Total foraging area of the Great Barrier Reef breeding
seabird
community
The total foraging area of the Great Barrier Reef breeding seabird
community was estimated at 2,941,000 km2 by global
foraging circles, of which 197,000 km2 was predicted
unsuitable foraging habitat and excluded following
transferability-supported refinement (Table 5). There were large
differences in the total area of unsuitable habitat excluded from
foraging circles between species due to radius size (Table 3), proximity
of GBR colonies (overlap of foraging circles; Fig. 5a) and model
transferability (Fig. 5b). We saw the greatest refinement in species
with large foraging radii (e.g. wedge-tailed shearwater long trips,
frigatebirds, tropicbirds) and species which had better model
transferability (e.g. terns, Fig. 5b). The better local prediction of
GBR colony-specific models over multi-colony models afforded locally
tracked species greater transferability-supported refinement (Fig. 5).
This led to greater total unsuitable habitat exclusion when locally
tracked sites were well represented and transferability was higher (e.g.
wedge-tailed shearwater short trips compared to brown booby and masked
booby; Fig 5; Table 5). By contrast, noddies had the highest number of
breeding colonies but unsuitable foraging habitat could only be excluded
at one site with local tracking because their multi-colony model
transferability was too poor (AUC<0.5) for foraging circle
refinement. Sooty terns were the only species for which we were unable
to use transferability-supported refinement for any foraging circles due
to their poor multi-colony model transferability and absence of GBR
tracking data. Refined foraging circles on the GBR produced through
transferability-refined refinement had high refinement confidence, with
a 96% average probability of including known foraging areas (Table 5).
This was higher than the 89% average from our global validation
exercise (Fig. 4) due to the contribution of locally tracked GBR
colonies, which all had 100% inclusion of known foraging areas in
refined foraging circles.
After transferability-supported refinement had been completed, six
modelled species still contained some colonies with refined foraging
circles that exceeded 100,000 km2 (Fig. 5b). Using
area-limited refinement to drop these refined foraging circles below
100,000 km2 excluded a further 1,629,000
km2 of predicted unsuitable foraging habitat from the
community (Table 5). However, this came at a cost of reducing refinement
confidence, with the average inclusion of known foraging areas in
refined foraging circles dropping from 97% to 55% in these species.
Sooty tern, tropicbirds and frigatebirds all required large area-limited
refinement to meet the 100,000 km2 limit, greatly
reducing their refinement confidence (16-40% inclusion; Table 5).
Wedge-tailed shearwater long trips required the greatest refinement to
meet the area limit and highlighted the power of tracking data from the
GBR; refined foraging circles delineated by the 90thpercentile of habitat suitability dropped predicted inclusion of known
foraging areas to 6% in multi-colony models but only to 87% in
colony-specific models (Table 5). Area-limited refinement of
wedge-tailed shearwater short trips and red-footed booby foraging
circles to meet the 100,000 km2 limit was achieved
with minor reduction in known foraging area inclusion (77-95%).
Applying the most appropriate foraging circle refinement approach to
each modelled species (none for sooty tern, area-limited for
wedge-tailed shearwater and red-footed booby, and
transferability-supported for the remainder) created a network of
refined foraging circles for the breeding seabird community of the GBR
that balanced size against refinement confidence (Fig. 6).