43 research outputs found
Forage nutritive quality in the serengeti ecosystem:the roles of fire and herbivory
Get PDFFire and herbivory are important determinants of nutrient availability in savanna ecosystems. Fire and herbivory effects on the nutritive quality of savanna vegetation can occur directly, independent of changes in the plant community, or indirectly, via effects on the plant community. Indirect effects can be further subdivided into those occurring because of changes in plant species composition or plant abundance (i.e., quality versus quantity). We studied relationships between fire, herbivory, rainfall, soil fertility, and leaf nitrogen (N), phosphorus (P), and sodium (Na) at 30 sites inside and outside of Serengeti National Park. Using structural equation modeling, we asked whether fire and herbivory influences were largely direct or indirect and how their signs and strengths differed within the context of natural savanna processes. Herbivory was associated with enhanced leaf N and P through changes in plant biomass and community composition. Fire was associated with reduced leaf nutrient concentrations through changes in plant community composition. Additionally, fire had direct positive effects on Na and nonlinear direct effects on P that partially mitigated the indirect negative effects. Key mechanisms by which fire reduced plant nutritive quality were through reductions of Na-rich grasses and increased abundance of Themeda triandra, which had below-average leaf nutrients
Microclimate Preferences of the Grey-Headed Flying Fox (Pteropus poliocephalus) in the Sydney Region
Get PDFThe population size of the grey-headed flying fox (Pteropus poliocephalus) has decreased dramatically as a result of a variety of threatening processes. This species spends a great proportion of time in roosting large social aggregations in urban areas, causing conflict between wildlife and humans. Little is known about why these bats choose to roost in some locations in preference to others. Roost selection by cave-dwelling bats can be greatly influenced by microclimatic variables; however, far less is known about microclimate selection in tree-roosting species despite the direct management implications. This study aimed to determine the microclimate characteristics of P. poliocephalus camps. Temperature and humidity data were collected via data-loggers located both in six camps and the bushland immediately adjacent to the camps in the greater Sydney region. We found significant differences between the microclimate within the camps and the surrounding bushland. In general, areas within the camps had a greater variance in temperature and humidity than the alternative locations. We hypothesise that camps may be specifically located in areas with high microclimate variance to accommodate a range of individual preferences that vary depending on demography
Fire and the reasons for its influence on mammalian herbivore distributions in an African savanna ecosystem
No full textFire is an abiotic factor which has long played a role in savanna and grassland ecosystems. Fire causes reductions in plant vegetation height and biomass and increases in plant nutrient content. Mammalian herbivores are attracted to post-fire burned areas and this attraction has largely been attributed to increases in plant nutrient content. However, because of the reduction in vegetation height and subsequent increase in sighting distance that fire causes, burned areas might also be safer habitats from predators. This dissertation investigates how fire influences the distribution of herbivores and carnivores in the post fire landscape. The results show that generally smaller sized herbivores prefer burned areas while larger sized herbivores do not. Both vegetation nutrient quality and vegetation height play a role in explaining this preference for burned areas. The role of predator avoidance as one possible explanation for herbivore preference of burned areas is further supported by data showing that lions ( Panthera leo ) do not prefer burned areas and do not kill more in burned areas despite increases in prey availability in these areas. To further investigate the potential impact of predators on herbivore use of burned areas, field studies were conducted on the vigilance behavior of Thomson\u27s gazelles ( Gazella thomsonii ) in burned and unburned areas before and after exposure to a model cheetah ( Acinonyx jubatus ). Prior to cheetah presentation there was no difference in vigilance between the two habitats. However, right after cheetah presentation and removal vigilance levels were lower in burned areas than in unburned areas, indicating that Thomson\u27s gazelles perceive burned areas to be safer habitats. Lastly, this dissertation explores three alternative hypotheses for herbivore preference of burned areas; (1) to avoid disease carrying and behavior changing invertebrates, (2) because burned areas are warmer microclimates, or (3) to obtain minerals from the ash. Of these three hypotheses only the ingestion of ash to obtain minerals is supported as a reason for herbivore preference of burned areas, but this would only be for time periods shortly after burning. The results from this dissertation contribute to our understanding of how and why fire influences herbivore distributions
Alternative stable states and spatial indicators of critical slowing down along a spatial gradient in a savanna ecosystem
No full textAim: Theory suggests that as ecological systems approach regime shifts, they become increasingly slow in recovering from perturbations. This phenomenon, known as critical slowing down CSD], leads to spatial and temporal signatures in ecological state variables, thus potentially offering early indicators of regime shifts. Indicators using temporal dynamics have been empirically validated in laboratory microcosms and other well-mixed systems, but tests of spatial indicators of regime shifts at large spatial scales in the field are rare due to the relative absence of high-resolution data and difficulties in experimental manipulations. Here, we test theoretical predictions of CSD-based spatial indicators using large-scale field data from the Serengeti-Mara grassland-woodland system. Location: Serengeti-Mara ecosystem, Tanzania and Kenya. Time period: Year 2000 Major taxa studied: Vegetation Method: We used a space-for-time substitution method to empirically test the validity of CSD-based spatial indicators, i.e., we computed indicators along a spatial in lieu of temporal] gradient of ecological states. First we used a model of vegetation dynamics to determine if our space-for-time substitution method was appropriate. Then we tested for CSD-based spatial indicators using high-resolution spatial vegetation 30 m] and rainfall 2.5 km] data from the Serengeti-Mara ecosystem. Results: Our model predicts that CSD-based indicators increase along a spatial gradient of alternative vegetation states. Empirical analyses suggest that grasslands and woodlands occur as alternative stable states in the Serengeti-Mara ecosystem with rainfall as one of the potential drivers of transitions between these states. We found that four indices of CSD showed the theoretically expected increasing trends along spatial gradients of grasslands to woodlands: spatial variance, spatial skewness, spatial correlation at lag-1 and spatial spectra at low frequencies. Main conclusions: Our results suggest that CSD-based spatial indicators can offer early warning signals of critical transitions in large-scale ecosystems
