A new problem is sprouting up inside the country’s most celebrated wildlife conservation area, the Chitwan National Park.

It’s an invasive weed—parthenium.

The invasive weed is spreading at an alarming rate inside different parts of the park and if not controlled on time, it is projected to be a cause of concern for the wildlife.

According to Baburam Lamichhane, a conservation biologist, parthenium (Parthenium hysterophorus) is spreading fast in various parts of the park.

“During this year’s rhino census, we found that the weed had spread to different parts of the park, mainly in rhino habitats,” said Lamichhane, who is also the chief of the Biodiversity Conservation Centre, Sauraha, under the National Trust for Nature Conservation. “The invasive plant has reached areas where it was not seen before.”

Invasive plants are non-native species and show a tendency to spread out of control, with the potential to negatively affect the local ecosystems. Invasive alien species are considered to be one of the biggest threats to biodiversity as they cause adverse impacts on native plants and the overall ecosystem, can also cause huge economic losses and harm to the environment and human health.

According to the United States Department of Agriculture, invasive species have contributed to the decline of 42 percent of US endangered and threatened species, and for 18 percent of US endangered or threatened species these invasive species are the main cause of their decline.

According to Lamichhane, the weed has already engulfed five to seven percent of the total rhino habitats, which he says is a worrying sign.

Parthenium, also known as carrot grass, is native to North America. The annual herb can grow upto 1-1.5 metre tall, with many branches developing from its top half and an erect stem. It invades disturbed, bare areas and pastures.

“Loose and exposed soil with adequate light is favourable for its growth. A newly tilled piece of land, for example,” said Lamichhane. “During the rhino count, the grass had recently been cleared and the land was still dry, so this invasive plant had not fully developed. When we clear bushes as part of grassland management or for making trails or roads, it lays the ground open creating a favourable condition for its unrestrained growth.”

According to Lamichhane, one parthenium plant can produce around 30,000 seeds and that can easily spread out in all directions, resulting in its massive spread.

Once it spreads over a large area, it can cause massive damage to the local environment and other native species.

In 2000, parthenium was identified as one of the ‘100 most invasive species in the world’ by the IUCN. Since then, it has become a serious agricultural and rangeland weed in parts of Australia, Asia, Africa and the Pacific Islands, where it affects the production of crops and livestock.

Studies have shown that parthenium has emerged as a major weed in field crops in over 45 countries and in Australia it causes yield losses worth millions of dollars every year.

As per estimates, parthenium costs the Australian beef industry $16.5 million and cropping industries several million dollars per year.

In Nepal, this is not the first time the weed has been reported inside the Chitwan National Park.

“The first time the weed was reported was in 2012-13. But at that time, the weed was found in small patches of land, mostly along the roadsides,” said Lamichhane. “So it was not a big headache then. But since last year, it’s growth has been rapid. Now, it has emerged as a serious problem in the park.”

Before parthenium, Chitwan National Park had to deal with another alien invasive species—Mikania micrantha. The species had grown so aggressively that it was considered one of the major challenges for rhino conservation in the Chitwan National Park.

At one point, Mikania micrantha, which was first reported inside the park in 1963, was responsible for 44 percent degradation of rhino habitats inside the Chitwan National Park. This weed suppressed the growth of fodder grass and prevented the regeneration of trees, according to a 2o16 study.

In 2010, then prime minister Madhav Kumar Nepal had travelled to Chitwan to take part in a Mikania micrantha uprooting drive.

“Mikania micrantha has somehow come under control in recent years. The invasive species is naturalised in the local environment where it is under attack from other insects and fungi, slowing down its fast growth,” said Lamichhane. “Besides, it also has to compete with other local plant species for its survival, therefore its invasiveness has decreased. In 2008, 2011, 2015 and 2021, when we conducted rhino censuses, we found that the spread of Mikania micrantha had reduced markedly.”

During the latest rhino census, we found that mikania cover had come down to around 35 percent of the rhino habitats.

However, experts like Lamichhane fear parthenium can cause similar damage and havoc if not contained on time.

“Grasslands provide food for the wild animals. However, when native green plant species have to compete with invasive plants like mikania or parthenium, these invasive plants win,” said Lamichhane. “Such a situation means no food for the animals as they don’t feed on such weeds.”

Chitwan National Park, which is home to Nepal’s biggest rhino population, has managed to increase its rhino numbers in the latest rhino census that concluded in April. The one-horned rhino population inside the park stood at 694, an increase of 89 rhinos from 605 rhinos in 2015. The growth has been attributed to conservation efforts and improved habitats among other factors.

The existing seven percent coverage of parthenium may look trivial for now but it can emerge as a problem similar to mikania in future if not managed properly, warns Lamichhane.

“Uprooting is the only way to get rid of the weed. But that would not be possible for large areas. Therefore at least in the animal habitats and grasslands, the weed should be uprooted without any delay. To remove the weed from larger areas, a special plan should be worked out.”