Ecotones: Nature’s own dramatic shades of transitions


Definition and Characteristics

Ecotones (Eng. Eco + Gk. Tonos, meaning tension) are areas of steep transition between ecological communities, ecosystems, and/or ecological regions along environmental or other gradients (for example, anthropogenic gradients, like agricultural lands and forests). These transition areas occur at varying spatial scales and range from natural ecotones between ecosystems and biomes to human-generated boundaries. They range from mountain tree lines over small and large areas to transitions between large biomes and specific ecoregions (eg, Mediterranean and arid), estuaries (freshwater and marine ecosystems) savannahs (forest and grassland ecosystems), etc.

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As the name suggests, ecotones are areas of high specific tension, encompassing all flora and fauna. The vegetation and wildlife that can be found here are in many cases very specific, such that they cannot be found in the intersecting ecosystem areas. This high tension is said to root from the high adaptability and resilience shown by the species as responses to the rapidly changing conditions in each of the intersecting ecosystems, that subsequently affect the conditions of the ecotone. The constant struggle for survival at a very high rate in these areas impart very interesting characteristics to these ecological areas. Various studies have shown that species richness, diversity, and abundances tend to peak in ecotonal areas, as compared to the adjoining biomes, though exceptions to these patterns occur. Evidence suggests that ecotones may also be speciation hotspots where new forms evolve. This can be related to the cases of parapatric speciation, where new species are formed from older species upon entering new adjoining niches. All these characteristics of ecotonal regions come under the broad definition of ‘edge effect’, and those species which cannot be found in the adjoining Eco biomes, except only along the edges of them, i.e., the ecotonal area are called ‘edge species’.

Image credits: Deep Green Permaculture. Species richness in ecotones.

As such, ecotones deserve high conservation investment, potentially serving as speciation and biodiversity centers. As populations in ecotones are potentially pre-adapted to changing environments, they are in most cases, more resistant to climate change, biotic invasions, invasive species, and other environmental changes. Because ecotones are often small in spatial extent and within this small area they are relatively rich in biodiversity, with the resident populations adapted to change, their conservation may be actually a very cost-effective strategy towards global biodiversity conservation.

Image credits: Studywrap


Over the recent decades, ecotone monitoring has surged due to another interesting fact. Owing to their super adaptable responses, the ecotonal species are more than often, very distinct indicators of ecosystem changes, like temperature, salinity, soil acidity, and oxygen richness levels. With rising awareness about global climate change and its repercussions and observable effects, the ecotonal species have increasingly been seen as barometers of climate change effects observable over varying local areas.
Anthropogenic ecotones are also existent along forest clearings (forests and agricultural landscapes). Many insect and plant populations peak in these specific areas. Human interventions over millennia have generated newer ecological areas, flagging the concept of natural plasticity and the existential vitality of nature.
What concerns us now, however, is the degradation of some of the best-known ecotonal areas of the world. I would highlight here as an example the specific case of the Sundarban marsh, one of the largest marshlands across the globe.


The Sundarbans Ecosystem

Sundarbans is a mangrove area in the delta formed by the confluence of the Ganges, Brahmaputra, and Meghna Rivers in the Bay of Bengal. It spans from the Hugli River in India's state of West Bengal to the Baleswar River in Bangladesh's division of Khulna. This marshy littoral tract spanning over a total area of around 10,000, experiencing tidal waves at regular intervals from the Bay of Bengal, has provided conducive hydrogeomorphological conditions for the development of flourishing wild mangrove ecosystem with highly diversified floral and faunal species. The estuarine ecosystem maintains a linkage between the terrestrial and marine ecosystems here. Agricultural landscapes dotted with freshwater pond ecosystems can be found beyond the limits of the virgin forest. The combination of so many ecosystems in the proximity of the Bay of Bengal has made all of them fragile whenever exposed to the onslaught of natural hazards like cyclone and tidal surge or subjected to over-exploitation of the wild and aquatic resources by human society thus disturbing the delicate balance of nature. Consequently, all three dimensions of biodiversity- species, genes, and ecosystem are suffering from the negative impact of their onslaught.
The Sundarbans’ complex ecosystem in a broader sense, represents an ecotone, with the Sundarbans marsh functioning as an overlapping area among many different natural as well as anthropogenic ecosystems flanking the area. Its rich biodiversity and dwindling populations of numerous species have resulted in its gaining of conserved status, spread across both countries. Since 2019, it is also recognized as a Ramsar site. It is home to many unique species of mangroves, like Goran and Sundari, the tree that gives Sundarbans its name.

Image credits: Anamitra Anurag Danda, Observer Research Foundation

Man-wildlife conflict

The Sundarban marsh is the prime source of livelihood for many people residing in the deltaic islands, who mostly are prawn cultivators and honey collectors, apart from the people fully employed in various levels of the tourism industry, from boatmen to caretakers to hotel employees. The islanders face hostility from nature as well as from wild animals and apex predators residing in the forests and creeks. Until very recently, the news of a honey collector or fisherman getting attacked by the Bengal tiger (the apex predator of the area) used to be quite frequent. With the betterment of administrative function, awareness campaigns, and education on dealing with man-wildlife conflict, such incidents have now become uncommon, although, the news of some small fisherman getting pulled away by a tiger might not reach today’s newspaper headlines. Human settlement and encroachment in the area have led to tiger attacks upon the islander’s homes, resulting in preying of pet chickens, goats, and cattle, and sometimes, very unfortunately, human babies in some of the rarest cases.

Image credits: Trekkerpedia. Fisherwomen of Sunderbans.

Ecological quotient

The Sunderbans are the primary line of defense for the Bengali population residing in both the countries sharing the great delta marsh, against the raging Bay of Bengal-sourced cyclonic storms that hit mostly every year, pillaging the coastal areas and destroying everything in their path. The mangrove trees form the defense that acts as the life-saving barrier for people residing on the mainland coasts. Although these are mostly resilient, they take time to grow back. Every successive cyclone in back-to-back years has damaged the endangered mangroves to an extent that it would be very long into the future when the delta marsh regains its forest cover. Moreover, increasing human settlement has led to small to medium level forest clearance in the area, to create living spaces and tourist shelters.


Biodiversity loss

What is more disheartening is that the main mangroves of the area are all listed under the Endangered category of flora in India. These include Kakra, Golpata, Sundari, Goran, Keora, Ohundhal, and Passer. The endangered fauna includes Estuarine Crocodile, Gharial, Olive back Loggerhead Turtle, Common Batagur, Gangetic Softshell Turtle, Water Monitor, Yellow Monitor, Indian Monitor, Python, and Olive Ridley turtle. The Two horn Rhino and water-loving wild buffalo have already become extinct in the nineteenth century. Gradually, the hog deer and barking deer, Indian Muntjac, and softshell turtle have also suffered from extinction. The exquisite Gangetic dolphin is rare now in the area which once used to be its safe habitat. The Govt. of West Bengal’s website on Sundarbans Affairs very precisely states the reason for such rampant extinction and decrease in biodiversity, which reads, ‘Various anthropogenic factors are responsible for such a state of affairs. These endangered species are falling prey to the greed of sawmill owners and poachers of the wild animals on one hand and to the hunger of people, etc on the other.’ This massive Sundarbans clearance had started way back in the British era when huge stretches of land were reclaimed from the marsh to create town settlements, and the urbanization has henceforth continued. Even then, the actual residents of the deltaic islands are far from urbanized. Many travelers have given personal accounts on how hard the residents of the villages spread across the deltaic islands toil to meet their daily expenses, with some islands still lacking basic existential amenities like drinking water taps and electricity.

Image credit: Depositphotos. Residents and tourists wait for their boats at an average ferry of Sundarbans.

The Govt. of West Bengal lists 40 species of mammals, 45 species of reptiles, 150 species of birds, 200 species of fish, 40 species of crabs, 25 species of prawn, and 6 species of frogs as residents of the Indian side of the littoral marsh forest, which also includes at least 64 different varieties of mangrove species. After years of sheer neglect, the mighty Sundarbans ecotone, rich in biodiversity, had finally been recognized for its ecological treasure-trove, and granted the status of a biosphere reserve in 1989, after it was recognized as a world heritage center in 1987. Being a natural home to the Royal Bengal Tiger, it is also a big center of tiger conservation, under India's Project Tiger scheme.

Biologically speaking, diversified relationships among the flora and fauna govern the Sundarbans forest ecosystem. A symbiotic relationship is maintained between Brackish tidal waves, saline-alkaline soil, and the mangrove species. Herbivores compartmentalize the forest horizontally and vertically depending on their food and nesting habits. Habitats are vertically divided into strata, where deer occupy the lowest stratum, monkeys stay in the middle, and avifauna dominate the topmost branches of the trees. Since they are not mutual competitors, they usually maintain peaceful coexistence. In ecotones, competition for food and shelter is very prominent, while deer and tiger maintain an eternal prey-predator relationship here.



Image credits: Zee News

The Sundarbans has seen it all - from being a chaste forest with a few local forest-dependent human communities, suddenly encroached by a large populace of refugees incoming to India after the Bangladesh Liberation War of 1971, to being the location of a very heavily criticized allegedly organized grave political mass murder, to residential and tourist settlements at an exponential rate, apart from being lashed by deadly cyclones year after year. The Sundarbans still stands strong, trying to rebuild itself and its lost pristine ecosystem, its ecotonal characteristics of adaptivity and resilience showing in full colors.

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