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Why model species distribution?
• Complements BHUs for conservation planning,
helping develop irreplaceability ratings
• Supplements data for known distribution of
species.
• Useful for discovering environmental factors that
determine distribution.
• Particularly useful for extrapolating effects of
environmental/climate change.
• Useful for estimating potential susceptibility of
areas to alien invasives.
Species Envelope
The areas in which Protea
nitida is known to occur
Source:
Proteas of Southern Africa
Why use GARP?
• Implemented for many purposes in recent
years, such as modelling invasive species
(Peterson & Robins, 2003), infectious diseases, (Peterson
& Shaw, 2003), and global warming (Thomas et al, 2004)
• Robust and powerful (Stockwell & Noble, 1992; Stockwell
& Peters, 1999).
• Needs no absence data, and little a priori
knowledge.
• Little independent testing of the algorithm has
been done, and should be carried out (Peterjohn,
2001)
Test for difference between specialist and generalist
species (H0: no difference; two sample t-test, df = 8)
The effects of the number of
modelling points used of the
effectiveness of GARP
The relationship between Type I and Type II errors
(false positives and false negatives) in GARPmodelled distributions
Implications of Type I and Type II errors in
alien invasive distribution modelling
TYPE I ERROR (false positive)
• Predicts species presence in
sites at which it may not be
found.
• Overestimates range,
increasing the risk priority of
the species.
• This leads to inclusion of
sites of little or no
importance for conservation.
• May falsely skew
conservation priorities
toward elimination of plant
species with low potential
impact.
TYPE II ERROR (false negative)
• Failure to predict
environmental suitability in a
site at which the species is
known to occur.
• May therefore underestimate
the overall susceptibility to
invasion.
• Leads to exclusion of sites of
possibly vital conservation
value.
• Skews conservation priorities
away from alien species with
a potentially high invasive
impact .
The incidence of Type II errors (false negatives)
related to the number of modelling points used
Conclusions
• GARP-derived species layers have some potential
use for conservation planning.
• There is little difference observed in the ability of the
model to predict specialist and generalist species,
although some tendencies to overestimate range for
specialist species have been observed.
• The conservation objectives for a species should be
borne in mind – in general it is better to err on the
side of caution and choose a model that minimizes
false negative (Type II) errors.
• Modelling for species at a finer scale may prove
challenging, because obtaining environmental layers
at the specified scale can be difficult and local
environmental conditions may vary (micro-climate),
and local the extent of field work required may be
prohibitive.
Acknowledgements
• Richard Knight
• Tony Rebelo and the Protea Atlas Project
• The Beit Trust