by Rohan Nath
Considering the rate with which many landscapes, including freshwater ecosystems, around the world is being converted by humans; it is important to understand the evolutionary potential of endangered species. The race between the surrounding countries to harness water extraction and hydropower propels the South Asian Rivers into a threat. The most endangered freshwater river dolphin in the world- the Ganges River dolphin (Fig. 1) is found in the Ganges-Brahmaputra-Meghna and Karnaphuli River Basin (Fig. 2) in India, Nepal and Bangladesh. However, several human development projects and activities like dams and barrages as well as natural factors hamper the ecology of the Ganges dolphin and alter their habitat. Numerous factors are likely to put the population of Ganges dolphin at risk of destabilization and extinction.
Dams and water-related projects in the main branch and the tributaries of the Ganges Brahmaputra-Meghna and Karnaphuli River Basin create a flow-regulating barrier that obstructs the movements of the dolphin population. This results in small, local subpopulations that disrupt the potential of the dolphin population for evolution.
Fig. 1. The Ganges river dolphin (Platanista gangetica gangetica). Image Source: Mansur / WCS Bangladesh / Braulik et al., doi: 10.1111/mms.12801.
Fig. 2. The Ganges-Brahmaputra-Meghna and Karnaphuli River Basin in South Asia and the location of the major dams that isolate the dolphin population into several small groups. Image Source: Paudel, S., & Koprowski, J. L. (2020). Factors affecting the persistence of endangered Ganges River dolphins (Platanista gangetica gangetica). Ecology and evolution, 10(6), 3138-3148.
Why is evolution important for the Ganges dolphin?
Under the serious threats of extinction, the dolphin population needs to adapt to changing environmental conditions which may threaten their existence. Therefore, evolution is the mean by which the population can cope up with environmental stresses.
Effects of Dams on the Population of Dolphins
i. Modification of Physical Habitat
The dams lead to habitat loss and fragmentation resulting in the isolation of small groups of Ganges dolphin with limited geographic range. Disturbances in any hydrophysical habitats like required depth can potentially reduce or eliminate the reproductive success of the Ganges dolphin. Further research is necessary to identify the other hydrophysical habitats to increase the conservation and survival of the dolphin population. The rate of local extinction has increased in the upstream range of the Ganges River. Considering the high risk in these small isolated populations, it is important to develop a plan to work on the conservation of these endangered species.
The government of India declared the Ganges dolphin as the national aquatic animal and developed the Conservation Action Plan for the Ganges dolphin. However, the government also constructed dams and structures at international borders leading to an alteration in the Ganges-Brahmaputra-Meghna and Karnaphuli River Basin, risking the population of the species they declared as the national aquatic animal.
WCS dolphin conservation project in Bangladesh and Vikramshila Gangetic Dolphin Sanctuary, Bhagalpur District of Bihar in India are some of the river dolphin-based conservation projects.
The Farakka Barrage in West Bengal divides the Ganges dolphin global population into several local subpopulations. The geographical limitation of the subpopulation of the Ganges dolphin puts them at a higher risk of extinction. This usually occurs through phenomena like reduction of genetic variability and inbreeding depression that decreases the genetic diversity and fitness of the population. The number of small local subpopulations is further increased with an addition of a new dam. Considering that the extinction of species occurs quicker in freshwater than in terrestrial ecosystem, the increase in inbreeding and decrease in genetic heterozygosity makes the local extinctions of Ganges dolphin seem inevitable.
iii. Behavioural ecology of the Ganges dolphin
Anthropological influences are likely to affect the specialized circadian rhythm of the Ganges dolphin concerning habitat selection like depth profile selection for foraging and reproduction. This in turn can affect the life-history stages and functional ecology of these species. The Ganges dolphin uses the cyclic range of water levels and seasonally moves between the mainstream and tributaries. For example, the Ganges dolphin is stimulated to migrate to other tributaries when there is a high-water flow in the mainstream. Anthropogenic structures like hydropower dams or development structures can regulate the water level and present as a false environmental cue, leading to a dysfunction of the functional ecological behaviour of the dolphin.
iv. Human-dolphin conflicts
The primary cause of endangerment and extinction of the Ganges dolphins is due to their interactions between artisanal fisheries in the Ganges-Brahmaputra-Meghna and Karnaphuli River Basin. The endangerment of the Ganges dolphin can be attributed to the dietary and diel activity and spatial and temporal overlap with the fisheries. The factors leading to the fisheries and Ganges dolphin interaction needs to be assessed for effective management. Therefore, the driving factors like spatial overlap, dietary competition and behavioural distractions need to be further researched both qualitatively and quantitatively to manage the coexistence between the river dolphin and fisheries.
v. Implications for future management
Genetic tools cannot be applied to explore the viability of the Ganges dolphin population in the Ganges-Brahmaputra-Meghna and Karnaphuli River Basin because of the limitation of resources and conservative policies. It is essential to develop a regional intergovernmental project that promotes the investigation of genetic viability and factors associated with the risk of extinction using genetic-based research. For genetic monitoring, it is important to consider the use of non-invasive tools, like environmental DNA. It is suggested to integrate census data with genetic data for the accurate prediction in the population trend of the Ganges dolphin. Proper capture and handling techniques might make it possible to improve the genetic stability by translocation of individuals among subpopulations.
It is tremendously difficult to predict the extinction using a single ecological factor due to the synergistic effect of several other factors. A better understanding of management purposes and conservation could be reached if we integrate demographics, genetics, and environmental factors in future studies. Restoration and preservation of essential surfacing and foraging habitats and maintenance of minimum stream flow can prevent the further decline in the population of the Ganges dolphin.
1. Paudel, S., & Koprowski, J. L. (2020). Factors affecting the persistence of endangered Ganges River dolphins (Platanista gangetica gangetica). Ecology and evolution, 10(6), 3138-3148.