Intraspecific Diversity of Congeneric Species Along an Elevational Gradient

Abstract

Latitudinal and elevational gradients provide a powerful natural framework to study how species interactions with changing environmental conditions shape ecological patterns across space. Elevational transects have much steeper environmental gradients than latitudinal transects. For example, the temperature changes by ~5 °C for every 750 m of elevation (within a few kilometres) while the same change occurs across 9 degrees of latitude translating (1000 km). Therefore, elevational populations of a species have a higher likelihood of locating their optimal environment with minimal dispersal limitations along a slope. Contact with habitat to which a species is not adapted can either limit its range or edge populations may get locally adapted and extend the range of the species. Both these responses will be reflected in patterns of intraspecific diversity, differentiation, and abundance along elevation. This short spatial distance can facilitate certain kinds of research (that are difficult with latitudinal studies) and also make for easier logistics.

We investigated patterns of intraspecific diversity and differentiation along a large elevational gradient in the eastern Himalayas at Eaglenest Wildlife Sanctuary (EWS) in Arunachal Pradesh, using DNA markers from a large sample of bush frogs of the genus Philautus. The high abundance of Philautus and its easy detection made it an excellent target species for our study. We used large fragments (6-7 kb) of mitochondrial DNA (mtDNA) to achieve a higher degree of accuracy in the estimation of intraspecific diversity and detection of differentiation between recently diverged taxa. We sequenced the DNA of 172 Philautus individuals using Nanopore’s MinION.

We developed a novel consensus-calling technique based on Bayes’ theorem to estimate the error on each individual consensus-call and to avoid using those with poor reliability. The DNA analysis showed that the cryptic Philautus community along our transect consisted of 3 species which neatly segregated along elevational regions of EWS (Khellong = low elevation, Sessni = mid-elevation, Bompu = high elevation).

We found the intraspecific diversity to be highest for mid-elevation Sessni species. We suggest that this is because the mid-elevations are shielded from the extreme conditions that the edge species must experience during climate cycle extrema, resulting in diversity loss for both the low-elevation Khellong- and high elevation Bompu species. We also noted a correlation between genetic distance and elevation distance for the high elevation Bompu species. The high elevations are known to be stressed environments and likely impose a high degree of adaptation for survival. This result is perhaps the shortest geographical distance (less than 3 km) over which clinal local adaptation has been detected.

Following a clue in the zones of overlap of the 3 frog species, we explored the width of range overlap in 107 congeneric bird species pairs along the same transect. We found that there is considerable overlap in the ranges of congeneric species. We also detected a correlation between width of range overlap on the one hand, and morphological traits like differences in body mass, wing length and beak width. These results suggest that the high diversity in the Himalayas may be facilitated by niche differentiation as a result of differences in functional traits.