Seasonal variation and the influence of environmental gradients on Batrachochytrium dendrobatidis infections in frogs from the Drakensberg mountains
Meyer, Leon Nicolaas
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The Batrachochytrium dendrobatidis fungus has been implicated in the decline of many frog species as well as the extinction of some throughout the world. Apart from this, declines in some amphibian populations are also caused by variations in temperature. It has been proposed that the cause of the decline or apparent extinctions of at least 14 high elevation species of the Australian tropics were due to B. dendrobatidis. The main aim of this study was to determine the effect of seasonal variations on B. dendrobatidis infections and the influence these have on frog populations in the Drakensberg Mountains in South Africa. In one part of this study, frog populations from different altitudes in the Royal Natal National Park and Mont-aux Sources in the Drakensberg region were monitored; Hadromophryne natalensis from low altitude sites and Amietia vertebralis from high altitude sites. Batrachochytrium dendrobatidis was detected in the field by using a 10x hand lens and in the laboratory with a compound microscope. No mortality has yet been observed in H. natalensis, but A. vertebralis is disease-susceptible and die-offs do occur. Most of the mortalities have therefore occurred at high altitudes where temperature levels vary from cold to moderate. This pattern of susceptibility with regard to altitudinal gradient is reflected in case studies from the Australian and American tropics. Although B. dendrobatidis is prevalent throughout the year at both high and low altitudes, prevalence levels peak in winter and spring. It is important for conservation strategies of montane amphibian communities to determine whether the observed mortalities constitute evidence of actual declines or whether these can be regarded as part of natural fluctuations in population size. Although no declines have been observed as yet, the chance exists that declines could occur because A, vertebralis is susceptible to the pathogen. Another part of this study was conducted with emphasis on the breeding behaviour of A. vertebralis which is a semi-aquatic, high-elevation frog endemic to the Drakensberg Mountains and the Lesotho highlands. This species breeds in slow-flowing streams and associated pools with sandy bottoms. Published data indicates that breeding occurs after the first spring rains in September and continues until March. The objective of this part of the study was to gain insight into the breeding biology of A. vertebralis by studying empirical data gained from its tadpoles. Tadpoles were collected on a bimonthly basis over a two-year period for staging and measurement. Casual observations on adults and egg clutches were also documented. Contrary to what has been documented, amplecting A. vertebralis pairs were observed as early as July; however, this could be an indication that they are opportunistic breeders. Tadpoles of different lengths and stages were collected throughout the year, supporting the notion that these frogs have an extended breeding season or that the breeding season is correctly described in the literature, but the development of the tadpoles takes place over an extended period of time. A preliminary study was conducted on the distribution of B. dendrobatidis along an altitudinal transect. Frogs were collected and DNA swabs were taken of each specimen and analysed with qPCR sequencing. Infection was found at every site across the transect except for one. Altitude did not play an influential role in infection levels of this pathogen. Rainfall had a negative correlation with prevalence at some stages when floods occurred, otherwise prevalence increased gradually according to rainfall. Temperature did influence prevalence infections, but a consistent pattern according to correlation with prevalence infections was not observed. In conclusion, chytrid has a widespread distribution across southern Africa and has no preference to infect only certain species. Most of the species that have been sampled were found to have been infected.
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