Climate-related range shifts in Arctic-breeding shorebirds

RUNNING HEAD

Range shifts in Arctic-breeding shorebirds

ABSTRACT

Aim: To test whether the occupancy of shorebirds has changed in the eastern Canadian Arctic, and whether these changes could indicate that shorebird distributions are shifting in response to long-term climate change
Location: Foxe Basin and Rasmussen Lowlands, Nunavut, Canada
Methods: We used a unique set of observations, made 25 years apart, using general linear models to test if there was a relationship between changes in shorebird species’ occupancy and their Species Temperature Index, a simple version of a species climate envelope.
Results: Changes in occupancy and density varied widely across species, with some increasing and some decreasing. This is despite that overall population trends are known to be negative for all of these species, based on surveys during migration. The changes in occupancy that we observed were positively related to the Species Temperature Index, such that the warmer-breeding species appear to be moving into these regions, while colder-breeding species appear to be shifting out of the regions, likely northwards.
Main Conclusions: Our results suggest that we should be concerned about declining breeding habitat availability for bird species whose current breeding ranges are centred on higher and colder latitudes.

KEYWORDS

Avian, Climate tracking, Conservation, Ecology, Global change, Polar, Poleward shifts, Range dynamics, Re-distribution, Wader

INTRODUCTION

Over the past century, many species have shifted their distributions in response anthropogenic influences. One increasingly important driver of distributional shifts is climate change, with species moving towards higher latitudes and higher elevations in response to a warming climate (Parmesan and Yohe 2003; Chen et al. 2011). For example, the northern limit of birds’ ranges measured by the North American Breeding Bird Survey shifted northward at a rate of 2.35 km/yr between 1967 and 2002 (Hitch and Leberg 2007). Likewise, butterfly and moth ranges have expanded northward in Finland (Mikkola 1997), Great Britain (Hill et al. 2002) and across Europe (Parmesan et al. 1999). These changes in species’ range limits are an important measure of how species are redistributing in response to climate change. Patterns of species density and community composition are shifting as well, creating novel ecological communities (Devictor et al. 2012; Kampichler et al. 2012; Lurgi, López, and Montoya 2012). Estimating the distribution of species has become a very active field of research, responding to concerns about how accelerating global environmental change will reshape the world’s ecosystems (Guisan and Thuiller 2005).
Identifying shifts in distribution in response to climate change requires long-term and large-scale species data, but the regions where the climate is changing fastest are often those where such data are sparse, making it impossible to directly measure shifts in species distribution (Shirey et al. 2021; Daskalova et al. submitted). The Arctic is a case in point. Here, temperatures are rising three times faster than the global average (AMAP 2021). Yet, even for birds, a taxon comparatively well monitored globally, the Arctic lacks consistent monitoring that would allow for rigorous analyses of climate effects on bird abundances and distributions (Smith et al. 2020; Aronsson et al. 2021). Research in remote Arctic locations is logistically complicated and expensive, costing on average eight times more than similar studies at a southern location (Mallory et al. 2018). More populated areas can support citizen science programs such as eBird (Johnston et al. 2021) and large-scale surveys such as the Christmas Bird Count or various regional and national Breeding Bird Survey programs offer immense quantities of distributional data that can be used to directly measure distribution shifts (Curley, Manne, and Veit 2020; Devictor et al. 2008; Lindström et al. 2013). However, in the Arctic, no regularly repeated, range-wide surveys upon which to base assessments of distributional change in the Arctic have yet been completed.
Despite the challenges, information about species distributions in the Arctic will be increasingly important for supporting conservation policies and protected areas being developed to protect northern species from increasing human presence and a rapidly warming climate. Melting ice is likely to lead to increases in shipping and resource extraction (Arbo et al. 2013). Arctic species are also particularly vulnerable to climate change due to three unique geographic factors that are leading to an “Arctic squeeze” which has the potential to dramatically limit the capacity of Arctic species to adaptively shift their ranges (Vincent 2020; Meltofte et al. 2007). First, the surface area of the Earth decreases as latitude increases towards a fixed end point at the pole, limiting options for northern expansion of habitats (Gilg et al. 2012). Second, northern expansion of terrestrial habitats cannot occur in regions that are bordered to the north by the Arctic Ocean; in many locations there is no more northerly landmass available for terrestrial species to expand into (Wauchope et al. 2017). Third, the southern border of tundra habitat is moving northwards, as shrubs and trees also shift northwards in response to climate change, encroaching into the open habitats preferred by many tundra-breeding species (Martin et al. 2017; García Criado et al. 2020).
In this paper, we assess if the regional population trends could indicate whether the distributions of breeding birds are shifting in the Canadian Arctic. We focus here on shorebirds, the most abundant and diverse group of birds in many tundra habitats. These species are likely to be particularly sensitive to climate change because of their highly migratory life history, as Arctic-breeding shorebirds undertake long, energetically expensive migrations, only have a short window available for breeding in the Arctic, and depend on ecological synchronicities with their invertebrate prey (Galbraith et al. 2014). Surveys providing an index of shorebird abundance during their migrations through southern Canada and the United States suggest that shorebird populations have experienced pronounced declines in the past 50 years, including all of the species studied here (Bart et al. 2012, Smith et al. 2020, Smith et al. submitted). These declines are often attributed to habitat loss and degradation at migratory stopovers and non-breeding sites (Thomas, Lanctot, and Szekely 2006), but given that climate change in the Arctic is expected to be rapid and severe, there is concern that environmental changes to shorebird breeding habitats may increasingly cause additional stress in these declining populations (Galbraith et al. 2014). Several previous studies have improved our understanding of the current distribution of Arctic-breeding birds. However, there have been no analyses of long-term, broad-scale monitoring data to address questions about potential changes in distribution on the Arctic breeding grounds, as a consequence of climate change.
We used a unique set of observations, made 25 years apart across 50,000 km² of mid-Arctic tundra habitats, to test whether the occupancy of shorebirds has changed over time, and whether these changes could indicate that distributions are shifting in response to long-term climate change. These data were collected as part of the Arctic Program for Regional and International Shorebird Monitoring (PRISM), an unprecedented Arctic-wide survey that will eventually track changes in the population size, trends and distribution of shorebirds (Bart and Johnston 2012). The observed summer temperature in northern Canada has increased by 1.6°C between 1948 and 2016 (Zhang et al. 2019). We therefore predicted that at mid-Arctic latitudes, species associated with warmer Low Arctic breeding habitats should be moving into the regio and observed more frequently, and species associated with colder High Arctic habitats should be moving out of the region and observed less frequently (Jiguet et al. 2010) (Figure 1). To test this prediction, we represented species temperature associations using the Species Temperature Index (STI), a simple version of a species climate envelope. Given that the population trends for these species are negative, we were interested to look at overall trends in survey counts in these regions to give context to any potential distribution shifts.
The STI is the long-term average temperature experienced by individuals of a species across their breeding range (Devictor et al. 2008). While species distributions are much more complex than simple climate relationships, this index has been a useful approach for describing how population trends and demography of bird populations are responding to climate change (P. Gaüzère et al. 2020; Princé and Zuckerberg 2015; Godet, Jaffré, and Devictor 2011). The collective contributions of individual species responses can give an indication of how the ecological community is responding to change (Curley et al. 2022; Pierre Gaüzère et al. 2019). Species with low STI consistently show more negative population trends in response to high temperatures (Pearce-Higgins et al. 2015). We thus predicted a positive relationship between STI and the temporal change in shorebird occupancy, with occupancy increasing for warmer-breeding species and occupancy decreasing for colder-breeding species over 25 years. Given that the large-scale population trends for these species are negative (Bart et al. 2012, Smith et al. 2020, Smith et al. submitted), for any species that have positive regional trends, this can be interpreted as reflecting distributional change, rather than change in population-level abundance.

METHODS