Determining future invasive plant threats under climate change: a decision tool for managers

Adaptation Research Grants Program
Lesley Hughes
Macquarie University
Year Started: 
New South Wales

Executive summary from final report:

Over the last decade, the potential for anthropogenic climate change to affect the distribution, physiology and management of established, invasive species has emerged as a major area for ecological research (Dukes & Mooney, 1999; Hellman et al., 2008). Given the significant economic burden and environmental consequences of invasive species (see Pimentel, 2002) the push to understand how changing climate regimes may alter the dynamics of invasions is unsurprising. However, in the rush to understand how well-established invaders may respond, few studies have focussed on the potential for changing climates to facilitate new invasions, or to enable presently noninvasive naturalised species to become invasive either more rapidly in situ, or invade new areas. 

Even if protocols such as Australia’s Weed Risk Assessment (WRA) system succeed in preventing the entry of new invaders, the next generation of invaders are already present as naturalised, but not yet invasive species. Little attention has so far been focused on this large pool of species and our understanding of invasion risk remains incomplete. Understanding how naturalised species may respond to climate change is thus an urgent goal for invasion ecologists and land managers and there is value in developing a prioritisation scheme that assesses current ‘sleeper species’ in order to gain a better understanding of how weeds will respond to future climatic change.

Naturalised species are non-native organisms that have formed self-sustaining populations that have not yet spread significantly through the landscape from their introduction foci (Richardson et al., 2000). Following their introduction, around 10% of non-native species progress to naturalisation and a similar proportion from this pool go on to become serious invaders (Williamson & Fitter, 1996), although recent data suggests that these percentages may be significantly higher e.g. >20% of naturalised plants in New Zealand are now recognised as major weeds (Williams & Timmins 2002). Progression through these three states (introduced, naturalised, and invasive) is conceptualised as a linear process where species pass through a series of abiotic and biotic barriers (including disturbance regimes), leading to invasion success or failure or in a worst case scenario, driving ecosystem change (Floyd et al., 2006). For example, the transition from introduced to naturalised may involve overcoming barriers to reproduction, (e.g. recruiting local pollinators) (Stokes et al. 2006; Milbau & Stout, 2008), climate (e.g. spreading into areas of suitable climate), and disturbance regimes (e.g. the enhancement or suppression of a previously natural fire regime).

Dubbed the ‘invasion-continuum’, this model of plant invasions has become a unifying tool for researchers over the last decade. In a recent review, Richardson & Pysek (2012) highlighted the fact that much research has focused on the final transition of the invasion continuum – when species have become widespread and have discernible impacts on native communities. However, as abiotic conditions continue to change under human influences it is increasingly likely that a new suite of plant invaders will emerge and that these species are likely to already reside in the pool of naturalised plants, given the sheer numbers of species involved (e.g. in Australia this pool represents about 3,000 plant species). Therefore, there is a clear need to better understand the potential for naturalised, but not yet invasive species, to emerge as new threats or transformer species.

In Australia, almost 30,000 non-native plant species have been introduced since European settlement and approximately 10% of these are recognised as naturalised (Randall, 2007). Whilst some of these naturalised plants pose significant threats to biodiversity, primary production and human health (e.g. the Weeds of National Significance), the majority have yet to become invasive. Some of these ‘sleeper weeds’, are almost certain to pose a latent threat to native biodiversity (Groves et al., 2005; Groves, 2006). In a global context, Australia has the highest number of naturalised plant species of any biogeographic region (Richardson & Pysek, 2012). 

This pool of naturalised plants is taxonomically distinct from the Australian flora (e.g. 26% of naturalised plants come from plant families not native to Australia, based on comparative data from the Australian Plant Census; and their establishment has therefore led to the ongoing homogenisation of Australia’s unique flora with other regions of the world. In addition, the number of newly naturalised plant species continues to grow annually. For example, Groves & Hosking (1998) found that over a 25-year period, 295 species, or approximately 12 new plants each year, were recorded as naturalised

We assessed the potential for climate change to affect the distribution of 292 naturalised, but not yet invasive alien plants in Australia. We developed both continent and state/territory-level spatial assessments to identify regions under both current and future climates where the risk of invasion is most concentrated. Overall, we found that the southern coastal regions and Tasmania are at the highest risk, with arid areas at least risk. In addition to the detailed species-based assessment, we also developed a preliminary, point-based prioritisation scheme that ranks species so that detailed weed risk assessments can focus on those species that pose the largest threat now and in the future. 

We applied the prioritisation scheme under two levels of spatial assessment and found results differ significantly between states and territories and at a national level. A major output of this project has been the creation of a dedicated website ( that includes a searchable database for use as a decision tool for land managers so they can more effectively and efficiently prioritise resources to cope with emerging weed threats.

View the final report