Update on the global abundance and distribution of breeding Gentoo Penguins ( Pygoscelis papua)

TL;DR: The assessment identifies South Georgia and sub-Antarctic islands in the Indian Ocean as being the most critical data gaps for this species and suggests that the global population has increased by approximately 11% since 2013, with even greater increases along the WAP.

Abstract: Though climate change is widely known to negatively affect the distribution and abundance of many species, few studies have focused on species that may benefit. Gentoo Penguin (Pygoscelis papua) populations have grown along the Western Antarctic Peninsula (WAP), a region accounting for ~ 30% of their global population. These trends of population growth in Gentoo Penguins are in stark contrast to those of Adelie and Chinstrap Penguins, which have experienced considerable population declines along the WAP attributed to environmental changes. The recent discovery of previously unknown Gentoo Penguin colonies along the WAP and evidence for southern range expansion since the last global assessment in 2013 motivates this review of the abundance and distribution of this species. We compiled and collated all available recent data for every known Gentoo Penguin colony in the world and report on previously unknown Gentoo Penguin colonies along the Northwestern section of the WAP. We estimate the global population of Gentoo Penguins to be 432,144 (95th CI 338,059 – 534,114) breeding pairs, with approximately 364,359 (95th CI 324,052 – 405,132) breeding pairs (85% of the population) living in the Atlantic sector. Our estimates suggest that the global population has increased by approximately 11% since 2013, with even greater increases (23%) along the WAP. The Falkland Islands population, which comprises 30% of the global population, has remained stable, though only a subset of colonies have been surveyed since the last comprehensive survey in 2010. Our assessment identifies South Georgia and sub-Antarctic islands in the Indian Ocean as being the most critical data gaps for this species.

Summary

INTRODUCTION

• While attention has focused largely on species at risk due to the impacts of climate change, less attention has focused on species that may benefit.
• In contrast, the majority of Indian Ocean colonies, approximately 16% of the global population, are rarely surveyed and represent areas of significant uncertainty for both abundance and trends.
• To update their understanding of Gentoo Penguin abundance and distribution, the authors compiled all census data available to estimate the current global abundance and distribution of breeding Gentoo Penguins including new ground count survey data from previously unknown colonies along the northwestern section of the WAP and on the Danger Islands.

METHODS

• The majority of the Gentoo Penguin distribution falls within the area governed by the Convention for the Conservation of Antarctic Marine Living Resources , which is divided into three statistical areas: Area 48 (Atlantic Ocean sector), Area 58 (Indian Ocean sector) and Area 88 (Pacific Ocean sector).
• Gentoo Penguins are found throughout Area 48 (including the Antarctic Peninsula, the South Shetland Islands, the Danger Islands, the South Sandwich Islands, the South Orkney Islands, and South Georgia) and Area 58 .
• The South Sandwich Island census data were collected via direct ground counts and using satellite imagery to estimate the aerial extent of the colony (as described in Lynch et al. 2016 ).
• The authors assumed a truncated [0,∞) Normal distribution for census errors and propagated observation error to total and regional abundances by drawing from the corresponding Normal distribution (n=1000) centered on each individual count with a standard deviation appropriate for count accuracy (See Online Resource 1).
• All analyses were performed in the R computing environment (R Development Core Team 2017).

RESULTS

• Colony specific census data, where available, are detailed in Online Resource 1.
• The authors report on 14 colonies in the Danger Islands and along the northern coast of Joinville Island unknown at the time of the last global review of Gentoo Penguin abundance and distribution (Lynch 2013 ; see Online Resource 1 for full details).
• There has been rapid population growth at these sites , and there are currently 1,789 breeding pairs in this expansion zone as of 2018.
• These colonization events have occurred in a cascading latitudinal pattern such that the most recent colonies are at the southernmost end of the species current range.

Population growth and range expansion on the Antarctic Peninsula

• The estimated 11% increase in the total global Gentoo Penguin population to 432,144 breeding pairs since the last global estimate published in 2013 can be attributed largely to increases on the Antarctic Peninsula, which has experienced warming air temperatures, increasing precipitation, and declines in the extent, seasonal duration, and thickness of sea ice (Trivelpiece et al.
• One of the more interesting aspects of Gentoo Penguin population growth and range expansion on the Antarctic Peninsula is that these trends are in direct contrast to population trends of Adélie and Chinstrap Penguins in the same region.
• Aerial photographs of Noble Rocks in the Neumayer Channel and Moot Point in the Penola Strait, both taken in 1956, show these regions as previously covered by terrestrial ice sheets .

Population changes in Indian Ocean Sector

• Though the last comprehensive survey of the breeding Gentoo Penguins on the Kerguelen Archipelago was conducted in 1985, the colonies on Courbet Peninsula, Penn Island, and Longue Island, have been regularly monitored since the 1980's.
• Lescroél and Bost (2006) reported an approximately 30% drop in breeding numbers from the mid 1980's to the mid-2000s on the Courbet Peninsula, which they suggested was the result of inshore food shortages where Gentoo Penguins typically forage (Lescroél and Bost 2005) .
• A 2018 survey found that the colony had recovered, surpassing its 1985 breeding population size by 9% (Weimerskirch pers. comm.) .
• In contrast, Possession Island, part of the Crozet Archipelago, was also surveyed in 2018 and found to have declined in breeding population size by 32% (Weimerskirch pers. comm) .
• With the exception of Courbet Peninsula, these population trends suggest an overall decline among colonies found in the Indian Ocean sector, likely related to declines in food availability due to environmental changes.

ACKNOWLEDGMENTS

• The authors would like to thank Ron Naveen and IAATO member cruise companies for logistical support of opportunistic field surveys, as well as the Dalio Foundation for funding the Danger Islands Expedition.
• The authors would like to thank Alastair Baylis and Henri Weimerskirch for providing information on Gentoo Penguins on the Falkland Islands and several other sub-Antarctic islands.

Full text

1
REVIEW ARTICLE 1
2
Update on the global abundance and distribution of breeding Gentoo Penguins (Pygoscelis 3
papua) 4
5
Rachael Herman
1*
, Alex Borowicz
1
, Maureen Lynch
2
, Phil Trathan
3
, Tom Hart
4
, Heather 6
Lynch
1,5*
7
8
1
Department of Ecology and Evolution, Stony Brook University, Nicholls Road, Stony Brook 9
NY, USA 10
2
Vermont Fish & Wildlife Department, 3902 Roxbury Rd, Roxbury VT, USA 11
3
British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK 12
4
Department of Zoology, South Parks Road, Oxford, OX1 3PS, UK 13
5
Institute for Advanced Computational Sciences, Stony Brook University, Nicholls Road, Stony 14
Brook NY, USA 15
16
*Corresponding authors: Rachael Herman, rachael.herman@stonybrook.edu; Heather Lynch, 17
heather.lynch@stonybrook.edu 18
19
20

2
ABSTRACT 21
Though climate change is widely known to negatively affect the distribution and abundance of 22
many species, few studies have focused on species that may benefit. Gentoo Penguins 23
(Pygoscelis papua) have undergone population growth along the Western Antarctic Peninsula 24
(WAP), a region accounting for ~30% of their global population. These trends are in stark 25
contrast to the Adélie Penguin (P. adeliae) and the Chinstrap Penguin (P. antarcticus), which 26
have experienced considerable population declines on the WAP attributed to environmental 27
changes. The recent discovery of previously unknown Gentoo Penguin colonies on the WAP and 28
evidence for southern range expansion since the last global assessment in 2013 motivates this 29
review of the abundance and distribution of this species. We compiled and collated all available 30
recent data for every known Gentoo Penguin colony in the world and report on previously 31
unknown Gentoo Penguin colonies along the Northwestern section of the WAP. We estimate the 32
global population of Gentoo Penguins to be 432,144 (338,059 – 534,114) breeding pairs, with 33
approximately 364,359 (95
th
CI: 324,052 – 405,132) breeding pairs (85% of the population) 34
living in the Atlantic sector. We estimate that the global population has increased by 35
approximately 11% since 2013, with even greater increases (23%) along the WAP. The Falkland 36
Islands population, which comprises 30% of the global population, has remained stable, though 37
only a subset of colonies have been surveyed since the last comprehensive survey in 2010. Our 38
assessment identifies South Georgia and sub-Antarctic islands in the Indian Ocean as being the 39
most critical data gaps for this species. 40
41
Keyword: Gentoo Penguins, range expansion, global census, Antarctic Peninsula, glacial retreat, 42
sea ice 43

3
INTRODUCTION 44
45
Climate change is widely known to affect the distribution and abundance of species, with some 46
taxa experiencing range retractions and extinctions and others experiencing latitudinal shifts in 47
response to warming conditions (Thomas et al. 2006; Hickling et al. 2006; Chen et al. 2011; Pecl 48
et al. 2017). While attention has focused largely on species at risk due to the impacts of climate 49
change, less attention has focused on species that may benefit. One such species is the Gentoo 50
Penguin (Pygoscelis papua), whose populations along the rapidly warming Antarctic Peninsula 51
(Vaughan et al. 2003) have not only undergone population growth since the early 2000s, but 52
have expanded their breeding range southwards with the establishment of many new breeding 53
colonies (Lynch et al. 2013). 54
These trends stand in stark contrast to their sister species, the Adélie Penguin (P. adeliae) 55
and the Chinstrap Penguin (P. antarcticus), which have experienced considerable population 56
declines on the Western Antarctic Peninsula (WAP), the principle drivers of which are still being 57
debated (Forcada and Trathan 2009; Trivelpiece et al. 2011; Lynch et al. 2012). Current 58
hypotheses for population increases and range expansion of Gentoo Penguins in this region 59
include: plasticity in their breeding phenology (Lynch et al. 2011), generalist foraging strategies 60
and a flexible trophic niche (Polito et al. 2015; Herman et al. 2017; McMahon et al. 2019), and 61
breeding habitat preferences (Cimino et al. 2013) vis a vis warming temperatures and declines of 62
sea ice (Stammerjohn et al. 2008) throughout the waters around the WAP. 63
Gentoo Penguins are one of the most widespread of the penguin species, with a 64
circumpolar breeding distribution and a wide latitudinal range stretching from 46°00’ S in the 65
Crozet Islands (2,350 km SSE of Madagascar) to 65°16’ S on the Antarctic Peninsula (Lynch 66

4
2013). Current taxonomy of Gentoo Penguins identifies two subspecies, P. papua papua and P. 67
papua ellsworthi, found north and south of 60°S, respectively (Dinechin 2012; Clements et al. 68
2017). However, recent genomic evidence suggests Gentoo Penguins found on South Georgia 69
are more closely related to the southern subspecies and there is a deep divergence of Indian 70
Ocean populations compared to other Gentoo Penguin colonies in the Atlantic sector, suggesting 71
taxonomic revisions may be required at the subspecies level (Levy et al. 2016; Vianna et al. 72
2017; Clucas et al. 2018; Pertierra et al. 2020). 73
Since the last global assessment of approximately 387,000 breeding pairs (Lynch 2013), 74
many new data have been collected across the Gentoo Penguin’s distributional range. While the 75
WAP population has been generally increasing, these trends are not homogenous, as some 76
colonies have experienced recent declines (Petry et al. 2018; Dunn et al. 2019). In addition, the 77
population in the Falkland Islands, where approximately one third of all Gentoo Penguins breed, 78
increased overall by 105% between 2005 and 2010 (Baylis et al. 2013), though a subset of 79
annually monitored colonies have since declined (Crofts and Stanworth 2019). In contrast, the 80
majority of Indian Ocean colonies, approximately 16% of the global population, are rarely 81
surveyed and represent areas of significant uncertainty for both abundance and trends. 82
To update our understanding of Gentoo Penguin abundance and distribution, we 83
compiled all census data available to estimate the current global abundance and distribution of 84
breeding Gentoo Penguins including new ground count survey data from previously unknown 85
colonies along the northwestern section of the WAP and on the Danger Islands. This updated 86
population assessment allows us to identify gaps and associated priorities for future research, and 87
forms the basis for our forecasts of continued range expansion along the WAP. 88
89

5
METHODS 90
The majority of the Gentoo Penguin distribution falls within the area governed by the 91
Convention for the Conservation of Antarctic Marine Living Resources (CCAMLR), which is 92
divided into three statistical areas: Area 48 (Atlantic Ocean sector), Area 58 (Indian Ocean 93
sector) and Area 88 (Pacific Ocean sector). Gentoo Penguins are found throughout Area 48 94
(including the Antarctic Peninsula, the South Shetland Islands, the Danger Islands, the South 95
Sandwich Islands, the South Orkney Islands, and South Georgia) and Area 58 (including Marion 96
and Prince Edward Islands, Crozet Island, Heard Island, and Kerguelen Islands (Figure 1). 97
Outside the CCAMLR area, Gentoo Penguins have a large breeding population on the Falkland 98
Islands, and smaller populations on Martillo Island in Tierra Del Fuego, Argentina, and 99
Macquarie Island in the southwestern Pacific Ocean. 100
Much of the census data from the Antarctic Peninsula and South Shetland Islands are 101
collected via opportunistic vessel-based field surveys (Lynch et al. 2013), though several 102
additional surveys near permanent stations have been published (Petry et al. 2018; Smagol et al. 103
2018: Dunn et al. 2019). Most surveys were conducted via manual ground counts of individual 104
nests, counting individual nests in panoramic photos taken from the ground or from an offshore 105
vessel, or counting individual penguins from photographs captured by drone imagery (Borowicz 106
et al. 2018). The majority of surveys used in this assessment are precise to within 5% (i.e., they 107
are Accuracy 1 counts, following Croxall and Kirkwood 1979). 108
The South Sandwich Island census data were collected via direct ground counts and using 109
satellite imagery to estimate the aerial extent of the colony (as described in Lynch et al. 2016). 110
The Falkland Island data are from the most recent island-wide census from Baylis et al. (2013) 111
and from an annual census of a subset of colonies by the Falkland Islands Seabird Monitoring 112