Published on January 22, 2008
The Role of Anthropogenic Fire and Livestock Grazing in Sustaining Avian Diversity in East African SavannasNathan Gregory1 and Ryan Sensenig21. Department of Ecology and Evolutionary Biology, Princeton University. Princeton, New Jersey 08544, USA. Email: [email protected] 2. Plant Sciences Department, University of California at Davis. One Shields Avenue, Davis, California 95616, USA: Avian abundance and diversity increase on burned plots, and these areas attract a unique assemblage of birds. These effects vary with the size of the disturbance. Bird species richness in bomas is higher than in burns of similar size, and some species show strong preferences for bomas over other habitats. In general, most of the species observed show strong preferences for disturbed habitats, and only some Ploceus weaver species, queleas, rattling cisticolas, and some shrub-loving species are specifically attracted to undisturbed sites. The prevalence of undisturbed habitat may be a relatively recent phenomenon resulting from post-colonial human impacts on fire and grazing regimes. The lack of birds specializing in these areas points to the key role of disturbance in maintaining avian diversity in East African savannas. Conservation of bird diversity will require the incorporation of both types of disturbance into management strategies. In Africa, savanna ecosystems are sustained through the interacting effects of fire and grazing, and these disturbances have long been influenced by humans (Werner 1990, Roques et al. 2001). Land use changes since European settlement have resulted in the suppression of large-scale wildfires, but there is growing interest among land managers in using controlled burning as a tool to improve grazing conditions for livestock and native ungulates. The increased use of prescribed fire is likely to change the savanna landscape and, by extension, its flora and fauna. We are investigating the effects of a modified disturbance regime on avian abundance, diversity, and community composition by asking the following questions: In a system where normal disturbance regimes have been altered, how do the practices of controlled burning and livestock grazing (bomas) affect avian diversity, community composition, and abundance? How do the effects of these disturbances compare? What are the mechanisms driving these changes? As land use change has been identified as the most important factor threatening global biodiversity, particularly in savanna ecosystems (Sala et al. 2000), it is essential to develop an understanding of how savannas function and how human impacts can be used to good advantage. Determining the manner in which anthropogenic disturbance influences community dynamics has important management implications and can shed light on the mechanisms underlying the maintenance of diversity. Fire suppression has made grazing one of the dominant forces structuring the landscape specifically in the form of bomas, thorn-scrub corrals, built by pastoralists. These structures house livestock at night, and once abandoned, these areas, denuded and enriched with dung, become productive grazing lawns with distinct plant communities that persist for decades (Stelfox 1986, Augustine 2003). Recently abandoned bomas have a very distinctive structure and are roughly 9 hectares in size (Figure 7). Introduction and Research Questions Funding: Princeton University EEB Princeton PEI/ STEP Wildlife Conservation Society Teresa Heinz Scholars for Environmental Research David A. Gardner Magic Project American Museum of Natural History Cooper Ornithological Society Thanks to: David Wilcove Ryan Sensenig Hussein Mohamed Dan Rubenstein Claire Kremen Michaela Hau Andy Dobson Nick Georgiadis Mpala Research Center and staff This study is taking place on three ranches in the Laikipia District of central Kenya (Figures 1 and 2). Since at least the 1950s, fire has been actively suppressed, and in its absence, bomas serve as the major landscape modifier. The experimental plots are located in Acacia drepanolobium-dominated habitat of homogeneous structure and soil type. Study Area Methodology This part of the study is a component of the Scale and Fire Ecology Project (S.A.F.E.), a collaboration that includes Ryan Sensenig of the University of California at Davis and Isaac Kimathe of the University of Nairobi. Experimental plots are organized into treatments that combine burns of three sizes (81 hectares, 9 hectares, and 1 hectare) with control plots of equal area. This design is replicated four times. To measure bird diversity and abundance, we employ the fixed-width transect method (Bibby 2000). Each plot has two transects that begin in the center, run to the corners, and extend 300 meters into the matrix (Figure 3). Observers record the locations of every bird detected within 50 meters on either side of the transect. Figure 1: Map of Kenya with Laikipia District shown in red Figure 2: Land use map of the Laikipia District. White areas are large, private commercial ranches. The approximate study area is shown in red. Acknowledgements Fire: Effects on Avian Abundance, Diversity, and Community Composition Figure 3: An example of a controlled burn plot with transects appearing in red. Results Figures summarize results from data obtained from June through August. Significantly higher densities of birds occur on 9 and 81 hectare burned sites (Figure 4). Figure 4: 9 and 81 hectare sites host greater densities of birds relative to controls Burned plots also host a slightly more diverse bird community than controls. While species-area curves indicate that richness in controls and burned plots do not differ significantly, abundance influences diversity measures. The abundance-based coverage estimator indicates that burned plots are more diverse for both 81 hectare plots (Figure 5) and 9 hectare sites (Figure 9). A similar pattern is observed in the winter. Figure 5: Species accumulation curves for 81 hectare burned and control sites indicate no significant difference in species richness. ACE values (large diamonds) show that burns are more diverse. Community composition differs between burns and undisturbed areas. For those species encountered at least 10 times, most displayed a strong preference for burned sites (Figure 6). Only red-billed queleas, rattling cisticolas, and some Ploceus weavers show a clear preference for the undisturbed habitat. Figure 6: Percent occurrence of species observed at least 10 times on burned vs. control sites during the summer. Bomas and Fire: Comparative Effects on Avian Abundance, Diversity, and Community Composition Methodology Survey methods are the same as for the fire component of the study. Transects begin in the center of the boma and proceed 300 meters into the matrix. 8 bomas are being surveyed. Results Bomas host densities of birds similar to burned sites (Figure 8) but are more diverse than both burned sites and controls (Figure 9). This pattern holds during the winter. Figure 9: Species accumulation curves for bomas, 9 hectare burns, and control sites. Large diamonds are ACE values. Bomas are the most diverse of the three treatments. Figure 10: Preferences of species observed at least 10 times during the summer for all three habitats. Mechanisms Figure 8: Comparison of bird density between controls, 9 hectare burns, and bomas As is the case with burned areas, some species of birds show strong preferences for bomas (Figure 10). There are some size-related differences as well. For instance, northern white-crowned shrikes show no preference between 81 hectare burned or control sites but show a clear preference for 9 hectare burns relative to controls. In the winter, migrants show strong preferences for disturbance (Figure 11). References Augustine, D. J. 2003. Long-term, livestock-mediated redistribution of nitrogen and phosphorus in an East African savanna. Journal of Applied Ecology 40:137-149. Bibby, C. J. 2000. Bird census techniques, 2nd edition. Academic Press, London ; San Diego, CA Roques, K. G., T. G. O'Connor, and A. R. Watkinson. 2001. Dynamics of shrub encroachment in an African savanna: relative influences of fire, herbivory, rainfall and density dependence. Journal of Applied Ecology 38:268-280. Sala, O. E., F. S. Chapin, J. J. Armesto, E. Berlow, J. Bloomfield, R. Dirzo, E. Huber-Sanwald, L. F. Huenneke, R. B. Jackson, A. Kinzig, R. Leemans, D. M. Lodge, H. A. Mooney, M. Oesterheld, N. L. Poff, M. T. Sykes, B. H. Walker, M. Walker, and D. H. Wall. 2000. Biodiversity - Global biodiversity scenarios for the year 2100. Science 287:1770-1774. Stelfox, J. B. 1986. Effects of Livestock Enclosures (Bomas) on the Vegetation of the Athi Plains, Kenya. African Journal of Ecology 24:41-45. Werner, P. A. 1990. Ecological Determinants of Savannas - Abiotic and Biotic. Journal of Biogeography 17:401-402. Figure 11: Preferences of species observed at least 10 times during the winter for all three habitats. Bars outlined in yellow indicate palearctic migrants. Conclusions The Role of Anthropogenic Fire and Livestock Grazing in Sustaining Avian Diversity in East African Savannas Nathan Gregory1 and Ryan Sensenig2 1. Department of Ecology and Evolutionary Biology, Princeton University. Princeton, New Jersey 08544, USA. Email: [email protected] 2. Plant Sciences Department, University of California at Davis. One Shields Avenue, Davis, California 95616, USA Figure 12: Arthropod biomass by treatment. Disturbance appears to negatively impact food resources such as arthropod abundance (Figure 12) which suggests that habitat characteristics may be key to structuring avian communities. However, further experiments are underway to disentangle the confounding effects of predation by birds on arthropod communities.