Browsing by Subject "Conservation Biology"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Open Access
    Effects of temperature on gular fluttering and evaporative water loss in four sympatric cormorants in southern Africa
    (2014) Campbell, Greg Thomas; Cook, Timothée; Sherley, Richard; Ryan, Peter G
    Climate change continues to cause rising air and sea surface temperatures and changes in precipitation patterns across the globe. Many seabirds will be challenged by increasing temperatures because they must balance conflicting adaptations for dealing with cold environments when foraging and hot environments when nesting. Heat stressed seabirds often gular flutter for thermoregulation, a behaviour that is effective for dissipating heat but expensive in terms of evaporative water loss. This study examined gular fluttering behaviour of four species of southern African cormorants, crowned ( Microcarbo coronatus ), Cape ( Phalacrocorax capensis ), bank ( Phalacrocorax neglectus ), and white-breasted ( Phalacrocorax carbolucidus) cormorants. Results show that gular fluttering is influenced by temperature, body position and body size. Gular fluttering increases with temperature and larger cormorant species spend a greater proportion of time gular fluttering for a given temperature. Threshold temperatures for initiating gular fluttering are lower for large than for small cormorant species. Proportions of time spent gular fluttering are higher when birds are sitting than when crouching over the nest. Water loss shows the same pattern as gular fluttering, with the larger species estimated to lose a higher percentage of their daily water intake. Larger cormorant species can lose as much as 40% of their daily ingested water after eight hours of gular fluttering. These findings indicate that temperature increases from climate change will likely have serious direct impacts on nesting cormorant colonies in southern Africa. Gular fluttering could increase by as much as 25% by 2100 under current projected temperature increases, and increases in water loss could reach nearly 10%. Some species may shift their breeding dates to compensate for increasing temperatures, but if breeding activities are timed to coincide with peaks in their main prey specie s, this may result in poorer diets or increased competition from other species.
  • Thumbnail Image
    Item
    Open Access
    How do trees die following low intensity fires: Exploring the hydraulic death hypothesis
    (2014) Nel, Jacques Adriaan; West, Adam
    The mechanism by which trees die following a low intensity fire is poorly understood. Traditionally, cambial necrosis (Carbon starvation hypothesis) has been used to explain post-fire tree mortality, however, this does not explain why some trees die within days following a fire. To address this, the hydraulic death hypothesis argues that post-fire tree mortality is a result of a reduction in hydraulic conductance rather than the necrosis of cambium. There are a number of proposed mechanisms by which hydraulic failure can occur during a fire: firstly, plume-induced cavitation as a result of high vapor pressure deficit (VPD) in a fire-plume has been shown to reduce hydraulic conductance. Secondly, changing surface tension as water is heated has also been shown to increase the chance of cavitation. The final mechanism is a reduction in conductance as a result of direct xylem vessel deformation due to the visco-elastic properties of xylem walls (lignin). In order to determine the relative importance of each proposed mechanism, stems of Kiggelaria africana and Eucalyptus cladocalyx were exposed to 70 and 100„aC in two treatments designed to isolate the effect of each mechanism. An oven treatment was used as a surrogate for a fire-plume in order to demonstrate VPD-induced cavitation and a water bath treatment (transpiration inhibited) was used to demonstrate xylem deformation (along with microscopy). This was possible because post-treatment flushing was indicative of the initial cause of the reduction as cavitation is reversible while deformation is permanent. The data was then explored using a Hydraulic Death model we created based on a xylem conductance model from literature. The results showed that VPD-induced cavitation as well as deformation are able to reduce hydraulic conductance in trees exposed to fire, however, E. cladocalyx showed higher loss of conductance at 65„aC than K. africana and deformation was only seen to occur in water bath treatments and only in K. africana. Here we propose that a chain of events provides a mechanism for slowing the rate of heating in branches exposed to a fire-plume and that cavitation plays a protective role. Model exploration implied that vulnerability segmentation is responsible for preventing fire-plume induced runaway cavitation. This is in agreement with the ¡§safety valve hypothesis¡¨, however, rather than drought stress, it is a fire-plume which causes the cavitation. It was also found that E. cladocalyx was able to prevent deformation because of xylem vessel characteristics (thick vessel walls) and not bark properties. We propose that the necrosis of cambium and phloem leading to the inability to refill cavitated vessels is the actual cause of mortality in trees exposed to low intensity fires. The ability to refill is dependent on water availability and carbohydrate content, which is highly sensitive to drought. As resprouters store water and carbohydrates in lignotubers and stems, they are less sensitive to pre-fire conditions. However, the survival of cambium and phloem is essential to the refilling process and thus the mechanism for reducing heat transfer, bark properties as well as xylem characteristics work in combination to determine persistence after a fire.
  • Thumbnail Image
    Item
    Open Access
    The influence of rainfall seasonality and climate change on the demography of Aloe Dichotoma, a long-lived succulent tree from semi-arid southern Africa
    (2014) Gallaher,Kirsten Sarah Leilani; Hoffman, Timm; Rebelo, Anthony G; Jack,Sam
    In the arid and semi-arid western parts of southern Africa, Aloe dichotoma Masson, a long-lived succulent tree species, is thought to be responding to anthropogenic climate change. However, differences in response across its distribution are likely to be related to rainfall seasonality. This study investigated change in ten Aloe dichotoma populations within winter and summer rainfall zones in South Africa and southern Namibia. Using repeat photography over a timespan of approximately 30 years, demographic patterns, population dynamics (including mortality, recruitment and overall population change) and growth were assessed and modelled with climatic variables. Long-term patterns of recruitment and longevity were also investigated by using individual plant growth data to reconstruct recruitment histories for each population. Finally, the influences of climatic conditions on recruitment were evaluated by superimposing historical rainfall and temperature data. Differences in response between rainfall zones are clearly evident throughout. Generalised linear models revealed lower mortality, higher recruitment and positive population change at winter rainfall sites, while summer rainfall sites showed negative population change. Growth data revealed more rapid growth in height of juveniles than adults, and slower growth in height in the winter rainfall zone, most likely related to differences in tree architecture. It is evident that biotic and anthropogenic factors such as herbivory, nurse plants and theft are likely to moderate observable patterns as opposed to driving them. Recruitment modelling suggested that A. dichotoma attains a maximum age of 300 to 350 years, and revealed recent recruitment peaks in the winter rainfall zone and peaks around the turn of the 19th century in the summer rainfall zone. Changes in temperature and rainfall are likely to be the main drivers. Rising temperatures within the last century may have driven increased recruitment and low mortality in the cold-limited winter rainfall zone, while decreasing rainfall within the summer rainfall zone, combined with increasing temperatures, may explain this region's low recruitment and high mortality. The advancement of knowledge of broad spatial and temporal patterns in A. dichotoma and the likely causes, coupled with fine-scale future studies, will enable more detailed prediction of the species' response to future global change.