biodiversity and functioning of tropical grasslands and savannas they may well become much more widespread and influential in the future Therefore it seems prudent to review the current status of plant invasions in the savanna biome

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