Our observations in this study indicate that in spite of the isolation caused by a barrier imposed by the Girijapuri barrage, the genetic differentiation between gharial populations at Girwa and Chambal is low. Moreover, an admixture of gene pool as a consequence of intermixing facilitated by the captive rear and release program could have occurred in the two populations. The gharial populations at Chambal and Girwa had similar sized founder populations and received similar external augmentations. Yet the Chambal population has increased substantially in population size and shows higher recruitment rates in various age classes of gharial compared to Girwa population (Table 4; Fig. 2)12.
Gharial habitat in Girwa is limited to a 20 Km stretch of the reservoir12. While there has been an increase in population size, it remains skewed towards adult animals12. Small size classes such as yearlings and hatchlings show negligible recruitment in the resident population12. It is believed that the Girijapuri barrage functions as a one way exit for resident gharials of Girwa. Barrage gates are opened twice annually, once for maintenance and second time during monsoons. Gharials that are flushed downstream, especially small sized individuals, are unable to enter and recruit in the population after the gates close. Due to a lack of terrestrial locomotion gharials are unable to bypass the barrage through land, as observed in mugger crocodiles. As a consequence, gharials at Girwa are geographically isolated in terms of genetic exchange with other gharial populations. However, under project Crocodile, common rearing of gharial eggs sourced from Girwa and Chambal have resulted in genetic mixing of the two populations32. Evidence of gene migration between the two populations despite no habitat connectivity can only be explained by the intermixing at Kukrail32. It also indicates recruitment of some of the released animals in the resident Girwa population and hence the low inter-population differentiation.
All wild populations of gharial have been intensively restocked33 but there is paucity of genetic studies of extant gharial populations6. Jogayya et al.14 studied the genetic diversity of captive populations of gharials using microsatellite markers with samples collected from zoos in India. However, analyses of genetic diversity in wild populations of gharials using the same microsatellite markers reported lower levels of heterozygosity (Table 3)32,34. Jogayya et al.14 had employed only 32 samples that were sourced from gharials separated temporally by decades and with no recent genetic exchange. Consequently, when the same markers were tested on wild breeding populations in Girwa and Chambal there was reduced heterozygosity number of alleles32,34. We observed similar levels of heterozygosity as previously reported32. Higher values of observed heterozygosity over expected values points to admixture of isolated populations, which is true in case of Girwa and Chambal. But, marker G13_5 was monomorphic for Chambal samples and marker G13_7 showed a null allele frequency of 20% in Girwa samples. Therefore, our data set (3 markers and 40 samples) may reflect bias and provide insufficient evidence to comment upon the population genetic structure. But, similar levels of heterozygosity were reported for gharials of Chambal by both Sharma et al.32 and Singh et al.34. Further, there was similar heterozygosity, low population differentiation and genetic migration between the Girwa and Chambal populations possibly due to intermixing during captive rearing program32. Although Sharma et al.32 have reported that the non-specific marker TGE2 is monomorphic, our results indicate that it is polymorphic (Table 1).
Founder populations play a key role in population dynamics of resultant future captive and wild populations by influencing the genetics35–38. Project Crocodile in India assisted in restocking and recovery of gharial. However, genetic screening of the captive or wild populations were never part of the program. Despite evidences of breeding and increase in population sizes in at least three gharial populations from Girwa, Chambal and Ramganga Rivers respectively, genetic diversity of these populations was largely overlooked. This has been the case for several reintroduction and restocking programs globally39. Restocking into depleting populations or new habitats can facilitate establishment or recovery of released species, and possible gene flow, or it can cause sudden imbalance of trophic structure and genetic failure in terms of loss of genetic structure, genetic diversity, loss of genetic adaptations, admixture, hybridization etc39. Genetic screening of captive and wild animals has been reported in several crocodile species (Alligator sinesis40; C. siamensis15,41−42; C. moreletii43; C. porosus42; C. rhombifer44. This has proven useful in identifying pure bred, genetically diverse and in some case hybrid animals.
Habitat and threats play a key role in population genetics in the wild. Habitat connectivity influences dispersal and movement of crocodiles, thereby affecting the availability of breeding adults and mating opportunities (e.g., gharial populations in Betwa, Ken, Mahanadi, Ramganga Rivers). Populations in limited and saturated habitat may employ compensatory mechanism (adult animals regulating recruitment of young sized animals)45,46. Killing of adult breeding crocodiles through illegal hunting or fishing may reduce breeding efforts and genetic inputs in subsequent breeding cycles (e.g., Son River gharial population5). Gharial population in Chambal is distributed over a stretch of 450 + km of the river and gharials nest on at least 44 locations5,31. In Girwa, the population is restricted to ~ 20 km upstream of barrage and gharials nest at only 3 locations of which two are actively maintained by habitat restoration interventions47. Further, Chambal population has 90 + male gharials compared to only 6–8 males in Girwa5. However, despite substantial differences in habitat, both populations show low genetic differentiation. This could be an outcome of smaller founder population and intensive restocking from a common rearing centre32.
A majority of studies focussed on genetic screening of captive crocodile populations globally have revealed low genetic variability and inbreeding43,48−49. This is due to employing of smaller founder populations from the wild48. Release of genetically related captive animals in the wild further aggravates the issue of inbreeding and low genetic variability43,49. Crocodiles have long life spans and breed regularly if circumstances are favourable. Consequently, the breeding animals in captive programs are often the only source of eggs for a considerably long duration and hence reproducing similar genetic variation in released animals49. Eggs collected from restocked wild populations or ranching sites are also genetically similar to the captive populations. Wild populations isolated due to geographical barriers e.g., inaccessible marshes, river basins, can show high inter population differentiation3,50. However, release of genetically similar captive animals can homogenize these differentiated populations. These factors make it critical for genetic screening of founder populations and released animals.
Anthropocene has contributed to various positive and negative impacts on wild species of flora and fauna. While on one side, human activity pushes wild species populations towards extinction, on the other through conservation initiatives anthropogenic interventions have recovered several species from near extinction. Often these initiatives are able to recover the population sizes, but not the original population dynamics of the species. This is because even though the population sizes are augmented, the habitat is degraded and is unsuitable to sustain the released animals and under anthropogenic threats. For instance, gharial recovery plan in Nepal is facing the stress of excess number of captive gharials without potential release habitat8,51. This is due to captive animals growing beyond viable release age, poor recruitment and breeding in released population, skewed population structure and most importantly lack of uniform monitoring and funding to support the recovery program51.
Due to limited habitat size and absence of natural gene flow, and genetic mixing during captive release program, gharial population at Girwa may be considered as a genetically similar sub sample of the larger gharial population at Chambal. However, release of captive bred animals can contribute in increasing genetic diversity, decreasing fixation of loci and reducing effects of genetic drift induced differentiation, provided the released animals are screened for their genetic structure3,52. Low heterozygosity in crocodiles is attributed to longevity, late maturation and generational overlaps53. However, traits such as longevity and delayed sexual maturity can result in retention of moderate genetic diversity in wild populations while masking effects of population decline such as genetic drift and low effective population sizes. Considering the life history traits of crocodiles, long term genetic assessments combined with population data should be prioritized in crocodile conservation. Lack of an initial baseline genetic assessment in case of gharials in India leaves a gap in understanding the effects of genetic mixing via restocking, however a detailed assessment of all the extant captive and wild population could provide answers to this question.
Our observations from this study as well as from previous works suggest that at Girwa anthropocene has indeed facilitated gene flow to an isolated gharial population via external augmentation and overcome genetic isolation imposed by a man-made structure that served as geographic barrier to natural population interaction.