Giraffe abundance and demography in relation to food supply, predation and poaching

Transcription

1 Giraffe abundance and demography in relation to food supply, predation and poaching Megan Strauss PhD Candidate, Ecology, Evolution & Behavior Graduate Program University of Minnesota This report was prepared for the Explorers Club in accordance with the terms of the 2010 Exploration Fund Award.

2 Summary: This dissertation research investigates the population ecology of giraffe in Tanzania's Serengeti National Park; with an emphasis on how food supply, predation and poaching drive giraffe abundance and demography. The project aims to answer several questions: Why has the Serengeti giraffe population declined in the last thirty years despite an apparent increase in the giraffe's food supply? How do giraffe drive woodland dynamics? What limits and regulates giraffe populations? To answer these and other questions, I rely on modeling and analysis of two field datasets from Serengeti--including a study done by R. Pellew in and a contemporary study from Use of Exploration Fund Award: The Exploration Fund Award was used to support the collection of field data in July - November A portion of the award was directed to aerial counts of giraffe in three areas of Serengeti: Seronera (240km 2 ), Kirawira (210 km 2 ) and Bologonja (175 km 2 ). These counts provide critical information about changes in giraffe density in Serengeti. A total of nine aerial counts were completed in 2010 with the assistance of the TAWIRI-Messerli Veterinary Programme and the Frankfurt Zoological Society. The three October counts, done with the assistance of the Frankfurt Zoological Society, were funded with the Exploration Fund Award. Figures 1-3 and the accompanying text illustrate the counting methodology. Figure 1. Megan Strauss with pilot Felix Borner of the Frankfurt Zoological Society and the Cessna 182 aircraft that was used for the October aerial counts. The observers for the October counts were Megan Strauss and Daniel Rosengren of the Serengeti Lion Project. Photo courtesy of Daniel Rosengren. 2

3 Figure 2. Megan Strauss on the way to conduct an aerial giraffe count in the Kirawira study area. The counting method is as follows. In a given study area, the aircraft is flown along flight lines at 1 km intervals at a flying height of ft and a speed of approximately 160 km/hr. Each observer--one seated on each side of the plane--counts all giraffe detected within a 500 m- wide strip. Photo courtesy of Daniel Rosengren. Figure 3. Giraffe as seen from the air in the Kirawira study area. While giraffe in open spaces are easy to detect those in dense woodlands can be well hidden. A correction factor is applied to account for giraffe that may be missed during the counts. Photo courtesy of Daniel Rosengren. 3

4 Table 1. Mean giraffe abundance in the three study areas in and , with 95% confidence limits. An undercounting error of 15% was assumed for raw counts from These surveys show a dramatic decrease in giraffe density in Serengeti between and today. Several additional surveys are planned in 2011 to increase the sample size and monitor giraffe population trends (from Pellew 1983) (this study) Seronera 352 ± ± 28 Kirawira 554 ± ± 45 Bologonja 248 ± ± 25 In addition to aerial counts, the Exploration Fund Award was used to maintain and repair the project Land Rover, on loan from the Serengeti Lion Project. The Land Rover was used daily for the collection of giraffe demography, behavioral and movement data and for biweekly and monthly ground census work. An off-road vehicle like a Land Rover is a necessity for tracking giraffe in difficult terrain. The vehicle withstood a lot of wear during the course of field research and required constant maintenance and repairs. Figure 4. Megan Strauss collecting demography data from the project Land Rover. Giraffe herds could often be approached within m. Almost 800 individual giraffe have been identified and catalogued during the field seasons. By tracking these individuals, we can learn about movement, birth and death rates and social associations. 4

5 Future: With the assistance of the Explorers Club and other funding institutions, the fieldwork portion of the project is mostly complete. I am in the process of analyzing the field data and developing models to answer the questions posed in my dissertation project. The results, which will be published in scientific journals, will advance our understanding of giraffe regulation and of the complex role the giraffe plays in ecosystem dynamics. Conservation Outcomes: Giraffe numbers across Africa have declined steeply in the last 30 years and we now believe that there are less than 100,000 giraffe left in Africa. This dissertation work will provide key information on giraffe status in Serengeti. Poaching appears to be an increasing threat to giraffe. This study will examine the effects of poaching on giraffe demography and abundance. In addition, this study assesses several traditional and novel survey methods for giraffe that could be widely used for research and long-term monitoring of giraffe populations. References: Pellew, R.A The giraffe and its food resource in the Serengeti. II. Response of the giraffe population to changes in the food supply. Afr. J. Ecol. 21, Statement of Expenses: Total amount spent in 2010: $14,243 Amount awarded by Explorers Club: $2000 Use of Exploration Fund Award: Item Expense Vehicle maintenance and repairs $1127 Aerial surveys with FZS $869 Total: $1996 Megan would also like to thank the following organizations for supporting this project: 5