Invasive ‘aliens’

THERE has been an unchecked increase in the density of Lantana camara, an alien invasive woody shrub, in the Western Ghats. “Alien invasive” is a term used to describe plants that are moved out of their natural habitats and end up exploiting the local biota in their new environment. Introduced in India in 1807, Lantana camara is originally from South and Central America and has the dubious distinction of being one of the world’s most invasive species. The growth is worrisome as the Western Ghats are a biodiversity hot spot.

A recent study has analysed the density of Lantana in relation to the factors that influence its spread in the tropical forest of Mudhumalai in Tamil Nadu. The researchers used data over a period of 18 years as many invasives are known to lie benign for an extended period and then exhibit a population explosion. The study looks at two biotic factors—competition from native plant species and the basal area of trees (as trees have been shown to limit the growth of invasives). It also looks at two abiotic factors—fire and moisture. The densities of Lantana, Chromolaena odorata (an alien woody shrub) and Helicteres isora (a native woody shrub) were monitored over the years. Interestingly, while the frequency of lower density patches of Lantana decreased over time, the frequency of very dense patches increased from two per cent in 2000 to 18 per cent in 2008, indicating that Lantana can rapidly colonise areas once its presence is above a certain threshold density.

The study, conducted by Geetha Ramaswami and R Sukumar from the Centre for Ecological Sciences, Indian Institute of Science, Bengaluru, was published on October 22 in PLoS One. While the study correlates external factors to the spread of Lantana, a shorter study by the same authors at the same plot during 2008-10 had found that contrary to expectation, Lantana did not affect densities of most woody plants. The authors believe that Lantana could also be viewed as an additional part of the landscape rather than just as an intruder. However, Ayesha Prasad from National Centre for Biological Sciences, Bengaluru, whose research suggests that Lantana does impact local species compositions, says, “Site-specific factors play an important role in influencing the outcome of such experiments.”

The long-term study by Ramaswami and Sukumar is valuable in understanding Lantana dynamics, says Ankila Hiremath of Ashoka Trust for Research in Ecology and the Environment, New Delhi. Although the reasons Lantana is such a successful invader are not fully understood, understanding the environmental factors that influence Lantana should help in better management of the plant.

Darwin’s naturalisation hypothesis states that a newly introduced species is likely to thrive in an environment where its close relatives are absent. Much data has been published on the topic, with some studies supporting it and others demonstrating exactly the opposite. A study by researchers from the University of California, Davis, US, published online in PNAS on October 14, disproves the hypothesis by comparing the genetic relatedness of invasive and non-invasive alien thistles of a single tribe (a plant classification between ‘family’ and ‘genus’) with the existing local flora in the California Floristic Province. The study found that among introduced thistles, the ones that are closely related to local flora are likely to be invasive, while the less related ones are largely benign. The issue remains unresolved. Petr Dostal from the Institute of Botany, Academy of Sciences of the Czech Republic, Pruhonice, who has looked at the impact of invasive plants over time, says that though interesting the study does not test the mechanisms driving the larger success of more related thistles.

While the debate continues, ecologists and forest managers continue to look for more effective ways to handle invasive plants like Lantana and thistles.

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