4. DISCUSSION
Through our observations at sea in the Chagos Archipelago we have
identified spatio-temporal trends in seabird distribution. Our multiyear
survey spanned a broad range of environmental conditions and has enabled
us to identify geomorphic and oceanographic conditions driving seabird
distribution, including a possible association between regional
abundance and the IO Dipole oscillation. Accounting for unbalanced
sampling effort, we have identified seabird hotspots within the BIOT
MPA, complementing previous investigations in the southwest and southern
IO (Mannocci, Laran, et al., 2014; Hyrenbach et al., 2007). We have
modelled the spatially explicit impact of islands with and without rats
on seabird distribution, and have identified areas of net increase in
abundance under an archipelago-wide rat eradication scenario.
Information on seabird hotspots, sensitivity to climate oscillation, and
how eradication can result in distribution shifts has critical
implications for tropical seabird conservation and for island
restoration strategies.
4.1 Drivers of seabird distribution within the Chagos
Archipelago
The distribution of the Laridae, dominated by the brown noddy, was
strongly related to depth and chlorophyll-a, with modelled abundance
concentrated around islands, and over shallow water (<100 m).
Laridae in the Chagos Archipelago have previously been observed to be
mostly lagoonal foragers (Carr, pers. comm), consistent with their
modelled distribution here. Our results are also consistent with
previous observations of the brown noddy and lesser noddy leaving and
returning to islands during the same day in the breeding season,
suggesting they are not long-distance or multiday foragers (Surman,
Nicholson, & Ayling, 2017; Jaquemet, Le Corre, & Weimerskirch, 2004).
In line with theoretical expectations due to their small size (Harper &
Bunbury, 2015), the Laridae were the group most sensitive to rats,
resulting in elevated abundance near rat-free islands.
The Sulidae showed a primarily oceanic distribution, with pronounced
hotspots east of the Diego Garcia and west of the Peros Banhos atolls,
and along the Lakshadweep-Maldives-Chagos ridge. Sulidae distribution
has traditionally been thought to be positively associated with areas of
high productivity and elevated chl-a concentration (Ballance, Pitman, &
Reilly, 1997; Jaquemet, Le Corre, Marsac, Potier, & Weimerskirch, 2005;
Weimerskirch, Le Corre, Jaquemet, & Marsac, 2005). In contrast, Mendez
et al. (2017) identified a negative correlation between red-footed
boobies and chl-a. Mendez et al. (2017) collected data on the foraging
behaviour of red-footed boobies using tracking devices in five colonies
along the equator (Galapagos Islands, Mozambique Channel, New Caledonia
and the IO) permitting a more robust understanding of factors that
determines distribution. They concluded that across the pantropical
range of the red-footed booby, distribution is closely driven by intra
and interspecific competition for prey. As chl-a concentration was not
prognostic here, our results were consistent with Mendez et al. (2017).
Sulidae were sensitive to rats although less so than Laridae,
aggregating in greater abundance near rat-free islands.
Procellariidae were evenly distributed across the BIOT marine protected
area, preferring warmer (>30° C), and less productive
waters (<0.20 mg. m-3 chl-a). Our
observations were consistent with those of Mannocci, Laran, et al.
(2014) in the western IO, reporting greater number of shearwater in
warmer waters (>29.5° C) and lower number in productive
areas (> 0.37 mg m-3). The Procellariidae
were the group the least sensitive to island proximity and, in contrast
to the Laridae and Sulidae, appeared to increase in abundance with
distance from islands. A potential reason for this is that
Procellariidae typically have a wider foraging range from nesting
colonies (i.e. 480 km; King 1974) than both Sulidae (i.e. 67.5 km; Young
et al. 2010) and Laridae (i.e. 80 km; Harrison & Stone-Burner, 1981;
King, 1974). Procellariidae return to burrows only at night; that their
distribution appears independent from islands may be due to a
predominantly scattered and remote distribution during the day (Dias,
Alho, Granadeiro & Catry, 2015). Although we found no significant
effect of rat presence on Procellariidae distribution, many
Procellariidae species are particularly vulnerable to invasive land
predators (Smith, Polhemus, & Vaderwerf, 2002), because they nest in
ground burrows (i.e. shearwaters). The pelagic behaviour and large
foraging range of this family (up to 3500 km in the Seychelles; Catry,
Ramos, Le Corre & Phillips, 2009), which our sampling range could have
failed to capture, may mask any distribution shift related to rat
presence.
Constant competition over prey is expected to lead to a prey depredation
zones around colonies, otherwise known as The Ashmole’s Halo (Ashmole,
1963). The Halo is expected to vary as a function of colony proximity,
size, and bird foraging range (Birt, Birt, Goulet, Cairns &
Montevecchi, 1987). Within the Chagos Archipelago, many islands are less
than 100 km apart and are clustered close together (< 20 km
between islands) within atolls. The range to which both the abundant
Laridae and Sulidae distribution radiates out from islands (i.e. 100.7
km and 176.0 km respectively), makes it therefore very likely that
neighbouring colonies overlap in distribution and therefore compete (in
line with Mendez et al. 2017), unless individuals from different
colonies express different behaviour to minimise foraging overlap
(Wakefield et al. 2013). The wide distribution range and lower abundance
of the Procellariidae makes it likely that the competition pressure
resulting from colony proximity is less pronounced than for the other
families (Gaston, Ydenberg & Smith, 2007).
4.2 Implication for rat eradication programmes
Past eradication efforts in the Chagos Archipelago include a failed
attempt on Eagle Island (Meier, 2006) and successful attempts on Îles
Vache Marine, du Sel and Jacobin (Harper et al., 2019). The latter
attempts focussed on small islands to test the feasibility of
eradication and appropriate methodologies on a small scale. Island
rodent eradication is increasingly recognized as a powerful strategy for
the preservation and recovery of avian populations (Brooke, et al.,
2018; Jones et al., 2016; Lavers, Wilcox, & Donlan, 2010). However,
eradication is technically challenging and expensive (Warren, 2018),
requiring the application of toxic rodenticide posing a risk to humans,
livestock, pets, and wildlife (Pickrell, 2019; Van den Brink, Elliott,
Shore & Rattner, 2017). Eradication is more likely to fail in the
tropics, with high mean annual temperatures and constant precipitation
(Russell & Holmes, 2015), and in the presence of land crabs and coconut
palms (Holmes, et al., 2015), making a programme in the Chagos
Archipelago challenging. Eradication on the largest island of Diego
Garcia is likely to be particularly complex and expensive as it is
inhabited (Harper & Carr, 2015).
Our analysis has revealed that gains in seabird distribution at sea
following eradication are spatially and family-specific, and that
eradication on larger islands will yield greater distribution gain.
This, combined with the presence of thresholds in distribution, suggests
that prioritising remote islands (> 50 km from nearest bird
colonies) will impose less spatial competition at sea to the
recolonizing seabirds. From the perspective of our study, we propose
that eradication should prioritise Île Sud-Est and Île Lubine in Egmont
Islands, Île du Coin and Île Pierre in Peros Banhos Atoll, Île Boddam in
Salomon Islands and Eagle Island in the Great Chagos Bank, in order to
minimise potential overlap in distribution, and therefore competition,
between recovering colonies. We note that these recommendations are on
the basis of factors explored in this study only and that there are
multiple factors that dictate the feasibility, success and approach to
rodent eradication. Our results here aim to be useful to feed into a far
wider set of considerations, with the ultimate aim of eradicating rats
from all islands in the archipelago, in order to achieve full
conservation impact.
4.3 Influence of Climate
Oscillation
Our multiyear time series enable us to test for distribution effect of
climatic oscillations at the inter-annual scale. We observed similar
abundance trends for Sulidae and Procellariidae during the six years of
sampling, with both families decreasing initially, followed by an
increase. The abundance of both families appeared therefore to closely
match the strength of the Dipole Mode Index, with the Procellaridae
doing so to a lesser degree. We detected no correspondence between
seabird and the ENSO index. Our results are consistent with current
understanding regarding the influence of the dipole of higher trophic
levels in the Indian Ocean. For example, Kumar, Pillai & Ushadevi
(2016), identified a positive association between IO tuna productivity
and the Dipole Index. While in the southern IO, Albatross
breeding-success has been correlated with the Dipole Index (Rivaland et
al., 2010). While knowledge of mechanisms driving these patterns is at
present limited, our results add to a fledgling body of research on the
importance of the Dipole on IO megafauna.
Previous studies may give some clues to possible mechanisms why seabird
abundance in the Chagos Archipelago is higher during positive Dipole
events, although these must remain speculative. There is, for example,
evidence that equatorial upwelling in the IO are more pronounced and
that westerly winds decrease in intensity during positive Dipole events
(Du & Zhang, 2015). In the Chagos Archipelago, it is possible that a
drop in regional upwelling require seabirds to forage further afield,
leading to a drop in regional abundance. This would be consistent with
understanding regarding other climate oscillations such as ENSO, which
is known to influence forage species productivity (Lehodey, Bertignac,
Hampton, Lewis, & Picaut, 1997), with implications for the demography
and foraging behaviour of higher trophic levels (Champagnon, Lebreton,
Drummond & Anderson, 2018). For example, Sprogis et al. (2018), found
that the common bottlenose dolphin (Tursiuops trunctatus )
migrated offshore during strong ENSO years, possibly due to a lack of
inshore prey. Use of telemetry and satellite tracking is currently being
deployed on red-footed boobies in BIOT (Carr, pers. comm.), which will
enable mechanisms to be explored in more detail. The greater sensitivity
of Procellaridae to both oceanographic variables and to the Dipole
suggests this family may be the most vulnerable to global environmental
change.
Any linkage between the Dipole and mobile megafauna are likely mediated
by multiple trophic links (Oro, 2014). As foragers commensal with
subsurface predators, seabirds could be impacted by the Dipole both
directly, for example, by a reduction in forage species abundance, and
indirectly, by an increase in tuna abundance (Maxwell & Morgan, 2013;
Kumar et al., 2016). It is beyond our scope to distinguish these
processes here, however we are currently expanding our analysis of
seabird distribution to include data on subsurface prey and predator
abundance collected simultaneously to the seabird observations, using
midwater baited videography (Letessier, Bouchet & Meeuwig, 2017;
Letessier et al., 2019).
4.4 Concluding remarks
Seabird abundance and distribution at sea in BIOT is driven by
geomorphology and oceanographic conditions. Our modelled distribution
complements previous efforts elsewhere in the IO and our time-series has
enabled us to identify potential interannual variability related to
climate oscillation. Seabird populations are vulnerable to both climatic
variability and interactions with human activities (Paleczny et al.,
2015). Environmental variability is predicted to increase globally under
climate change scenarios (Allen et al., 2014), and evidence suggest that
global warming variability may decouple the Dipole from upwelling in the
western IO (Watanabe, Watanabe, Yamazaki, Pfeiffer & Claereboudt,
2019). Identifying how inter-annual processes like the IO Dipole drives
seabird distribution, where human activities are limited, is valuable
for identifying long-term strategies for seabird protection.
To our knowledge, this is the first attempt at predicting the response
of seabird distribution to a rat-eradication scenario. We have
demonstrated areas of net gain in distribution and have predicted new
hotspots at sea after a rat-eradication programme. There is considerable
impetus for eradicating invasive species on islands (Brooke et al.,
2018; Jones et al., 2016; Lavers et al., 2010). In addition to practical
considerations such as cost and probability of success, eradication
programmes should identify where eradication can have the greatest
conservation impact and ecological footprint. We have successfully
identified multiple factors that should influence conservation
activities, which is particularly important for seabirds, whose niche
extends beyond terrestrial breeding colonies.