Glacier retreat on South Georgia and implications for the spread of rats

Abstract

Using archival photography and satellite imagery, we have analysed the rates of advance or retreat of 103 coastal glaciers on South Georgia from the 1950s to the present. Ninety-seven percent of these glaciers have retreated over the period for which observations are available. The average rate of retreat has increased from 8 Ma-1 in the 1950s to 35 Ma-1 at present. The largest retreats have all taken place along the north-east coast, where retreat rates have increased to an average of 60 Ma-1 at present, but those on the south-west coast have also been steadily retreating since the 1950s. These data, along with environmental information about South Georgia, are included in a new Geographic Information System (GIS) of the island. By combining glacier change data with the present distribution of both endemic and invasive species we have identified areas where there is an increased risk of rat invasion to unoccupied coastal regions that are currently protected by glacial barriers. This risk has significant implications for the surrounding ecosystem, in particular depletion in numbers of important breeding populations of ground-nesting birds on the island.

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Antarctic Science 22(3), 255–263 (2010) &Antarctic Science Ltd 2010 doi:10.1017/S0954102010000064
Glacier retreat on South Georgia and implications for the
spread of rats
A.J. COOK
1
, S. PONCET
2
, A.P.R. COOPER
1
, D.J. HERBERT
1
and D. CHRISTIE
3
1
British Antarctic Survey, NERC, High Cross, Madingley Road, Cambridge CB3 0ET, UK
2
South Georgia Surveys, PO Box 756 Stanley, Falkland Islands FIQQ 1ZZ
3
Government of South Georgia and the South Sandwich Islands (GSGSSI), Government House, Stanley, Falkland Islands
acook@bas.ac.uk
Abstract: Using archival photography and satellite imagery, we have analysed the rates of advance or
retreat of 103 coastal glaciers on South Georgia from the 1950s to the present. Ninety-seven percent of
these glaciers have retreated over the period for which observations are available. The average rate of
retreat has increased from 8 Ma
-1
in the 1950s to 35 Ma
-1
at present. The largest retreats have all taken place
along the north-east coast, where retreat rates have increased to an average of 60 Ma
-1
at present, but those
on the south-west coast have also been steadily retreating since the 1950s. These data, along with
environmental information about South Georgia, are included in a new Geographic Information System
(GIS) of the island. By combining glacier change data with the present distribution of both endemic and
invasive species we have identified areas where there is an increased risk of rat invasion to unoccupied
coastal regions that are currently protected by glacial barriers. This risk has significant implications for
the surrounding ecosystem, in particular depletion in numbers of important breeding populations of ground-
nesting birds on the island.
Received 5 August 2009, accepted 10 December 2009
Key words: ecosystem, GIS, invasive species, sub-Antarctic
Introduction
The sub-Antarctic island of South Georgia lies between
35850'–388W and 548–54855'S, just south of the Polar Front
(Fig. 1). It is approximately 170 km long and up to 40 km
wide. The landscape is mountainous, with eleven peaks
above 2000 m, the highest being Mount Paget at 2934 m.
Glaciers, ice caps and snowfields cover over 50% of the
island, leaving a narrow coastal fringe of vegetation that is
snow-covered in winter. The island’s location in the
Southern Ocean makes it an important breeding site for
an estimated 30 million pairs of seabirds, notably penguins,
albatrosses and petrels, and over 3 million fur seals and
100 000 elephant seals. Its rich biodiversity makes the
island unique and of worldwide importance.
South Georgia also has a long history of human exploitation,
starting soon after its discovery by Captain James Cook in
1775, and during the first half of the 20th century it supported
several whaling stations. During this period rats (Rattus
norvegicus (Berkenhout)), reindeer (Rangifer tarandus L.),
mice (Mus musculus L.) and a number of invasive species of
plants and invertebrates were introduced to the island, either
accidentally or deliberately (Frenot et al. 2005). Of these
invasive species, rats pose the greatest threat to birds, as they
are active predators of the eggs and young of burrow-nesting
petrels (e.g. common diving petrels (Pelecanoides urinatrix
exsul Salvin), South Georgia diving petrels (Pelecanoides
georgicus Murphy & Harper), Antarctic prions (Pachyptila
desolata Gmelin) and blue petrels (Halobaena caerulea
Gmelin)) and ground-nesting passerines and waterfowl (e.g.
the South Georgia pipit (Anthus antarcticus Cabanis) and
yellow-billed pintail (Anas georgica georgiana Gmelin)) (Pye
& Bonner 1980, Moors 1985, Prince & Poncet 1996, Poncet
2000). Rats are restricted principally to coastal areas of tussac
and Festuca grasslands (Pye & Bonner 1980, Poncet 2000,
Pasteur & Walton 2006). These are the dominant plant
communities on the island, and cover a large proportion of the
non-glaciated coastal areas of South Georgia.
Previous glaciological research carried out on South
Georgia has shown a general pattern of glacial advance and
retreat over time: a period of advance in the late 19th
century Little Ice Age was followed by a recession, then
another advance in the early 20th century and finally the
current period of recession (Hayward 1983, Clapperton
et al. 1989, Gordon & Timmis 1992, Gordon et al. 2008).
The climate records from South Georgia (recorded at King
Edward Point from 1905 until 1988, and subsequently from
2001 until 2008) show that in the early 1900s the summer
temperatures were relatively high (average 4.98C), lower
between the 1920s to the 1940s (average 4.38C), and higher
from the 1950s to the present (average 5.18C) (Fig. 2).
Early last century, most of South Georgia’s glaciers
reached the sea, subdividing the coastline into a series of
discrete areas of potential rat habitat. Each area was
bounded by an ice barrier that would have been impassable
to rats (Robertson & Gemmell 2004). These barriers
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protected large parts of the coast of South Georgia from
colonization by rats. This study presents evidence of
widespread retreat of these glaciers, and examines the
impact this may have on rat distribution on the island.
South Georgia environment data
The South Georgia Geographic Information System (SGGIS -
www.sggis.gov.gs), created by the British Antarctic Survey
(BAS) for the Government of South Georgia and South
Sandwich Islands (GSGSSI), contains data about the wildlife
of South Georgia, its human history and changing physical
environment. The web-based database is used to aid effective
environmental management of the island and for analysing the
combined datasets for patterns of change.
Many of the layers included in the GIS were from datasets
compiled for a recent BAS 1:200 000 scale map (BAS Misc
12A 2004), including topographic features, bathymetry and
toponymy. Other layers, such as vegetation, were interpreted
and digitised from Landsat images, with many additions based
on personal knowledge by S. Poncet. Survey information also
contributed to the datasets, for example, habitat boundaries
were interpreted from data collected during the South Georgia
Breeding Birds Survey 1985–88 and an Environmental
Baseline Survey carried out in 1999–2002 (Scott & Poncet
2003). The wildlife data, including the locations of penguin
colonies, albatrosses and ground-nesting birds, were based on
surveys carried out by S. Poncet and BAS between 1985 and
2007 (e.g. Poncet et al. 2006).
Knowledge about invasive species on South Georgia is
primarily anecdotal due to the remoteness and scale of the
location. The data held in the SGGIS for rat distribution
are based on presence/absence information collected during
the South Georgia Breeding Birds Survey 1985–88 and
subsequent surveys (Poncet 2000). The rat distribution
layer in the GIS represents areas where rats could
potentially live long-term, forage or use as access routes
to adjoining areas of suitable habitat. Polygons were
created for rock and vegetation below 200 m, these being
considered to be areas containing suitable habitat for rats.
This height was chosen based on the evidence that there is
little or no tussac grass above 200 m and that above this
height the temperature is too low for rats to survive for long
enough to transit from one area to another.
As part of the GIS, we compiled a dataset that showed the
changing positions of 103 coastal glacier fronts on South
Georgia, using aerial photographs and satellite images dating
from the 1950s to the present. We measured the changing
positions of the glacier fronts to give results for both changes
in overall length and in rates of retreat. Using the GIS we
analysed these changes alongside rat presence data gathered
from field surveys.
Glacier front changes
Data sources and method
The approach for mapping glacier front changes on South
Georgia was the same as that described in detail in Cook
et al. (2005).All of the ice fronts were mapped onto a
common satellite image base: Landsat ETM1Path:206
Row:098, 7 February 2003. All available relevant sources
were examined and where glacier fronts were visible, these
were digitised. Metadata included a reliability rating based
on the accuracy of the original source. The sources
consisted primarily of Royal Navy aerial photographs,
which were flown at frequent intervals from the 1950s to
the present. Satellite image scenes were also used,
including SPOT imagery (1990), Landsat ETM1(2003),
Envisat ASAR (2004–06), ASAR WSM (2008), and
Quickbird (2006–08). For the earlier years, a 1957
Directorate of Overseas Surveys (DOS) map of South
Georgia (scale 1:200 000), and panoramic hand-held
photographs taken on survey expeditions between the
1950s and 1970s were used. The source material did not
Fig. 1. South Georgia location map.
Fig. 2. Average summer temperatures on South Georgia
(recorded at Grytviken, Thatcher Peninsula) (data from
Turner et al. 2004).
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allow the capture of ice front changes at uniform intervals,
resulting in an irregular number of ice fronts and time
periods between each of the glaciers measured. In order to
make an unbiased comparison between the glaciers, the ice
fronts were analysed and averaged into 5-year time
intervals before rates of change were calculated (see
Cook et al. 2005 for method).
The resulting database contains the frontal positions for
103 coastal glaciers on South Georgia. These are defined as
glaciers that terminate on or near the coast, and include ice
fronts whose source may consist of more than one glacier.
It does not signify the total number of glaciers on the
island, but it is a comprehensive study of all glaciers on
South Georgia for which there is source material available.
Patterns of glacier change
The glacier changes are presented in Fig. 3. Of the 103
coastal glaciers measured on South Georgia, 99 (97%) have
retreated since their earliest recorded position in this study.
It should be noted that the earliest records varied from 1938
to 1995, but the majority (84%) were from the 1950s. The
majority (64%) of glaciers have only retreated by between
0 and 500 m since their first observations. There are
significant differences between glaciers along the north-
east coast of the island and the south-west coast. Of the 103
Fig. 3. Change in glacier length since earliest records (typically1950s). NB names refer to glaciers of significance: those that have
advanced, or those that have retreated over 1 km.
Fig. 4. Mean rates of change across all glaciers since 1950s.
Number of glaciers contributing to average is shown in red.
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glaciers, the majority (65) occur along the south-western
shore. These are smaller and narrower than the 38 glaciers
which are spaced out along the north-eastern coast. A total
of fifteen glaciers have retreated by over 1 km, and ten of
these lie on the north-eastern coast. Of these, two glaciers
stand out as having retreated most: Neumayer Glacier by
4.4 km since 1957, and the ice front fed by Ross and Hindle
glaciers by 2.14 km since 1960. Only four ice fronts have
advanced: Fortuna Glacier (64 m), Harker Glacier (37 m),
Novosilski Glacier (142 m) and a small unnamed tributary
glacier flowing towards Newark Bay (840 m). The locations
of these do not appear to be in any pattern, as all four are
widely distributed across the island. There are large
differences in change rates between these glaciers and
they show no clear temporal trends. The unnamed glacier in
Newark Bay is unique in that it has recently shown
unusually high advance rates. However, this is based on
only three data sources between 1992 and 2003.
Due to the differences in first and last observations
between glaciers, a more accurate representation of glacier
changes over time is based on rates of retreat. The rate of
retreat for all 103 glaciers has increased from (on average)
8ma
-1
in the late 1950s, to 35 Ma
-1
at present (Fig. 4),
revealing an accelerating rate of retreat since the 1990s.
The recent rapid increase in the average rate is mainly
driven by large increases in retreat rates of glaciers on the
north-east side of the island, which are currently showing
an average of 60 Ma
-1
retreat. Of these, some individual
glaciers have shown particularly great changes, e.g.
Neumayer Glacier has increased from 3 Ma
-1
retreat in
the late 1950s to 384 Ma
-1
retreat at present (Fig. 5).
The glaciers along the south-west coast of the island,
however, are significantly different in their rate of change.
This region is defined in this study as the coastline between
Cheapman Bay and Drygalski Fjord (see Fig. 6a for
placenames mentioned in text). Here it is more exposed and
the climate is colder, windier and wetter than the leeward
north-east side. The dissimilar weather patterns caused by
orographic effects can explain the differences in the scale of
response between glaciers on each side of the island (Gordon
& Timmis 1992, Gordon et al. 2008). The glaciers on the
south-west side have been in retreat slowly since the 1950s;
this retreat remained at a constant rate of approximately
8Ma
-1
(Fig. 4) until the late 1990s but may now be gradually
increasing, although on a much smaller scale than on the
north-east side of the island (currently 12 Ma
-1
). The north-
east glaciers results largely correspond to those in another
recent study of 36 glaciers on South Georgia by Gordon et al.
(2008), although our comprehensive study shows that the
south-west glaciers are retreating, contrary to previous beliefs
that many were stable or advancing (Gordon et al. 2008).
The glacial retreat over the past half-century coincides
with the recent period of climate warming that began in the
1950s (Fig. 2). Acceleration in retreat rates of glaciers on
the north-east coast has occurred in the past decade as the
climate has continued to warm, and although the glaciers
on the south-west side have been slow to respond, their
retreat rates may now also be on the increase. The change
in mass balance of glaciers is attributed to many other
factors including topography, catchment area, glacier width
and flow dynamics. This study gives an overview of the
main trends observed, but other factors must be taken into
account when considering the responses of individual
glaciers (e.g. Oerlemans 1989).
Fig. 5. a. Neumayer Glacier front positions since 1955.
b. Neumayer Glacier mean rates of change since 1955.
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Impact of glacier retreat on the ecosystem
Invasive species on South Georgia
There are a number of explanations for the presence of
invasive species in different regions of South Georgia. The
original cause of introduction was the carriage of rats and
mice on sealing and whaling vessels, which frequently
visited South Georgia between 1775 and 1965 (Poncet
2000). Since their initial introduction, flotsam and sea ice in
sheltered bays may have enabled rats to reach rat-free
areas, and although swimming is a less likely method of
invasion (Pye & Bonner 1980), rats are known to have
swum up to a distance of at least 30 m in sheltered waters at
South Georgia (S. Poncet, observation). Rats colonize and
thrive in the suitable habitats on the island. Data held in the
SGGIS reveals that 8.6% (306 km
2
) of the area of South
Georgia (3542 km
2
) is vegetated (i.e. classified as sparse,
partial or full cover) and of this, 73% (223 km
2
)israt-infested.
Fig. 6. a. Overview map of South
Georgia showing placenames
mentioned in text, plus vegetation
patterns and land surface. b. The
spatial distribution of rats and pipits on
South Georgia.
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The occupied areas are primarily along the northern coast,
and on the southern coast to the west of Holmestrand,
where the climate conditions are more amenable than the
exposed southern coast. Tussac grass is essential for the
survival of rats on South Georgia (Pye & Bonner 1980,
Poncet 2000), as it is the principal component of their diet
and provides nesting material and shelter.
Rat predation on birds and invertebrates that inhabit
tussac areas is well documented (Pye & Bonner 1980,
Moors 1985, Prince & Poncet 1996, Poncet 2000). Surveys
at South Georgia have shown that pipits and small
burrowing petrels cannot co-exist with rats (probably due
to predation on eggs and chicks) and the spread of rats is
the probable cause of depletion in numbers of these ground-
nesting birds on the island (Pye & Bonner 1980, Prince &
Poncet 1996, McIntosh & Walton 2000, Poncet 2000,
Pasteur & Walton 2006). The current distribution of rats
is shown in Fig. 6b, alongside the distribution of pipits
and areas of vegetation cover. It should be noted that
mice probably also affect the ecosystem, as field visits by
S. Poncet to Nun
˜ez Peninsula have found that there are
mice but no rats present in this vegetated region and yet
pipit populations are lower than expected.
Rats do not currently occupy all sections of the coast on
South Georgia, because several factors limit their spread. The
absence of rats along the south coast can be largely attributed
to its harsh climate, which results in an unfavourable habitat
for rodents. Rats introduced to this region during the ‘Little
Ice Age’ in the late 18th century (Headland 1984) may not
have been able to survive, due to the severity of the climate
and the limited food resources. However, with the recent
increase in average temperature, there may be areas of this
south coast that would now be a habitable environment for
rats. This stretch, covering approximately two thirds of the
southern coast, is currently occupied by pipits and remains rat-
free. Whether it is likely to remain so is discussed in the
following section.
Glacial barriers to invasive species
There is much evidence to suggest that glaciers are
extremely effective dispersal barriers (Holdaway 2001,
Robertson & Gemmell 2004). Analysis of the glacier front
changes in relation to the other environmental data in the
SGGIS reveals a strong spatial link between the presence of
rats and the retreat of glaciers.
The two key areas when considering management of rat
migration are at the head of King Haakon Bay, near the west
end of the island, and in Drygalski Fjord, at the south-east
extremity of the island (Fig. 6a). These locations are at either
end of the rat free south-west coast and so the glaciers in
these regions are currently acting as barriers to rat migration.
The results show, however, that they have retreated by over
1 km in the past 50 years, and are still undergoing retreat
(Figs 7 & 8). Continuing glacial retreat will expose beach
(even if only at low tide), create an access route for rat
migration to adjoining rat-free areas and lead potentially to
extermination of local populations of pipits and burrowing
petrels. To determine the likelihood of this scenario we assess
the two areas in more detail. In each case, several factors are
considered including the rate of retreat of the glacier,
glaciological factors such as the long profile of the glacier,
indications that its bed at the snout is below or at sea level,
and the nature of the vegetation near the glacier barrier.
Case study 1: King Haakon Bay
In King Haakon Bay, Briggs Glacier at the head of the bay
separates the area that is colonized by rats from rat free
terrain (Fig. 8a). Cape Rosa and Nun
˜ez Peninsula to the
south is a major breeding area for pipits and other burrow-
nesting birds. Briggs Glacier has retreated more than 1 km
Fig. 7. Changes in Briggs Glacier (King Haakon Bay) and
Risting/Jenkins Glacier (Drygalski Fjord).
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since 1958, but since 2000 little change is visible, and
features of the glacier front are consistent from image to
image. The profile of the glacier, based on a Digital
Elevation Model derived from Shuttle Radar Topography
Mission (SRTM) interferometric radar, is convex upward
from the snout, i.e. it is steeper near the snout than further
inland. The glaciological indicators suggest that this glacier
is currently grounded at or only slightly below sea level. Ice
thickness can be estimated from surface slope (Paterson
1981, p. 86) and conservative assumptions give ice
thicknesses in the region of 50 m between the coast and
the 100 m contour. The unchanging nature of the ice front,
the estimated ice thickness and the convex upward long
profile all suggest that the rapid retreat from 1958–2000
took place in parts of the glacier that were grounded below
sea level, but since 2000 the glacier has become grounded
at or near sea level. There are also several smaller glaciers
on the south shore of King Haakon Bay to the west of
Briggs Glacier. They are all less than 1 km wide and are
retreating, and like Briggs Glacier, their snouts appear to be
Fig. 8. a. King Haakon Bay.
b. Drygalski Fjord.
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grounded at or near sea level. Although vegetation is sparse
on the steep south side of the bay (as indicated in the
vegetation layer in the SGGIS, derived from the normalized
difference vegetation index from a Landsat ETM1image),
there may still be sufficient isolated patches of tussac grass
for rats to use as ‘stepping stones’. Even minor further
retreat of Briggs Glacier and glaciers on the south coast of
King Haakon Bay will result in a land pathway to the
currently rat-free Cape Rosa area.
If invasion occurs, the next major barrier to rat dispersal
would be at Esmark Glacier in Holmestrand. Although
there is beach along the snout of the small northern tongue
of the glacier, the southern half of Esmark Glacier is an
actively calving glacier front. In contrast to Briggs Glacier,
Esmark Glacier shows continuing retreat. The long profile
of the glacier is also lower than that of Briggs Glacier.
These factors suggest that Esmark Glacier is still grounded
below sea level, and that there is no immediate danger of
rats being able to pass this barrier.
Case study 2: Drygalski Fjord
In the region around Drygalski Fjord (Fig. 8b), at the other
extremity of the rat-free area of the south coast of South
Georgia, glacier barriers may not be the only factor
inhibiting the spread of rats into currently rat-free areas.
Larsen Harbour was extensively used by whalers and
sealers, providing many opportunities for rats to be
introduced. However, there is very little suitable tussac
habitat suitable for rats, and the area is currently rat free.
The nearest area occupied by rats is the eastern shore of
Drygalski Fjord, and further migration up into the fjord is
probably constrained as much by the steep, barren terrain
and absence of tussac as it is by glacial barriers. Risting
Glacier and Jenkins Glacier at the head of Drygalski Fjord
form the major glacier barrier in this region, and in
common with other major glaciers, both have shown an
accelerating rate of retreat since the mid-1990s. Ice
thickness estimates from surface slope indicate ice
thicknesses in the region of 100–200 m, so these glaciers
are based at or below sea level. Continued retreat will not,
therefore, provide a land pathway for rats for some time.
The immediate conclusion is that the combination of
glaciological and ecological factors will continue to
provide an effective barrier to the migration of rats west
of Drygalski Fjord. A greater danger could be presented if
the extent of tussac habitat increased due to regional
warming; in this case the glacial barriers would become the
primary barrier to migration of rats.
Rat management implications
This study has highlighted that one consequence of glacier
retreat on South Georgia is the risk of rats spreading into
previously rat-free areas. The presence of mice on Nunez
Peninsula shows that the habitat is suitable for small
rodents to survive and breed, therefore the absence of rats is
not because they cannot survive there but because they
have not yet been introduced into the area. This is
conclusive evidence that Briggs Glacier is currently
acting as a barrier to rats from the north. Our results
show that this retreating glacier front is currently grounded
at or near sea level; associated with this is a high risk of a
gateway opening up to allow rats to spread into a currently
rat-free region. The glaciers in Drygaslski Fjord do not
appear to currently pose a risk and so action is less critical
in this region at present. Although there is no evidence to
show that the coast in the vicinity of Briggs Glacier has
already become accessible to rats, this study highlights this
region as a priority for taking action on the prevention of
rat spread. Several courses of action are possible, including
the installation of sentinel stations (i.e. gnawsticks) along
the coastline west of Briggs Glacier in order to act as an
early warning device and to establish current rat
distribution in proximity to the glacier margin. Although
the timescale for further glacier retreat above sea level
cannot be predicted from this study, it highlights this region
as a priority for rat eradication or erection of man-made
barriers and suggests action should be taken before further
ice barriers to rat migration are lost.
Conclusion
The coastal glaciers on South Georgia show a trend of
accelerating retreat over the past fifty years, with the most
rapid increase occurring in the past decade. This has occurred
simultaneously with the recent period of climate warming
that began in the 1950s. The most dramatic changes have
occurred along the north-eastern coast, where ten of the
thirty-eight glaciers have retreated by over 1 km in the past
50 years. Of these, Neumayer Glacier has retreated the most:
4.4 km since 1957. The rates of retreat of these glaciers have
also increased, from 8 Ma
-1
in the 1950s to 60 Ma
-1
since
2005. Although the glaciers along the north-east coast were
known to be retreating, our study shows that those along the
south-west side have also been retreating throughout the past
half-century. Only two of the sixty-five glaciers in this region
have advanced. The average rate of retreat of the south-west
glaciers throughout this time has been 8 Ma
-1
, with a slight
increase since 2000.
In terms of environmental management, the results for
the south-west coast glaciers are of greater significance
than those on the north-east coast. Almost all of the
habitable northern coastal regions are already occupied by
rats and other invasive species. The area of concern is the
rat-free south coast, where glaciers previously thought to be
stable or advancing are in fact retreating. An increase in
rates of retreat of these glaciers has potential to open up
new regions for colonization, with consequent increased
predation of important breeding populations of ground- and
burrow-nesting birds.
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Examination of the retreat rates and long-profiles of
glaciers in two key regions on the south coast reveals that
the continuing retreat of glaciers in one of these regions, the
King Haakon Bay area, presents a real risk of rat spread. Here
a land-bridge is likely to form at the snout of Briggs Glacier,
allowing potential rat incursion to the area of land to the
south, initially as far as Esmark Glacier. The Drygalski Fjord
region is of less immediate concern as the glaciers and
habitats there remain effective barriers to rat spread, although
this may change if current warming trends continue.
Climate change, resulting in more amenable conditions
in previously hostile environments (providing better habitat
and more food), is a significant factor in opening up new
areas for rat colonization. Glacial retreat, however, will
provide the pathway to these regions and must be taken into
account when calculating the risk of rat spread on South
Georgia.
Acknowledgements
We are very grateful for the three reviewers for their
helpful comments and suggestions, which helped us to
improve this paper.
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