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