Researchers have developed a new scientific method to estimate the time lag between the introduction of an alien species and its extensive spread. For New Zealand weed species, the study estimated that the average lag was around 20-30 years. Although this may seem to be a long time, seemingly rare but fast-spreading species could become tomorrow's harmful weeds.
Invasions of alien species tend to follow a pattern: once introduced into a new region, the species remains at a low level for several years followed by a phase during which the numbers increase markedly. This time it takes for the alien species to start increasing or spreading is known as the 'lag-phase'. Scientists have proposed several explanations for the lag-phase, including genetic adaptation, but there are concerns that it could simply be due to biases in data sampling. For example, once an alien species is considered a potential invasive threat, a subsequent increase in search effort might make it appear as if there has been an increase in the species. A great deal of research has identified lag-phases, but little has been done to investigate if they are sampling artefacts.
The research was the first attempt to develop a statistical model that quantifies the lag-phase and subsequent spreading phase in plant invasions. The approach was described as 'piecewise', in that it used different statistical models for the different phases.
The research quantified the lag phase of real plants by applying the model to 105 weeds from a herbarium (collection of preserved plant species) in New Zealand. Nearly all the weeds demonstrated a lag-phase, suggesting that it was not an artefact of sampling. The lag phase averaged around 20 to 30 years; four per cent of species had lag-phases greater than 40 years with some extending for nearly 100 years. Examples of species with the longest lag phases were Scotch broom (97 years) and common elder (93 years).
There was a strong relationship between the rate of spread of species and the number of recorded incidences of the species, proving that rapidly spreading species tend to become established rather than dying off after a quick growth. This means that some alien species whose numbers are currently at a seemingly low level could become harmful weeds once they have passed through the lag phase and if they have a high rate of spread. Rate of spread could provide an early warning indicator to identify potentially problematic species. However, its use may be limited since, by the time sufficient evidence has been gathered to quantify rate of spread, the species may already be widely distributed.
The researchers pointed out that the lag-phase in the study was shorter than other estimates of lag-phases of weeds in Australia and Germany. This could be due to a number of reasons. Firstly New Zealand weeds are mainly herbaceous species that have shorter lag-phases than more woody species found in other countries, and secondly, the existence of maximum lag-phases are constrained by the fact that many plant introductions only occurred in the last 150 years when Europeans colonised the region.
Source: Aikio, S., Duncan, R.P. & Hulme, P.E. (2010) Lag-phases in alien plant invasions: separating the facts from the artefacts. Oikos. 119:370-378.