La Trobe University botanist John Morgan is a member of a 36-strong global research team that recently shot holes in one of the most common theories about how invasive species establish themselves at the expense of native plants.
The work, published in the journal of Ecology Letters, has significant implications for weed risk assessment and biosecurity.
Dr Morgan’s part of the study was carried out in alpine grasslands at Falls Creek in Victoria and was the only Australian field site in the global initiative.
Invasive plants are a serious environmental, economic and social problem. They lead to loss of native biodiversity and damage the functioning of ecosystem, such as the re-cycling of nutrients.
With climate change, gaining knowledge about plant spread and behaviour has become more urgent.
Dr Morgan says predicting the success of invading species has always relied on the assumption that these plants are more abundant in their new settings than they are in their native communities because they behave in a special way.
One example is the highly invasive weed English Broom. This has infested large parts of eastern Australia and New Zealand.
‘It was thought the absence of natural enemies that kept the species in check in Europe, combined with its ability to fix its own nitrogen, gave it competitive advantages over the native species in its new range,’ he says.
Instead, the global research team’s comprehensive ‘Home and Away’ study, of 26 species at 39 locations on four continents, found little difference between plant numbers on their introduced and their native ranges.
‘We discovered that increases in species abundance are, in fact, unusual,’ says Dr Morgan.
The study was carried out in New Zealand, Switzerland, the USA and UK, as well as in Australia, Canada, China and Germany.
It is part of a cooperative global experiment called the ‘Nutrient Network’ which comprises 51 sites dominated by herbaceous plants.
Dr Morgan says many of the invasive plants studied – 12 grass and 14 broad-leafed species – had been introduced for social, economic and environmental reasons, such as soil stabilisation, pasture improvement, horticulture and agriculture.
All but three were now officially classified as weeds by their new host countries.
However, only a few species were found to conform to the ‘abundance assumption’. Grasses tended to be fairly abundant both at home and away, while broad-leafed plants were generally low in abundance, or even lower, away than at home.
The study also challenges another very common assumption about introduced plants – namely that they thrive and grow bigger in their new ranges. It found that many species were actually larger at their native sites.
Dr Morgan says Australia’s most widely used biosecurity screening process – the Australian Weed Risk Assessment – takes into consideration a species’ history, climate and distribution, weed status elsewhere, undesirable traits, biology and ecology.
‘On the basis of our findings, the success of a plant on its native range is probably a good indicator to predict its spread at introduced sites – a criterion which currently is not included in our biosecurity screening program,’ he says.
The study was carried with funds from the US National Science Foundation’s Division of Environmental Biology.