Abstract
The provision of artificial sources of water for domestic stock in arid Australia has resulted in high levels of grazing over extensive areas, and the impact on native fauna is likely to be widespread. However, there is little direct evidence of the effect of grazing on lizards, and particularly on the relationship between climate variability and grazing in detennining the distribution and abundance of lizard assemblages. This study investigates the effect of grazing on ground-active lizards in a highly variable climate. Populations of ground-dwelling lizards were monitored at eight sites between September 1993 and December 1995. The study sites were in mulga (Acacia aneura) tall shrubland on a pastoral property in central Australia, where sites were categorised as having high and low levels of grazing based on their distance from artificial water points for cattle.Because lizards were sampled solely by means of pitfall traps, I conducted an experiment to test the hypothesis that ground cover affects the efficiency of pitfall traps in capturing lizards. Reducing ground cover had no effect on the subsequent rates at which lizards were captured, compared with control sites, or rates of capture before grass was cut. I conclude that pitfall-trapping is a valid technique for comparing lizard populations in mulga shrublands which are subject to different levels of grazing within the range of vegetation covers used in this study.
Twenty-three species and 2249 individual lizards were captured during the study. For six frequently captured species, patterns of activity and abundance and variation in growth and body condition (a measure of mass in relation to body length) were considered in detail. The time when lizards were active in different years was variable and did not correspond closely to seasons, except for geckos. Three diurnally active species (the skinks Ctenotus leonhardii and Ctenotus schomburgkii and the agarnid Ctenophorus nucha/is) delayed spring activity considerably after a long dry period in 1994, which coincided with low rates of plant growth, but after rain in early 1995 lizards maintained high levels of activity into autumn and winter. Growth rates and body condition of lizards also varied over the period of study among these species. Body condition was low in spring 1994 during the dry period. In 1995, when rainfall was higher than average, activity levels were high and body condition improved. Growth rates were also higher in 1995 when lizards were active for longer, compared with 1994, but growth rate per unit time active was comparable between the two years. In contrast to the diurnal lizards, the activity of three gecko species (Diplodactylus conspicillatus, Diplodactylus stenodactylus and Rhynchoedura ornata) was more consistently correlated with temperature, and activity was generally restricted to the warmer months of each year. Body condition of geckos did not differ between years. Correlations between the activity of each species of lizard and environmental variables, including temperature, invertebrate biomass and plant growth, differed according to the time scale considered and when each species was active in a particular year. It appears that temporal patterns of activity are crucial in determining body condition and rate of growth of lizards at different times because they determine how effectively lizards can make use of available resources.
The agarnid Ctenophorus nucha/is was found almost exclusively at sites that were close to artificial water points, with high levels of grazing. These sites were characterised by comparatively large areas of open ground. In contrast, the total number of skinks captured was greater at sites with low levels of grazing in nine of eleven species and the abundance of skinks as a family group was significantly higher at sites with low levels of grazing. Ctenotus leonhardii and Ctenotus schomburgkii, were generally more abundant at sites with low grazing levels, but this trend varied with time. Differences in skink abundance between sites with different grazing levels were least apparent in the year when activity was delayed.
Vegetation cover was measured twice during the study. The relative cover of ground vegetation among sites was not consistent between years, but despite this variation, the abundance of skinks was positively correlated with the cover of ground vegetation at different sites in spring 1994 and 1995. In the dry year (1994), when abundance of lizards was low, there was no significant difference in the number of skinks captured, or the cover of ground vegetation, between sites with high and low levels of grazing. In the wet year ( 1995) the cover of vegetation was higher at sites with low levels of grazing and the abundance of skinks was also higher at these sites. Hence, in this environment, differences in skink abundance between sites with different grazing levels appear to be related to differences in ground vegetation and are not always perceptible because the cover of vegetation is highly variable over time. The two species of Ctenotus also had higher body condition at sites with low levels of grazing compared to at heavily grazed sites at the end of 1995 but there were no differences in body condition with respect to grazing level in 1994. There were no significant
differences in the abundance or body condition of nocturnal species between sites with different levels of grazing at any time, and the abundance of these species was not correlated with vegetation cover.
Cover from vegetation at canopy level was higher at sites with low levels of grazing, and shaded micro-habitats were more common across space and time at these sites. Shrub canopy and ground vegetation may both provide important sources of cover for diurnally active lizards when temperatures are very high, and differences in the shelter provided by plants at sites with different levels of grazing may be significant for these species. The skinks captured in this study are widely foraging species and thus are exposed to a range of thermal micro-habitats which vary according to vegetation cover. These skinks probably also spend a considerable amount of time foraging within and around clumps of grass. Agamids, like the skinks captured in the study area, are heliotherntic, diurnally active lizards, but their physiology, foraging mode and thermoregulatory behavior differ considerably from the skinks. Agamids are less likely to be adversely affected by high temperatures and some species probably benefit from large areas of open ground if suitable foraging perches are available. Groundactive geckos and pygopods, in contrast, may not be affected by grazing in mulga shrublands, because the effect of vegetation cover on the thermal environment is largely irrelevant for nocturnal species.
This study has identified clear differences between nocturnal and diurnal lizards in their response to variable rainfall and in their abundance at sites with different levels of grazing. Further work is required to deterntine whether these differences are consistent under other climatic sequences and in other habitats. Specific focus on the relationship between the abundance of some lizard species and the cover of vegetation is also necessary. Grazing by cattle has resulted in considerable change to habitats in the central Australian region. This study has shown that patterns in the landscape, particularly variations in ground cover, which result from concentrated grazing around artificial water points affect the distribution and abundance of some lizard species. Based on these findings I recommend that the current diversity of habitats be maintained until further information is available, and particularly that the areas which are currently remote from water be preserved.
| Date of Award | May 1999 |
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| Original language | English |
| Supervisor | Keith Christian (Supervisor) |