Browsing by Author "Retief, Kirsten"

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    Are foraging patterns of aerial predators influencing diurnal activity of their prey?
    (2011) Retief, Kirsten; Simmons, Robert E
    There is often a reciprocal relationship between predators and their dominant prey. This results in continuous co-evolution, where the prey selects more efficient ways to avoid predation and the predator selects more efficient ways of catching prey. The aim of this study was to determine if aerial predators (raptors) influence diurnal activity patterns of their prey (mice). Sampling of both raptors and mice was done simultaneously during autumn, winter and spring at two different study sites in three different habitats on South Africa's arid west coast. Sixty PVC mouse traps were checked every half hour throughout all daylight hours and raptors were observed using scan sampling every two minutes. A total of 1228 rodents were trapped consisting mainly of the diurnal mouse, Rhabodmys pumilio and a total of 197 hours was spent observing hunting behaviour of four raptor species: Black and African Marsh Harriers (Circus maurus and C. ranivorus), Black-shouldered Kite (Elanus caeruleus) and Rock Kestrel (Falco rupicolus). All birds hunt rodents on a regular basis and all were beginning to breed towards the end of my study. Results showed significantly higher small mammal activity during winter and in habitats with dense vegetation. Diurnal small mammal activity peaks appeared to shift randomly between sites, habitats and seasons, thereby avoiding predation through unpredictability of activity. Small mammal activity patterns deliberately avoided peak foraging times of raptors during winter at the Pelican site; however this relationship was not linked to any climatic variables to explain how mice avoid high predation periods. Raptors selected peak foraging times dming periods of high small mammal activity at the Dune site during winter; however wind speed and cloud cover were correlated with both raptor foraging and small mammal activity, thereby influencing the relationship between the predator and the prey. In conclusion, no reoccurring diurnal peaks in small mammal activity were seen across the three seasons and this may have been an anti-predation strategy, because predators would find it more difficult to predict high prey activity. However the raptors it seemed were selecting periods of high mice activity at one site when weather conditions allowed.
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    Can stable isotopes and radiocarbon dating provide a forensic solution for curbing illegal harvesting of threatened cycads?
    (Wiley, 2014-09-17) Retief, Kirsten; West, Adam G.; Pfab, Michele F.
    Cycads in South Africa are facing an extinction crisis due to the illegal extraction of plants from the wild. Proving wild origin of suspect ex situ cycads to the satisfaction of a court of law is difficult, limiting law enforcement efforts. We investigated the feasibility of using multiple stable isotopes to identify specimens removed from the wild. Relocated and wild specimens from the African genus Encephalartos were sampled: E. lebomboensis and E. arenarius. 14C analysis indicated that a ±30 year chronology could be reliably obtained from the cycads. For E. arenarius, pre-relocation tissue was consistent with a wild origin, whereas tissue grown post-relocation was isotopically distinct from the wild for 87Sr/86Sr and δ15N. For E. lebomboensis, δ34S, δ18O and 87Sr/86Sr were different between relocated and control plants, consistent with the >30 years since relocation. Our findings demonstrate the potential for a forensic isotope approach to identify illegal ex situ cycads.
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    Cycad forensics: tracing the origin of poached cycads using stable isotapes, trace element concentrations and radiocarbon dating techniques
    (2013) Retief, Kirsten; West, Adam; Pfab, Michele
    In South Africa, the greatest threat to cycad populations is the illegal removal of wild plants for landscaping purposes and collector's gardens, resulting in declining populations that are already threatened with extinction. Radiocarbon dating in conjunction with stable isotopes and trace element concentrations could provide the essential evidence needed in court when prosecuting poachers and collectors. The aim of this study was to determine the feasibility of using radiocarbon dating, stable isotopes and trace element concentrations to identify specimens removed from the wild. To test this method, we sampled two individuals of Encephalartos lebomboensis and E. arenarius, with known wild origins and relocation histories (1946 and 1992 respectively), and compared these to individuals from the same wild location that had not been relocated (controls). Vascular rings were sampled using an increment corer from the upper, middle and lower sections of the cycads, which were cut into sections so that the inner, middle and outer rings for each core could be sampled. These samples were reduced to cellulose for δ18O and δ13C isotope analysis. Leaf bases were sampled from the upper, middle and lower sections of the cycad stem using a hammer and chisel, and petioles were cut from the cycad to represent the most recent growth. These samples were analysed for the following stable isotopes δ13C, δ15N, δ34S, 87Sr/86Sr, 206Pb/207Pb and 208Pb/207Pb, and the concentration of trace elements were analysed for the leaf bases. The means and variance were compared between the relocated and control specimens and a principal component analysis was done. The radiocarbon ages of E. lebomboensis tissues were more recent than expected, dating from 2006 to 1979, suggesting that this method would only be appropriate for specimens relocated within the past 6 to 30 years. Consistent with this, our δ18O and δ13C in cellulose, as well as the δ34S,87Sr/86Sr ratio and principal component analysis indicated that the relocated E. lebomboensis specimen (relocated 66 years ago) was significantly different from the control, implying that the stem of the relocated specimen did not grow in the wild. In contrast, the relocated E.arenarius specimen (relocated 20 years ago) was isotopically indistiguishable from the control in older tissues indicating a common origin, but was significantly different for the upper leaf bases, as well as a shift in δ18O cellulose for the upper vascular rings, indicating that the plant has been relocated. Thus, the use of radiocarbon dating and stable isotopes showed great potential as a forensic technique for cycads and suggestions for future development of this technique are given.
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    Influence of fire severity in fynbos plant communities and mode of regeneration in the southern Cape Peninsula
    (2011) Retief, Kirsten; Hoffmann, Timm
    Fire severity influences species diversity and alters species habitat requirements, therefore it is important to understand what effect severity has on fynbos communities. The smallest twig diameter of burnt skeletons was used as a severity index and compared between two fire events for 9 permanently marked plots on the southern Cape Peninsula. Species diversity, richness and composition were analysed for one year and eight years after a fire event in 2000 and 3 years after a fire event in 2008. Twig diameters for Erica, Leucadendron, Rhus and Acacia species were significantly greater for the 2008 fire than the 2000 fire event, indicating that the fire was more severe in 2008. The ratio of reseeders to resprouters changed from 8:2 in 2001 and 2008 to 7:3 in 2011 post the second fire event. Thus resprouters were not negatively affected by increased fire severity. Ericoid reseeders appeared to favour more intense fires. Succession of fynbos communities had a greater influence on species diversity, richness and composition than fire severity. Thus fire severity does not influence fynbos communities and factors that influence species survival between fire events, such as succession or alien plant invasions, may be more influential than fire severity.