Invasive species: The biological pollution or green cancer

In recent centuries, Earth has seen rapid growth in population of a single species, humans. Humans have explored every nook and corner, deep into the Earth to the depths of Oceans, and even beyond the Earth to space. And this rapid growth has resulted in an increase in trade throughout the globe. During this trade and expansion, humans have been introducing species to new habitats, intentionally or unintentionally. According to the Intergovernmental Platform on Biodiversity and Ecosystem Services (IPEBS), over 37,000 alien species have been introduced through human activities worldwide, with 37 percent of these reported since 1970—attributed to the increased global trade and human movement.

Not all these introduced species become invasive, but many are capable of or forcing their adaptation to the new ecosystem/habitat, making them a potential invasive species. Once the exotic becomes invasive, they grow rapidly and become dominant changing the overall ecosystem dynamics. In most cases, their growth and invasion is supported by the absence of a natural enemy. For the same reason, they are termed biological pollution or green cancer. More than 3,500 species, including 6 percent of alien plants; 22 percent of alien invertebrates; 14 percent of alien vertebrates; and 11 percent of alien microbes, have become invasive, threatening the native biota, ecosystem functioning, human health, and the economy. The IPBES Global Assessment Report (2019) highlights invasive alien species as one of the five most important direct drivers of biodiversity apocalypse, contributing to 60 percent of global extinctions and being the sole cause of 16 percent of recorded animal and plant extinctions. Besides the ecological loss, indigenous communities who are directly dependent on ecosystem services are directly affected by invasive alien species.

Exotic fauna in Nepal

It is plants which are among the major focus when it comes to alien species in Nepal. In Nepal, there are over 182 naturalized alien floral species reported, 27 of which have become invasive. Four of them are among the world’s 100 most invasive species. In contrast, there is limited data on exotic faunal species in Nepal, mostly due to sparse research. A research conducted by Prem Bahadur Budha in 2015 documented 64 alien faunal species in Nepal. The list is mostly dominated by animals introduced intentionally for commercial farming including livestock breeds, fisheries, aquaculture, and for pets and ornamental purposes. However, several alien animal species have invaded different ecosystems and threatened native animal species but are understudied. Some of the reported alien faunal species are among the world’s 100 most invasive species such as African giant land snail, Western Mosquito fish (Gambusia affinis), Mozambique Tilapia, Rainbow trout (Onchorhynchus mykiss), Brown trout (Salmo trutta) and Terrestrial flatworm (Platydemus manokwari), etc.

Contributing factors

In Nepal, many exotic fish species are introduced for augmenting fisheries for their high economic value and rapid breeding, and some for ornamental trade. Some are introduced illegally by farmers into fishery ponds. Currently, Nepal has at least 16 exotic fish species (www.fishorg.com), with some studies reporting up to 19.  These species often escape into natural water systems due to floods or unintentional dumping by aquarists, posing a threat to native species.

A great irony is that some of these species have been reared in natural ecosystems too. For instance, Tilapia fish have been released in large natural lakes such as Phewa, Begnas, Rupa Lake of Pokhara valley, Indrasarobar and other parts. Tilapia is listed among the “100 of the World’s Worst Invasive Alien Species”. An investigation from Begnas Lake found the introduction of alien species led to a 42 percent reduction in native fish. Another study reported native fish catch in Phewa Lake decreased to 13.7 percent of the total, with exotic species making up over 86 percent of the catch. Additionally, a study in Kailali district revealed that 80 out of 101 natural lakes were being used for exotic fish farming, mostly carp.

Despite these ecological risks, there is an increasing trend of importation of exotic species. However, these introductions are questionable. It is not clear whether any scientific assessment has been conducted before introducing these species.

Ornamental fishery

Ornamental fish trade also contributes to the introduction of invasive species. Aquariums housing ornamental fishes and other aquatic animals have become increasingly common, with annual import of ornamental fishes in Nepal rising from 318.1 metric ton (Rs 13.3m) in 2009 to 1233 metric ton (Rs 153.4m) in 2017. A recent study recorded 57 exotic fish species in Kathmandu’s aquarium shops, a notable increase from  27 exotic fish species reported back in 2019.

One concerning example is the presence of Vermiculated Sailfin Catfish (Pterygoplichthys disjunctivus) from Lohandra River, Morang likely introduced through ornamental pet trade. The species is known for rapid growth, high reproduction rates, and adaptability to various habitats, which facilitates its establishment in natural ecosystems. A total of 43 individuals were collected from two sites along the Lohandra, indicating a likelihood of successful reproduction in the wild. A social media video last month showed a local fisherman capturing this species in the nearby Rate Khola, Morang, suggesting its potential spread.

In addition, invasive alien species like red-eared slider (Trachemys scripta elegans) have been found in Nepal’s water bodies, possibly through accidental escape or intentional release from aquariums. Unfortunately, this species is listed among “100 of the World’s Worst Invasive Alien Species” and poses a significant threat of becoming invasive if not regulated.

Existing legal instruments

The Kunming-Montreal Global Biodiversity Framework aims to reduce the rate of introduction and establishment of invasive alien species by 50 percent by 2030. Nepal’s National Biodiversity Strategy and Action Plan (NBSAP) (2014-2020) recognized invasive species as a major threat to biodiversity and native species but focused primarily on plants. The NBSAP prioritizes formulation and implementation of a plan to regulate the introduction and expansion of invasive aquatic fauna by 2018 (WB-B8), but progress on this priority action remains unclear. The Invasive Alien Species Management Strategy has been drafted in 2016 (2074) but hasn’t been approved yet.

Preventing the introduction of non-native species to the natural ecosystem is the most effective measure to avoid invasions. Besides, preparedness, early detection and rapid response are effective at reducing invasion rate by alien species. In addition, eradication can be successful and cost-effective when the invasive populations are small and slowly spreading. There is need for strict measures to prevent intentional or unintentional release of non-native faunal species and to enhance awareness among public and merchants of ornamental fish about risks of exotic invasive species. Strengthening the institutional capacity, capacity of customs and quarantine departments is also crucial for controlling invasion and spread of alien species. Studies have reported that about 15 native fish species of Nepal are ornamental or have the potential to become ornamental fishes. Prioritizing these native fish species for ornamental market trade can help reduce the import of potentially invasive exotic species.

Nepal’s aquatic ecosystem is already jeopardized by infrastructure constructions in the name of beautification, blockage and damming projects, pollution and climate change. Extensive trade and introduction of exotic species has further augmented the threats to aquatic ecosystems. Urgent action is needed to prevent the introduction of alien faunal species and protect Nepal’s biodiversity.