Diversity and Distributions, (Diversity Distrib.) (2011) 17, 1015–1029 S BIODIVERSITY Impacts of invasive Australian acacias: PECIAL ISSUE REVIEW implications for management and restoration 1 2 3 David C. Le Maitre *, Mirijam Gaertner , Elizabete Marchante , Emilie-Jane :H Ens4, Patricia M. Holmes5, Anı´bal Pauchard6, Patrick J. O’Farrell1, Andrew M. Rogers2, Ryan Blanchard1,2, James Blignaut7 and David M. Richardson2 UMAN 1Natural Resources and the Environment, ABSTRACT - CSIR, P. O. Box 320, Stellenbosch 7599, South MEDIATED INTRODUCTIONS OF Aim The biophysical impacts of invasive Australian acacias and their effects on Africa, 2Centre for Invasion Biology, Department of Botany and Zoology, ecosystem services are explored and used to develop a framework for improved Stellenbosch University, Private Bag X1, restoration practices. 3 Matieland 7602, South Africa, Centre for Location South Africa, Portugal and Chile. Functional Ecology, Department of Life Sciences, University of Coimbra, Apartado Methods A conceptual model of ecosystem responses to the increasing severity 3046, 3001-401 Coimbra, Portugal, 4Centre (density and duration) of invasions was developed from the literature and our for Aboriginal Economic Policy Research, The knowledge of how these impacts affect options for restoration. Case studies are A Journal of Conservation Biogeography Australian National University, Canberra used to identify similarities and differences between three regions severely affected 0200, ACT, Australia, 5Environmental by invasions of Australian acacias: Acacia dealbata in Chile, Acacia longifolia in Resource Management, City of Cape Town, Portugal and Acacia saligna in South Africa. Berkley Road, Maitland 7404, South Africa, Results Australian acacias have a wide range of impacts on ecosystems that A 6Facultad de Ciencias Forestales, Universidad increase with time and disturbance, transform ecosystems and alter and reduce USTRALIAN ACACIAS de Concepcio´n, Chile and Institute of Ecology and Biodiversity (IEB), Chile, 7Department of ecosystem service delivery. A shared trait is the accumulation of massive seed Economics, University of Pretoria, Pretoria banks, which enables them to become dominant after disturbances. Ecosystem 0002, South Africa trajectories and recovery potential suggest that there are important thresholds in ecosystem state and resilience. When these are crossed, options for restoration are radically altered; in many cases, autogenic (self-driven and self-sustaining) recovery to a pre-invasion condition is inhibited, necessitating active intervention to restore composition and function. — Main conclusions The conceptual model demonstrates the degree, nature and A GLOBAL EXPERIMENT IN BIOGEOGRAPHY reversibility of ecosystem degradation and identifies key actions needed to restore ecosystems to desired states. Control and restoration operations, particularly active restoration, require substantial short- to medium-term investments, which can reduce losses of biodiversity and ecosystem services, and the costs to society in the long term. Increasing restoration effectiveness will require further research into linkages between impacts and restoration. This research should involve scientists, practitioners and managers engaged in invasive plant control and restoration programmes, together with society as both the investors in, and beneficiaries of, more effective restoration. and Distributions Keywords *Correspondence: David Le Maitre, Natural Acacia, biodiversity loss, biological invasions, degradation ecology, ecosystem Resources and the Environment, CSIR, P. O. Box 320, Stellenbosch 7599, South Africa. functions, ecosystem services, impact mechanisms, invasive species, seed banks, E-mail: [email protected] soil nutrients. Diversity DOI: 10.1111/j.1472-4642.2011.00816.x ª 2011 Blackwell Publishing Ltd http://wileyonlinelibrary.com/journal/ddi 1015 D. C. Le Maitre et al. suggest that improved restoration demands clearer insights INTRODUCTION into the trajectories and processes leading to degradation or Many invasive plant species are able to transform ecosystems altered ecosystem functioning. We further propose that such (Richardson et al., 2000; Ortega & Pearson, 2005), resulting in improved insights can be developed by collating insights on losses of biodiversity, altered ecosystem functioning and a Australian Acacia invasions from different parts of the world. changed capacity to provide services (Vitousek et al., 1997; This study therefore seeks to develop a conceptual under- Pimentel, 2001; Levine et al., 2003; Didham et al., 2007; standing of ecosystem changes driven by invasive Australian Pejchar & Mooney, 2009; Vila` et al., 2010). Management acacias. We focus on three regions where problems are most interventions that address these impacts are underway in many acute and where most information is available. We review the parts of the world. They include measures to prevent impacts of acacia invasions, on both biophysical and ecosystem introductions, efforts to detect and eradicate new invaders, services, drawing on published information and the insights of biological control and various efforts aimed at mitigating the authors. We then develop a conceptual model linking impacts (Pysˇek & Richardson, 2010; Wilson et al., 2011). Many degradation to restoration and apply this to Australian acacias programmes adopt a passive approach to restoring invaded based on case studies on three continents. By drawing out systems and simply aim to remove the existing invaders and cross-continental similarities and differences, we synthesize limit or prevent their regeneration. This approach often fails to insights to show how knowledge of the range and complexity achieve the desired outcome of a functional ecosystem of impacts can be used to direct restoration towards desired dominated by native species (D’Antonio & Meyerson, 2002; outcomes. Hulme, 2006; Reid et al., 2009; Blackwood et al., 2010). Practical problems that prevent the achievement of goals IMPACTS OF ACACIA INVASIONS ON include ‘secondary invasions’ – the rapid replacement of the BIOPHYSICAL FEATURES AND ECOSYSTEM removed invasive species by others that capitalize on distur- SERVICES bance caused by the control operations. Resource alterations caused by the invasive species, the management intervention or This section addresses both the biophysical impacts (those combinations of these also often complicate or thwart which affect ecosystem structure and function) and ecosystem restoration efforts (Galatowitsch & Richardson, 2005; Buckley, service impacts (those where the biophysical impacts also affect 2008; Young et al., 2009). ‘Legacy effects’ – long-lasting the generation and delivery of ecosystem services to society). changes in ecosystem structure such as increased soil nutrient Invasive Australian acacias, like many other invasive species, levels that persist following the removal of the invasive species have a wide range of impacts including a number that interact – are another major problem (D’Antonio & Meyerson, 2002; in a synergistic fashion (Fig. 1; specific impact studies sum- Marchante et al., 2009). The result is that many control and marized in Table S1). Acacia species have been shown to restoration efforts have unplanned and undesirable outcomes induce simultaneous changes in the above- and below-ground and do not achieve sustainable mitigation of the impacts communities, microclimates, soil moisture regimes and soil caused by invasive species. We contend that better mitigation nutrient levels (Fig. 1; Marchante et al., 2003, 2008a; Yelenik of impacts caused by invasive plant species demands an et al., 2004; Werner et al., 2010; Gaertner et al., 2011). Many improved understanding of the interacting factors that gener- changes are directly attributable to key traits of Acacia species: ate such impacts, and recognition that control and restoration their rapid growth rates and ability to out-compete native measures must explicitly address the fundamental drivers of plants (Morris et al., 2011); their capacity to accumulate high such impacts and their effects. biomass; large, persistent seed banks; and their capacity to fix At least 23 Australian Acacia species (a group of 1012 species nitrogen (Yelenik et al., 2007). These features enable them to in Acacia subgenus Phyllodineae native to Australia; see Miller dominate competitive interactions with native species. Many of et al., 2011 for taxonomic details) are major or emerging the abiotic changes and biotic responses to them are tightly invaders in many parts of the world (Castro-Dı´ez et al., 2011; linked and may advance simultaneously rather than sequen- Richardson & Rejma´nek, 2011; Richardson et al., 2011). They tially (Hobbs et al., 2009), as does the progression from have a range of ecological and socio-economic impacts (Le structural to functional impacts (Fig. 1). Maitre et al., 2000; De Wit et al., 2001; Marchante et al., 2003, The impacts of Australian acacias on biodiversity and 2008a,b; Gaertner et al., 2009; Hellmann et al., 2011; March- ecosystem properties and functions also affect the delivery of ante, 2011; Rascher et al., 2011). The range and magnitude of ecosystem services and the benefits that society derives from the impacts of existing invasions are becoming more severe, them. Ecosystem services include: supporting services (e.g. soil and similar impacts are likely to emerge in other areas where formation); regulatory services (e.g. water flow and nutrient invasive acacias were