Ecology

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Flexibility in metabolic rate in a small Afrotropical bird Zosterops virens.
(2014) Thompson, Lindy Jane.; Downs, Colleen Thelma.; Brown, Mark.
The scientific literature contains hundreds of studies on avian basal metabolic rate (BMR), many of which assumed that BMR was fixed for each species. Yet those from the last few decades have shown avian BMR to be a flexible trait, changing temporarily and reversibly in response to numerous environmental variables. Given that birds from lower latitudes are relatively understudied compared with temperate and Holarctic species, and that seasonal trends in BMR of southern hemisphere birds are not well understood, we looked at seasonal variation in BMR of a small Afrotropical bird, the Cape white-eye (Zosterops virens), over two years, and found that small birds may reverse the direction and amplitude of seasonal change between years. We also looked at circannual rhythm in avian resting metabolism (RMR), and found that peaks and troughs in resting metabolic rate (RMR) may not necessarily correspond with peaks and troughs in ambient temperature, suggesting that some of the confusion regarding the direction and magnitude of seasonal change in avian BMR may be caused by timing of seasonal measurements. Since we were using captive birds for my work, and since captivity may have an effect on avian BMR, we compared the BMR of freshly wild-caught birds with that of long-term captives housed in outdoor aviaries. The captive birds had higher BMR, giving weight to the argument that some physiological data of captive birds should not be used as representative of wild conspecifics, however the direction of seasonal change was similar in freshly wild-caught and long-term captive birds. Along the same vein, acclimation to laboratory conditions, experimental procedure, and different thermal environments, may also affect avian BMR, and thus before we started the final experiment, Cape white-eyes were acclimated to two different thermal regimes, with no change in RMR over an eight-week period, although there was an increase in body mass over the first three weeks, presumably due to the captive diet being of higher quality than a wild one. These results suggested that in some instances, small birds that are freshly wild-caught may not need to be acclimated in terms of their metabolism, before respirometry trials begin. Finally, given that anthropogenic climate change is anticipated to eclipse all other threats to biodiversity, and since many current predictive models pay no heed to metabolic flexibility of birds, we investigated the effect of a 4°C increase in housing temperature on resting metabolism of the Cape white-eye. This temperature increase is equivalent to that predicted for the range of this species by 2080, and therefore gives an indication of the effect of a sustained increase in mean surface air temperature. The results showed only a marginal difference in various metabolic parameters, suggesting that these birds may cope with the mean temperature increase predicted for their range in the coming decades. Together, these results highlight the importance of considering phenotypic flexibility when studying avian resting or basal metabolic rate. This has special implications for seasonal studies that implicitly assume that summer and winter measurements provide snapshots of the maximum and minimum RMR of which birds are capable, and for comparative studies, which may incorporate metabolic data from both wild and captive populations, or from study birds that were acclimated for different periods.
Managing nesting hawksbill turtles (Eretmochelys imbricata) on a small tropical island.
(2018) Gane, Julie Alexandra.; Downs, Colleen Thelma.; Brown, Mark.
Although human impact has caused the hawksbill turtle (Eretmochelys imbricata) to be on the brink of extinction, conservation efforts are on the increase and management strategies are being re-defined to manage the species towards rapid population increases. Numerous conservation measures have been applied and tested, with varying levels of success, to minimize threats and increase hatchling recruitment success rates. Conservation management is imperative for the survival of this species and has been shown to be easiest and most feasible at the nesting and hatching stages. On Cousine Island where this study took place, a turtle monitoring and management program was initiated in 1994 to protect nesting females and incubating eggs at the nesting and hatching stages of the lifecycle. Controversial conservation measures such as the handling of eggs, relocating clutches and application of crab proof barriers, have been applied on an ad hoc basis to minimize the high levels of mortality from natural erosion and crab predation. The merit of these practices, such as the translocation of egg clutches and the implementation of these specific barrier methods has never to our knowledge been scientifically tested using controlled experimentation. The first aim of this study was to find the most effective crab barrier method for increasing hatchling recruitment success and minimising predation rates with limited negative consequences. The second aim was to decipher crab density trends in order to offer a proxy for the number of crabs a nest might be exposed to within an area and develop beach profiles along the entire nesting beach to provide an expression of beach morphology to minimise nest loses from erosion. Finally, we analysed 10 years of nesting ecology and hatching success data with the aim of providing nesting and hatching trends to inform management decisions on Cousine Island into the future. Results across this study indicated that management measures can have both positive and negative effects on the nesting ecology and hatchling recruitment success in particular. We found that hawksbill clutches incubating without crab barriers are susceptible to losing on average a third or more eggs to crab predation than those with a crab barrier in place. Nests which were protected with netting had significantly higher hatchling recruitment success rates and nests protected with fencing had significantly lower predation rates than control nests. The use of netting, however, had a substantial cooling effect which can potentially distort natural sex ratios. Results across the entire study indicated that spatial distribution and environmental variations have an effect on embryonic development, hatchling fitness, sex determination, hatching success and the risk of predation. The hatchling recruitment success on Cousine Island was also found to be affected by nest losses each season from seasonal beach erosion. Beach dynamics were also found to be cyclical and we discovered that the nesting beach is prone to higher levels of erosion than accretion which created significant changes to beach width across the season. This study showed that when focusing turtle conservation measures at the nesting site, hatchling recruitment success can be increased by minimising mortality at the egg and hatchling stage. The aim in the long-term is to assist with the analyses of local and global population dynamics, deciphering threats and minimising the threat of extinction.
Urban ecology of the crowned eagle stephanoaetus coronatus in KwaZulu-Natal, South Africa.
(2015) McPherson, Shane Cameron.; Downs, Colleen Thelma.; Brown, Mark.
Urban environments comprise a complex and dynamic landscape, and urban sprawl is irreversibly transforming large areas of land globally. Increasingly, the need for incorporating ecosystem services into urban landscapes provides opportunities for green-space to benefit biodiversity and indigenous wildlife. Enhancing urban green-space maximises indigenous biodiversity and provides conservation value, and can also benefit people by enriching their experience and awareness of nature. Large charismatic species can stimulate awe and interest as emblematic representatives of the wilderness. As the global population becomes ever more urban, this enriches the human experience. The crowned eagle (Stephanoaetus coronatus) is a large predatory raptor and a threatened species that is increasingly known to inhabit the Durban Metropolitan Open Space System (D’MOSS), within eThekwini municipality, KwaZulu-Natal (KZN), South Africa. This research investigated the ecology of the crowned eagle in the urban environment and suggests opportunities for enhancing the urban landscape for conservation benefits. Globally, dramatic land use change typical of urbanisation negatively affects biodiversity, especially for top predators. The D’MOSS design faces the challenge of conserving biodiversity in a regional hotspot in the face of rapid urban growth in one of Africa’s major commercial hubs. Understanding habitat use of keystone and apex species provides urban planners with an opportunity to integrate biodiversity in a growing city. Consequently, we investigated habitat use and nest site selection of crowned eagles on various spatial scales within this urban mosaic. Unexpectedly the inter-nest distances were small in this human-dominated landscape. However, breeding sites were not evenly distributed through the landscape and were closely associated with natural forest, while nest trees were most frequently in patches of exotic large riverine Sydney blue gum (Eucalyptus saligna, Smith 1797) within the D’MOSS planning zones. Crowned eagles showed a strong tendency to avoid informal settlement areas, however they were tolerant of proximity to established formal settlements and occupied dwellings. Further, home range and habitat selection were investigated with GPS telemetry, albeit with a limited sample size (n =5) due to the limitations of abundance and dispersion of this apex predator. The 350 km2 urban core study area comprised a matrix of mainly formal settlements (44%), and DMOSS green space areas (29%). The study area was occupied by up to 22 active breeding pairs of crowned eagles. We documented a mean (n = 4) annual home range of 13 km2 (hull100%) containing 6.3 km2 of territory per pair (LKDE HLSCV 95%). These relatively small home ranges for a large eagle included shared territorial boundaries. Rapid replacement of vacancies at breeding sites suggests a saturated population. Habitat selection within the home range, thresholds of critical habitat, exotic trees, and correlation with DMOSS show the importance of pockets of indigenous forest in this urban mosaic landscape. These forests are fragmented and fragmentation increases the available edge habitats and landscape heterogeneity, potentially enhancing resource availability for crowned eagles in a highly modified landscape. The presence of remnant patches of mature Eucalyptus was more preferred than monotypic timber plantation stands. Consequently, continued protection of the D’MOSS system, and a considered approach to management of Eucalyptus are required for the persistence of the crowned eagle in this landscape The study of diet is pivotal in understanding a species, particularly for quantifying a predatory raptors’ economic niche and potential for human-wildlife conflict. In close association with urban development, the local population of crowned eagles has the potential to be a concern to the safety of domestic stock and pets. Time-lapse cameras were positioned at urban nest sites (n = 11) to identify the prey composition during breeding, particularly in regards to taxa with human associations. This was the first use of this technique for this species. The numerical proportion of avian prey, particularly hadeda ibis (Bostrychia hagedash) pulli, was several times greater than any previous diet description. The methodology used and the abundance of hadeda ibis in these urban environments are potential contributing factors. Rock hyrax (Procavia capensis) was the primary prey and where hyrax were unavailable, the diet composition was broader and included more vervet monkeys (Chlorocebus pygerythrus). Domestic stock comprised only 6% of the identifiable prey. Contrary to popular belief, no dogs (Canis familiaris) and few cats (Felis catus) were delivered to the nest by breeding eagles in this study. In situ pet attacks are most frequently attributed to juvenile and immature crowned eagles in winter and spring. Attacks on pets by crowned eagles, especially on small dog breeds, although relatively rare have a substantial influence on human-wildlife conflict and public perceptions. Pet attacks are generally attributed to juvenile and immature crowned eagles during periods of limited resources, particularly winter and during dispersal in the juveniles’ first spring. Negative social perceptions have resulted in persecution (n = 5), one of the main causes of recorded injury and mortality to crowned eagles in the region. Gunshot persecution, electrocution and collisions with anthropogenic structures have the greatest impact on juvenile and immature survival in the region. We provide management recommendations regarding various categories of crowned eagle human-wildlife interactions. Collaboration of wildlife authorities with NGO’s and public stakeholder input creates an environment for successful crowned eagle conservation and management of human-wildlife conflicts. Public awareness is an important aspect to the sustainability of the urban crowned eagle population. This study demonstrates that urban mosaic landscapes can provide conservation benefits for the crowned eagle. The land planning strategies enacted in Durban can guide urban expansion in tropical forest biomes to enhance indigenous biodiversity in urban mosaic landscapes in Africa, and globally.

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Browsing Ecology by Author "Brown, Mark."