- Around 65 per cent of India’s natural areas are suitable for invasion by alien plant species, according to the National Tiger Conservation Authority.
- A new study suggests that megaherbivores weighing over 1,000 kilograms, can help improve biotic resistance and mitigate alien invasions in ‘midproductive’ ecosystems, like floodplains.
- Experts say more detailed studies are needed to assess which invasive species megaherbivores feed on, to learn more about the kind of control they exert over an ecosystem.
This year’s tiger census report, published by the National Tiger Conservation Authority, carried a worrying statistic – 22 per cent of India’s natural areas were found to be invaded by high-concern alien invasive plants, and another 65 per cent was found to be suitable for invasion in the future.
Alien plant invasions are a chronic problem, not only in India but across the world. Alien invasive plants reproduce rapidly and outcompete native species, causing biodiversity loss and altering ecosystem functioning. Once invasive species are embedded within a system, removing them is challenging and expensive. “Invasives are by definition very good at spreading. A lot of the time, invasive species form alliances with native dispersers, which helps them spread quickly,” Ankila Hiremath, adjunct senior fellow at Bengaluru-based Ashoka Trust for Research in Ecology and the Environment (ATREE), explained. The NTCA’s report estimates removing alien invasive plants of high concern will cost at least Rs one trillion.
A common way invasive plant species like Lantana camara are dealt with, especially in protected areas, is by uprooting and burning them – a human-intensive process that is both time-consuming, expensive, and difficult to scale in areas overrun by the species. Other methods involve biological control – using insects and other organisms to “control” invasive populations, and chemical control with the use of herbicides.
A new study by Indian and Danish researchers suggests that large herbivores weighing over 1,000 kilograms, such as elephants, rhinos, wild water buffaloes, and gaurs, can offer a solution in the management of alien invasive plants in certain contexts. “Megaherbivores have historically shaped vegetation patterns, and they’re uniquely positioned to do this because they’re not regulated by predation,” said Ninad Mungi, an ecologist and researcher at Aarhus University, and lead author of the paper. The paper observed interactions between native and invasive plants with megaherbivores across 12 ecoregions in the country, placing camera traps in 26,838 places.
The study found that across all ecosystems, areas with megaherbivores had on average 13 per cent higher native plant cover and 17 per cent lower invasive cover compared to areas without them. But these results aren’t uniform, and the complex interactions between large herbivores and plants call for greater scrutiny before a solution can be proposed, experts say.
Findings of the report
Large herbivores can influence plant growth in an ecosystem through various behaviours, including uprooting and trampling on plants and consuming them in large quantities. Unlike their smaller counterparts, large herbivores are more likely to be tolerant of alien species in their diets because of the sheer amount they need to consume, the study says.
Their consumption and trampling habits can spur feedback loops depending on the level of invasion already occurring in an ecosystem, the study suggests. Large herbivores may not be able to effectively control alien invasions in systems where native plant dominance is already diminished, since such settings don’t offer the kind of nutritional diversity needed to support their diets. This, the study says, can trigger an “alien plant release feedback,” where the declining forage resources could further reduce the ecological carrying capacity of herbivores, and promote invasive alien plant growth.
On the other hand, large herbivores can also reduce alien plant invasions because of their eating habits, particularly in systems with native plant dominance. “Abundant and diverse native plants could in turn promote megaherbivore populations, thereby establishing an ‘alien plant control feedback,” says the study.
The study looked at 12 tropical and subtropical ecoregions in India and divided them into four categories: Evergreen and semi-evergreen forests (considered ‘productive’ ecosystems), floodplains and deciduous forests (moderately productive), dry savannahs (dry and less-productive systems) and those that are severely invaded by alien species.
The researchers then observed the relationships between megaherbivores, native plants and invasives across these landscapes. They tested whether ecosystems with higher megaherbivore abundance were richer in native plants and less invaded, and if the relationships between megaherbivore occurrence and native versus alien plant patterns “were influenced by the type of ecosystem, the level of protection and various other ecological factors.” It also sought to test whether the general influence of megaherbivores on plant richness and cover varied across the ecosystem productivity and whether the traits of invasive plants themselves affected the relationship between megaherbivores and plants.
“We found that human disturbances are the most defining trait for the presence and prevalence of invasives. Areas with competition from native plant species with a high density of natives had fewer invasives, like in wet evergreen and rainforests,” said Mungi. “Across all these landscapes and conditions, our study found that megaherbivores seemed to have the biggest impact in moderately productive ecosystems, like savannahs and floodplains.”
In the Brahmaputra floodplains, the study found that average native plant cover was on average 37 per cent higher, even reaching up to 63 per cent in the presence of megaherbivores, while invasive cover was 52 per cent lower, dropping up to 31 per cent. This “indicates that herbivory may have a large-scale, key role in these systems,” says the study.
The study presents an interesting idea worth exploring, said Hiremath, but according to her, more convincing research is needed to prove that megaherbivores are consuming alien invasive plants, and if they are, then which ones. “From the observed pattern alone it is difficult to say whether there are more native plants because there are more megaherbivores, or whether there are more megaherbivores because there are more native (palatable) plants,” she said.
A 2021 study by ATREE researchers found that, contrary to mitigating invasive alien plants, elephants were one of the key mammals responsible for the spread of Senna spectabilis, an invasive tree found abundantly in Wayanad, Kerala.
“We did not observe elephants eating the Senna or Lantana plants (which forms dense thickets ), but elephants did eat the fruits of Senna which helped in seed dispersal and subsequently their spread,” said Anoop N R, a researcher with ATREE and author of the study in Wayanad. “The Senna fruit wasn’t the main part of the elephants’ diets, rather it was something they ate because it was present in the areas they roamed.”
Meanwhile, the Indo-Danish study found that in areas where the presence of thicket-forming alien invasive plants like Lantana exceeded 40 per cent, “the general megaherbivore effect was less than expected from our model.” Such plants have characteristics which “can resist herbivory through physical hindrance, growing tall and escaping herbivory or by being unpalatable, together diminishing the impact of megaherbivores and resulting in alien plant release feedback,” the study says.
Whether mega herbivores feed on alien invasive plants depends on the species itself, and whether they are causing a problem in a certain ecosystem or not, said Anoop. “There are some 40 invasive alien plants identified in Kerala, but it is species like Lantana, Senna and Prosopis julifora that are creating a problem because no herbivore will eat them and they are not controlled,” he said.
The need for more detailed studies
A 2016 study from the Brahmaputra floodplains found that excluding grazing and herbivory resulted in higher growth of woody invasives and promoted heterogeneity among grass species. It found that the combination of rotational grazing by medium and large herbivores and removal and burning of invasives worked best for their management.
“This is one of the few studies that isolated the control megaherbivores have over invasive plants. More studies like this are needed across India so we can learn more about the extent to which they can act as a control over particular ecosystems, and with which invasive plant species,” said Mungi.
The Indo-Danish study makes a case for protecting unfragmented habitats and “reinstating herbivory to historically high levels” in mid-productive ecosystems, where the impacts of megaherbivores are likely to be highest.
The study’s findings could have global relevance too. Similar studies in Africa have found that wild herbivores can help improve biotic resistance to invasions. Three long-term studies in Kenya tested the effects of wild herbivores on the density of exotic invasive cacti and found that they were significantly lower in plots accessed by megaherbivores, compared to those that weren’t.
In the Indian context, Hiremath agrees that finding more evidence is necessary to learn more about the role megaherbivores can play. “One possible next step could be to see if fencing areas and excluding herbivores supports these observations.” she said, adding, “if megaherbivores are feeding on invasive plants, it would also be useful to name the species that they are consuming.”
Banner image: A rhino grazing in Kaziranga National Park, Assam. Photo by Vishnu K/Wikimedia Commons.