Browsing by Author "Mohanty, Nitya Prakash"
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- ItemThe invasive Indian bullfrog Hoplobatrachus tigerinus on the Andaman Islands : evaluating drivers of distribution, density, and trophic impact of an early stage invader(Stellenbosch : Stellenbosch University, 2019-04) Mohanty, Nitya Prakash; Measey, John; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.ENGLISH ABSTRACT: The global spread of humans and their activities change movement patterns of other species, by limiting or enhancing their movement and consequently their distribution. Biological invasions occur when species are moved beyond their natural range by human activities to a new range, where the species reproduce and spread. These biogeographic changes now occur with rapidity on large scales due to accelerating global trade and transport. Amphibians are an emerging group of invaders, with increasing global frequency of invasive populations. Invasive amphibians have considerable ecological impact on the recipient system mediated through toxicity, competition, predation, and probable disease transmission. The level of ecological impact by invasive amphibians is comparable to that of invasive fish and birds. However, only a limited number of species have been well-studied for their invasion dynamics, limiting understanding and management. The Indian bullfrog Hoplobatrachus tigerinus, a large dicroglossid frog (snout to vent length: up to 160 mm), is native to the Indian sub-continent. Despite the high likelihood of invasion success for the bullfrog, based on species-traits and human-interaction, its invasion process has not been assessed. This study aimed to understand four major aspects of the Indian bullfrog’s invasion on the Andaman Islands, where it has recently been introduced: i) distribution and dispersal, ii) impact of adults iii) impact of carnivorous tadpoles, and iv) invasion dynamics and efficacy of potential management strategies. Finally, the thesis aimed to assess v) the bullfrog’s global invasion potential and status of all extra-limital populations. I used a novel approach to reconstruct the Indian bullfrog invasion of the Andaman Islands, combining public surveys and field surveys in a formal analytical framework. The bullfrog occurred in at least 62% of the sampled sites spread over six islands, a dramatic increase to the previously known invaded range. The bullfrog was most likely introduced in early 2000s, and its exponential expansion has occurred since 2009. ‘Contaminants’ of fish culture trade and intentional ‘release’ were reported to be the primary pathways of introduction and post-introduction dispersal, facilitating introductions from the Indian mainland and inter-island transfers. The use of public surveys in a systematic framework adds a complimentary tool to the existing methods for reconstructing invasions. I assessed the diet of the invasive Indian bullfrog and two co-occurring native frogs (genus Limnonectes and Fejervarya) to assess the impact of adult bullfrogs. Vertebrates made up the majority of the bullfrog’s diet in terms of volume, whereas, invertebrates were numerically dominant. I only found a significant dietary overlap between the bullfrog and individuals of the genus Limnonectes. Prey size electivity was governed by body size of the three species. This intensive study on a hitherto unassessed genus of invasive amphibians contributes to the knowledge on impacts of amphibian invasions. To assess the impact of the larval (tadpole) stage of the Indian bullfrog on endemic anurans of the Andaman archipelago, I carried out a mesocosm experiment with larval bullfrogs, the Chakrapani’s narrow-mouthed frog, Microhyla chakrapanii, and the Andaman tree frog, Kaloula ghosi. Predation by bullfrog tadpoles resulted in no survival of endemic tadpoles, with all individuals being consumed within a three-week period. In contrast, the single-species treatments of M. chakrapanii and K. ghosi led to a survival of 90% and 62% respectively. This predation impact is likely to translate to population declines in anurans which co-occur with and breed in similar habitats as the bullfrog. The study is timely as the rapidly expanding invasion is likely to affect other native anurans including many anuran genera that are awaiting formal taxonomic re-assessments. Further, the findings augment the limited existing knowledge on the impact of amphibian invaders with carnivorous larvae. I developed a model to evaluate the effect of human-mediated translocations, natural dispersal, and demography on the invasion dynamics of the Indian bullfrog. I combined an age-structured demographic model with a gravity model of human influence, in a spatially explicit modelling context. Human influence had a positive effect on spread rates, facilitating both between island and within island movement of the bullfrog. Interestingly, the model predicted an overriding effect of human influence on origin of the invasion. Based on the modelled predictions, I recommend immediate deployment of screening mechanisms between islands (especially for the hitherto uncolonized Baratang and Long Island). Understanding invasions with frequent human-mediated translocations in the extra-limital range, can benefit from the modelling approach developed in this study, which allows for utilization of surrogates of human influence. Finally, I assessed the profile of the Indian bullfrog as a potentially emerging invasive species. Apart from the focal study area of the Andaman archipelago, I could only confirm another successful invasion on Madagascar. Reported populations on Maldives and Laccadive Islands do not have recent substantive records for validation; Thailand and Cuba have captive individuals and do not have confirmed populations in the wild. An environmental niche model identified isothermality, high precipitation, and human modification as factors conducive for bullfrog occurrence. I assigned the species a standardized score of ‘Moderate’ for ‘socio-economic impact’, on account of reduction in human activities of poultry keeping and threat to aquaculture. Similarly, ‘environmental impact’ was assigned a score of ‘Moderate’, based on documented population extirpations of native anurans under experimental conditions. Overall, the Indian bullfrog is likely to increase it extra-limital range by spreading to the Nicobar Islands and in new locations of Madagascar and the Andaman Islands. I identified the Nicobar Islands, Mascarene Islands, Malaysia and Indonesia, and East Africa to be likely recipients of new introductions. Screening at points of entry is likely to be effective for small islands, such as the Andaman and Nicobar archipelagos, due to the relatively low human traffic they experience. The thesis used a suit of methodological approaches to understand the invasion dynamics of the Indian bullfrog and generated novel insights that are transferable to other taxonomic groups and contexts. The findings have theoretical and applied implications for biological invasions and population ecology in general.
- ItemThe terrestrial life of sea kraits : insights from a long-term study on two Laticauda species (Reptilia: Squamata: Elapidae) in the Andaman Islands, India(Wildlife Information Liaison Development Society, 2018-10-26) Tyabji, Zoya; Mohanty, Nitya Prakash; Young, Erina; Khan, Tasneem; Voris, Harold K.Sea kraits forage in water and return to land to digest their prey, mate, slough, and lay their eggs. The temporal terrestrial patterns in encounter rate and behaviour of two species of sea kraits Laticauda colubrina and L. laticaudata were studied over four years at the New Wandoor beach in the southern Andaman Islands. The encounter rate of L. colubrina was found to be 20 times higher than L. laticaudata, and sea kraits were observed to prefer the natural refuge that the microhabitat of uprooted trees provide. Additionally, nesting observations are presented that emphasize the need to promote the conservation of these crucial terrestrial habitats.
- ItemWhat's for dinner? Diet and potential trophic impact of an invasive anuran Hoplobatrachus tigerinus on the Andaman archipelago(PeerJ, 2018-10-02) Mohanty, Nitya Prakash; Measey, John; Colman, BenENGLISH ABSTRACT: Amphibian invasions have considerable detrimental impacts on recipient ecosystems. However, reliable risk analysis of invasive amphibians still requires research on more non-native amphibian species. An invasive population of the Indian bullfrog, Hoplobatrachus tigerinus, is currently spreading on the Andaman archipelago and may have significant trophic impacts on native anurans through competition and predation. We carried out diet analyses of the invasive H. tigerinus and native anurans, across four habitat types and two seasons; we hypothesized that (i) small vertebrates constitute a majority of the H. tigerinus diet, particularly by volume and (ii) the diet of H. tigerinus significantly overlaps with the diet of native anurans, thereby, leading to potential competition. We assessed the diet of the invasive H. tigerinus (n = 358), and individuals of the genera Limnonectes (n = 375) and Fejervarya (n = 65) and found a significant dietary overlap of H. tigerinus with only Limnonectes. Small vertebrates, including several endemic species, constituted the majority of H. tigerinus, diet by volume, suggesting potential impact by predation. Prey consumption and electivity of the three anurans indicated a positive relationship between predator-prey body sizes. Individuals of H. tigerinus and Fejervarya chose evasive prey, suggesting that these two taxa are mostly ambush predators; individuals of Limnonectes chose a mixture of sedentary and evasive prey indicating that the species employs a combination of ‘active search’ and ‘sit and wait’ foraging strategies. All three species of anurans mostly consumed terrestrial prey. This intensive study on a genus of newly invasive amphibian contributes to knowledge of the impact of amphibian invasions, and elucidates the feeding ecology of H. tigerinus, and species of the genera Limnonectes and Fejervarya. We also stress the necessity to evaluate prey availability and volume in future studies for meaningful insights into diet of amphibians.
- ItemWhy have a pet amphibian? insights from youtube(Frontiers, 2019-03-04) Measey, John; Basson, Annie; Rebelo, Alexander D.; Nunes, Ana L.; Vimercati, Giovanni; Louw, Marike; Mohanty, Nitya PrakashThe desire to own a pet amphibian is growing, and with it a growth in amphibian trade and in negative impacts on native populations, including disease transmission and invasive amphibian populations. We know very little about how or why people choose amphibians as pets, but amphibian owners share large numbers of videos on freely accessible platforms, such as YouTube. We aimed to use videos of captive amphibians to determine which species are kept, their life-history stage and the types of videos uploaded. We watched and categorized 1,162 videos by video type, type of amphibian behavior and amphibian taxonomy (superfamily, family, and species). We used data on the amphibian trade from the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), on conservation status from the International Union for Conservation of Nature (IUCN) red list, and on potential environmental impact from published Environmental Impact Classification of Alien Taxa (EICAT) records, to determine potential conflicts of owning pet amphibians. We recorded 173 captive species in 847 videos with a taxonomic overrepresentation of salamandroids and pipoids, and an underrepresentation of ranoids and plethodontoids. When compared to videos of wild amphibian species, videos of captive animals featured disproportionate amounts of adults feeding, being handled and moving. The videos watched had a smaller proportion of threatened amphibian species, but a higher proportion of invasive species, than would be expected by chance, with the proportion present in CITES appendices (18%) being non-significant. We suggest that such data can be used to profile potential pets for trade and attempt to avoid conflicts with threatened and highly impacting alien species.