Think about it. The origin story of the brown trout in India dates back to just over 100 years. It was introduced to the cold waters of the Kashmir Valley by the British. The fish has since flourished in the erstwhile state. It has also snagged a spot on the International Union for Conservation of Nature’s (IUCN’s) 100 of the world's worst invasive alien species (IAS) and emerged as one of the biggest threats to the indigenous snow trout and 150 other native fish species found in the Himalayan waters.
An internal study by the Wildlife Institute of India pins the blame on climate change. “This particular breed inhabits cold waters, but rising temperatures are pushing native fish species northwards, leading to a conflict with the brown trout. It is a conflict that the brown trout will dominate,” says Arjun Singh, who has been working towards the conservation of the snow trout in the Kashmir region for over a decade.
All species (alien and native) are affected by environmental change. Most IAS, however, are more adept at taking root, multiplying and altering the ecosystem to suit them. Climate change gives them a further edge and compounds their impact. It facilitates their spread and establishment, and creates new opportunities for them to become invasive, especially in our waterbodies. Here’s how:
Deep impact
Mean global temperatures have risen above 1 degrees Celsius since pre-industrial times. Apart from warmer temperatures, climate change has also resulted in more extreme weather events, such as droughts, floods, cyclones and faster-rising sea levels.
Waterbodies are being constantly altered, too. Their environment is affected by changing temperatures, salinity, chemistry, sea levels, ice content and weather patterns. It can make previously unsuitable habitats more suitable for IAS, accelerating their proliferation.
For example, the waters of the Ganga are predicted to see a mean annual temperature rise of 1 degree Celsius to 4 degrees Celsius between 2010 and 2050. Warmer waters will allow existing non-native species such as the common carp, Nile tilapia and African catfish to move into new areas.
Changing conditions can also make waterbodies less suitable for some native species. This year’s first-ever census of Kerala’s Ashtamudi lake found that 85 per cent of the 51 species present in the lake are marine varieties. It included the super-invasive Charru mussel. Researchers attribute the reason to a decline in freshwater flow from the Kallada river due to climate change.
That apart, changes in the course of rivers can lead to the introduction and increase in invasive species. “Sudden floods can trigger the production of fish at the wrong time. Some species can become stressed due to environmental changes, resulting in weaker immune systems and lower reproductive success, while others flourish and become competitive. An increase in water temperature can also result in a change in sex ratios, causing alterations in the time of spawning and migration,” says Vidya Karnik, an independent biologist, who has spent a decade studying the Kaveri river.
Winner takes all
In our country, the Zoological Survey of India lists 157 animal species as invasive. It includes 99 marine species and 19 species of freshwater fish. To become a successful invader, according to the Convention on Biological Diversity, an alien species must tick several boxes. It must out-compete native organisms for food and habitat, spread through its new environment, increase its population, and harm ecosystems in its introduced range.
Extreme climatic events accelerate this process. Cyclones, floods and droughts transport IAS to new areas and decrease the resistance of habitats to invasions. For example, a survey after the 2018 floods in Kerala found 30 alien fish species in its 44 rivers and 53 reservoirs. Some, such as the arapaima (native to the Amazon and Essequibo basins of South America) and alligator gar (from North America), it turns out, had escaped from aquaculture facilities in the state.
Once established, IAS proliferate fast and take over the ecosystem. In 2005, the waters of the Yamuna harboured only negligible quantities of Nile tilapia (native to Africa and West Asia). In two years, it had increased to about 3.5 per cent of the total fish species. Today, it is among the dominant species in several parts of the river.
The know-how
To counter these challenges, we must have a better understanding of how climate change affects invasive species. Serious knowledge gaps, however, still exist. There is, for example, limited data available on IAS in the north-eastern states. The life cycle of alien species and the impact of climate change on them is rarely monitored. In many regions, basic environmental data is not available.
“All these factors make it difficult for conservationists and policymakers to anticipate and prevent IAS from finding a foothold in newer areas,” says Singh.
Even having this information may not be nearly enough. According to the Centre for Biodiversity Policy and Law, successful control has so far been established only in a few IAS and complete eradication is not possible in all cases. In the case of aquatic-invasion management especially, it states, prevention is the only cost-effective option available. “Simply put, in this case, prevention is better than cure,” says Singh.
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