In 2018, biologists observed the invasive plant Parthenium, known locally as congress grass, establishing itself in grasslands of India’s Pobitora National Park.

Invasive species threaten protected areas in Assam state, and herbivores like the greater one-horned rhinos that live within them, by crowding out the native plants animals rely on for food.

Each of Assam state’s four rhino reserves currently faces threats from invasive plants including Parthenium, Mimosa, Mikania and water hyacinth.

Experts are contemplating the use of several strategies to tackle invasive plants, including manual removal and the introduction of biological control agents such as the Mexican beetle that feeds on Parthenium.

On a gloomy afternoon in May 2018, conservation biologist Bibhab Talukdar was visiting Pobitora National Park in India’s Assam state. As he rode an elephant into the grasslands at the core of the park he was met with an alarming sight: a luxuriant growth of a low, bushy herb with small green leaves and creamy white flowers. Locally known as congress grass, this plant, Parthenium hysterophorus, is native to the Americas and an alien species in India.

Talukdar, the chair of the International Union for Conservation of Nature’s (IUCN) Asian Rhino Specialist Group and CEO of the NGO Aaranyak, says he was worried because Parthenium is a highly invasive plant with a history of damaging local ecosystems in more than 20 countries around the world. Australia is the prime example, with vast swaths of native rangelands and summer crops under pressure from the weed.

If its spread in Pobitora isn’t checked in time, Talukdar says he believes Parthenium could spell trouble for the park’s 102 greater one-horned rhinoceroses (Rhinoceros unicornis) by smothering the native species they feed on.

It’s not clear how and when exactly Parthenium arrived in India. The popular theory is that the seeds arrived in the 1950s, piggybacking on consignments of grains imported as part of a U.S. government food-assistance program. The weed was first reported growing on trash heaps in 1956 in Pune, in the state of Maharashtra.

Although Parthenium was first spotted in Assam in the 1980s, it had not previously been reported in the core areas of Pobitora. Nine years ago, when Talukdar visited the park as part of a team of researchers that studied the status and distribution of invasive plants in the rhino habitat, such Parthenium growths weren’t observed. “Its arrival in the park seems to be recent,” he says.

Silent stranglers

Parthenium is just the latest addition to half a dozen invasive plants colonizing rhino ranges across Assam, the Indian state that harbors more than two-thirds of the world’s one-horned rhino population in four protected areas.

According to a 2011 article in the journal Pachyderm, the already established invasive plants in Assam’s rhino ranges are Mimosa invisa, Mikania micrantha (the “mile-a-minute” vine), Chromolaena odorata (Siam weed) and Ipomoea carnea (pink morning glory). All were identified in India’s Fifth National Report on Biological Diversity in 2014 as invasive plant species responsible for harming local ecosystems.

“You’ve got to add Parthenium, Eicchronia carssipes [water hyacinth] and Lantana camara to the list,” says Iswar Chandra Barua, a professor of agronomy at Assam Agricultural University in Assam’s Jorhat district.

Barua, who has been studying the dynamics of phytoinvasion in Assam for over a decade now, says all four of Assam’s rhino reserves — the national parks of Kaziranga, Orang, Pobitora and Manas — are currently reeling under attack from these invasive plants.

Each presents a unique danger, Barua says. Siam weed and water hyacinth are “driver species,” capable of decimating native plants without any assistance from accompanying environmental change; Parthenium and Mimosa possess tremendously high competitive ability, known as allelopathy, which inhibits the growth of indigenous plants in areas colonized by these weeds; Ipomoea stems form thick mats that block natural water flows, making it the most troublesome aquatic weed after water hyacinth, which is notorious for clogging water bodies; Lantana exudes poison from its roots, killing off native flora; Mikania produces around 40,000 seeds every year and colonizes forest areas at a very rapid pace by choking and smothering grasses and tree saplings.

“Hundreds of indigenous plants, including the rhino’s favorite fodder species, are under direct pressure from these invasives,” Barua says.

Different parks, different problems

In Kaziranga National Park, home to 2,413 rhinos per the 2018 census, Mimosa can be seen overrunning natural grasslands in several ranges. However, the situation has improved since the early 2000s, when the problem was more acute. Barua credits the natural floods that have hit the park in recent years. “During the annual flood, the entire park remains inundated for days and sometimes weeks. This works somewhat as a natural check on the growth of Mimosa,” he says.

However, a potential threat lurking on the park’s doorstep is Ludwigia peruviana (Peruvian water primrose). A 2017 study led by Barua found that this semi-aquatic weed has damaged already around 200 square kilometers (77 square miles) of marshland plant communities in Karbi Anglong district, where part of Kaziranga is located. He says Ludwigia will likely infest Kaziranga marshlands if no measures are taken to check its spread. “Kaziranga National Park is located pretty close to Ludwigia-infested areas. As such it could just be a matter of time for the weed to sneak into the park’s marshlands. If we don’t step up vigil now, it could be all over inside Kaziranga in a couple years.”

In Orang National Park, home to 101 rhinos, Mimosa appears to be the most severe current threat to the ecosystem. From 1987 to 2008, Orang saw its wet alluvial grassland shrink by 12.8 percent, and dry savanna grassland and degraded grassland increase by 9.25 percent and 6.51 percent respectively, mainly due to the impact of Mimosa, according to a 2011 study on land cover change in the park.

Manas National Park is also experiencing a phytoinvasion, with Chromolaena and Mikania infesting large patches of forest. With a density of 9.4 to 15.1 plants per square meter, according to a 2004 survey, Chromolaena is overrunning areas of already-degraded vegetation along the southern border of the park, while Mikania is colonizing riverine grassland patches and forest edges.

In Pobitora National Park, the biggest threat is Ipomoea, primarily a colonizer of grasslands. The spread of Ipomoea inside the park has sparked competition among indigenous grass species and the weed for space and nutrients. In addition, as Talukdar observed, Parthenium is an emerging challenge. Over the last year, the weed has been seen spreading alarmingly along the park’s grassland areas.

Effects on rhinos

Rhinos are already suffering as Assam’s ecosystems are hit by these invasive plants. “First, they choke and smother native plants on which rhinos feed, leading to shrinkage of fodder in protected areas,” Talukdar says. “Second, when species such as Mimosa, which is thorny, take over grasslands, it becomes difficult for rhinos to graze, which pushes the animals to stray out of the safety of the park and thus makes them vulnerable to poaching.”

Recognizing these threats, the New Delhi Declaration on Asian Rhinos 2019, signed by five Asian rhino range nations on 28 February at the Indian capital, has mandated research on invasive species threatening rhino reserves along with other habitat parameters.

A 2015 report by the Wildlife Institute of India (WII) holds invasive plants responsible for the increase in rhino straying incidents in Pobitora. The report says the rapid growth of invasive plants including Ipomoea, and the accompanying depletion of native fodder species in the park, has led to an increase in crop-raiding incidents by rhinos in villages on the park’s fringes.

“Because the invasive weeds are choking out grasslands, rhinos are increasingly straying out to forage on crops in neighboring farmlands,” says Mukul Tamuly, the forest range officer at Pobitora.

Moreover, some invasive plants contain toxic compounds that can harm rhinos and other herbivores that ingest them. “Lantana camara, for example, contains pentacyclic triterpenoids, a hepatotoxic compound known as lantadene. If rhinos feed on lantana leaves, it could inflict significant liver injuries on them,” Tamuly says.

Fighting invasive weeds

Plants that become successful invaders are fast-growing and highly adaptable to new environments. When they are transported to ecosystems where their natural predators are not present, such species can rapidly overrun indigenous plants.

“Take, for instance, the case of Parthenium,” says D.J. Rajkhowa, a senior agronomist and joint director of the India Council of Agricultural Research (ICAR), Nagaland Center. “In alien environments such as that of India it is free from its natural enemies like the Mexican beetle, Zygogramma bicolorata, that feeds only on Parthenium leaves. It gives the weed a competitive edge and more resources to grow, reproduce, spread and eventually outcompete indigenous plant species.”

Due to their higher phenotypic plasticity — the ability to develop differently in different environments — invasive plants can adapt to environmental fluctuations. This helps them spread and establish in new areas. Rajkhowa cites the example of Mikania, which is now seen growing in a range of habitats from terrestrial to littoral.

Moreover, many invasive plants are cryptic in nature and have the ability to go undetected for years, or their damaging impacts are not immediately clear, making it difficult to control these species.

Given these challenges, there are three methods to control invasive plants: mechanical removal; spraying chemicals; and using biological control agents.

The mechanical removal method, which involves hand weeding, is the simplest way, but is labor-intensive, expensive and has to be repeated consistently. It is widely used in Assam’s rhino reserves, but has yielded little results. “We keep uprooting Mimosa plants at regular intervals, but they sprout up again at a very rapid speed,” says Chakrapani Rai, the forest range officer at Orang National Park. “We’ve also applied controlled burning. Even that’s turned out to be futile. The weed’s spread seems to be unstoppable.”

Spraying chemicals is considered unviable in protected areas as it could have disastrous effects on wildlife and the ecosystem.

The biological control method, a far more complicated process that entails releasing organisms that feed on the targeted species, is touted as a less dangerous and potentially effective method. India’s Project Directorate of Biological Control (PDBC), a government agency based in the city of Bengaluru, is tasked with the responsibility of ensuring the hazard-free introduction, handling and release of biological control agents.

Attempts to use biological controls against Parthenium have achieved mixed results in India: the biocontrol agent for the plant, the Mexican beetle, has shown successful results in Bengaluru, but has failed in Delhi.

Rajkhowa says the best way to deal with the problem is to develop a plan that combines all available methods. “In developing an integrated weed management plan, biological control should be the key component. Till suitable biocontrol agents are found, the emphasis should be on mechanical removing.”

So far biological control agents haven’t been used in Assam. But Rajkhowa says he thinks it’s a strategy that should be considered. “Given the state’s moderate climatic conditions, it might just work.”

But the biocontrol method has its own risks: an organism introduced as a biocontrol agent to contain a particular invader may decimate other species as well, turning into an invader itself. The most infamous example of biocontrol going wrong is Australia’s cane toad catastrophe. Cane toads were introduced in 1935 to control cane beetles in Queensland’s sugarcane crops. But in the span of a few decades they spread all over Australia, causing severe damage to local ecosystems.

Assam Agricultural University’s Barua says he favors exploring other alternatives. While he, too, endorses the idea of an integrated weed management plan, he says such efforts should prioritize exploring ways to use invasive plants as raw materials for local industries.

There has been some success on this front. Water hyacinth has been used to make an array of products such as mats and bags; the craft now employs about 3,500 local artisans in northeastern India. Silkworm rearers in Karbi Anglong have started using Mikania as a host plant for Eri silk, for want of the primary host plant, castor (Ricinus communis). Research shows there is also potential to commercially exploit Mikania to this end, which could in turn help control the weed. Bodo tribal weavers near Manas National Park are experimenting with using Chromolaena as a natural dye, while lantana is being used as a raw material for furniture — an initiative that has seen some success in controlling the weed.

“If such initiatives find commercial footing,” Barua says, “they’ll not only contribute toward effective weed management but also boost local livelihoods.”

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