The dispersal ability, performance and population dynamics of Cape Xenopus frogs

De Villiers, Francois Andre (2016-03)

Thesis (MSc)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: Dispersal is an important life history trait which is present in most organisms, and can affect the population structure, such as survival and age structure, of a species. An organism’s dispersal ability is related to the individual’s performance ability, which is ultimately determined by the individual’s morphology. Amphibians have been classically viewed as poor dispersers, but studies have shown that there are some exceptions. However, these studies represent terrestrial frog species and lack aquatic frog species. Frogs in the genus Xenopus are principally aquatic, and have been known to make use of water corridors for dispersal, however overland dispersal has been observed. There are two Xenopus present in the south Western Cape; X. gilli and X. laevis. In this study I compare and determine the dispersal ability, performance and morphology of X. gilli and X. laevis. I hypothesize that X. laevis will outperform X. gilli in both dispersal and relative performance. Two sites were used in this study; an area east of Kleinmond and the Cape of Good Hope section of TMNP (CoGH). In Kleinmond both X. laevis and X. gilli were marked with PIT tags, whereas only X. gilli was tagged in CoGH. Jumping and swimming performance was obtained by filming the frogs at 240fps, and wet and dry endurance was measured in a 4m track. Dispersal distances between two ponds were represented by Euclidean distances based on recapture of individually marked animals. The results show that X. laevis was the better jumper and swimmer of the two species. All performance trails that included jumping were found to be related to the leg morphology of the two Xenopus species. There was no difference in the dispersal distance of the two species, which was mirrored by the lack of difference in dry endurance; however Xenopus laevis had the highest proportion of individuals dispersing. Population structure is an important tool to determine the state of a given population. In a species that experiences high juvenile predation it is expected that the age structure would be skewed towards the older individuals and that the juveniles will have a low survival rate. Xenopus laevis poses a threat to X. gilli through predation, competition and hybridization. I compared the survival and age structure of X. laevis and the two X. gilli populations. I hypothesized that X. laevis will have a negative effect on the survival and age structure of X. gilli. Recaptures at Kleinmond and CoGH recurred between three to six weeks. The second phalange of the outer toe of 20 males and 20 females of X. laevis and X. gilli at both sites were sectioned. The number of lines of arrested growth (LAG) were recorded as a proxy for age. All frogs that were recaptured in 2015 were photographed again and growth was determined. The results indicate that Kleinmond X. gilli had the lowest survival (0.319), which was accredited to competition and predation by X. laevis, whereas CoGH X. gilli had the highest survival (0.562). Age structure was not determined possibly due to the presence of artificial water-bodies and consequently the lack of LAG production in some individuals.

AFRIKAANSE OPSOMMING: Verspreiding is 'n belangrike lewensgeskiedenis eienskap wat teenwoordig is in meeste organismes. Prestasie vermoë is verwant aan die individu se verspreiding vermoë, wat uiteindelik geaffekteer word deur die morfologie van die individu. Amfibieë word oor die algemeen as swak verspreiers beskou, maar studies het getoon dat daar 'n paar uitsonderings is. Verspreiding vermoë word oor die algemeen gebestudeer in terrestrielle padda en nie akwatiese paddas nie. Paddas van die genus Xenopus is hoofsaaklik akwaties en maak meestal van waterverspreiding gebruik, alhoewel oorland verspreiding waargeneem is. Daar’s twee Xenopus teenwoordig in die suid Wes-Kaap; X. gilli en X. laevis. In hierdie studie vergelyk en bepaal ek die verspreiding, prestasie vermoë en morfologie van X. gilli en X. laevis. Twee areas word in hierdie studie gebruik; Kleinmond en die Kaap van Goeie Hoop deel van TMNP (CvGH). In Kleinmond was beide X. laevis en X. gilli ingeent met 'n PIT merker, terwyl slegs X. gilli gemerk was in CvGH. Spring en swem prestasie is verkry die paddas teen 240fps af te neem en beide nat en droë uithouvermoë is gemeet deur die paddas rondom 'n 4m te jaag baan. Verspreiding afstande tussen twee damme is verteenwoordig deur Euklidiese afstande. Die resultate toon dat X. laevis die beter springer en swemmer van die twee spesies was. Alle prestasie eienskappe wat spring ingesluit het, was verwant aan die been morfologie. Daar was geen verskil in die verspreiding vermoë tussen die twee spesies nie, wat aan die gebrek in verskil in droë uithouvermoë geakkrediteer word. Xenopus laevis het die hoogste persentasie van individue wat versprei het gehad. Die oorlewing en ouderdom struktuur is belangrike fasette om bevolking struktuur te verstaan. Hier word twee hipotetiese bevolkings aangebied; een gesonde bevolking en een met 'n bevolking met eensydige predasie druk. In die tweede bevolking sal die oorlewing van jeugdiges verwagtend laag wees en die ouderdom struktuur sal na verwagting swyk na volwassenes. Xenopus laevis hou ‘n bedreiging in vir X. gilli deur predasie, kompetisie en verbastering. Ek vergelyk die oorlewing en ouderdom struktuur van X. laevis en die twee X. gilli bevolkings. Die paddas was in Kleinmond en CvGH elke drie tot ses weke gevang en hervangs was in binêr aangeteken. Die tweede kootje van die buitenste toon van 20 mannetjies en 20 wyfies van X. laevis en X. gilli in beide areas was gesny. Die aantal lyne van stuitgroei (LVS) is aangeteken as 'n aanduiding van die ouderdom. Al die paddas wat in 2015 gevang is was weer afgeneem en groei is bepaal. Die resultate dui daarop dat Kleinmond X. gilli die laagste oorlewing (0.319)het, wat aan die kompetisie en predasie deur X. laevis geakkrediteer. Die ses maandelikse oorlewings skatting van X. laevis was 0.381. Ouderdom struktuur kon nie vasgestel word as gevolg van die teenwoordigheid van 'n kunsmatige water-liggame en gevolglik is die gebrek aan LVS produksie.

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