3. RESULTS
3.1 Seabird sightings
In total, 15,063 seabirds were observed during the six expeditions
between 2012 and 2017 (Figure 2) across all samples. Seven families were
recorded: Laridae (noddies and terns), Sulidae (boobies), Procellariidae
(shearwaters and petrels), Phaethontidae (tropicbirds), Fregatidae
(frigatebird), Hydrobatidae (northern storm-petrels) and Oceanitidae
(southern storm-petrels). Observations were dominated by the Laridae
(8,817 individuals), Sulidae (5,057 individuals) and Procellariidae (872
individuals). These families were retained for further distribution
modelling (Figure 2). For the Laridae, Sulidae, and Procellaridae,
individuals per sample was highest in 2013 (33.1 ind. per sample), 2017
(24 per sample, and 2012 (16.3 ind. per samples), respectively and
lowest in 2015 (16 ind. per sample), 2014 (3.2 ind. per sample) and 2013
(no ind.), respectively.
3.2 Predictive Modelling
3.2.1 Oceanic drivers of
distribution
Total deviance explained for each GAM was 33.7% for Laridae, 46.3% for
Sulidae and 21.2% for Procellariidae. Seabed depth explained 42.90%
and 33.64% deviance in the distribution of Laridae and Sulidae
respectively (Figure 3a and 3e). The only other geomorphometric variable
that was important for Sulidae distribution was slope (28.68%, Figure
3f). In terms of oceanographic variables, sea level anomaly influenced
the Procellariidae (Figure 3c, 3g and 3i), explaining 31.24% of the
deviance. Sea surface temperature and chlorophyll-a were important
variables for Laridae (13.11% and 42.60%) and for Procellariidae
(24.31% and 23.97%, Figure 3b, 3d, 3h and 3j) but not the Sulidae.
Sulidae and Procellariidae showed high yearly variability (28.68 and
20.47% deviance respectively) consistently decreasing in abundance from
2012 to 2015, an increasing from 2015 to 2017 (Figure 4).
3.2.2 Spatial Predictions within
the Chagos Archipelago
Spatial model predictions for Laridae and Sulidae distribution revealed
a strong seabed depth signature (Figure 5a, b, d, and e), while
Procellariidae distribution was more uniform with higher abundance
levels near land and towards the northwest of the Archipelago (Figure 5c
and f). Laridae abundance was pronounced over shallow seabeds
(< 1000 m) in proximity to islands and atolls (Figure 5a, d).
Sulidae abundance was more pronounced in pelagic and deeper areas
(Figure 5b, e), and in areas with intermediate slope (c. 15º, Figure
5b). Both Laridae and Sulidae distribution was pronounced along the
Lakshadweep-Maldives-Chagos ridge. Model prediction uncertainty was
spatially and family specific (Appendix S2).
3.2.3 Response to rat
presence
Rat-free BRT models explained more deviance (Sulidae ≈ 71%, Laridae ≈
37%, and Procellariidae ≈ 20%) than rat-infested models (Sulidae ≈
59%, Laridae ≈ 31%, and Procellariidae ≈ 12%). Distance to a rat-free
or rat-infested island was important for Laridae and Sulidae (39.3 -
48.5%, 32.5 – 36.1% respectively) compared with the Procellariidae
(0- 14.6 %). Deviance explained by Distance to a rat-free island was
consistently higher than that by Distance to rat-infested island
(Laridae ≈ 48%, Sulidae ≈ 36%, and Procellariidae ≈ 15%). Conversely,
deviance explained by the area of the nearest island was higher in the
rat-infested models (Laridae ≈ 8%, Sulidae ≈ 11%, and Procellariidae ≈
5%; Figure 6a – 6g).
All seabird families were sensitive to the proximity to rat-free
islands, as revealed by broken stick regressions (Figure 6g – 6i).
Breaking points (BP) indicated the threshold to which the nearest
island, whether infested or not, influenced the distribution of
seabirds. Thresholds BP in the effect of islands were apparent for all
families. The BP for Laridae was at 47.5 km for rat-free islands [CI
44.8, 50.2] and at 63.6 km for rat-infested islands [CI 59.1,
67.9]. The BP for Sulidae was at 16.51 km [CI 11.9, 21.1] for
rat-free islands and 60.54 km for rat-infested islands [CI 54.0,
67.3]. No effect of rat presence on nearby islands was detected for
the Procellaridae family, as the BP showed no significant difference in
the CI between rat-free island (BP 82.9 km [CI 82.9, 96.4]) and
rat-infested islands (BP 73.5 km [CI 68.6, 78.4]). Threshold
breaking points are reported in Appendix S3 (Table S3).
The presence of rats on nearby islands reduced the abundance of all
seabird families (Figure 7a – 7f). This pattern was more pronounced for
Laridae (Figure 7b and 7e). Following rat eradication all seabird
families increased in abundance (Figure 7). The gain was most pronounced
near larger islands (Figure 7g – 7l). This was particularly pronounced
for Laridae (Figure 7 k). Gains for Procellariidae were minor and
uniformly distributed (Figure 7l).