Tag: Australia

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Understanding Alien Plant Invasions

Alien plants are everywhere – but not all invaders behave the same

a group of yellow flowers
Photo by Brittany Lee on Unsplash

Alien plant invasions are accelerating worldwide, posing serious threats to biodiversity and costing billions in management. A recent study – led by David Gregory as part of his Masters at King’s and in collaboration with Matt White from the Victorian government – sheds light on how these invasions unfold across landscapes and why growth form matters when predicting and managing risk.

The research, conducted in Victoria, Australia, analysed data from more than 7,600 vegetation surveys spanning five decades. It found that 69 per cent of surveyed plots contained alien species, which made up 22 per cent of all recorded plant species. Forbs (broad-leaved herbs) were the most common invaders, followed by graminoids (grasses and similar) and woody plants. Yet the patterns of invasion were far from uniform.

Using boosted regression trees – a machine-learning approach well suited to ecological data – the team modelled how environmental, biotic and human factors influence both the presence and dominance of alien plants. Abiotic conditions, particularly temperature and rainfall, emerged as the strongest drivers overall, explaining up to 76 per cent of variation in invasion risk. Summer maximum temperature was a consistent predictor across all growth forms, with occupancy rising sharply above 23°C.

Human activity also played a major role. Areas with intensive land use, such as urban centres and agricultural zones, showed the highest levels of invasion. Alien forbs and graminoids were especially prevalent in these disturbed landscapes, often reaching more than 70 per cent cover in towns and cities. Alien woody plants were less widespread but still more likely to occur in urban areas than in intact forests.

Interestingly, the relationship between vegetation cover and invasion differed by growth form. Alien forbs and graminoids were more likely to occupy sites with high vegetation cover, but their proportional cover tended to decline as native vegetation increased – a sign of strong competition. Woody invaders, by contrast, were negatively associated with woody vegetation cover, suggesting that dense tree cover offers resistance to colonisation.

Spatial predictions confirmed these trends. Alien forbs had a high probability of occurring almost everywhere, even at higher elevations, though their cover remained low in alpine regions. Alien graminoids were largely confined to lowland areas dominated by human activity, while woody invaders were the most restricted, reflecting lower seed dispersal and availability and lower habitat suitability.

A global challenge


These findings resonate far beyond Australia. Invasive alien plants are among the top five drivers of biodiversity loss globally, according to the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES).

They disrupt ecosystems, alter fire regimes and threaten food security. Economic costs are staggering – estimated at more than US$400 billion annually worldwide – and rising as trade and travel expand. Climate change compounds the problem by creating conditions that favour invaders, while land-use change accelerates their spread.

Understanding invasion dynamics at scale is therefore critical for global conservation strategies.

The implications for management are clear. Maintaining and restoring native vegetation is critical to limiting alien plant dominance, particularly after disturbances such as wildfire – a growing risk under climate change. Urban expansion and agricultural intensification will likely increase invasion pressure, making strategic land-use planning essential. Grouping species by growth form, as this study does, offers a practical way to prioritise control efforts without building hundreds of single-species models.

Alien plant invasions are complex, shaped by climate, land use and ecological interactions. But by recognising both shared drivers and growth-form-specific patterns, we can design more effective strategies to protect ecosystems. Growth-form-based models provide a tractable, widely understood tool for science and policy – a step towards smarter, landscape-scale management of one of the most pressing environmental challenges of our time.

Read more:

Gregory D, White M, Catford JA (2025) Similar drivers but distinct patterns of woody and herbaceous alien plant invasion. NeoBiota 103 31–52. https://doi.org/10.3897/neobiota.103.164914

Article originally posted on KCL’s Spheres of Knowledge

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Weed or feed? New pasture plants intensify invasive species risk

Pasture plants gone rogue: Phalaris is highly invasive but new varieties are still being developed
Pasture plants gone rogue: Phalaris is highly invasive but new varieties are still being developed

To meet increasing demands for livestock production, agribusinesses around the world are breeding new varieties of pasture plants. Unfortunately, many of the plant characteristics promoted for use in pasture – higher growth rates, greater resistance to disease, higher tolerance of environmental extremes and higher reproduction – are shared by invasive species. Coupled with the fact that many pasture species are already highly invasive, this effectively means that agribusiness may be inadvertently breeding “super weeds”, which farmers then spread across the landscape.

And, just to make matters worse, this increased weed threat is going largely unchecked: even countries with leading biosecurity do not consider the weed risk posed by plant varieties that are developed within-country.

But all is not lost!

As described in a new PNAS paper led by Don Driscoll, there are various ways in which this problem can be fixed.

Read more about this issue in Nature, The Conversation and ESA’s Hot Topics or see more about it in Don’s video.

Invasive Gamba grass, planted for pasture, can increase bush fire intensity five fold
Invasive Gamba grass, planted for pasture, can increase bush fire intensity five fold

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Flow regulation and drought drive riparian plant invasion

Ecologists, like epidemiologists, are often confronted with the challenge of trying to determine causality by piecing together bits of information observed in nature. When the presence or absence of a species at a site is affected by the characteristics of the environment and community, the availability and dispersal success of propagules, stochastic events and the peculiarities of the species itself, it can be very difficult to isolate the likely mechanisms that lead to the occurrence – or lack thereof – of a particular species, especially when the influential factors are highly correlated.

The joys of being in control. (Source: thecampaignworkshop.com)
The joys of being in control.
(Source: thecampaignworkshop.com)

Experiments are obviously made for getting around such problems; by controlling and isolating one factor at a time, the relative importance of different factors can be quantified. However, experiments are not always possible, desirable or ethical. Take plant invasions along rivers, for example: they occur at large spatial and temporal scales; many factors may drive the invasion process; introducing and augmenting the supply of invasive species is unpalatable and likely prohibited; plus, river environments are very hard to control and manipulate, as any manager will tell you. So, if we are limited to potentially confounded survey data, how can we more effectively identify the drivers of plant invasion so that we know which factors to target in weed management?

In a paper recently published in Diversity and Distributions, my colleagues and I contend that incorporating data about species characteristics into survey-based approaches provides an additional line of evidence that can be used to improve inferences drawn from patterns. We illustrate how using information about environmental gradients, species distributions and species characteristics can increase understanding of ecological phenomena – here, riparian plant invasion, which can help inform management responses.

Using this approach, we find that, of four hypotheses examined, hydrological modification (indicated by flood magnitude) most likely drives invasion in River Murray wetlands. Flow regulation may inhibit native species adapted to the historical hydrological regime, facilitating exotic species with different environmental ranges. A symptom of environmental change, invasion may have been exacerbated by drought, although it is unclear why.

By hitching a ride on walkers' shoes and boots, exotic plants may even invade places like this. Columbia River Valley, Oregon. (Source: JAC).
By hitching a ride on walkers’ boots, exotic plants may even invade places like this.
Columbia River Valley, Oregon. (Source: JAC).

There was no indication that human-increased propagule pressure or colonisation ability facilitated invasion. Exotic cover was unrelated to proximity to towns, recent flood frequency and cattle grazing intensity. Additionally, similar proportions of exotic and native species were used in cultivation and, despite a higher proportion of exotics being known weeds, weed status was unrelated to exotic species occupancy. Overall, colonisation ability was unrelated to species’ origin or response to water depth and hydrological change. Although exotics had higher specific leaf area and shorter longevity (indicative of higher colonisation ability), they had heavier (not lighter) seeds and did not differ in height from natives.

Based on our findings, we conclude that (i) using environmental flows to reinstate mid-range floods and (ii) augmenting the propagule supply of native species with characteristics suitable for modified conditions may help limit invasion in these wetlands.

For more, have a look here or drop me a line and I’ll send you a copy. I’d be delighted to hear any thoughts, comments or queries that you may have.

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The divine glory of our nation’s capital

It may come as a shock to some people, but I am delighted to say that Canberra is now my home.

For those who aren’t so familiar with Australian snobbery, Canberra (along with our South Australian sister, (R)Adelaide) tends to get a pretty bad rap. Boring; nothing to do; weird; where are the milk bars?!

While I nod my head appreciably at the last comment (and, dare I say, groovy wine bars?!), the first few seem to come from people who a) have never been to Canberra, b) came once during primary school to visit Parliament House or c) have very poor taste.

My response to this:

  • fast, flowing mountain bike trails winding through beautiful grassy woodland all of ten minutes from Canberra’s CBD;
  • Two hours to the coast, two hours to the mountains;
  • And then there are the after work strolls with kangaroos, kookaburras, cockatoos, wallabies, rosellas…

    What I did at the weekend: Canberra to Kosciuszko by bike. Photo by Milly Brent

Clearly, I am a “nature lover” (surprising, I know) and Canberra offers “nature” in droves. As well as being great for one’s physical and mental wellbeing, this also presents some great work opportunities as the field really isn’t that far away. To illustrate, I’ll briefly introduce a couple of the field sites where some of my colleagues in the Fenner School of Environment and Society work.

Mulligans Flat–Goorooyarroo Woodland Experiment

MulligansExperiment fig
Response variables being studied in the woodland experiment: 1 Dead wood; 2 Birds; 3 Invertebrates; 4 Vegetation; 5 Reptiles; 6 Fungi; 7 Bettong reintroduction; 8 Brown Treecreeper reintroduction; 9 Kangaroos; 10 Small mammals; 11 Litter, soil and soil microbes; 12 Exclusion of feral pests. See link to the left for source info.

Located in a couple of nature reserves 15 km north of central Canberra, the Mulligans Flat–Goorooyarroo Woodland Experiment is a partnership between the Australian National University, the ACT Government and CSIRO. The aim of the project is to find ways of improving box-gum grassy woodland for biodiversity and the experiment manipulates and monitors a whole raft of factors (see figure above).

One of the many exciting aspects of this experiment is the reintroduction of the Tasmanian Bettong (Bettongia gaimardi) – it has been extinct from the mainland of Australia for 80 years.  Regarded as an ecosystem engineer, it will be interesting to learn what effects the Bettong has on the ecosystem.

Tumut Fragmentation Study

Based in the Buccleuch State Forest 100 km west of Canberra, the idea for the Tumut Fragmentation Experiment was sparked when David Lindenmayer was flying from Canberra to Melbourne. Peering out of the plane window, David saw an area of native forest that had been cleared for a radiata pine (Pinus radiata) plantation. Rather than just bulldozing the whole lot of it though, patches of native forest had been left. Representative of the original forest, these patches varied in size from half a hectare to 200 hectares thus providing a great way to study effects of forest fragmentation on biodiversity.

The Tasmanian Bettong (Bettongia gaimardi). Photo by JJ Harrison (jjharrison@facebook.com)
The Tasmanian Bettong (Bettongia gaimardi). Photo by JJ Harrison (jjharrison@facebook.com)

In all, the Conservation and Landscape Ecology group at Fenner run seven large-scale longitudinal field studies, all located in south eastern Australia. Long-term, large-scale ecological studies are pretty rare in Australia, yet provide incredibly valuable insights because many ecological processes occur at the landscape-scale, it can take a long time for ecosystems to respond to certain actions and it can also be very hard to detect ecological responses when background levels of variability are so high (just think of weather patterns versus climate change). A major impediment to establishing long-term studies is the fact that most grants last for only a few years. While there is increasing support and appreciation of long-term studies (the merits of which are nicely illustrated by the Long Term Ecological Research Network in the US), many researchers rely on passion, strong working relationships and cheap labour (i.e. their own) to maintain such research.

I have been in the Fenner School at the Australian National University for a few months now and I am just loving it. Although I am still employed by the University of Melbourne and retain strong links with the Quantitative and Applied Ecology research group, I will be based here for the duration of my grant and hopefully, fingers crossed, beyond that.

If you are ever in town, or are keen to visit, please drop me a line. We could even go to Parliament House.

Parliament House. Source: Milly Brent
Parliament House. Source: Milly Brent

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Managing the Murray’s water to combat invasive species

An invasive exotic species, Sagittaria platyphylla, dominating a wetland in Cobrawonga, Victoria (2008)

About this time last year, I wrote an article for H2O Thinking, a water management magazine published by eWater (until recently the eWater CRC). While the turnaround time is nothing to envy, the piece found its place on the web earlier this week.

In the article, I focus on two questions that anyone* who has spent any time along a river will surely have asked:

Why are river banks, floodplains and floodplain wetlands so susceptible to alien species invasion?

And what can we do about it?

Well, I’m not going to give the game away, but lets just say that the words “flow” and “regulation” do make an appearance. Click here for more scintillating reading (?!).

A very nice wetland near Cobram, Victoria (2007)

*OK, anyone like me would surely have asked…

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New paper: Predicting novel riparian ecosystems in a changing climate

Image
A bit of a stretch, but I couldn’t resist including this image!

I was fortunate to attend a workshop hosted by the National Climate Change Adaptation Research Facility last year that focused on riparian ecosystems under climate change. Among the various discussions at the workshop, some colleagues and I started discussing how riparian ecosystems might be affected by climate change and ways in which their abiotic and biotic characteristics are likely to change. It soon became clear that envisioning future ecosystems is no easy task, so we set about trying to come up with an approach by which to do so. 

We present our approach in a paper that has recently been published in the journal Ecosystems. Based around four recommendations, we present the approach in the first part of the paper.  We then use four case studies from contrasting environments to illustrate the approach and to determine:

–        Whether certain characteristics make some ecosystems more susceptible to climate-induced shifts in community structure than others; and

–        Which aspect of climate change seems to have the greatest effect on community structure and therefore should be a research priority.

Focusing on changes in community structure, we use qualitative process models to predict likely abiotic and biotic changes in four case study systems: tropical coastal floodplains, temperate streams, high mountain streams and urban riparian zones. We concentrate on functional groups rather than individual species and consider dispersal constraints and the capacity for genetic adaptation.  Our scenarios suggest that climatic changes will reduce indigenous diversity, facilitate non-indigenous invasion (especially C4 graminoids), increase fragmentation and result in simplified and less distinctive riparian ecosystems.

Compared to models based on biota-environment correlations, process models built on mechanistic understanding (like Bayesian belief networks) are more likely to remain valid under novel climatic conditions. We posit that predictions based on species’ functional traits will facilitate regional comparisons and can highlight effects of climate change on ecosystem structure and function. Ecosystems that have experienced similar modification to that expected under climate change (e.g. altered flow regimes of regulated rivers) can be used to help inform and evaluate predictions.

While the paper centres on Australian riparian zones experiencing climate change, the approach can be applied to ecosystems in other biomes that are subject to environmental change.

The paper is now online early; you can find the abstract and link here.

Please drop me a line if you’d like a copy.

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Project kick-off: propagule pressure, functional traits, resource availability and plant invasion

As mentioned in a previous post, I was lucky enough to be awarded one of the inaugural ARC Discovery Early Career Researcher Awards (DECRA) late last year. I officially started my DECRA research in April, so I thought it was time that I introduce it – albeit rather briefly.

In essence, I am planning to investigate the susceptibility of native vegetation edges to alien plant invasion using quantitative and experimental approaches.  The project will contain both theoretical and applied elements and will primarily examine plant invasion through a community ecology lens (or is it community assembly through an invasion lens??!).

I’ll specifically be looking at the combined (and interactive) effects of species traits, resource availability and propagule pressure on invasion success using Bayesian meta-analysis, causal modelling and a field experiment.  As stated in my grant application, “disentangling effects of alien species’ seed supply, high resource availability (light, water, nutrients) and species’ traits on invasion will indicate their relative influence on plant invasion and community assembly.  As a result, new knowledge will be gained on the efficacy of invasive species prevention and control by indicating which invasion pathways to target, and under what conditions.”

The project will run for three years and I’ll be splitting my time between Australia and the US to achieve it. The plan is to work with CEED/NERP folk on the more quantitative aspects of the project while in Australia (principally with people like Brendan Wintle, Cindy Hauser, Mick McCarthy and Peter Vesk in the QAEcology group at Melbourne Uni, but also with Phil Gibbons and David Lindenmayer at the Australian National University; more on that later). I’ll conduct the experiment at Cedar Creek Ecosystem Science Reserve in Minnesota working with David Tilman.  I’m planning to spend two months at the University of Minnesota this year (July-August) and then 6 months for the following two years (roughly April-Sept/Oct).  As a lover of warm weather, an endless summer comes as an added bonus!