Black Rat (Rattus rattus) Population Densities
on Navy Support Facility Diego Garcia, British
Indian Ocean Territory
Diego Garcia is part of the United Kingdom in
the British Indian Ocean Territory (BIOT)
Small atoll, ~39 miles from tip to tip
Predominately Coconut Forest
Some Mixed Forest
Rats can have devastating effects on
island ecosystems
• Prey on native birds: adults, chicks and eggs
depending on the species
• Prey on native invertebrates and smaller
vertebrates
• Impact native plants by seed consumption
• Human health impacts
• Rats on Diego Garcia are common.
• Often observed during daytime.
• Population densities had never been
estimated and rat diet had never been
assessed.
Snap Trapping Grids in Coconut and
Mixed forest
• Coconut forest trapping grid: 144 traps (12 x 12),
with 20 meter spacing between lines and traps.
• Area covered = 220 meters x 220 meters, 4.84
hectares (11.95 acres)
• Mixed forest trapping grid: 121 traps (11 x 11),
with 10 meter spacing between lines and traps.
• Area covered = 100 meters x 100 meters, 1
hectare (2.47 acres)
• Rats captured on the mixed forest grid were
necropsied for diet, health and reproductive
condition.
Traps attached to trees, ~1 meter off of the ground to mitigate crab
interference. Baited with coconut and peanut butter. Traps checked and rebaited (if needed) in the early morning and late afternoon.
A problem with grid trapping: animals come from off of the grid area to be trapped
but don’t know how far off. Therefore, the area sampled by the trapping grid is larger
than the grid dimensions. Need to know the area sampled to calculate a density
(animals per unit of area)
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trapping grid
So, assess daily movement of rats with thread bobbins. Glue on,
release and follow the thread. Estimate home range or daily
movements and add an appropriate size “buffer strip” to the grid
dimensions to estimate the sampling area of the grid.
Results
• A total of 914 rats were captured on the coconut forest grid
over 11 days of trapping and 125 rats were captured on the
mixed forest grid over 8 days of trapping.
• Abundance estimate for the coconut forest grid was 1139
rats (95% confidence interval: 1075-1229) and 142 rats
(95% confidence interval: 132-167), for the mixed forest
grid. Data analyzed on program MARK as a removal project.
• Thread bobbin data (7 animals) showed very small home
ranges with a mean of 705m² (27m x 27m).
• Adjusted sampling area (grid size plus buffer strip was 6.1
hectares for coconut forest grid and 1.6 hectares for the
mixed forest grid.
• Density estimate in coconut forest is 187/ha
(95% Confidence Interval: 176-201) and 88/ha
(95% Confidence Interval: 82-104) in mixed
forest.
• 81% of rat stomachs contained coconut, along
with other vegetation, animal tissue (possibly
geckos) and 67% had coconut exclusively.
Coconut eaten by rats
Intsia bijuga, native hardwood tree,
seeds eaten by rats
At both sites ~30% of the population was active during
the day.
Coconut Forest
80
70
60
50
Daytime captures
40
Nighttime captures
30
20
10
0
1
2
3
4
5
6
7
8
9
10
11
• Mixed Forest
30
25
20
Day captures
15
Night captures
10
5
0
1
2
3
4
5
6
7
8
Conclusions
• The mixed forest grid was trapped out in less
time due to a higher density (4x higher) of traps.
• The densities are high enough that a segment of
the population (30%) is now diurnal, an example
of resource partitioning (night shift and day shift).
• The high population densities are fueled by
abundant coconut.
• The sheer number of rats points to environmental
impacts but don’t know what at this stage.
• Rat eradications on islands are becoming more
common and might be possible on Diego Garcia