Spatial ecology of fire:
Herbivore preferences for burns of varying size and patchiness
Ryan Sensenig
INTRODUCTION
Graduate Group in Ecology, University of California, Davis
RESULTS
While most species showed some preference for burned areas, smaller bodied
animals showed the most dramatic response (Figure 1). Hare dung was 25 X’s
more abundant in burns than unburned areas, Grants and Impala dung 6 X’s more
abundant, and oryx and hartebeest 2-3 X’s more abundant. However, dung of the
largest species (zebra, eland, and elephant) were at greater densities in unburned
controls than in burned sites.
Burn Preference as a Function of Body Size
mean dung in burn / mean dung
in unburned control plots
100
Grazers & Mixed Feeders
Pure Browsers
R2 = 0.90
Hare
10
Impala
Grant
Steinbuck
1
Warthog
Hartebeest
Zebra
Eland
Cattle
Hartebeest
1
Giraffe
Eland
Cattle
Zebra
100
1000
10000
Species preferences for patchy versus continuous burns varies with body size
(Figure 2). The smallest species showed preferences for patchy burns, as did the
larger antelope (oryx and hartebeest). Grants gazelle and impala showed
preferences for continuous burns. The largest species showed no preference
between patchy, continuous, or unburned areas.
Preference for burn pattern related to body size
2
patchy
1.5
81
Figure 3. Ratio of dung in burn sites to dung in control
plots for different burn sizes. Colored lines indicate
species where burn sizes are significantly different (p =
0.05, white filled dots are different.)
CONCLUSIONS
These findings suggest that grazers respond to burns differently depending
on their body size and burn scale. Differences may relate to predator
avoidance strategies; small species require cover for hiding and larger
species select open areas with increased visibility. Alternatively, larger
animals may not be able to obtain enough food in burns and require
unburned grass nearby (oryx, hartebeest, zebra).
This study suggests that burning will benefit the smaller species in Laikipia
the most, and that burns need not be large to have a significant impact.
This may be an important realization when balancing the use of fire with
preservation of whistling thorn for elephants and other browsers. Further
analysis is ongoing to understand how visibility, grass quality, and rainfall
affect grazing selection of burned areas.
ACKNOWLEDGEMENTS
1
continuous
0.5
0
9
As shown in Figure 3,
grazers seem to select
burn sizes related to
their body size. Hare
had highest dung
densities in the smallest
burns and Grants
gazelle the highest in 9
ha burns. Eland,
hartebeest, and giraffe
had highest dung
means in 9 ha burns,
though less
dramatically. The
largest species avoided
burns and showed a
slight tendency to select
the largest burn sizes.
Burn Size (ha)
Increasing burn
“preference”
10
Avoid
burns
0.1
Elephant
METHODS
Average dung per 40 m2
Grant
Giraffe
Figure 1. Relationship between body size and overall preference for burned
sites (patchy and continuous combined). Pure browsers, giraffe and
steinbuck, are shown in orange and were excluded from the regression.
Grazer use of burned sites was assessed by counting dung pellets in
burned and unburned areas. The first survey was completed in
April/May, about 1 month after the burn, and 2 additional surveys in
June and August.
10
Elephant
log body mass (kg)
CONTINUOUS
Prefer
burns
1
1
PATCHY
Hare
Oryx
0.1
During March of 2004 and 2005 we burned more than 480 hectares
across five different properties in Laikipia (Jessel, Ol Pejeta, Mpala,
Segera, and Sweetwaters). We completed a total of 18 different burns
which varied in their size (81, 9 and 1 hectares). Eleven of these burns
were “continuous” burns and seven were “patchy” burns as shown
below.
100
mean dung in burned areas /
mean dung in control plots
Large herbivores are known to select burn sites for improved grazing
efficiency, enhanced nutrient quality, and increased visibility. Few
studies have addressed (a) the magnitude of attraction for species of
varying body size and (b) whether the scale of the burn affects their
response.
Burn Size Preferences
unburned
Figure 2. Mean dung piles in patchy burns, continuous burns, and unburned
control plots from 3 surveys. (MANCOVA, Wilk’s Lambda = 0.008)
Special thanks to Jessel, Mpala, Ol Pejeta, and Segera Ranches. Support and assistance
from Peter Jessel, Giles Prettejohn, JohnHenry Ruggieri, Richard Vigne and Ken
Wreford-Smith has been vital to the project and is greatly appreciated. Assistance
from MRC, Nathan Gregory, and Isaac Kimathi has been invaluable. Thanks to James
Ekiru, Patrick Etelej, John Mpaiyan, and Callistus for their hard work.
Funding provided by a National Science Foundation
Graduate Research Fellowship, a National Science
Foundation Dissertation Improvement Grant, UC
Davis, the Global CRSP, and a NSF to Truman P.
Young.