The Role of Mega-Herbivores in Combating Invasive Plants
An elephant makes inroads into golden crownbeard (Verbesina encelioides), which originates from North America and spreads in India, where it is a nuisance for e.g. arable farmers. The plant is also invasive in Denmark. Credit: Suryoday Singh Mann
Heavy herbivores like elephants and buffaloes are effective against invasive plants, concludes a new Danish/Indian study based on data from the world’s largest survey of wildlife. But you don’t need elephants to get the same effect elsewhere.
Large herbivores can play a crucial role in preserving local ecosystems by consuming and trampling invasive plant species that threaten biodiversity.
At first glance, one might think these herbivores would also target native plants. However, native plants have evolved alongside these large animals for thousands of years, making them resilient to such interactions, unlike invasive species.
This is the conclusion of a new study from Aarhus University and the Wildlife Institute of India, which has just been published in the scientific journal, Nature Ecology & Evolution. The study shows great potential for using large herbivores as a natural weapon to prevent invasive plants from out-competing native species.
At least it works in India, where the researchers gathered their data. More specifically from the world’s largest wildlife survey using camera traps, which takes place every four years, as well as India’s extensive monitoring programme for plants. (Read about the survey in the fact box at the bottom of this article.)
However, the researchers point out that the results are also relevant for areas that do not have herbivores quite as large as those in India. More about this later.
Horse and water buffalo in a rewilding project in Geding-Kasted Mose near Aarhus, Denmark. Credit: Peter F. Gammelby, Aarhus University
The study is based on what the researchers call mega-herbivores, i.e. animals weighing more than one metric ton. In India, these are elephants, rhinos, wild water buffalo, and Indian bison (the largest and heaviest bovine in the world).
The study demonstrates a positive correlation between the number of mega-herbivores and the balance between native and invasive plant species: Where there are many mega-herbivores, there are also many native plants and fewer invasive plants.
And vice versa. In places where invasive species predominate, there are few or no mega-herbivores.
Except in some areas in India where invasive plant growth has become so tall and dense that mega-herbivores cannot get to it.
A herd of gaur or Indian bison “caught” by a camera trap. Gaur is the world’s largest living bovine species, and can weigh well over a ton. Credit: AITE 2018, NTCA-WII, India
The significance of these findings lies in the fact that the United Nations identifies invasive species as a top threat to global biodiversity.
These invasive species, which include various animals, plants, and fungi that are not indigenous to an area, often harm native biodiversity. Efforts to combat these invasive species have cost over USD 120 billion globally in the past half-century, with limited success.
The sheer size of mega-herbivores means they consume vast quantities of various plant species. And they are used to eating many different plant species, even species with less nutritional value, because they simply cannot afford to be picky. Therefore, they are more likely to include unfamiliar plants in their diet.
The research team could have included smaller species of herbivores in the study, but their roles in local ecosystems are more complex; they are also on the menu for tigers and leopards. Elephants etc. are not.
And so we return to how we can apply the study in countries without elephants, rhinos, etc.
You don’t need mega-herbivores to keep invasive plant species at bay – slightly less large and medium-sized species can have similar effects.
“Although some of the cattle released in rewilding projects in Europe can grow to more than a tonne, animals weighing less than a tonne can have similar effects. In Hungary, it’s been shown that water buffalo drive away the invasive giant goldenrod; a species that is also a problem in Denmark. Scottish Highland cattle are also used in Denmark to keep down rosehip bushes; an Asian species that is often considered problematic in Danish nature,” says Professor Jens-Christian Svenning from Aarhus University.
The study’s lead author, Ninad Avinash Mungi, is a postdoc at Aarhus University, and he stresses that the size of grazing animals is not decisive in the fight against invasive species.
“You can easily use a mixture of large, medium, and small herbivores. Deer, buffalo, cattle, and horses work well together in rewilding projects, and together they can also target different invasive plant species. This also makes efforts more flexible and resilient,” he says, and continues:
“It’d be a really good idea to carry out a large-scale European biodiversity survey like the one in India, which holds the Guinness World Record. Europe has more money to invest in nature and nature restoration.”
It is no exaggeration to call the Indian survey the world’s largest. With 26,838 camera traps, it has earned a place in the Guinness World Records, and it even involves field work, with tens of thousands of participants on foot covering a forest area totaling 381,200 km2.
The survey is carried out every four years. The main purpose is to find out how the country’s tiger populations are faring, but the motion sensors in the cameras detect much more than the striped predators. Thousands of elephants, rhinos, wild water buffaloes and Indian bison (the world’s largest species of wild cattle) have also appeared in the almost 35 million photos. The enormous volumes of data also include large amounts of vegetation and dung samples.
Reference: “Megaherbivores provide biotic resistance against alien plant dominance” by Ninad Avinash Mungi, Yadvendradev V. Jhala, Qamar Qureshi, Elizabeth le Roux and Jens-Christian Svenning,31 August 2023, Nature Ecology & Evolution.DOI: 10.1038/s41559-023-02181-y
