The impact of Xenopus laevis predation on aquatic ecosystems

Thorp, Corey James (2017-03)

Thesis (MSc)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: Predator-prey interactions are dynamic and the ability to predict their impact on prey species has become an important aspect in ecology. One method to predict the impact of a predator species on a prey population is by analysing the predator’s functional response. However, predators are not all functionally similar and may differ intraspecifically. Predators are also not limited to prey from other species as they can cannibalise vulnerable individuals within their own population. The African clawed frog (Xenopus laevis) is a predator with a broad diet, known to consume multiple prey species, including its congeners. They are notorious cannibals with populations consisting of different sized conspecifics. They occur in sympatry with several congeners including the endangered X. gilli which are thought to be under threat through competition, hybridisation and predation from X. laevis. In this study, I investigated the role of predator size on the functional response of X. laevis predators using mosquito larvae (Culex pipiens) as a common prey. I also investigated the threat of X. laevis predation on X. gilli using choice and no-choice experiments to evaluate the relative vulnerability of X. laevis and X. gilli larvae to X. laevis predation. For the functional response experiments, predators were classified by size into small (15-30mm snout-vent length), medium (50-60mm) and large (105-120mm) size classes. Predator-prey interactions were filmed in order to compare handling time and attack rate to the functional response model. In the choice and no-choice experiments, both X. laevis and X. gilli larvae species were collectively and separately exposed to treatments with the presence or absence of a predator. Results showed that the functional response of X. laevis predators change with size: small predators were found to have a Type II response, while medium and large predators had a Type III response. Both functional response and behavioural data showed an inversely proportional relationship between predator attack rate and predator size. Small and medium predators had the highest and lowest handling time, respectively. That the functional response was found to change with the size of predator suggests that predators with overlapping cohorts may have a dynamic impact on prey populations. Therefore, predicting a predator’s impact from the functional response of a single size-matched predator experiment may be a misrepresentation of the predator’s potential impact on a prey population. Results from the choice and no-choice experiments showed that large X. gilli showed a significantly higher vulnerability to X. laevis predation compared to small X. laevis. Large and small X. laevis larvae, and same size X. gilli and X. laevis larvae showed no significant differences in relative vulnerability. Behaviour may be a factor in contributing to large X. gilli larvae’s vulnerability to X. leavis predation, and this will likely have negative implications for the population structure of the endangered X. gilli. Keywords Attack rate; cannibalism; feeding; functional response; habitat; handling time; predator; size; vulnerability

AFRIKAANSE OPSOMMING: Roofdier-prooi interaksies is dinamies en die vermoë om te voorspel die impak daarvan op prooispesies het 'n belangrike aspek in die ekologie word. Een metode om die impak van 'n roofdier spesies op 'n prooibevolking voorspel is deur die ontleding van funksionele respons die roofdier se. Maar roofdiere is nie almal funksioneel soortgelyk en kan intraspecifically verskil. Roofdiere is ook nie beperk tot prooi van ander spesies as hulle kan cannibalize op kwesbare individue binne hul eie bevolking. Die Afrikaanse klou kikker (Xenopus laevis) is 'n roofdier met 'n breë dieet, bekend om verskeie prooi spesies, waaronder die conge verteer. Hulle is berug kannibale met bevolkings wat bestaan uit verskillende grootte indringing. Hulle kom in sympatry met verskeie conge insluitend die bedreigde X. gilli wat gedink moet word bedreig deur die kompetisie, verbastering en predasie van X. laevis. In hierdie studie ondersoek ek die rol van roofdier grootte op die funksionele reaksie van X. laevis roofdiere met behulp van muskietlarwes (Culex pipiens) as 'n algemene prooi. Ek ondersoek ook die bedreiging van X. laevis predasie op X. gilli behulp keuse en geen keuse eksperimente om die relatiewe kwesbaarheid van X. laevis en X. gilli larwes om X. laevis predasie te evalueer. Vir die funksionele reaksie eksperimente, is roofdiere geklassifiseer volgens grootte in klein (15-30mm snoet-vent lengte), medium (50-60mm) en groot (105-120mm) grootte klasse. Roofdier-prooi interaksies verfilm om die hantering van tyd en aanval koers te vergelyk met die funksionele reaksie model. In die keuse en geen keuse eksperimente, was beide X. laevis en X. gilli larwes spesies gesamentlik en afsonderlik blootgestel aan behandelings met die teenwoordigheid of afwesigheid van 'n roofdier. Resultate het getoon dat die funksionele reaksie van X. laevis roofdiere verander met grootte: klein roofdiere is bevind dat 'n Tipe II reaksie het, terwyl medium en groot roofdiere n Tipe III reaksie gehad. Beide funksionele reaksie en gedrag data toon 'n omgekeerd eweredig verhouding tussen roofdier aanval koers en roofdier grootte. Klein en medium roofdiere het die hoogste en laagste hantering tyd, onderskeidelik. Dat die funksionele reaksie is gevind om te verander met die grootte van roofdier dui daarop dat roofdiere met oorvleuelende kohorte n dinamiese impak op prooi bevolkings kan hê. Daarom, die voorspelling van die funksionele reaksie van 'n enkel-grootte ooreenstem roofdier eksperiment kan 'n wanvoorstelling van potensiële impak van die roofdier se op 'n prooibevolking wees. Resultate van die keuse en geen keuse eksperimente het getoon dat 'n groot X. gilli het 'n aansienlik hoër kwesbaarheid vir X. laevis predasie in vergelyking met klein X laevis. Groot en klein X. laevis larwes, en dieselfde grootte X. gilli en X. laevis larwes het geen betekenisvolle verskille in relatiewe kwesbaarheid. Gedrag kan 'n faktor in die bydrae tot kwesbaarheid groot X gilli larwes se X. laevis vasgemaak predasie wees, en dit sal waarskynlik negatiewe gevolge vir die bevolking struktuur van die bedreigde X gilli. Trefwoorde Aanval koers; kannibalisme; voeding; funksionele reaksie; habitat; hanteringstyd; roofdier; grootte; kwesbaarheid

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