The invasive Indian bullfrog Hoplobatrachus tigerinus on the Andaman Islands : evaluating drivers of distribution, density, and trophic impact of an early stage invader
| dc.contributor.advisor | Measey, John | en_ZA |
| dc.contributor.author | Mohanty, Nitya Prakash | en_ZA |
| dc.contributor.other | Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology. | en_ZA |
| dc.date.accessioned | 2019-02-11T12:52:22Z | |
| dc.date.accessioned | 2019-04-17T08:24:30Z | |
| dc.date.available | 2019-02-11T12:52:22Z | |
| dc.date.available | 2019-04-17T08:24:30Z | |
| dc.date.issued | 2019-04 | |
| dc.description | Thesis (PhD)--Stellenbosch University, 2019. | en_ZA |
| dc.description.abstract | 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. | en_ZA |
| dc.description.abstract | AFRIKAANSE OPSOMMING: Die wêreldwye verspreiding van mense en hul aktiwiteite verander bewegingspatrone van ander spesies, deur die beperking of bevordering van hul beweging en gevolglik hul verspreiding. Biologiese invalle kom voor wanneer spesies oor hul natuurlike bevolkingsreeks verplaas word deur menslike aktiwiteite na 'n nuwe reeks, waar die spesies voortplant en versprei. Hierdie biogeografiese veranderinge vind op groot skaal plaas teen haas as gevolg van versnelde wêreldhandel en vervoer. Amfibieë is ʼn groep wat toenemend op ʼn globale vlak indring in nuwe omgewings Uitheemse amfibieë het aansienlike ekologiese impak op die inheemse ekostelsel wat deur toksisiteit, kompetisie, predasie en waarskynlike siekteoordrag veroorsaak word. Die vlak van ekologiese impak deur indringende amfibieë is vergelykbaar met dié van indringende visse en voëls. Slegs 'n beperkte aantal spesies is egter goed bestudeer vir hul indringdinamika, wat begrip en bestuur beperk. Die Indiese brulpadda, Hoplobatrachus tigerinus, 'n groot dicroglossid padda (neus tot kloaka lengte: tot 160 mm), is inheems aan die Indiese subkontinent. Ten spyte van die hoë waarskynlikheid van indringersukses vir die brulpadda, gebaseer op spesie-eienskappe en menslike interaksie, is sy invalproses nie geassesseer nie. Hierdie studie het ten doel om vier hoofaspekte van die Indiese brulpadda se inval op die Andaman-eilande te verstaan, waar dit onlangs bekendgestel is: i) rangskikking en verspreiding, ii) die impak van volwassenes iii) die invloed van karnivoor paddavisse, en iv) indringdinamika en doeltreffendheid van potensiële bestuur strategieë. Uiteindelik het die proefskrif gemik op die evaluering van v) die brulpadda se wêreldwye invalpotensiaal en status van alle buite-limietbevolkings. Ek het 'n nuwe benadering aangewend om die Indiese brulpadda inval van die Andaman-eilande te herbou, en die opname van openbare opnames en veldopnames in 'n formele analitiese raamwerk te analiseer. Die brulpadda is teenwoordig in minstens 62% van die steekproewe wat oor ses eilande versprei is, 'n dramatiese toename in die inval streek. Die brulpadda is waarskynlik vroeg in die 2000's bekendgestel, en die eksponensiële uitbreiding het sedert 2009 plaasgevind. 'Verontreiniging' van viskultuurhandel en doelbewuste 'vrylating' is aangewys as die primêre paaie van indringing en na-indrining verspreiding, fasiliteer dus van die indringing uit die Indiese vasteland en tussen-eiland oordragte. Die gebruik van openbare opnames in 'n sistematiese raamwerk voeg 'n komplimentêre instrument by die bestaande metodes om invalle te herbou. Ek het die dieet van die uitheemse indringende Indiese brulpadda en twee mede-voorkomende inheemse paddas (genus Limnonectes en Fejervarya) geëvalueer om die impak van volwasse brulpaddas te bepaal. Vertebrate het die grootste deel van die brulpadda se dieet in terme van volume uitgemaak, terwyl ongewerwelde diere numeries oorheersend was. Ek het net 'n beduidende dieet oorvleueling tussen die brulpadda en individue van die genus Limnonectes gevind. Prooi grootte en tipe is bepaal deur die liggaam grootte van die drie spesies. Hierdie intensiewe studie oor 'n tot dusver onbeoordeelde genus van indringende amfibieë dra by tot die kennis oor die impak van amfibiese invalle. Om die impak van die larwe (paddavis) stadium van die Indiese brulpadda op endemiese amfibieë van die Andaman-eilandgroep te assesseer, het ek 'n mesokosm-eksperiment uitgevoer met brulpadda larwe, die Chakrapani se smalmondige padda, Microhyla chakrapanii, en die Andaman boompadda, Kaloula ghosi. Predasie deur brulpaddas het gelei datgeen endemiese paddavisse oorleef nie, al die paddavisse was binne ʼn tydperk van drie weke opgeëet. In teenstelling hiermee het die enkel-spesies behandelings van M. chakrapanii en K. ghosi gelei tot 'n oorlewing van onderskeidelik 90% en 62%. Die bevolkingsdalings van die inheemse amfibieë is moontlik as gevolg van kombinasie van dìe predasie impak en soortgelykte broeihabitatte. Die studie is tydig, aangesien die vinnig groeiende inval waarskynlik ander inheemse amfibieë sal beïnvloed, insluitende baie Anuran genera wat op formele taksonomiese herbeoordelings wag. Verder bevind die studie die beperkte bestaande kennis oor die impak van amfibiese indringers met karnivoor larwes. Ek het 'n model ontwikkel om die effek van mensgemedieerde translokasies, natuurlike verspreiding en demografie oor die indringdinamika van die Indiese brulpadda te evalueer. Ek het 'n ouderdom gestruktureerde demografiese model gekombineer met 'n swaartekragmodel van menslike invloed, in 'n ruimtelike eksplisiete modelleringskonteks. Menslike invloed het 'n positiewe uitwerking gehad op verspreidingsyfers, wat beide tussen eiland en binne die eilandbeweging van die brulpadda fasiliteer. Interessant genoeg het die model 'n oorheersende uitwerking van menslike invloed op die oorsprong van die inval voorspel. Op grond van die gemodelleerde voorspellings, beveel ek aan onmiddellike implementering van siftings meganismes tussen eilande (veral vir die tot dusver ongekoloniseerde Baratang en Long-eiland). Die verstaan van invalle met gereelde mensgemiddelde translokasies in die buite-limietreeks kan baat vind by die modelleringsbenadering wat in hierdie studie ontwikkel is, wat die gebruik van surrogate van menslike invloed moontlik maak. Uiteindelik het ek die profiel van die Indiese brulpadda beoordeel as 'n tot dusver onbekende en moontlik ontluikende indringerspesie. Afgesien van die fokusarea van die Andaman-eilandgroepl, kon ek net nog 'n suksesvolle inval op Madagaskar bevestig. Gerapporteerde populasies op Maldive en Laccadive-eilande het nie onlangse inhoudelike rekords vir bevestiging nie; Thailand en Kuba het gevangenes en het nie bevolkings in die natuur bevestig nie. 'n Omgewing-nismodel het isotermie, hoë neerslag en menslike aanpassing geïdentifiseer as faktore wat bevorderlik is vir die voorkoms van brulpaddas. Ons het die spesie 'n gestandaardiseerde telling van 'Gematigde' vir 'sosio-ekonomiese impak' toegeken aan die hand van die vermindering van menslike aktiwiteite van pluimvee en bedreiging vir akwakultuur. Net so is 'n omgewingsimpak 'n telling van 'Matig' toegeken, gegrond op gedokumenteerde bevolkings uitdrywings van inheemse amfibieë. Gevolglik, die Indiese brulpadda sal waarskynlik die buite-limietreeks verhoog deur na die Nicobar-eilande en op nuwe plekke van Madagaskar en Andaman-eilande te versprei. Ek het die Nicobar-eilande, Mascarene-eilande, Maleisië en Indonesië geïdentifiseer, en Oos-Afrika is waarskynlik ontvangers van nuwe inleidings. Sifting by intreepunte sal waarskynlik effektief wees vir klein eilande, soos die Andaman- en Nicobar-eilandgroepe weens die relatief lae menslike verkeer wat hulle ervaar. Die proefskrif het 'n variasie metodologiese benaderings gebruik om die indringdinamika van die Indiese brulpadda te verstaan en nuwe insigte te skep wat oordraagbaar is aan ander taksonomiese groepe en kontekste. Die bevindings het teoretiese en toegepaste implikasies vir biologiese indringers en populasie-ekologie in die algemeen. | af_ZA |
| dc.format.extent | 145 pages : illustrations, maps | en_ZA |
| dc.identifier.uri | http://hdl.handle.net/10019.1/106030 | |
| dc.language.iso | en_ZA | en_ZA |
| dc.publisher | Stellenbosch : Stellenbosch University | en_ZA |
| dc.rights.holder | Stellenbosch University | en_ZA |
| dc.subject | Biological invasions | en_ZA |
| dc.subject | Trophic ecology | en_ZA |
| dc.subject | Species distribution modelling | en_ZA |
| dc.subject | Indian bullfrog (Hoplobatrachus tigerinus) -- Andaman Islands | en_ZA |
| dc.subject | UCTD | en_ZA |
| dc.title | The invasive Indian bullfrog Hoplobatrachus tigerinus on the Andaman Islands : evaluating drivers of distribution, density, and trophic impact of an early stage invader | en_ZA |
| dc.type | Thesis | en_ZA |