Eichhornia crassipes, commonly known as common water hyacinth, is an aquatic plant native to the Amazon basin, and is often a highly problematic invasive species outside its native range. It was introduced in Bengal in India because of its beautiful flowers and shapes of leaves, but turned out to be an invasive weed draining oxygen from the water bodies and resulted in death of many fish. Fish is a supplement food in Bengal, and because of the fish scarcity in Bengal caused by Eichhornia, the water hyacinth is also called "Terror of Bengal

Exotic species, also known as invasive species, are non-native species that are introduced into an ecosystem and have the potential to cause harm to the native flora and fauna. In the case of water bodies in India, one of the exotic species that has become a menace is Eupatorium odoratum.

1. Eupatorium odoratum:
Eupatorium odoratum, also known as the Ageratum Conyzoides or Mexican devil, is a highly invasive plant species native to Central and South America. It was introduced to India as an ornamental plant but has since spread to various water bodies, including rivers, lakes, and wetlands.

2. Rapid spread and growth:
Eupatorium odoratum is notorious for its rapid spread and growth. It is a perennial plant that reproduces through seeds and vegetatively through its creeping stems. It forms dense mats on the water surface, choking out native aquatic plants and disrupting the natural balance of the ecosystem.

3. Negative impact on biodiversity:
The presence of Eupatorium odoratum in water bodies has a negative impact on biodiversity. It outcompetes native aquatic plants for resources such as sunlight, nutrients, and space, leading to a decline in native plant species. This, in turn, affects the entire food chain as many organisms rely on native aquatic plants for their survival.

4. Altered ecosystem dynamics:
Eupatorium odoratum alters the ecosystem dynamics of water bodies. Its dense mats can block sunlight from reaching the water surface, reducing photosynthesis and oxygen production. This can lead to a decrease in dissolved oxygen levels, which is essential for the survival of aquatic organisms. Additionally, the dense mats can impede water flow and cause stagnant conditions, further degrading the ecosystem.

5. Economic and ecological impacts:
The presence of Eupatorium odoratum in water bodies has significant economic and ecological impacts. It can clog irrigation canals, water intake pipes, and hydropower infrastructure, leading to reduced water flow and increased maintenance costs. It also affects the fishing industry as native fish species may decline due to habitat destruction and altered food availability.

In conclusion, Eupatorium odoratum, or Mexican devil, is an exotic species that has become a menace to many water bodies in India. Its rapid spread, growth, and negative impacts on biodiversity and ecosystem dynamics make it a significant threat. Efforts are being made to control its spread and mitigate its harmful effects on aquatic ecosystems.