Figure 3: Identity Test showing niche overlap of the pair are outside the 95% confidence interval
Discussion
The niche difference has been clearly visible from the study. Four general factors are opined to be important causes of niche conservatism which are natural selection, gene flow, pleiotropy, and lack of variability (Wiens, 2004c; Wiens, 2004a). I believe, the lack of niche conservatism in this case could have been an interplay of some of these factors. The new ecological condition in Indus may not have been one that reduce their fitness or population growth. Otherwise, natural selection would favour traits that keep individual inside the niche if there was a cost of reduction of fitness or population growth (Holt and Gaines, 1992; Holt, 1996). Contemporary genetic exchange has been deemed unlikely since the populations are separated by several hundred kilometers of land at their closest point and the dispersal through brackish water has been deemed highly improbable (Braulik et al. , 2015). If there was a consideration of the dispersal abilities of species than it could radically change interpretations of niche similarity or difference (Peterson, 2011). A clear genetic differentiation between the two geographically isolated populations have been found through mtDNA study and there has been speculations that additional comparative investigations may allow recognition of these isolated populations as distinct taxa (Braulik et al. , 2015).
Likewise, non-conservatism can be achieved by any change of the three BAM (biotic, abiotic, movement) factors (Pearman et al. , 2008). The abiotic conditions includes the climate, physical aspects that impose physiological limits on species’ ability to persist in an area. The biotic factors are the set of interactions with other species modifying the species’ ability to maintain populations. Movement is the parts which are accessible to the species in some ecological sense. It is not clear as to the changes in which factors resulted in the non-conservatism in the present study but movement bears a special significance by initially separating the species through drainage capture and then creating of barrier through land and brackish water limiting their ability to interact.
While generally underutilized, SDM in combination of phylogenetics has already made contribution on species delimitation (Carstens et al. , 2013). In a study by Raxworthy et al. (2007) they used ecological niche modeling for studying species delimitation. The study elevated three subspecies to species rank using a combination of morphological, niche study and mitochondrial DNA. The additional comparative study as suggested by Braulik et al. (2015), should thus be conducted. An erroneous classification can lead to potentially inappropriate conservation management decisions for threatened species. The additional study could help to ascertain the taxonomic status of the South Asian Dolphins. This is fundamental for insights into the biology and evolution of species and effective conservation management (Huet al. , 2020). Carstens et al. (2013) recommend that supposition about species limits should use methods which is an integration across many data types and analyses. It is rather wise to fail to delimit species than to falsely delimit entities. The study clears the air on niche conservatism of the species and it is time to focus on the genetic evidence of the taxas.
Conclusion
The South Asian Dolphins show very little niche overlap but this article by no means ascertains the existence of two separate species of South Asian dolphins. But very little niche overlap were seen and their niches were not found to be equivalent. A lot of factors have been speculated on why the niche non-conservatism has been witnessed such as lack of contemporary genetic exchange. The study simply aids in the discussion that further efforts are needed to ascertain its taxonomic status. Correct species delimitation could have a lot of implications for the conservation of these Endangered taxas. Hence, I recommend further genetic study to affirm this claim.
Statement of data achieving
The supplementary data and OBIS-SEAMAP (http://seamap.env.duke.edu/) provides a complete reference of presence records.
Acknowledgements (optional)
I would like to thank Dr. Dan Warren for entertaining my queries on ResearchGate regarding the niche equivalency test. I would also like to thank Mr. Aashish Mishra for reviewing my manuscripts for formatting and grammatical errors. A special acknowledgement to the Majhi (indigenous fishing tribe) in Koshi River who’s snatched catch from the protected area made a deep impact on me to study about Ganges River Dolphins.