"A successful invasion is a rare event,' says Suresh Babu, a researcher at the School of Environmental Studies, University of Delhi. An accepted thumbrule is that only one in 10 introduced species become naturalised, and only one in 10 among the latter actually turn invasive . This is because, as Babu puts it, "A species arriving in a new community faces a series of filters.' (see flowchart: What it takes to be invasive) Invasion usually occurs in three stages. A species is first transported to a new habitat. It must then establish itself there: a base population must manage to successfully reproduce itself. Naturalised in this way, the species population explodes and so turns invasive.
Even so, what makes for a successful invasion? Out of 250 species in the genus Mikania, how is Mikania micrantha capable of smothering new habitats? Imported and introduced, reportedly, in West Bengal in the early 20th century, it was initially used in Assam to cover airstrips during World War II. It then moved from there and now is the biggest threat to tea plantations, apart from badly affecting pineapple, banana, ginger, acacia and rubber plantations. How is it able to do so?
For one, this mile-a-minute weed could possesses what scientists call phenotypic plasticity. As O R Reddy, joint director, plant pathology, plant protection quarantine, Union ministry of agriculture, explains, "It helps the invader to have resilience to environmental stress, the ability to adjust to different environmental conditions like prosipis juliflora displays. It shows wide range of spread across India.' Greater genetic variability also helps. As Babu outlines, a plant must have the following traits to become invasive:
It has long-lived seeds for discontinuous germination
It grows rapidly from vegetative to reproductive stage
It is capable of very high seed output when environmental conditions are favourable
It produces seed continuously throughout the growth period and in a range of environmental conditions
It is built to disperse seeds over short and long distances
It has a strong potential to compete with other species
Usually, if a species is present in small numbers, its breeding options are low. This genetic bottleneck might cause it to become extinct. But invasive species manage the bottleneck with flourish, and after establishment can explode in numbers. New research published in the journal Conservation Biology suggests a factor called propagule pressure might be crucial to "invasiveness': the number of individuals introduced, and the number of times a population is released into the new habitat.
But how is a species able to outperform locally adapted, indigenous species? R M Callaway, in a paper published in the journal Science, provides some answers. First, some species may be intrinsically better competitors because they evolved in a more competitive environment. Second, the absence of enemies