Results - A geographical analysis of melanistic cordylid lizard

By Julie Foster,2014-10-25 18:29
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Results - A geographical analysis of melanistic cordylid lizard

Results for some of the student projects

    Project: Morphological and behavioral correlates of melanism (Dahné du Toit, Ph.D. student)

    As part of her Ph.D. study on melanistic cordylids, Dahné investigated the climatic conditions associated with the distribution of these species. A summary of the results is presented here.

    Most melanistic (black) cordylid lizard species occur in small populations along the west coast of South Africa and in the western parts of the Cape Fold Mountains. However, there are two exceptions. In contrast to the coastal or montane distribution of most of these species, Peers’Girdled Lizard (Cordylus peersi) is found in Namaqualand, and in contrast to the limited

    distribution ranges of most species, the Graceful Crag Lizard (Pseudocordylus capensis) has a

    relatively wide distribution. A detailed analysis of the geographical distribution of melanistic cordylid species, and a search for climatic correlates to explain the observed patterns of distribution,

    were conducted. A geographic

    information system (GIS) was used

    to determine altitude, minimum

    temperature, maximum temperature,

    potential evaporation, precipitation,

    solar radiation, fog, and cloud cover

    for each locality. Data were

    analyzed by means of a Principal

    Component Analysis (PCA).

    Melanistic species with restricted

    distribution ranges were

    characterized by a specific set of

    climatic variables, while the more

    widespread species spanned a wide

    range of climatic conditions. In

    general, there was a good correlation between the distribution of coastal and montane populations and cool climatic conditions. Coastal species were mainly associated with a high incidence of fog and minimum temperatures, while montane species were associated with cloud cover and rainfall. A dark body would absorb radiant energy more efficiently than a light body, and melanism is thus often considered an advantage in a cool environment. However, this correlation does not hold true for all melanistic species, as Pseudocordylus capensis and Cordylus peersi were associated with

    warmer climatic conditions. This suggests that these two species have a broader tolerance range for warm climatic conditions than other melanistic cordylids, or that these species occupy very specific habitats within their distribution range. A regional scale analysis of climatic variables will not reveal the climatic conditions associated with specific microhabitats. A fine-scale investigation of the microhabitats- and climates associated with these species was conducted in order to better understand the ecology of melanistic cordylids in warm climates. Results will be available soon.

    Project: Armadillo lizards and termites (Cindy Shuttleworth, M.Sc. student)

    Cindy recently completed her M.Sc. thesis and received her degree in December 2006. Several articles are being prepared for publication and will be posted as they become available. A summary of her work is presented here.

The role of the southern harvester termite, Microhodotermes viator, and several climatic

    parameters in the distribution of the group-living lizard, Cordylus cataphractus, was investigated.

    Microhodotermes viator is considered the most important prey item of C. cataphractus and

    termitophagy as the causative agent in the evolution of group-living in this species. One would therefore expect a high degree of correspondence in the ranges of C. cataphractus and M. viator.

    As climate will also play a role in the distribution of any

    species, various climatic variables were investigated to

    determine their influence on the distribution of C.

    cataphractus. Species distributions were visualized

    using the minimum polygon technique and the degree

    of overlap was determined using standard geographic

    information systems (GIS) techniques. A total of 52 C.

    cataphractus localities were investigated for the

    presence of termites. The climatic limits of the

    geographical distribution of C. cataphractus were

    investigated by means of three models, namely

    Classfication Trees, General Discriminant Analysis and

    Logistic Regression. Climatic data included mean

    annual precipitation; mean annual temperature; solar

    radiation per month; mean daily minimum temperature;

    mean daily maximum temperature; potential

    evaporation; and median rainfall. A topographic

    variable, altitude was also included. The range of C. cataphractus was completely included within

    the range of M. viator. Microhodotermes viator was included in the diet of C. cataphractus at 73%

    of the localities sampled within the lizard’s range. The current geographical range of C.

    cataphractus is mainly determined by two climatic factors, namely the low summer rainfall and high monthly solar radiation. The restricting role of both these factors can be directly linked to the

    group-living nature of C. cataphractus.

    If termitophagy were the overarching cause of group-living in C.

    cataphractus, then one would expect a close relationship

    between termite density and lizard density and termite density

    and lizard group size. These relationships were investigated at

    both a local and regional scale. For the local scale study, 25

    quadrates of 25×25m were plotted at a selected site, and for the

    regional scale study, ten 35×35m quadrates at sites throughout

    the lizard’s range were used. In each quadrate, a whole range of

    variables were recorded, the most important of which were lizard

    density, lizard group sizes, termite foraging port density, distance

    to nearest termite foraging ports, vegetation height and

    vegetation cover. It was found that the density of termite foraging ports determines C. cataphractus density. Vegetation height and cover furthermore effects crevice selection by C. cataphractus groups, probably because an unobstructed view is necessary to locate termite activity at foraging ports.

Possible differences in the use of termites by different sized groups of C. cataphractus during

    different times of the year were also investigated. Faecal samples, collected once a month at Eland’s Bay from small, medium and large groups from January 2005 to December 2005, were

    analysed for the presence of termite head material. It was found that large groups fed on termites

    to a greater extent than small groups during certain times of the year and there was a general tendency for this phenomenon throughout the year.

The results obtained in this study indicate that the southern harvester termite, M. viator, plays a

    central role in the ecology of the group-living lizard, C. cataphractus.

Project: The GCBC and reptile distributions (Anita Meyer, M.Sc. student)

    For her M.Sc. study, Anita is conducting a biogeographical analysis of reptile distributions and diversity. Preliminary results and maps are presented here.

    Good progress is being made with the sifting, correction and mapping of the reptile distribution records obtained from CapeNature’s State of Biodiversity (SOB) Database. The mapped records of each reptile species were translated to a grid system and subsequently the first preliminary analyses of the data were run. The following maps are representations of the reptilian species assemblages and the species richness throughout the study area (soutwestern cape area south of 31?S and west of 21?E). The particular maps show the results at a cell resolution of an eighth of a degree.

Map 1

    Preliminary map depicting species richness of reptiles in the soutwestern Cape region. The most direct way of representing species richness (or alpha diversity) is simply a species count per unit area. The darker the hue, the higher the number of species present within each cell. Cells with the highest species richness often coincide with reserve localities. Two of the areas with very low species richness are known agricultural areas (Sandveld and Overberg regions) with severe habitat destruction for which low species richness values are expected. The north-western region of the study area is known to be under-sampled and therefore the low species richness may not be a true indication of reptile diversity.

Map 2

    For the following map, cells were divided into five different groups on the basis of similarity in their species compositions with the aid of a hierarchical clustering program. Cells from each of the five groups were then mapped in a specific colour. The subsequent map is a useful visual aid showing that cells which share the same species assemblages cluster together. These computer generated clusters reflect distinct spatial faunal assemblages, such as a west-coast fauna, south-western cape fauna, cape fold mountain fauna and a more northern to inland fauna.

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