A review of a decade of lessons from one of the world’s largest MPAs: conservation gains and key challenges

Abstract

Given the recent trend towards establishing very large marine protected areas (MPAs) and the high potential of these to contribute to global conservation targets, we review outcomes of the last decade of marine conservation research in the British Indian Ocean Territory (BIOT), one of the largest MPAs in the world. The BIOT MPA consists of the atolls of the Chagos Archipelago, interspersed with and surrounded by deep oceanic waters. Islands around the atoll rims serve as nesting grounds for sea birds. Extensive and diverse shallow and mesophotic reef habitats provide essential habitat and feeding grounds for all marine life, and the absence of local human impacts may improve recovery after coral bleaching events. Census data have shown recent increases in the abundance of sea turtles, high numbers of nesting seabirds and high fish abundance, at least some of which is linked to the lack of recent harvesting. For example, across the archipelago the annual number of green turtle clutches (Chelonia mydas) is ~ 20,500 and increasing and the number of seabirds is ~ 1 million. Animal tracking studies have shown that some taxa breed and/or forage consistently within the MPA (e.g. some reef fishes, elasmobranchs and seabirds), suggesting the MPA has the potential to provide long-term protection. In contrast, post-nesting green turtles travel up to 4000 km to distant foraging sites, so the protected beaches in the Chagos Archipelago provide a nesting sanctuary for individuals that forage across an ocean basin and several geopolitical borders. Surveys using divers and underwater video systems show high habitat diversity and abundant marine life on all trophic levels. For example, coral cover can be as high as 40-50%. Ecological studies are shedding light on how remote ecosystems function, connect to each other and respond to climate-driven stressors compared to other locations that are more locally impacted. However, important threats to this MPA have been identified, particularly global heating events, and Illegal, Unreported and Unregulated (IUU) fishing activity, which considerably impact both reef and pelagic fishes.

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Vol.:(0123456789)
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Marine Biology (2020) 167:159
https://doi.org/10.1007/s00227-020-03776-w
REVIEW, CONCEPT, ANDSYNTHESIS
A review ofadecade oflessons fromone oftheworld’s largest MPAs:
conservation gains andkey challenges
GraemeC.Hays1 · HeatherJ.Koldewey2,3· SamanthaAndrzejaczek4· MartinJ.Attrill5· ShantaBarley6,7·
DanielT.I.Bayley8· CassandraE.Benkwitt9· BarbaraBlock4· RobertJ.Schallert4· AaronB.Carlisle10· PeteCarr3,11·
TaylorK.Chapple12· ClaireCollins3,11· ClaraDiaz5· NicholasDunn11,13· RobertB.Dunbar14· DannielleS.Eager5·
JulianEngel15· ClareB.Embling5· NicoleEsteban16· FrancescoFerretti17· NicolaL.Foster5· RobinFreeman11·
MatthewGollock2· NicholasA.J.Graham9· JoannaL.Harris5,18· CatherineE.I.Head11,19· PhilHosegood5·
KerryL.Howell5· NigelE.Hussey20· DavidM.P.Jacoby11· RachelJones2· SivajyodeeSannassyPilly21·
InesD.Lange22· TomB.Letessier11,23· EmmaLevy2· MathildeLindhart24· JamieM.McDevitt‑Irwin4·
MarkMeekan25· JessicaJ.Meeuwig23· FiorenzaMicheli4,26· AndrewO.M.Mogg27,28· JeanneA.Mortimer29,30·
DavidA.Mucciarone14· MalcolmA.Nicoll11· AnaNuno3,31· ChrisT.Perry22· StephenG.Preston19· AlexJ.Rattray1·
EdwardRobinson5· RonanC.Roche21· MelissaSchiele11· EmmaV.Sheehan5· AnneSheppard21,32·
CharlesSheppard21,32· AdrianL.Smith19· BradleySoule15· MarkSpalding33· GuyM.W.Stevens18·
MargauxSteyaert11,19· SarahStiel19· BrettM.Taylor25· DavidTickler7· AliceM.Trevail34· PabloTrueba15·
JohnTurner21· StephenVotier34· BryWilson19· GarethJ.Williams21· BenjaminJ.Williamson35·
MichaelJ.Williamson11,36· HannahWood11· DavidJ.Curnick11
Received: 10 June 2020 / Accepted: 28 September 2020
© Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract
Given the recent trend towards establishing very large marine protected areas (MPAs) and the high potential of these to con-
tribute to global conservation targets, we review outcomes of the last decade of marine conservation research in the British
Indian Ocean Territory (BIOT), one of the largest MPAs in the world. The BIOT MPA consists of the atolls of the Chagos
Archipelago, interspersed with and surrounded by deep oceanic waters. Islands around the atoll rims serve as nesting grounds
for sea birds. Extensive and diverse shallow and mesophotic reef habitats provide essential habitat and feeding grounds for
all marine life, and the absence of local human impacts may improve recovery after coral bleaching events. Census data
have shown recent increases in the abundance of sea turtles, high numbers of nesting seabirds and high fish abundance, at
least some of which is linked to the lack of recent harvesting. For example, across the archipelago the annual number of
green turtle clutches (Chelonia mydas) is ~ 20,500 and increasing and the number of seabirds is ~ 1 million. Animal tracking
studies have shown that some taxa breed and/or forage consistently within the MPA (e.g. some reef fishes, elasmobranchs
and seabirds), suggesting the MPA has the potential to provide long-term protection. In contrast, post-nesting green turtles
travel up to 4000km to distant foraging sites, so the protected beaches in the Chagos Archipelago provide a nesting sanctu-
ary for individuals that forage across an ocean basin and several geopolitical borders. Surveys using divers and underwater
video systems show high habitat diversity and abundant marine life on all trophic levels. For example, coral cover can be
as high as 40–50%. Ecological studies are shedding light on how remote ecosystems function, connect to each other and
respond to climate-driven stressors compared to other locations that are more locally impacted. However, important threats
to this MPA have been identified, particularly global heating events, and Illegal, Unreported and Unregulated (IUU) fishing
activity, which considerably impact both reef and pelagic fishes.
Introduction
The growing recognition that marine ecosystems are
threatened by biodiversity declines and habitat degrada-
tion (McCauley etal. 2015) has led to international calls
Responsible Editor: S. Shumway.
Reviewed by B. E. Lapointe and an undisclosed expert.
Extended author information available on the last page of the article

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for protecting the world’s ocean, including within Marine
Protected Areas (MPAs) (Convention on Biological Diver-
sity’s Aichi Target 11 https ://www.cbd.int/sp/t arge ts/;
Woodley etal. 2019). Negotiations at the United Nations
are also ongoing to establish a new international treaty
within which MPAs would be established in Areas Beyond
National Jurisdiction (ABNJs) (O’Leary etal. 2020). A large
body of research spanning over 50years demonstrates that in
general, MPAs lead to increases in biodiversity, abundance,
size and biomass (e.g. Ballantine 2014; Lester etal. 2009).
Importantly, there is also clear evidence of fisheries ben-
efits (Goñi etal. 2010; Harrison etal. 2012), well-being and
social benefits (Ban etal. 2019), and resilience afforded by
protection in the face of climate change (Mellin etal. 2016;
Roberts etal. 2017). While there are recognised limitations
(Devillers etal. 2015; Edgar etal. 2014; Giakoumi etal.
2018), impacts of protection are largely positive in coastal
ecosystems.
Very Large Marine Protected Areas (VLMPAs),
areas > 100,000km2, are fundamental to halting and revers-
ing ocean health declines and to meeting global targets. The
Aichi Target calls for a minimum of 10% of the world’s
ocean to be protected by 2020, a target that will not be met
with currently only 2.5% of the ocean’s surface in highly
protected MPAs (https ://www.mpatl as.org/; Sala etal. 2018).
Additionally, the 30 × 30 initiative, supported by the analysis
of O’Leary etal. (2016), suggests that a minimum of 30%
of the ocean should be in highly protected MPAs. Positive
conservation outcomes from large-scale protection are also
expected to generate positive social, economic and equity
outcomes with respect to food security and resource access
(Sumaila etal. 2015). However, the benefits of VLMPAs
remain debated and empirical studies evaluating their effec-
tiveness are essential. These studies have been limited due to
the relatively young age of VLMPAs; the first VLMPA to be
established was the Pacific Remote Islands National Marine
Monument in 2009 (MPA Atlas, https ://mpatl as.org/mpa/
sites /77043 95/). Significant challenge also exists in deliver-
ing conservation research in remote regions and on large
spatial scales that include offshore pelagic environments.
The British Indian Ocean Territory (BIOT) MPA was
proclaimed by the UK Government in April 2010. It is clas-
sified as a VLMPA at 640,000km2 and as an IUCN manage-
ment category 1a strict nature reserve (Day etal. 2019), with
effectively no permitted fishing. At the time of its designa-
tion, it was the largest contiguous highly protected MPA. The
MPA includes a range of habitats with deep oceanic areas
surrounding the shallow reef environments and reef islands
of the Chagos Archipelago. Its recognition as an important
site for conservation (reviewed previously by Sheppard etal.
2012) has helped drive a concerted programme of ongoing
studies to understand the outcomes of the MPA’s creation
and its importance for the species and ecosystems it hosts.
At the same time, the legality of this MPA has been chal-
lenged (Appleby 2015; United Nations 2019). Given both
the ongoing challenges to the BIOT MPA and the wealth
of recent studies, here we assess the knowledge gains over
the past decade regarding this MPA’s conservation value.
We also discuss the ongoing conservation challenges facing
the BIOT MPA that continue to require new and innova-
tive approaches and consider the implications of the lessons
learnt for marine conservation planning and management
more broadly across the globe.
Materials andmethods
Identifying case studies
Marine research in BIOT extends back to the 1970s but
has increased rapidly in the last 15years. Recently, much
of the research within the BIOT MPA has been coordi-
nated through the Bertarelli Programme in Marine Science
(BPMS). At the annual BPMS meeting in London (18–20
September 2019), programme-supported scientists were
asked to describe their key recent findings that highlight
either the conservation value or the challenges facing the
MPA. Experts who attended this meeting were also asked to
identify other individuals from around the world who should
be invited to participate in writing a review summarizing the
last decade of research on the BIOT MPA. The assembled
authors were able to provide comprehensive coverage of the
breadth of recent work that has taken place concerning the
BIOT MPA, including work on a range of habitats such as
shallow coral reefs and pelagic realms as well as a range of
taxa including fishes, seabirds and turtles. Case studies were
identified by taxonomic group, by habitat, or by ecological
question and then experts in each area prepared text describ-
ing their recent discoveries, which are synthesised below.
Background andoverview ofrecent scientic work
Of the 640,000km2 of the BIOT MPA, 19,120km2 is shal-
lower than 100m and the remainder is deep oceanic water
with maximum depths of > 5000m. The Chagos Archi-
pelago consists of discrete atolls with around 58 associated
islands, submerged banks, and an estimated 86 seamounts.
The Great Chagos Bank is described as the world’s largest
atoll structure, covering an area of 12,642km2 and water
depths down to about 90m (Fig.1). The land area of the
islands within the archipelago totals only 56 km2. These
islands are surrounded by shallow fringing coral reefs and
encompass lagoons with sheltered reefs, patch reefs, coral
outcrops and seagrass meadows. The BIOT MPA covers the
entire Economic Exclusion Zone (EEZ) with the exception
of Diego Garcia atoll and a three-nautical mile buffer around

Marine Biology (2020) 167:159
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it, noting that large parts of this atoll and waters receive
separate protection under multiple legal and other regulatory
controls (https ://biot.gov.io/). From the eighteenth century
until the 1970s, the archipelago was managed as a coco-
nut oil plantation. When the final plantations closed, the
archipelago was declared a military exclusion area, and the
remaining population was relocated (Wenban-Smith and
Carter 2017). Since then, commercial fishing—comprising
licensed pelagic longline and purse seine fisheries and a rela-
tively small-scale demersal fishery—was allowed up until
2010 at which point all legal commercial fishing ceased.
Local human impacts on the reefs within the MPA have
generally been minimal, but were significant on the islands
when previously settled. Approximately half of Diego Gar-
cia, which has the only current human settlement in the
archipelago, has been extensively altered for the creation of
a large military facility, with buildings and infrastructure,
including coastal modification, ports and anchorages.
The isolated and protected nature of the Chagos Archi-
pelago means that many human influences are minimal. This
limited human presence and remote setting of the BIOT
MPA provides a baseline to other systems more impacted
by anthropogenic pressures. All else being equal, it might
be expected that the MPA would result in positive species
and habitat conservation outcomes. There have been con-
siderable recent efforts, documented below, to quantify
species abundances for comparison with other areas in
the Indian Ocean, as well as assessing long-term changes
within the archipelago. This work has shown the value of
the MPA for sea turtles, pelagic and reef-associated fishes,
seabirds, invertebrates and key habitats, such as coral reefs
and seagrass beds (Fig.2). To assess patterns of movement
in relation to the MPA, a range of turtles, fishes and sea-
birds have been tracked using satellite (Argos and GPS),
acoustic telemetry and archival biologging packages. Coral
reef surveys have been conducted for four decades, thus
informing research on how climate change impacts these
Fig. 1 The Chagos Archipelago. Inset shows the general location
within the Indian Ocean and the MPA boundary (red). Main map
shows the archipelago which lies at the heart of the MPA. The five
atolls with land are in bold, versus selected submerged reefs and
atolls not in bold. Islands on the Great Chagos Bank include Danger
Island, Eagle Island, Three Brothers islands and Nelsons Island. Blue
shading indicates water shallower than approximately 100m
Fig. 2 The breadth of recent studies in the BIOT MPA. Recent work
in the BIOT MPA has used electronic tags to track the movements of
sea turtles, seabirds and fish. Pictured with tags attached a a green
turtle (Chelonia mydas) with a Fastloc-GPS Argos tag on the cara-
pace, b a red-footed booby (Sula sula) with a light-based geoloca-
tor tag on its leg, c a silvertip shark (Carcharhinus albimarginatus)
prior to being fitted with a long-term, internal acoustic transmit-
ter. d Habitat surveys using SCUBA and deployed instruments have
shown long-term changes in reef environments and water tempera-
ture. e Counting tracks on beaches has revealed long-term increases
in sea turtle nesting numbers. f Marine surveys have been extended
using technology such as Baited Remote Underwater Video Systems
(BRUVS) deployed in the open ocean or in shallow coastal areas. Pic-
tured in (f) silvertip sharks. Images courtesy a, e Nicole Esteban and
Graeme Hays, b Hannah Wood, c David Curnick, d Charles Shep-
pard, f Jessica Meeuwig

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ecosystems. Fish surveys on reefs and in pelagic areas with
stereo Baited Remote Underwater Video Systems (BRUVS)
have been used to describe species assemblages and rela-
tive abundance. More recently, detailed oceanographic stud-
ies have been undertaken to better understand the drivers
behind the biotic patterns and behaviours observed, while
remotely operated vehicles (ROVs) have been employed to
study the health and diversity of mesophotic reefs and how
they may act as refuges for shallow reefs. The temporal,
spatial and bathymetric extent of data is thus now signifi-
cant and increasing rapidly. In addition to these studies on
abundance, trends and movements, the MPA has allowed a
range of questions to be addressed on ecosystem function-
ing, movement ecology and animal behaviour in an environ-
ment relatively free of most human influences. At the same
time, patrols of the MPA provide indications of the extent of
Illegal, Unreported and Unregulated (IUU) fishing activity.
Review structure
We begin by examining the importance of the BIOT MPA
for coral reefs and coral-reef research. We then consider
work with taxa that has included tracking individuals and/
or census surveys including coral-reef fish, turtles, sea-
birds and pelagic fish. We then consider recent knowledge
gains regarding invertebrate fauna and mesophotic reefs.
We examine how the MPA has provided an environment
for seminal work on natural behaviours and ecological rela-
tionships in the absence of anthropogenic influences and we
consider how the physical oceanography of the region may
influence its ecological value. Finally, we highlight the key
threats the MPA faces, particularly climate warming impacts
on coral reefs and IUU fishing impacts on fish stocks.
Results
Importance oftheBIOT MPA forcoral reefs
andcoral‑reef research
The BIOT MPA represents a valuable reference site for
understanding coral community resilience in an ocean
where most reefs have undergone significant and continuing
declines in health. Although reefs in the Chagos Archipelago
have not been spared from the effects of large climate-driven
stressors (i.e. temperature driven coral bleaching), the MPA
has afforded protection from many of the local threats that
reefs face in other parts of the world such as destructive fish-
ing practices, local pollution, or sedimentation and eutrophi-
cation from anthropogenic land-based sources.
Data collected following the major coral bleaching
event of 1998 showed that despite its geographically iso-
lated position, the Chagos Archipelago was not immune
from widespread coral mortality, which extended to depths
of > 40m in some locations (Sheppard etal. 2012). How-
ever, most of the reefs recovered quickly and by 2012 coral
cover on reefs in the BIOT MPA averaged 40–50% (Fig.3a,
d), with juvenile coral densities of 20–60 colonies m−2
(Fig.3b) (Sheppard etal. 2017; Sheppard and Sheppard
2019). Thus, the reefs had largely regained coral cover levels
consistent with those documented prior to 1998 and coral
recruitment was clearly prolific. This high coral cover and
return of dominant branching and tabular species on many
fore reef sites supported high net positive carbonate budgets,
an important metric influencing reef growth potential and
the maintenance of habitat complexity (Perry etal. 2015).
Resultant estimates of average vertical reef accretion rates
on Acropora dominated reefs (4.4 ± 1.0mmyear−1) were
high in a global context, indicating that many of the reefs
would have the capacity to track projected future sea level
rise (Perry etal. 2018). For context it is important to note
that not all reefs in the wider region recovered as well or as
fast after the 1997–1998 bleaching event. For example, shal-
low reefs in the Maldives recovered to pre-bleaching states
by 2013–2014, albeit comparatively slowly and displaying
subtle changes in community composition (e.g. Morri etal.
2015), whilst in the Seychelles reefs followed more divergent
recovery trajectories. Some sites recovered well, while oth-
ers regime-shifted to macroalgal or rubble dominated states
with coral cover < 10% (e.g. Chong-Seng etal. 2014; Harris
etal. 2014; Graham etal. 2015). Regime-shifted sites had
negative carbonate budgets and reef accretion rates (Perry
etal. 2018).
It is clear that the absence of local impacts, provided by
the remoteness of the Chagos Archipelago and the presence
of the MPA, aided relatively rapid recovery of many reefs
compared to other Indian Ocean sites (Sheppard and Shep-
pard 2019). In particular, water quality is emerging as an
important factor shaping the response of corals and reefs
to heat stress (Wooldridge and Done 2009; D’Angelo and
Wiedenmann 2014; MacNeil etal. 2019; Lapointe etal.
2019; Donovan etal. 2020). Specifically, an increase in
nitrogen (especially nitrate) coupled with phosphorous limi-
tation, which are typical of land-based pollution, exacerbate
the effects of heat stress and prolongs recovery time follow-
ing bleaching events (Wiedenmann etal. 2013; Ezzat etal.
2016; Burkepile etal. 2020). The absence of such stressors
within the Chagos Archipelago is likely a key contributor
to the rapid recovery observed on these reefs compared to
other reefs within the region and within other MPAs (e.g.,
the Florida Keys National Marine Sanctuary and the Great
Barrier Reef Marine Park) (MacNeil etal. 2019; Lapointe
etal. 2019).
However, it is also relevant to note that these reefs have
not been immune from repeated disturbances over the last
decade. Localised outbreaks of crown-of-thorns starfish

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(Acanthaster planci) were observed in 2013, causing high
mortality of branching Acropora spp.. Further,White Syn-
drome disease was prevalent on many reefs in 2014 and
2015, causing widespread mortality of tabular Acropora
colonies (Wright 2016; Sheppard etal. 2017). Most signifi-
cantly, however, the reefs were again heavily impacted by the
recent global heat stress event, which caused back-to-back
coral bleaching and mortality in 2015 and 2016. Intensive
research efforts in BIOT over the last five years are providing
detailed insights into subsequent ecological changes across
a wide range of depths and habitats.
As after the 1998 event, widespread coral mortality
reduced average coral cover to around 10% in 2017, mainly
affecting reefs to a depth of 15m (Fig.3a, e) (Sheppard etal.
2017; Head etal. 2019). This decline in coral cover was
driven primarily by a ~ 90% decline in Acropora spp. cover
in shallow and mid depths, shifting community composition
from competitive to stress-tolerant taxa and leaving Porites
spp. as the dominant coral genus post-bleaching (Head etal.
2019; Lange and Perry 2019). In deeper water (20m+), the
largest losses were of foliacious forms. No evidence of coral
acclimation following 1998 can thus be inferred. Soft corals
have also been lost, especially on shallow reefs and seaward
facing exposed reefs, and now occupy less than 4% in the
15–25m depth range. Sponges showed an initial increase in
2018, especially in deep waters, but have declined to about
12% cover in 2019 (Sannassy Pilly etal. unpubl. data).
Despite the decrease in coral cover, fleshy macroalgae are
very rare, which may be attributed to absence of nutrient
stress from fertilizer and sewage runoff that negatively affect
reefs in many coastal areas (Fabricius 2005; Lapointe etal.
2019). The only life form to show a mean increase across
reefs are calcifying algae (especially Halimeda spp.), which
have increased from negligible values to 12% in shallow
waters and to 15–16% in deeper waters. Crustose coralline
algae cover has increased from 8% to around 25% in shallow
Fig. 3 Metrics of reef health on ocean-facing coral reefs across
the Chagos Archipelago. a Live coral cover (%) at different depths
1995–2019; b Juvenile coral densities (individuals m−2) at different
depths 2012–2019; c Coral carbonate production rate (kg m−2year−1)
and rugosity at 8–10m depth 2015–2020. All values are means ± SD.
Shaded areas represent major coral bleaching events. Photographs
show reef states in d 2015, e 2018 and f an example of young Acro-
pora spp. growing on a dead table coral in 2019. Note that 2020 data
in c are based on a subset of survey locations. Photographs: d Chris
Perry, (e, f) Ines Lange

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water and to around 20% in deeper waters in 2019 (Benkwitt
etal. 2019; Sannassy Pilly etal. unpubl. data). From a geo-
ecological perspective, the main consequence of the above
community changes has been a major decline in carbonate
production rates, which have dropped by an average of 77%
(Fig.3c). At the same time, mean reef rugosity declined by
16% (Fig.3c) and rubble cover doubled between 2015 and
2018 (Lange and Perry 2019).
Critical questions at present are whether the reefs will fol-
low the same recovery trajectories as after 1998, or whether
more divergent trajectories will occur in different sites and
locations (see section below on Key Ongoing Threats). The
presence of the BIOT MPA guarantees that recovery trajec-
tories will not be impeded by local stressors such as anthro-
pogenically-derived nitrogen enrichment and altered nutrient
ratios, which can exacerbate coral disease and bleaching and
has led to reef degradation in other protected areas, e.g. the
Florida Keys National Marine Sanctuary (Lapointe etal.
2019). Still, recovery potential will ultimately depend on
recurrence intervals and magnitudes of future heat stress
events.
Coral reef shes are much more abundant
thaninother Indian Ocean locations
The first underwater visual surveys of fish biomass and com-
munity structure in the Chagos Archipelago were conducted
on the outer reef slopes of the atolls in 2010, the year the
MPA was established. The archipelago had also been a de
facto MPA for reef fishes, with very limited reef fishing
since the 1970s (Koldewey etal. 2010). Fish biomass on
these reefs was six times greater than even the best-protected
smaller MPAs surveyed across eight other countries in the
Western Indian Ocean (WIO) (Graham and McClanahan
2013). Much of this biomass was made up of species tar-
geted by fishing elsewhere in the region, higher trophic level
species and larger body-sized fishes (Graham etal. 2013).
These species often have large home ranges (Green etal.
2015), making them vulnerable to fishing pressures outside
smaller MPAs. The trophic structure of fish communities
across the Indian Ocean changes dramatically with fishing
pressure (Barley etal. 2017, 2020) and in the Chagos Archi-
pelago forms a concave shape, with biomass accumulating
at the top and bottom of the trophic structure, allowing for
efficient energy transfer through the food-web (Graham
etal. 2017). The semi-pristine fish community allowed for
baselines in a range of community-level life history and
functional metrics, including maximum length, length at
maturity and abundance of top predators and grazers, to be
benchmarked across the region (McClanahan and Graham
2015; McClanahan etal. 2015), and regional-level manage-
ment priorities to be set (McClanahan etal. 2016).
The high biomass values and relatively intact community
structure have also been informative to global fish ecology
and fisheries studies. Along with some remote locations in
the Pacific, fish biomass and structure in the Chagos Archi-
pelago enabled estimates of unfished biomass for coral reefs
globally (MacNeil etal. 2015) and the functional structure
of semi-pristine fish communities to be established (D’Agata
etal. 2016). Globally, the reef fish biomass in the Chagos
Archipelago stands out as a ‘bright spot’, being greater than
would be expected based on the human and environmen-
tal conditions experienced alone (Cinner etal. 2016), with
indications that deep-water refuges and the natural flow of
nutrients may contribute to this high biomass (Graham etal.
2018). Further, the biomass and proportion of reefs with top
predators helped identify the key role of distance to markets
as a driver of resource condition inside and out of MPAs
(Cinner etal. 2018), as has been also observed for pelagic
species (Letessier etal. 2019). Reef fish otolith studies in
the region have revealed the effects of fishing pressure on
life spans and patterns of mortality of fishes in other loca-
tions across the Indo-Pacific (Taylor etal. 2019). Biochro-
nological reconstructions of growth histories of fish spe-
cies have furthermore helped to refine ecological feedback
loops between parrotfishes and habitat disturbance (Taylor
etal. 2020a) as well as decadal growth responses to oceano-
graphic conditions (Taylor etal. 2020b).
A climate resilient nesting sanctuary forturtles
fromacrosstheWestern Indian Ocean (WIO)
Green (Chelonia mydas) and hawksbill (Eretmochelys
imbricata) turtles nest in the Chagos Archipelago with both
species heavily exploited for two centuries prior to protec-
tion being introduced in 1968–1970, with the creation of
the MPA further reinforcing this protection (Mortimer etal.
2020). Ongoing census data have highlighted both regionally
important nesting populations as well as upwards trends in
abundance. For example, estimates of the annual number of
clutches across the archipelago for the period 2011–2018
are 6300 and 20,500 for hawksbill and green turtles respec-
tively, increasing 2–5 times for hawksbills and 4–9 times
for green turtles since 1996 (Mortimer etal. 2020). These
upward trends in nesting for both species presumably reflect,
at least in part, the fact that there has been no known human
exploitation of eggs or adults in the Chagos Archipelago
for ~ 50years. Regional estimates indicate that the Chagos
Archipelago accounts for 39–51% of hawksbill and 14–20%
of green turtle clutches laid across the entire south-western
Indian Ocean (Mortimer etal. 2020).
Satellite tracking of nesting green turtles in the Chagos
Archipelago has shown that they disperse widely across the
WIO at the end of their nesting season, which peaks during
June to October (Fig.4) (Hays etal. 2020; Mortimer etal.

Marine Biology (2020) 167:159
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2020). While some individuals travel to foraging grounds
around 80km away on the Great Chagos Bank, others travel
to foraging grounds 1000s of km away, for example, in the
Seychelles, Maldives and mainland Africa. The Chagos
Archipelago thus provides a key nesting sanctuary for adult
green turtles foraging across much of an ocean basin. Ongo-
ing work is assessing migration patterns in adult hawks-
bill turtles after their nesting season, which peaks during
October–February (Mortimer et al. 2020). These green
and hawksbill turtle tracking data are being used to inform
marine spatial planning broadly across the WIO, helping,
for example, to determine boundaries of protected areas in
the Seychelles. Investigation of foraging grounds within the
MPA have led to discoveries of extensive, deep-water sea-
grass meadows across the south-east Great Chagos Bank
(Esteban etal. 2018). Little is known about these newly dis-
covered habitats, but they appear to support abundant and
diverse fish communities (Esteban etal. 2018). As marine
mega-herbivores can act as indicators of the presence of sea-
grass meadows (Hays etal. 2018), future tracking of green
turtles in BIOT may increase knowledge of the distribution
of these important habitats broadly across the entire WIO.
In addition, immature hawksbill and green turtles foraging at
Diego Garcia are also being satellite tracked to assess their
patterns of space use.
Sand temperature monitoring has shown that the nesting
beaches at Diego Garcia are particularly climate resilient
with regard to incubation temperatures (Esteban etal. 2016).
The sex of sea turtle hatchlings is determined by the tem-
perature in the nest in the middle third of incubation. Around
the world there is concern that, with a warming climate,
populations are becoming increasingly feminised, as females
are produced at warmer temperatures. A lack of male hatch-
lings may ultimately lead to population extinction. At many
sites globally, hatchling production is already heavily female
skewed (Hays etal. 2014). However, at Diego Garcia, the
sand at nest depths is relatively cool, most likely because
of a combination of heavy rainfall and shading provided by
vegetation behind the nesting beaches. As a consequence
of these cool incubation temperatures, it is estimated that
hatchling sex ratios are currently balanced (Esteban etal.
2016). Hence, in scenarios of climate warming, excessive
feminisation of hatchlings will be much less likely to occur
in the Chagos Archipelago than at most other nesting sites
around the world. The Chagos Archipelago also supports
immature foraging green and hawksbill turtles and ongoing
work with drone surveys is estimating the size of these popu-
lations and their regional importance (Schofield etal. 2019).
The BIOT MPA protects globally signicant seabird
populations
Research in the Chagos Archipelago has reinforced the
important role seabirds play in tropical marine ecosystems.
The WIO has been estimated to support ~ 19 million seabirds
of 30 species, with the Chagos Archipelago supporting ~ 1
million (or 5% of the WIO total) individuals (Danckwerts
etal. 2014). However, their status and distribution required
updating, and until recently virtually nothing was known
about their at-sea distribution. A recent synthesis of seabird
Fig. 4 The value of the Chagos Archipelago for sea turtles. a The
archipelago provides a nesting sanctuary for green turtles that forage
at distant sites throughout the Western Indian Ocean. Tracks of 35
adult female green turtles are shown, with individuals equipped with
tags on nesting beaches on Diego Garcia and then dispersing widely
at the end of the nesting season. The extent of the MPA is indicated
by the blue hatched area. Stars denote the foraging locations of tur-
tles, i.e. the end-point of migrations where turtles remained for many
months before tags failed (modified from Hays et al. 2020). b The
significant positive trend (p < 0.01, r2 = 0.88) in the estimated num-
ber of green turtle clutches laid throughout the Chagos Archipelago.
Numbers are scaled relative to those estimated in 1995, i.e. abun-
dance in 1995 appears as one, to highlight the extent of the increase
(modified from Mortimer et al. 2020). Between 2011 and 2018, the
estimated mean number of clutches per year throughout the archipel-
ago was 20,500 (Mortimer etal. 2020)

Marine Biology (2020) 167:159
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159 Page 8 of 22
status and breeding distribution across the Chagos Archi-
pelago based on visits to all 55 islands, estimated 281,596
breeding pairs of 18 species (Fig.5a). Of these, 96% com-
prised three species, the sooty tern (Onychoprion fuscatus
70%), lesser noddy (Anous tenuirostris 18%) and red-footed
booby (Sula sula 8%) (Carr etal. 2020). Assuming 50%
breeding success, 281,596 breeding pairs (563,192 individu-
als) will produce 140,798 offspring, equating to ~ 704,000
breeding adults and immatures, or ~ 4% of the regional total
(Danckwerts etal. 2014). Current estimates are consider-
ably lower than those proposed by Danckwerts etal. (2014),
and there is strong evidence from early visiting naturalists
(Bourne 1886) and guano mining records (Edis 2004; Wen-
ban-Smith and Carter 2017) to suggest this is a fraction of
the historic breeding seabird populations. Yet, it is unclear
whether trends observed in BIOT are representative of the
WIO. Therefore, updated estimates from across the WIO
are now needed to reassess the status of breeding seabirds
for this region.
At-sea behaviour and distribution of one of the most
widely distributed and abundant species in the archipelago,
the red-footed booby, is being revealed through the deploy-
ment of GPS loggers on breeding adults. Tracking reveals
adults commute long-distances over relatively straight paths
to feed in deeper waters beyond the Great Chagos Bank
(Fig.5b) and suggests at-sea segregation as seen elsewhere
with seabirds from different colonies (Wakefield etal. 2013).
As the vast majority of individuals remained within the MPA
(Fig.5b), the lack of commercial fishing within the MPA
may help ensure high availability of forage fish and reduce
threats from fisheries bycatch. The restriction of suitable
breeding habitat due to the persistence of introduced rats and
associated abandoned coconut plantations across 95% of the
terrestrial landmass, remains a constraint to seabird recovery
and the MPA delivering its full potential as a seabird sanctu-
ary, although a feasibility study for eradicating rats across
the archipelago has recently been completed.
The large no‑take MPA encompasses important
pelagic wildlife
The relatively recent establishment of VLMPAs, combined
with the logistical and methodological challenges of sam-
pling remote, expansive regions means that empirical data
on the effectiveness of these MPAs for pelagic species are
currently limited and conclusions are sometimes conflicting.
Some studies suggest that MPAs are beneficial for mobile
species, with the benefits of MPAs increasing with size,
remoteness and age (Edgar etal. 2014). The BIOT MPA
therefore represents an excellent reference site for such
studies.
Since the establishment of the MPA, electronic tagging
studies have reported, albeit with relatively low numbers
and limited durations, higher than expected residency of
pelagic fish species, such as silky sharks (Carcharhinus
falciformis), sailfish (Istiophorus platypterus) and yel-
lowfin tuna (Thunnus albacares) (Carlisle etal. 2019).
The historical fishing record shows that large yellowfin
tuna have also been reported to occur in the archipelago
year-round (Curnick etal. 2020). Further, activity spaces
of all pelagic species tagged around the Chagos Archi-
pelago were significantly smaller than the extent of the
Fig. 5 Seabird abundance and movements. a Seabird species rich-
ness and abundance varies across the Chagos Archipelago. Data are
from breeding seabird counts on all 55 islands 2008–2018 (Carr etal.
2020). b Centrally placed red-footed boobies breeding on the Cha-
gos Archipelago largely forage within the MPA and show evidence
of colony-specific at-sea segregation. Data are from 192 individuals
at three colonies (DG Diego Garcia, 2016–18, n = 99; DI: Danger
island, 2019 n = 30; NI Nelson’s Island, 2018–2019, n = 63). Study
colony locations are marked with triangles and the grey line deline-
ates the MPA

Marine Biology (2020) 167:159
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Page 9 of 22 159
MPA, suggesting it may be large enough to provide a
refuge for extended periods of time (Carlisle etal. 2019).
Increased understanding of large pelagic species
around the Chagos Archipelago has also been informed
through the use of fisheries independent mid-water ste-
reo-BRUVS (Fig.2f). Assessments of pelagic richness
and biomass using mid-water stereo-BRUVs (in 2012,
2015 and 2016) showed variation among pelagic habitats
associated with atolls, seamounts and a deep-sea trench
(Meeuwig unpubl. data). This is consistent with historical
fisheries data that show high spatial heterogeneity in the
distributions of species such as yellowfin tuna (Dunn and
Curnick 2019). Pelagic richness and biomass around the
Chagos Archipelago are also relatively high compared to
global averages (Letessier etal. 2019).
The BIOT MPA was established for biodiversity con-
servation and not as a fisheries management tool. Studies
elsewhere have shown benefits to adjacent tuna fisher-
ies by VLMPA establishment (Boerder etal. 2017) and
residency behaviour in yellowfin tuna to remote locations
(Richardson etal. 2018). Yet a recent study of commer-
cial catch data found no direct evidence that indices of
yellowfin tuna abundance have improved in the areas
immediately surrounding the MPA (Curnick etal. 2020).
However, since the MPA’s establishment, mismanagement
of the yellowfin tuna fishery and a failure to adhere to
catch reduction measures (Andriamahefazafy etal. 2020)
has resulted in the stock being downgraded to “overfished
and subject to overfishing” since 2015 (IOTC-SC21,
2018). It is therefore not surprising that a single MPA
tiny in size compared to the fished region would be suf-
ficient to turn around such declines, arguing the need for
greater regional protection.
All pelagic shark species evaluated by the Indian
Ocean Tuna Commission (IOTC)—with the exception of
the blue shark (Prionace glauca) – have no or uncertain
stock assessments (IOTC–SC21 2018). Tracking stud-
ies have shown that pelagic sharks may travel across the
Indian Ocean to the BIOT MPA, providing further evi-
dence that the MPA may provide an important sanctuary
for this group (Queiroz etal. 2019). So, while tracking
data confirm sometimes protracted residence of pelagic
species within the BIOT MPA (Carlisle etal. 2019) and
BRUVs data show high pelagic species richness (Letess-
ier etal. 2019), benefits may also be partly negated by
overfishing in the surrounding region (IOTC–SC21 2018;
Curnick etal. 2020) and/or the ongoing IUU fishing
activity (see below). Combined, these initial studies sug-
gest that the BIOT MPA and its habitats could have con-
siderable benefits for pelagic wildlife, particularly in the
context of high fishing pressure in the region (Kroodsma
etal. 2018).
The BIOT MPA hosts exceptionally high cryptofauna
diversity
First estimates of the decapods in the Chagos Archipelago,
one of the most speciose cryptofauna groups on coral-reef
microhabitats (Stella etal. 2011), recorded 1868 individu-
als across 164 nominal species on 54 dead coral colony
microhabitats (Head etal. 2018). This number of species is
exceptionally high relative to similar studies in other loca-
tions (e.g. Preston and Doherty 1990; Plaisance etal. 2009;
Enochs and Moanzello 2012; Head etal. 2018) and com-
munity structure is unusual due to a prevalence of obligate
coral-dwelling decapods, such as Trapezia crabs (Head etal.
2015). Studies are now being undertaken across the archi-
pelago to identify the most important environmental drivers
of cryptofauna communities.
The BIOT MPA protects diverse mesophotic coral
ecosystems
Mesophotic coral ecosystems (MCEs) are typically found
at depths of 30 to > 150m (Turner etal. 2017). Much of
our knowledge of MCEs in BIOT is based on diver surveys
from the 1970s (Sheppard 1980) and a small number of brief
ROV surveys in 2016 (Andradi-Brown 2019). Building on
these studies, in late 2019, high-resolution multibeam and
a sophisticated ROV fitted with a HD camera were used to
conduct extensive surveys of both upper and lower meso-
photic communities from 30 to 150m at seven sites around
Egmont Atoll and Sandes Seamount. Preliminary analysis
has revealed diverse and abundant MCEs at all locations
surveyed, hosting communities of zooxanthellate sclerac-
tinian corals, soft corals, sea fans and sponges. A number
of scleractinian coral specimens were also sampled at mul-
tiple sites and depths during the surveys. Using molecular
techniques, work is ongoing to identify the species of corals
sampled and to assess genetic connectivity among shallow
and mesophotic reefs. Preliminary observations indicate that
the MCEs of BIOT offer huge potential in the level of diver-
sity they encompass and the extension of the shallow-water
reefs into deeper waters, which is especially pertinent given
recent bleaching events in the region (Head etal. 2019).
Thus, the BIOT MPA has significant value in protecting
extensive areas of diverse mesophotic coral ecosystems,
which have the potential to support both local and regional
shallow-water reefs in the face of climate change.
Long‑term protection preserves habitat
connectivity, natural behaviours andecological
relationships
Remote areas like the BIOT MPA can act as natural labo-
ratories that deepen our ecological understanding of reef

Marine Biology (2020) 167:159
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159 Page 10 of 22
ecosystems. The BIOT MPA is home to numerous species
of seabirds and mobile teleost and elasmobranch fishes that
play an important role in connecting discrete habitats. Due
to their proximity to deeper waters, the atoll ecosystems
are spatially heterogeneous and temporally dynamic with
resource availability continually shifting under the influ-
ence of diel and seasonal cycles, as well as oceanographic
processes. Quantifying connectivity across these seascapes
is important for understanding the degree to which popu-
lations should be treated and managed as distinct units
(Jacoby and Freeman 2016) and to uncover the functional
role that mobile species play in nutrient transfer (Williams
etal. 2018a), predation pressure (Heupel etal. 2014) or local
measures of biodiversity (Benkwitt etal. 2020).
Seabirds in the Chagos Archipelago forage in the open
ocean, far from the islands on which they roost and breed
(Fig.5). In doing so, they transfer large quantities of nutri-
ents from pelagic food webs to terrestrial systems. This path-
way of nutrient flow from seabird guano to coral reefs is
illustrated by elevated nitrogen signatures in terrestrial soils
and plants, benthic marine organisms, such as sponges and
algae, and marine consumers, including herbivorous dam-
selfish (Graham etal. 2018). These nutrient subsidies, in
turn, bolster the growth rates of individual coral-reef fishes,
and lead to enhanced biomass and ecosystem functioning
(including secondary productivity, grazing and bioerosion
rates) of entire fish assemblages (Graham etal. 2018; Ben-
kwitt etal. 2020). Contrary to anthropogenically-derived
nutrient inputs, which negatively affects coral physiol-
ogy and increase susceptibility to bleaching (Wooldridge
and Done 2009; Wiedenmann etal. 2013; D’Angelo and
Wiedenmann 2014; MacNeil etal. 2019; Donovan etal.
2020), naturally-derived nutrients provide nitrogen and
phosphorous in optimal ratios and can thus increase coral
growth (Shantz and Burkepile 2014; Savage 2019) and
may reduce susceptibility to heat stress (Ezzat etal. 2016).
Indeed, nutrient inputs from seabirds can also alter the
response of coral reefs to marine heatwaves, as demonstrated
in part by the proliferation of calcifying algae (e.g., crus-
tose coralline algae) around islands with abundant seabirds
following the 2015/2016 mass coral bleaching event in the
Chagos Archipelago (Benkwitt etal. 2019) (Fig.6).
Since 2013, a large network of acoustic receivers installed
across the archipelago, and annual deployments of both
acoustic and satellite tags, are beginning to reveal the extent
to which large mobile fishes utilise and link different areas
across atoll archipelagos (Carlisle etal. 2019; Jacoby etal.
2020). Acoustic tracking of grey reef and silvertip sharks,
both of which are a principal target of IUU fishing activity in
the BIOT MPA, has revealed a few key locations where con-
nectivity is unexpectedly high (Jacoby etal. 2020). A closer
look at the reef shark assemblage, using network analyses of
the telemetry data, reveals how these species play different
roles in connectivity across the MPA, with grey reef sharks
exhibiting more residential/site-attached behaviour, while
silvertip sharks have considerably more dynamic movements
(Carlisle etal. 2019; Jacoby etal. 2020). Interestingly, the
movement patterns, and thus connectivity of these sympa-
tric species, vary both diurnally and seasonally suggesting
both spatial and temporal segregation within the reef shark
assemblage, corroborating patterns observed through stable
isotope analyses in BIOT (Curnick etal. 2019).
Fig. 6 Benefits of rat-free islands to coral reefs. On rat-free islands in
the Chagos Archipelago, seabird guano supplies nutrients to the adja-
cent coral reefs. These nutrient subsidies, in turn, bolster the growth
rates of individual coral-reef fishes, leading to enhanced biomass and
ecosystem functioning. Additionally, these nutrient inputs from sea-
birds can also alter the response of coral reefs to marine heatwaves,
as demonstrated by responses to the 2015/2016 mass coral bleaching
event. Even though seabird nutrients did not enhance community‐
wide resistance to bleaching, they may still promote recovery of these
reefs through their positive influence on a calcifying algae (e.g., crus-
tose coralline algae) and b herbivorous fishes (modified after Benk-
witt etal. 2019)

Marine Biology (2020) 167:159
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For large-bodied, wide-ranging planktivores like reef
manta rays (Mobula alfredi), habitat selection is strongly
influenced by prey availability (Stewart etal. 2018). Telem-
etry and biologging approaches are beginning to show that
the reef manta rays found in the BIOT MPA frequently uti-
lise atoll ecosystems, sometimes with long-term site fidelity
and aggregation sites, such as at Egmont and Salomon atolls
(Carlisle etal. 2019; Harris 2019; Andrzejaczek etal. 2020).
Connectivity is greatly facilitated by dynamic reef manta
movements over frequent short-distances (< 10km) and
infrequent long-distance (> 200km) horizontal movements
as well as dives recorded as deep as 500m (Andrzejaczek
etal. 2020). Characterising the portion of the population that
is highly mobile will enable us to better understand drivers
of connectivity across the archipelago.
A range of unusual or rarely observed behaviours have
been studied in the Chagos Archipelago, which are likely
linked to its isolation. Examples include moray eels (Gym-
nothorax pictus) diurnally hunting shore crabs on land (Gra-
ham etal. 2009), day octopus (Octopus cyanea) hunting
cooperatively with fishes (Bayley and Rose 2020) and coco-
nut crabs (Birgus latro) predating on adult seabirds (Laidre
2017). All such behaviours are rarely seen, if at all, in highly
human-impacted systems elsewhere (Graham and McClana-
han 2013). Furthermore, parrotfish and surgeonfish in the
archipelago exhibit reduced ‘flight’ behaviour compared to
fished areas, showing either an inherited or learned effect of
wariness in response to fishing pressure (Januchowski-Hart-
ley etal. 2015). Protected or wilderness areas can therefore
provide a valuable window into the natural ecological inter-
actions and behaviours, which have otherwise disappeared
or been modified.
In remote systems such as the Chagos Archipelago, char-
acterised by high consumer biomass (Graham and McCla-
nahan 2013), general ecological theories can be tested about
relationships and behaviours. Such locations are ideal for
investigating what mechanisms maintain trophic structure,
drive variation in structure and complexity, and what the
implications are for individual behaviours, species interac-
tions, or food-web stability and productivity (McCauley
etal. 2012, 2018; Woodson etal. 2018). Current work in
the Chagos Archipelago has just begun to test such broader
ecological theories, for example, the biodiversity-ecosys-
tem function relationship (Benkwitt etal. 2020). Thus, not
only can remote MPAs like the Chagos Archipelago inform
conservation, but also contribute to broader basic ecology
research.
Understanding thephysical oceanography driving
biodiversity acrossthearchipelago
Deep oceanic flushing of cold water into the atolls across
the Chagos archipelago drives plankton distributions and
ecosystem functioning within the sheltered lagoons (Shee-
han etal. 2019). Seamounts are also particularly important
features within BIOT and include relatively shallow features
such as the Sandes and Swartz seamounts west of Diego
Garcia. Their biological significance has been suggested
from acoustic surveys during which backscatter indicated
100× higher biomass in close proximity to seamounts and a
“halo” influence of the seamount of approximately 1.8km
(Letessier etal. 2016). Recognised as a hotspot for pelagic
sharks (Tickler etal. 2017), studied seamounts exhibit inter-
nal lee waves that flush the summits with nutrient rich, cool
water (Hosegood etal. 2019). The steep and narrow sea-
mounts found throughout the archipelago, however, prohibit
the formation of Taylor Columns that are frequently cited as
the mechanism causing the local retention of nutrients and
the subsequent primary production over seamounts (Genin,
2004). Instead, the local generation of turbulent and ener-
getic currents associated with the lee waves are proposed
to encourage schooling behaviour of lower trophic levels
upon which sharks prey and thereby explain the correspond-
ing acoustic signature in biomass over the drop-off where
the internal wave impacts are most pronounced. Acoustic
surveys during 2019 over the slopes surrounding Egmont
Island, further confirmed that the intensification of biomass
is not limited to seamounts but extends to the steep slopes
surrounding islands and atolls throughout the archipelago
(Fig.7).
Key ongoing threats
Illegal shing poses amajor threat tovulnerable habitats
andspecies intheBIOT MPA
IUU fishing activity is a considerable challenge inside the
BIOT MPA. Historically, IUU occurred alongside a licensed
tuna fishery and it has persisted since the fishery closure in
2010 (Fig.8). From 2002 to 2018, the majority (78%) of ves-
sels have originated from Sri Lanka, although vessels from
south-west India are also active (12% of sightings). The Sri
Lankan vessels are medium-sized (10–15m) operating both
gill-net and long-line gears, often using illegal wire trace to
target sharks (MRAG 2015) (Fig.8).
Enforcement occurs primarily through use of the BIOT
Patrol Vessel, which is responsible for the detection and
apprehension of IUU fishing vessels within the MPA. Fer-
retti etal. (2018) estimated that 20–120 boats enter the area
annually. However, determining the actual level of IUU
threat is complicated by temporal and spatial variation in
patrolling effort. Although patrolling has occurred since
1996, patrol effort data have only been logged consistently
since December 2013. That notwithstanding, trends in IUU
vessel encounters suggest that the MPA’s implementation
has had little discernible impact on the IUU activity (Fig.8).

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Spatial and temporal analyses of all vessel encounters sug-
gest that suspected IUU is focused on the shallow reefs and
northern sectors (Fig.8) with peaks in activity in the months
of May–June and December (MRAG, unpublished data).
IUU fishing appears to have driven declines in some shark
populations within the MPA (Ferretti etal. 2018; Tickler
etal. 2019) and so may impair the MPA’s function as a ref-
uge for these species (Letessier etal. 2019). From the catch
data, Ferretti etal. (2018) estimated that between 1,745 and
23,195 sharks were caught between 1996 and 2015 within
the MPA. The number of sharks seen per scientific dive in
the archipelago reduced from ~ 4 in the 1970s to ~ 1 since
the mid-1990s (Graham etal. 2010). Recent re-surveys
(2018–2019) of the reef fish community structure and bio-
mass on the outer reef slopes at the same sites, using the
same methods, and by the same observer, have indicated
substantial declines in biomass (Graham etal. unpubl. data)
that have also been linked to a reported increase in reef fish
within confiscated catches (MRAG 2015).
Similar to the temporal surveys on the outer reef slopes,
substantial declines in reef fish and sharks were observed in
BRUVS surveys within the atoll lagoons between 2012 and
2016 (Meeuwig unpubl. data). Important exploited families,
such as serranids and lethrinids, decreased by 74% and 53%,
while coral feeding groups, such as chaetodontids, declined
by 37% (Meeuwig unpubl. data). Among the shark species,
whitetip reef sharks (Triaenodon obesus) declined by 81%
and 60% in relative abundance and size, respectively. The
grey reef shark declined by 76% in relative abundance and
by 4% in size. The tawny nurse shark (Nebrius ferrugineus)
reduced in relative abundance and size by 37% and 60%
(Meeuwig unpubl. data). These declines in relative abun-
dance and size were coincident with recorded poaching inci-
dents (MRAG 2015).
Currently, the BIOT Patrol Vessel has to balance patrol
activities, border protection, scientific research support, as
well as service and maintenance outside the territory. As
such, there have been recent efforts to improve enforcement
capacity through the trialling of additional technologies
within the MPA through the UK’s Blue Belt Programme
with a Technology Roadmap under development. Impor-
tantly, the continued threat from IUU fishing highlights the
need to improve monitoring and understanding of the human
dimensions (e.g. socio–economic drivers of illegal fishing)
of large MPAs which, although remote, are interconnected
within wider socio-ecological systems (Gruby etal. 2015).
Concerns have also been raised about the adequacy and
effectiveness of punitive measures, whereby risks of capture
Fig. 7 Use of sonar and cameras to reveal mid-water fauna. 38kHz
raw Sv echograms of submerged banks at a Sandes and b Egmont
(lower). Dense dark red echogram returns show the seabed and sec-
ond echo at Sandes, with aggregations of biomass (fish and zooplank-
ton) in shallower water, confirmed opportunistically using camera
drops. c and d cruise tracks showing seabed depth (with red showing
echogram portion. e and f camera validation of targets (Hosegood,
Williamson and Embling, unpublished data, 2019)

Marine Biology (2020) 167:159
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Page 13 of 22 159
combined with low costs associated with any arrest may still
leave IUU fishing as a viable option for some fishers.
Coral reefs intheChagos Archipelago are notimmune
tobleaching events
Reefs in the Chagos Archipelago have repeatedly been
impacted by global coral bleaching events, and the current
ecological condition of the reefs suggests they are presently
at a critical recovery stage. While coral cover is starting to
increase, structural complexity changes are likely to continue
for several years, as the remaining reef continues to degrade
due to intense external and internal bio-physical erosion.
Shallow reefs are increasingly covered by the bioeroding
sponge Cliona spp., decreasing the area suitable for new
coral settlement. Additionally, an outbreak of coralline fun-
gal disease has been observed in 2018, potentially impacting
coral recruitment further (Williams etal. 2018b). Indeed,
data from 2017 indicates that the density of newly settled
coral recruits (< 1year-old) has reduced by approximately
90% since 2013 (Fig.3b). Larger young corals (> 1year)
are present in greater numbers, though most are located
on unstable dead table corals or mobile rubble (Fig.3f),
and therefore are likely to experience high mortality rates
(Sheppard etal. 2017). Measured growth rates for several
coral species were also comparatively low in 2018–2019,
Fig. 8 The threat of Illegal, Unreported and Unregulated fishing. a
Heat-map of AIS activity from fishing fleets operating in the Brit-
ish Indian Ocean Territory area of interest (BIOT AOI) between 1
January 2014 and 31 December 2019. Fishing vessel identities were
confirmed and the activity shown is restricted to AIS transmissions
associated with speeds between 0.5 and 5 knots, speeds typically
associated with fishing operations and fishing activity at sea. The
extension and level of fishing activity is represented by positional
densities that vary from: black = no activity, transparent-green = lower
activity (low positional densities) to red/higher activity (hotspots).
Legal activity within 3 nautical miles of Diego Garcia (white cross)
and slow transits to and from port are not shown. The activity in the
northern MPA is produced by small-scale commercial fishing ves-
sels (fleet) transiting regularly at slow speed and shaping these lanes
between the northeast and northwest boundaries. However, these ves-
sels very frequently deploy fishing gears inside the MPA while on
transit and need to be accounted for within the overall fishing activity.
Overall, fishing activity is high and widespread through the adjacent
high seas. The east and west boundaries of the MPA show high risk
due to fishing activity encroaching and entering the marine protected
area, with short and repetitive incursions. Additionally, low positional
densities inside the south-west MPA are produced from infrequent
longer incursions. b Vessels suspected of IUU activity that were
either detained by authorities or escaped capture from 2002 to 2020.
The dashed line indicates MPA implementation (2010). Flag of ori-
gin indicated in legend, other = Indonesia, Mauritius, Japan, Taiwan.
Source: MRAG, unpublished data, 2020. c Location of detained or
escaped vessels suspected of IUU from 2002 to 2020. Numbers rep-
resent the number of vessels from that same site. The cross indicates
the location of Diego Garcia. Source: MRAG, unpublished data,
2020. d An example of a confiscated catch in the BIOT MPA (photo
Tom B Letessier)

Marine Biology (2020) 167:159
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159 Page 14 of 22
suggesting prolonged effects of heat stress on coral physiol-
ogy (Lange and Perry 2020). Since the late 1970s, several
coral species and key species assemblages in the Chagos
Archipelago have gone regionally or functionally extinct.
Although species diversity remains high at present, local
extinctions may increase in the future, following a spiral of
positive feedback through low recruitment and lack of suit-
able settlement substrate (Sheppard etal. 2020).
Importantly, the remote and protected nature of the BIOT
MPA has previously supported rapid coral community recov-
ery following widespread mortality in 1997–1998, giving
hope for future recovery (Sheppard etal. 2008). However,
it is unclear whether all reefs will restructure in the same
way that they did after 1998, whether recovery will be as
fast at all sites, or whether some sites may regime-shift to
other states. The return of Acropora spp. dominated com-
munities will be crucial to restore the key geo-ecological
functions of habitat complexity and carbonate production
that local reefs delivered pre-bleaching (Lange and Perry
2019). Ultimately, the primary control on coral-reef recovery
in the Chagos Archipelago will be the recurrence intervals
and magnitudes of future heat stress events. Unfortunately,
BIOT is predicted to see a large increase in the frequency
of annual severe bleaching events in the coming decades,
even under conservative emission scenarios (van Hooidonk
etal. 2016). Additionally, atmospheric nitrogen deposition is
projected to increase in the future, negatively affecting even
remote coral reefs (Chen etal. 2019).
Discussion
Future research directions forlarge MPA science
Here, we have shown how recent research in the BIOT MPA
has helped to identify not only its conservation benefits, such
as increased abundance of various species, habitat diversity
and resilience, but also the physical and ecological processes
that drive these benefits. Fundamental to these findings has
been the multi-year monitoring that has identified important
conservation successes, such as the increase in nesting turtle
numbers, the recovery of coral reefs following bleaching
and mortality, or the preservation of natural processes such
as seabird subsidies improving reef vigour. Global climate
change remains a huge threat to coral reefs, both within the
BIOT MPA and elsewhere (e.g. Bates etal. 2019), with the
frequency of temperature anomalies and extent of ocean
acidification likely to play key roles in dictating the type of
shallow reefs that survive into the future. Such monitoring
needs to be continued and expanded. Long-term monitoring
of mesophotic reefs will help identify if they are more resil-
ient than shallow reefs to global heat waves and if these deep
reefs help the recovery of bleached areas. It will also identify
if the encouraging trends of increased sea turtle nesting con-
tinue in the future as well as the impact of potential threats
to sea turtle and seabird nesting posed by rising sea levels.
Finally, long-term monitoring of pelagic species at BIOT
will also demonstrate the degree to which the MPA gener-
ates conservation benefits for mobile exploited species that
contribute to regional fisheries.
The BIOT MPA houses regionally significant fish assem-
blages that play an important role in the resilience of its coral
reefs to climate threats but that continue to be impacted by
IUU fishing. Future research should focus on improving the
understanding of the scale and nature of IUU fishing in the
MPA, as well as its drivers to assist with improved enforce-
ment and compliance. Targeted research is also needed to
develop efficient mechanisms to combat IUU fishing given
the huge area of the BIOT MPA poses significant logistical
challenges. Innovative methods to combat IUU fishing have
started to be implemented, often with methods tailored to
target the specific IUU fishery (e.g. Tickler etal. 2019) and
need expanding.
It is important to assess the extent of animal movements
in relation to MPAs so that threats to mobile species can be
identified and benefits of different sized protected areas can
be objectively assessed (Dwyer etal. 2020). Given that many
marine species may travel many thousands of km (Hays and
Scott 2013), even the largest protected areas, such as the
BIOT MPA, may sometimes not encompass the full extent
of marine animal movements. While a number of species
have been tracked (e.g. green turtles and red-footed boobies)
important knowledge gaps remain. For seabirds, their move-
ments outside the breeding season remain unknown. Initial
studies suggest that the BIOT MPA and its habitats could
have considerable benefits for pelagic fish. Yet, a challenge
remains to humanely capture and equip a large enough num-
ber of individuals to assess the overall patterns of movement
for pelagic fish species. Interestingly, some pelagic sharks
equipped with tags 1000s of km away off southern Africa,
have travelled across the Indian Ocean to the BIOT MPA
(Queiroz etal. 2019). So, for some taxa, tagging studies
conducted within the BIOT MPA might usefully be blended
with studies being conducted elsewhere to assess patterns of
space use across the Indian Ocean and more broadly (Bark-
ley etal. 2019). The huge value of such data-sharing in ani-
mal tracking studies has recently been emphasised (Sequeira
etal. 2019). In some areas, such as marine animal tracking,
routes by which data can drive conservation outcomes have
been identified (Hays etal. 2019) and the tracks of turtles
equipped in the Chagos Archipelago that migrate broadly
are already being used to help direct marine spatial planning
both in BIOT and the Seychelles.
Little is known about some important habitats in the
BIOT MPA. While coral reefs have been a focal habitat for
concerted research for some time, a depth limit of 25m is

Marine Biology (2020) 167:159
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placed on diving activities to minimise the risks in such a
remote location. Yet most of the Great Chagos Bank, the
world largest atoll structure, is between 25 and 100m deep.
Deeper areas are only starting to be explored with, for exam-
ple, the use of drop-down cameras and ROVs (remotely
operated vehicles). Furthermore, research in the BIOT MPA
to date has also been focussed on returning to sites previ-
ously surveyed, to build a robust, long-term time-series. Yet
this has resulted in the majority of the archipelago remaining
unexplored and under-studied, such as the seagrass beds on
the Great Chagos Bank. Here, there may be a very useful
synergy between animal tracking studies and habitat surveys,
with hot-spots of space use identified in tracking studies,
being used to direct in-situ habitat surveys, i.e. tracking ani-
mals helps identify areas of particular interest (Jacoby etal.
2020). An example here is the use of green turtles to identify
the location of seagrass beds on the Great Chagos Bank that
were hitherto unknown (Esteban etal. 2018).
Lessons learned ofrelevance toother VLMPAs
While the number of MPAs across the world is increasing,
their benefits continue to be debated (Edgar etal. 2014;
Bruno etal. 2019). Set against this backdrop, case stud-
ies showing the value of MPAs are important (Murray and
Hee 2019). One feature that is evident from much of the
recent research is the importance of long-term monitoring
throughout the system. It is well established how the value
of ecological time-series grows as the time-series lengthen
(e.g. see Edwards etal. 2010), allowing the drivers of long-
term changes and inter-annual variability to be more clearly
identified. It is therefore important for long-term monitoring
to occur in VLMPAs and that it embraces new technology.
Such monitoring allows assessment of the success of con-
servation actions and identification of emerging threats. For
instance, in the Florida Keys National Marine Sanctuary,
whilst highly protected zones have benefited fishes relative
to partially protected zones, this high level of protection has
had no impact on the rate of coral decline (Toth etal. 2014)
which is driven both by large scale factors such as poor water
quality and climate-related storms and bleaching.
That the BIOT MPA, despite its extreme remoteness,
remains subject to incursions of IUU fishing with a demon-
strable impact on biodiversity demonstrates the need for
more efficient mechanisms to combat IUU fishing. This may
be a common issue with remote MPAs and necessitates the
need for innovative methods to combat IUU fishing (Park
etal. 2020). For example, in the territorial waters around
French Islands in the Southern Ocean, radar detecting tags
carried by albatrosses are being used to detect large ships
operating illegally (Weimerskirch etal. 2019). Further, inter-
actions between large static MPAs and mobile fishing gears,
such as fish aggregation devices (FADS) (Bucaram etal.
2018) and industrial fishing fleets around their perimeters
(Kroodsma etal. 2018; Curnick etal. 2020) need to be better
understood. Given the huge fishing pressures in unregulated
high seas fisheries outside protected areas, the importance of
large MPAs for pelagic species protection has been stressed
(Queiroz etal. 2019). Yet, we emphasise that large protected
areas, such as the BIOT MPA, should not be considered as
a silver bullet, but rather in conjunction with wider sustain-
able and effective fishery management regulations to provide
the urgent conservation and management benefits needed
for pelagic predators. The recent developments to expand
the UN Convention on the Law of the Sea (UNCLOS) to
include a new legally binding instrument on the conservation
and sustainable use of marine life in Areas Beyond National
Jurisdiction (General Assembly resolution 72/249) are there-
fore encouraging.
In addition to studying a range of marine habitats within
MPAs, another important research direction is to better
quantify the connections between terrestrial and marine
environments. Although this research will take different
forms in the BIOT MPA and other remote VLMPAs com-
pared to smaller MPAs located closer to human population
centres, prioritizing research and encouraging management
across land-sea boundaries applies to all MPAs. Specifically,
land-based nutrient pollution plays a large role in declin-
ing coral health, especially when coupled with increasing
warming events (Wooldridge and Done 2009; Donovan
etal. 2020). As a result, there have been recent calls to bet-
ter regulate run-off from land adjacent to MPAs to miti-
gate continuing coral loss and enhance recovery following
bleaching events (Lapointe etal. 2019; MacNeil etal. 2019).
In contrast to these human-derived nutrients, natural nutri-
ent subsidies, such as those provided by seabirds nesting on
islands, may benefit coral reefs and enhance their resilience
to global heat waves (Graham etal. 2018; Benkwitt etal.
2019). Thus, while one research and management priority
within BIOT is the restoration of such natural nutrients (e.g.,
by eradicating invasive rats and restoring seabird popula-
tions), less remote MPAs will likely need to simultaneously
reduce human-derived nutrient run-off to have similar bene-
fits for coral reefs. Still, jointly managing terrestrial systems
in conjunction with MPAs may be broadly applicable, and
may increase the effectiveness of MPAs at conserving coral
reefs and other nearshore habitats.
Cutting across all the marine science work in the BIOT
MPA, an important goal is to maximise the translation of
the accumulated data into positive conservation outcomes, a
theme that pervades across MPAs more broadly (Lubchenco
and Grorud-Colvert 2015). The BIOT MPA was one of the
early wave of no-take VLMPAs implemented from 2006 to
2010 (with Papahānaumokuākea Marine National Monu-
ment, USA and Phoenix Islands Protected Area, Kiribati)
as countries worked to meet Aichi Target 11 of 10% ocean

Marine Biology (2020) 167:159
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159 Page 16 of 22
protection by 2020 under the United Nations’ (UN) Conven-
tion on Biological Diversity (CBD), later endorsed under
Sustainable Development Goal 14. Today, only 5.3% of the
world’s ocean is protected with 2.5% highly protected in
no-take MPAs (https ://mpatl as.or g/, accessed 26 May 2020).
However, the UK government is leading the 30-by-30 ini-
tiative, pushing for at least 30% of the global ocean to be
protected by 2030 with the hope that this goal will be ratified
at the 2020 CBD Conference of the Parties, now rescheduled
for 2021. Research from the BIOT MPA therefore provides
important insights to inform policy commitments around
ocean protection, including the need for greater regional
protection, as part of the actions identified to rebuild ocean
life (Duarte etal. 2020). Mechanisms to effectively achieve
this science to policy interface will be aided by the UN
Decade of Ocean Science for Sustainable Development
(2021–2030). The wealth of new information from ongo-
ing work in the BIOT MPA promises to help drive marine
conservation both within the MPA and more broadly, which
is, perhaps the most important legacy this work can leave.
Author contributions This manuscript was conceived by GCH and
ideas discussed and modified at a workshop led by HK and DC and
held in London during September 2019. GCH, DC, IDL, CTP, DMPJ,
HK, JJM, NG, NE, NLF and CEIH led the writing with all authors
contributing. GCH and DC assembled the text and led the initial editing
and all authors contributed to the final manuscript editing.
Funding Major support came from the Bertarelli Foundation as part of
the Bertarelli Programme in Marine Science. The Darwin Foundation
supported BRUVS work by JJM and TBL. TeachGreen supported sea-
bed BRUVS work (JJM). The Garfield Weston Foundation supported
work on the oceanography and mesophotic reefs by NLF, CD, KLH,
PH, CBE, BJW, EVS, MJA. Early coral reef and atoll work was sup-
ported by the Overseas Territories Environment Programme (CS, JT,
MS), and Darwin Initiative Project 19-027 (JT, HK, CS) and Selfridges
& Co. (HK). Additional funding for tags was provided by the JSF Pol-
litzer Charitable Trust, The Rufford Foundation and the Ernest Klein-
wort Charitable Trust through the Chagos Conservation Trust (DC).
Compliance with ethical standards
Conflict of interest The authors declare that they have no conflict of
interest.
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Aliations
GraemeC.Hays1 · HeatherJ.Koldewey2,3· SamanthaAndrzejaczek4· MartinJ.Attrill5· ShantaBarley6,7·
DanielT.I.Bayley8· CassandraE.Benkwitt9· BarbaraBlock4· RobertJ.Schallert4· AaronB.Carlisle10· PeteCarr3,11·
TaylorK.Chapple12· ClaireCollins3,11· ClaraDiaz5· NicholasDunn11,13· RobertB.Dunbar14· DannielleS.Eager5·
JulianEngel15· ClareB.Embling5· NicoleEsteban16· FrancescoFerretti17· NicolaL.Foster5· RobinFreeman11·
MatthewGollock2· NicholasA.J.Graham9· JoannaL.Harris5,18· CatherineE.I.Head11,19· PhilHosegood5·
KerryL.Howell5· NigelE.Hussey20· DavidM.P.Jacoby11· RachelJones2· SivajyodeeSannassyPilly21·
InesD.Lange22· TomB.Letessier11,23· EmmaLevy2· MathildeLindhart24· JamieM.McDevitt‑Irwin4·
MarkMeekan25· JessicaJ.Meeuwig23· FiorenzaMicheli4,26· AndrewO.M.Mogg27,28· JeanneA.Mortimer29,30·
DavidA.Mucciarone14· MalcolmA.Nicoll11· AnaNuno3,31· ChrisT.Perry22· StephenG.Preston19· AlexJ.Rattray1·
EdwardRobinson5· RonanC.Roche21· MelissaSchiele11· EmmaV.Sheehan5· AnneSheppard21,32·
CharlesSheppard21,32· AdrianL.Smith19· BradleySoule15· MarkSpalding33· GuyM.W.Stevens18·
MargauxSteyaert11,19· SarahStiel19· BrettM.Taylor25· DavidTickler7· AliceM.Trevail34· PabloTrueba15·
JohnTurner21· StephenVotier34· BryWilson19· GarethJ.Williams21· BenjaminJ.Williamson35·
MichaelJ.Williamson11,36· HannahWood11· DavidJ.Curnick11
* Graeme C. Hays
g.hays@deakin.edu.au
1 Centre forIntegrative Ecology, Deakin University, Geelong,
Australia
2 Zoological Society ofLondon, Regent’s Park,
LondonNW14RY, UK
3 Centre forEcology andConservation, College ofLife
andEnvironmental Sciences, University ofExeter,
PenrynTR109FE, Cornwall, UK
4 Hopkins Marine Station, Stanford University, PacificGrove,
CA, USA
5 School ofBiological andMarine Sciences, University
ofPlymouth, PlymouthPL48AA, UK
6 Minderoo Foundation, 80 Birdwood Parade, Dalkeith,
WA6009, Australia
7 School ofBiological Sciences, The University ofWestern
Australia, Crawley, WA6009, Australia
8 Centre forBiodiversity andEnvironment Research,
University College London, Bloomsbury,
LondonWC1H0AG, UK
9 Lancaster Environment Centre, Lancaster University,
LancasterLA14YQ, UK
10 School ofMarine Science andPolicy, University
ofDelaware, Lewes, DE19958, USA
11 Institute ofZoology, Zoological Society ofLondon, Regent’s
Park, LondonNW14RY, UK
12 Hatfield Marine Science Center, Oregon State University,
2030 SE Marine Science Drive, Newport, OR97365, USA
13 Department ofLife Sciences, Imperial College London,
Silwood Park, Ascot, UK
14 Earth System Science, Stanford University, Stanford,
CA94305, USA

Marine Biology (2020) 167:159
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159 Page 22 of 22
15 Harwell Innovation Centre, OceanMind, Building 173 Curie
Avenue, Harwell, DidcotOX110QG, UK
16 Department ofBiosciences, Swansea University,
SwanseaSA28PP, Wales, UK
17 Department ofFish andWildlife Conservation, College
ofNatural Resources andEnvironment, Virginia Tech,
Blacksburg, VA, USA
18 The Manta Trust, Catemwood House, Norwood Lane,
CorscombeDT20NT, Dorset, UK
19 Department ofZoology, University ofOxford,
OxfordOX13SZ, UK
20 Department ofIntegrative Biology, University ofWindsor,
OntarioN9B3P4, Canada
21 School ofOcean Sciences, Bangor University,
MenaiBridgeLL595AB, Wales, UK
22 Geography, College ofLife andEnvironmental Sciences,
University ofExeter, ExeterEX44RJ, UK
23 School ofBiological Sciences (M092), The University
ofWestern Australia, Crawley, WA6009, Australia
24 Civil andEnvironmental Engineering, Stanford University,
Stanford, CA94305, USA
25 Australian Institute ofMarine Science, Indian Ocean Marine
Research Centre, The University ofWestern Australia,
Crawley, WA6009, Australia
26 Center forOcean Solutions, Stanford University, 120 Ocean
View Blvd, PacificGrove, CA93950, USA
27 NERC National Facility forScientific Diving, Scottish
Association forMarine Science, Oban, UK
28 Tritonia Scientific Ltd., Dunstaffnage Marine Laboratories,
ObanPA371QA, UK
29 Department ofBiology, University ofFlorida, Gainesville,
FL32611, USA
30 P.O. Box1443, Victoria, Mahé, Seychelles
31 Interdisciplinary Centre ofSocial Sciences (CICS.NOVA),
School ofSocial Sciences andHumanities (NOVA FCSH),
NOVA University Lisbon, Avenida de Berna, 26-C,
1069-061Lisboa, Portugal
32 School ofLife Sciences, University ofWarwick,
CoventryCV47AL, UK
33 Conservation Science Group, Department ofZoology,
University ofCambridge, CambridgeCB23QZ, UK
34 Environment andSustainability Institute, University
ofExeter, Penryn Campus, PenrynTR109FE, Cornwall, UK
35 Environmental Research Institute, University
oftheHighlands andIslands, Ormlie Road,
ThursoKW147EE, UK
36 Department ofGeography, King’s College London,
LondonWC2B4BG, UK