Have ecological niches been conserved in evolutionary time for South Asian River Dolphins?
Anu Rai
The Thin Page Pvt. Ltd.
https://orcid.org/0000-0002-8778-9571
raianu191@gmail.com
Abstract
The niches of species that are geographically separated can either be conserved or diverge. This dynamic has been studied over time through Species Distribution Modeling and quantification of niche metrics. Here I have conducted this study for South Asian Dolphins. The Ganges River Dolphin and the Indus Dolphins are considered sub-species but there has been debate on this classification. The niche overlap was found to be very low meaning Indus Dolphin which most likely separated through the process of drainage capture has diverged from its original niche. The niche equivalency test also showed that there is a niche divergence is statistically significant. These dolphins could well be two separate species given their difference in niches but further study especially genetic studies are needed to affirm this. But the study highlights deviations from niche conservatism for the taxa due to significant niche shifts.
Keywords
South Asian Dolphins, Species Distribution Modeling, Niche overlap, Niche equivalence
Introduction
As species are separated geographically their ecological niches can be either be conserved or diverge (Wang et al. , 2017). During separation, competing species preferentially occupy habitats and microsites which bears a close resemblance with their previous habitat. This promotes stabilizing selection and is predicted to lead to niche conservatism (Webb et al. , 2002). The phenotypic plasticity on the other hand allows species to adjust to heterogeneous environment. Either case can result speciation with ecological conservatism restricting the gene flow between incipient due to geographical isolation (Peterson et al. , 1999) and divergence inducing reproductive isolation as a by-product of adaptation (Rundle and Nosil, 2005a). The failure to adapt to novel environmental condition when ancestral distributional ranges shift and break up results in vicariant speciation in niche conservatism (Wiens, 2004b; Shepard and Burbrink, 2008). In divergence, phenotypic traits through phenotypic plasticity (Price et al. , 2003) and genetic mechanisms (Rundle and Nosil, 2005b) are induced leading to ecological differences being secondarily accumulated among lineages.
The niche conservatism suggests that ecological differences accumulate after the speciation event (Peterson et al. , 1999) whereas niche divergence suggests locally divergent evolutionary responses by populations, result in divergent environmental tolerances because of selective pressures across the landscape (Parmesan, 2006). There is a widespread regard that closely related species are ecologically similar leading to assumption that niche conservatism exist by default (Losos, 2008). In a way, niches are always somewhat conserved there are no sister pairs one occurring in tropical rainforest and the other in deep sea vents (Wiens and Graham, 2005). However, niche conservatism is not ubiquitous (Rato et al. , 2015; Wellenreuther et al. , 2012; Raxworthy et al. , 2007) and there is a need to examine this on a case by case basis.
This dynamic has been studied over time through Species Distribution Modeling (SDM) and the quantification of niche similarities between or within species (Warren et al. , 2008a; Broennimann et al. , 2012; Warren et al. , 2010; Muscarella et al. , 2014; Heiblet al. , 2018). SDM studies also aid in defining species delimitation (Rissler and Apodaca, 2007; Raxworthy et al. , 2007). The ecological niche modeling estimates the potential geographical distributions of species based on environmental variables. Whether there is overlap in species niches helps to understand niche conservatism or divergence among taxa.
In the present study, I use recently developed methods for SDM and calculated if the niche overlap persist for South Asian River dolphins. The detection of niche conservatism has been made through niche equivalency (whether native and non-native niches are indistinguishable) (Warren et al. , 2008a) which prior were only inferred through underlying statistical and conceptual assumptions (Strubbe et al. , 2013). This study also helps in shedding light on some aspect of species delimitation but will overall concentrates on the aspect of niche study of the South Asian River dolphins.
The South Asian River dolphins are split into two subspecies namely Ganges River Dolphin (Platanista gangetica gangetica ) and Indus Dolphin (Platanista gangetica minor ). The former is found in the Ganges-Brahmaputra-Meghna (GBM) and Karnaphuli-Sangu (KS) river system and the later in the Indus river system. It has been theorized that the Indus dolphin diverted to Indus through the process of drainage capture some 550,000 years ago (Braulik et al. , 2015). Have the species conserved their niche or not is an important topic of study. This lack of clarity can impair scientific conservation management.
Morphologically the key difference between the two dolphins has been difference in tail length. Of nearly the species of same body length, the length of tail was shorter in GRD than ID (for 118 cm body length, 6.7 cm shorter tail length in GRD) (Kasuya, 1972). Pilleri et al.(1982) however, recognized two species of South Asian River dolphins based on differences in the prominence of the nasal crests, caudal height of the maxillary crests, length of the lower transverse processes of the sixth and seventh cervical vertebrae, blood protein composition and free-esterified cholestrin ratio in the lipids. But the arguments were deemed unconvincing, due to examination made on small sample of adult specimens and the absence of statistical analyses (Reeves and Brownell Jr, 1989). The morphological difference is however seen which made the present classification as subspecies.
These two dolphins were long regarded as identical until Pilleri and Gihr (1971) divided them into two species (P. gangetica andP. minor ). Kasuya (1972) reduced the two taxa to sub-species ofP. gangetica . This treatment is followed in the IUCN Red List of Threatened species. There is a need to study whether species have conserved their niche or diverged through evolutionary time scale so that it can aid in resolving the species delimitation. Both the species are classified as Endangered with some sub-groups even being classified as Critically Endangered such as the sub-population in Nepal (Jnawaliet al. , 2011).
Materials and methods