A team of Indian ecologists has found that completely eradicating the common weed, Lantana camara, one of the world’s worst invasive plant species, is going to be much harder than previously thought. This study finds that removing lantana poses a major challenge in the Indian landscape owing to its easy dispersal by fruit eating birds. A group of researchers from the Nature Conservation Foundation, Wildlife Institute of India, Jawaharlal Nehru University, Indian Institute of Science from India and CSIRO Land and Water, Australia jointly conducted the study.
Today, Lantana camara is ubiquitous in India. But it wasn’t until the 1800s, that it was introduced to India as an ornamental plant. This plant, with pretty pink and orange flowers has soon turned into a scourge. A hardy plant that adapts to a range of conditions, Lantana also breeds prolifically. It is avoided by plant eating animals and is also known to affect the growth and survival of native plant species. As a result, it has quickly become established as an invasive species, and poses a threat to agriculture and the biodiversity of India’s forests. Efforts to manage the spread of lantana involve uprooting or burning the plant.
The researchers undertook a systematic study of Lantana growth in two forests, the Biligiri Rangan Temple Hills Tiger Reserve and the Rajaji National Park, where forest personnel extensively manage Lantana invasion. Knowing that the Lantana plants bear fleshy fruits, which attract fruit eating birds to disperse the seeds, they then proceeded to examine if birds could fly the distances needed to disperse the lantana seeds from an area where it grows wild, to an area where it has been removed.
The answer was determined by the time taken for the seeds to pass through the bird’s gut once it has been eaten, and the distance the bird could fly from the spot where it ate the seed. If this spot was close enough, then the chances of the seed being ‘dropped’ by the bird into a cleared spot would be very high, depending on how long a seed could be held in its gut.
Through hours of patient observation, the team counted the different species of birds that visited Lantana bushes and those that ate the fruits. Among the feathered visitors, Bulbuls of the Pycnonotus genus were found to spend the longest time on bushes and also consumed a relatively large number of fruits. The researchers then fed a single bulbul bird with Lantana fruits for a controlled period of time and measured the time the seeds took to pass through the bird’s gut. Sometimes, obtaining good scientific data involves poking around in bird droppings!
To calculate the distance these birds could fly, the team tagged two fruit-eating species of birds commonly found amongst Lantana bushes with radio collars - a red-vented bulbul and two red whiskered bulbuls. Combining the readings from observed data and computer based modeling; they calculated if birds could indeed pass the seeds to distances sufficient to recolonize areas where lantana had been removed.
The calculations show that, about half of the number of seeds eaten by the bulbuls could be carried to a distance of 30.2 meters. A majority of the seeds were displaced within a distance of 170 m from the source plant. This distance is far more than the median distance between the source plants and the managed areas, which is a mere 20.6 meters.Thus the data shows that birds could indeed carry the seeds from the source plant to managed areas, where they could recolonize the source plant.
The researchers opine that locally controlling the Lantana population, rather than its total eradication would prove to be a more realistic goal, given the current scenario. Geetha Ramaswami, one of the authors of the study says, “our study shows that the movement pattern of bulbuls affects how far Lantana seeds reach. Most of the managed areas are going to be bang in the middle of the dispersal range of these seeds. In order to ensure the control of L. camara, one needs to account for how far it gets dispersed, and continue management action for as long as source plants and their dispersers exist!”
About the research:
This article is based on the paper “Dispersal by generalist frugivores affects management of an invasive plant”, which appeared in the journal Biotropica in July 2016
Authors: Geetha Ramaswami, Monica Kaushik, Soumya Prasad, Raman Sukumar, and David Westcott
Corresponding author: Geetha Ramaswami; e-mail: firstname.lastname@example.org