Invasive plants are a problem around the world, but are they just a nuisance or are they killers? So far there are no documented cases of native plants becoming extinct purely because of an alien plant invasion.

However, researchers Paul Downey and David Richardson argue in a paper published this month in AoB PLANTS, 'Alien plant invasions and native plant extinctions: a six-threshold framework', that traditional methods of modelling extinction do not work well for plants. Focusing purely on extinction can distract plant conservationists from growing problems. Instead they propose six thresholds that species cross before they become extinct.

"The main reason why there is no clear evidence of extinction that can be exclusively attributed to plant invasions is that invasions have not been around long enough" said Dave Richardson of the Centre for Invasion Biology (C-I-B) at Stellenbosch University, South Africa. "Our research shows that plant extinction is an agonizingly slow process" he adds.

"However, red flags are evident in numerous locations around the world - species that now exist in fragmented populations, with radically reduced opportunities to reproduce".

It's this difference in timescale between human or animal activity and plant activity that the authors identified as a key problem in traditional models of extinction.

According to Dave Richardson, "There's often a substantial time lag between the introduction of an alien species and the manifestation of impacts attributable to invasions of that species.

This results in an 'extinction debt' - effects that place species firmly on a trajectory towards extinction that takes time to become obvious. Whereas many invasive animals (notably predators on islands) have caused extinctions of native animals very soon after the invaders arrived, processes leading to extinction in plants are orders of magnitude slower."

"Also, proving that every last individual of a plant species has been lost (i.e. gone extinct) is extremely difficult, especially for species which have long-lived seed banks in the soil, or can regenerate from underground structures.

"The extinction of a species is the result of a sustained level of threat across the entire distribution of a species, over a prolonged period. Demonstrating that alien plants cause the extinction of native plants requires a series of conditions to be met, many of which are either not measured, or have not been examined for long enough."

The study by Downey (Institute for Applied Ecology, University of Canberra) and Richardson reviewed the evidence for impacts of alien plant invasions on native plant species worldwide. They identified six thresholds along the extinction trajectory of plant species affected by invasive plants.

1.Plants die quicker than they can be replaced by their offspring in some locations.

2.Plants disappear from some locations entirely, but potential offspring remain as'propagules', seeds or spores that could regenerate a new cohort of individuals.

3.Some locations lose both individual plants and their propagules. With no plants or seeds, this is a local extinction.

4.The last locations hosting a species lose their individual plants, but in some places seeds or spores remain in the soil.

5.The species is entirely lost in the wild with no individuals or propagules. The only survivors are held in botanic collections.

6.Extinction. The remaining plants are lost, and the remaining seeds or spores are no longer capable of becoming new plants.

Although Downey and Richardson found no evidence for extinctions driven solely by plant invasions, they found abundant evidence for progression along the extinction trajectory driven by the effects of invasive plants. Using these thresholds as guides could help conservation efforts massively.

"If we wait until we have sufficient evidence to show that extinctions are occurring, it will be too late to save a great number of species", Paul Downey explained.

"Also, land managers need to know when to undertake alien plant control to protect declining native plants before it is too late."

Moving to a plant-based model for plant extinctions will mean changing what kinds of data scientists collect in their fieldwork.

"The results of this work show that we must shift attention away from the end point of the extinction trajectory when assessing the impact of factors such as invasions on native plants, to give due consideration of the full series of processes that drive declines of populations of native species" said Downey.

"We need a radical overhaul of the indicators used to track the impacts of plant invasions".

Richardson added, "Using only information on extinction rates when assessing the impacts of plant invasions on native plant species is extremely short-sighted. There is absolutely no doubt that alien plant invasions are eating away at native plant biodiversity. Many native plant species - probably HUNDREDS of species - are precariously close to being functionally extinct and survive as the "living dead".

You can read the full paper free at AoB PLANTS