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Indirect effects of invasive
species removal devastate World
Heritage Islands
Bergstrom et al., 2009, Journal of
Applied Ecology
“Biological invasions can lead to
large-scale alterations in ecosystem
Control and eradication important and
Idea: Recover of indigenous species
through eradication
Unintended consequences through
removal of an alien transformer.
Little Barrier Island
Removal cats less breeding success of cooks petrel. Better after eradication of rats.
Little Barrier Island
Macquarie island
34 km long, cool maritime climate. Tundra like
vegetation. (Tussock grass, mega-herbs and
Rabbits 1878, Cats 60 years before.
Hyper-predation loss of two flightless bird
Management: Virus vectored by flea:
Rabbits populations decreased from 130
000 to 20 000.
2006 stop of virus spreading.
Hypothesis: Increased population due to
eradication of top-level predator.
Cat predation and rabbit population
Estimates of consumption by cats of rabbits,
rats and mice were calculated.
Based on cat daily food intake and
proportion of food items in guts.
Rabbit populations: mean rabbit densities
from 8 rabbit count areas.
Rabbit Population Change
Hypothesis: Population size increasing
due to eradication of Top predator.
But also virus and climate variation might
have influence.
Rabbit abundance: dependent variable
presence / absence of virus + (season mean
temperature and precipitation):
independent variable
Vegetation Change
In 2001 45 plots 5x5m, 2007 18 revisited.
Individual plant species cover as a percentage.
Use of remote sensing imagery
Satellite imagery:
Change Vector Analysis (CVA): Change measure
Difference in pixel intensity.
(100% from total absorption to total reflection)
Threshold for no change: 17%
Normalized Difference Vegetation Index
(NDVI): loss or gain of chlorophyll.
Results: Rabbit Population
Bergstrom et al. 2009
Results: Rabbit Population
virus present
cats present
Bergstrom et al. 2009
Results: Vegetation
15 of 18 sampled plots changed significantly
loss of species cover, gain of new (smaller) taxa
Bergstrom et al. 2009, supporting information
Results: Vegetation
Substantial island-wide vegetation change (36 % of island area)
NDVI Change Bergstrom et al. 2009, supporting information CVA Magnitude
• Rabbit numbers returned to pre control
• increased herbivory caused significant
damage (local and on landscape scale)
• Efforts to control rabbit population
reversed in only 6 years
➡ strong top-down control of rabbits by a
small cat population (ca. 160 adult cats)
• Further effects of cat extinction
– more rodents, further impacts on multiple
trophic levels
– rabbit abundance also influences other
species (e.g. destroying petrel burrows,
making them more vulnerable to attack by
– changes to soil structure & nutrient cycling
Implications for Management
• Appreciation of complex
• more thorough assessment of
ecological risks
– general biology of the target species
– interspecific interactions
– trophic interactions
• better modeling of expected results
D.M. Bergstrom, A. Lucieer, K. Kiefer, J. Wasley, L. Belbin, T.K. Pedersen,
& S.L. Chown. 2009. Indirect effects of invasive species removal
devastate World Heritage Island. Journal of Applied Ecology 46: 73–81.
Supporting Information to the article, retrieved from on
Questions ?