What makes a species 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 -- say, herbivores in the case of plants -- gives the non-indigenous species more resources to grow, reproduce and eventually out-compete native species. In this context, the case of the Chinese tallow tree (Sapium sebiferum) is extremely interesting: as it evolved in its introduced range, it began to show competitive ability far greater than it exhibited in its original habitat.
Indeed, invasive species can create and manage brilliant strategems to beat the competition. When a pathogen's host is already there, as mosquitoes were for the West Nile virus, it can tip the odds in favour of the invader, says Daniel Simberloff, professor at the University of Pennsylvania, USA.
A species may thrive because its natural enemies don't exist in the habitat it has arrived into. It may be adept at colonising a disturbed area, or quickly take over the fresh dirt turned up at a construction site, or a roadside. Some species sit around innocuously for years, only to then unleash themselves upon the ecosystem. The rogue Lantana camara exudes poisons through its roots, killing off its competitors. It also uses the colours of its flower petals -- yellow and red -- to advantage. As Uma Shanker, professor at the University of Agricultural sciences, Bangalore, explains, "Butterflies have a very high fidelity in moving from one flower type to the other. A butterfly visiting a red flower may not visit a yellow flower, but would only visit another red flower. Adapting to avoid this phenomenon, different lineages of Lantana have evolved." Other invasives go piggybacking. In the eastern US, non-native variety of fish aid bullfrog invasion by eating native dragonfly nymphs that would normally prey on bullfrog larvae. Indeed, evidence of such "aiding and abetting" between two non-native vertebrates is conclusive, according to a US Geological Survey study published in the journal Ecology Letters.
"Gauging the potential invasiveness of species is usually based upon comparing geographic origin, climatic and latitudinal ranges, taxonomic relations, past invasive performances, and assessing their reproductive biology and general ecology. But [such studies] are often erroneous because we do not appreciate the role of mutual interactions between species," says Uma Shanker. Shanker's colleague K N Ganeshaiah has recently completed a study to predict the movement of the deadly woolly aphid (see box on previous page: Wanted: more predictive research).
Sucessful invasion depends not only on the traits species have, but also on how susceptible the habitat is. Generally, islands face the highest risk. "Three generally accepted models to understand this susceptibility have emerged -- the fluctuating resource availability model, the enemy release hypothesis and Niche opportunity model," points out Babu. The first proposes that invasiveness depends on excess resources (energy) being available to be consumed. Such availability fluctuates over time; so, therefore, does a habitat's vulnerability to invasion. The enemy release hypothesis suggests that when a plant species is introduced to an alien habitat, herbivores and other natural enemies pay it a little less attention. So it is able to literally bloom. This hypothesis is predicated on three principles: natural enemies are important regulators of populations; they impact native species more than exotics; and plants capitalise on less regulation by the enemy. Lastly, the Niche opportunity model defines conditions that promote invasions: interactions between resources, natural enemies and the physical environment, and how these vary in time and space.
Whatever the path, an invasive plant invariably alters the habitat, affecting its health. It draws in more nutrients and moisture, so out-competing native species. It paves the way for further invasions by microorganisms, agricultural pests and human pathogens. The worst cases result in large-scale environmental transformation. Chemicals released from their roots or leaf litter may change the soil's chemistry; the chemical nature and decomposition rate of the leaf litter and other dead parts may alter the habitat's nutrient cycle. "Invasion by fast-growing tree species in catchments as well as aquatic weeds in water bodies drastically reduce the flow of streams and rivers, as in the case of water hyacinth which has infested water bodies in Assam alone," says R Ramani, senior scientist, Directorate of Biological Control, Bangalore.
Animals can be as incapacitating. The zebra mussel is today considered one of the worst aquatic invaders in the US. Originally found in Russia, it came to the US floating in the ballast water of a transatlantic freighter. In less than 10 years, it has spread to over 100 lakes. Able to live and feed in different aquatic habitats, this mussel is a prolific breeder (each female produces one million eggs per year). Its larva is the size of the diameter of a human hair, and so can spread easily via water currents. Adult zebra mussels are larger than two inches in length, and attach themselves to hard objects. They often stick to boats, and so travel from one water body to another, clogging municipal, industrial and power plant water supply systems, damaging boat engines and fouling boat hulls, killing native molluscs and eliminating fish spawning beds.
Damage by invasives is both ecological and economic. It's a challenge the world is waking up to, slowly.
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