Indicator #08
The Arctic Species Trend Index
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Michael J. Gill, Environment Canada, Circumpolar Biodiversity Monitoring Program of CAFF, Whitehorse, Yukon, Canada.
Christoph Zöckler, UNEP World Conservation Monitoring Centre, Cambridge, United Kingdom.
Louise McRae, Institute of Zoology, Zoological Society of London, London, United Kingdom.
Jonathan Loh, Institute of Zoology, Zoological Society of London, London, United Kingdom.
Ben Collen, Institute of Zoology, Zoological Society of London, London, United Kingdom.

8

Dramatic changes, such as sea ice loss, are projected to occur in Arctic ecosystems over the next century [1]. Understanding how the Arctic’s living resources, including its vertebrate species, are responding to these changes is essential in order to develop effective conservation and adaptation strategies. Arctic species that are adapted to these extreme environments are expected to be displaced, in part, by the encroachment of more southerly species and ecosystems [2]. Limited functional redundancy in Arctic ecosystems poses a particular risk as the loss of a single species could have dramatic and cascading effects on an ecosystem’s state and function [2]. Our current, mostly single species approach to monitoring with a bias towards charismatic species over functional species, limits our ability to detect and understand critical changes in the Arctic’s ecosystems. A broader and more integrated approach is needed to facilitate a better understanding of how Arctic biodiversity is responding to a changing Arctic and how these changes might reflect or counter global biodiversity trends.

For the first time, an index that provides a pan-Arctic perspective on trends in Arctic vertebrates is available. The Arctic Species Trend Index (ASTI)1, like the global Living Planet Index (LPI), illustrates overall vertebrate population trends by integrating vertebrate population trend data of an appropriate standard [3] from across the Arctic and over the last 34 years (with 1970 as the baseline2). This index not only gives a composite measure of the overall trend of Arctic vertebrate populations, but can also be disaggregated to display and investigate trends based on taxonomy, biome, region, period, and other categories. These disaggregations will ease the identification of potential drivers of these trends. Over time, tracking this index will help reveal patterns in the response of Arctic wildlife to growing

pressures, thereby facilitating the prediction of trends in Arctic species.

Population/ecosystem status and trends

A total of 965 populations of 306 species (representing 35% of all known Arctic vertebrate species) were used to generate the ASTI. In contrast to the global LPI [4], whose overall decline is largely driven by declines in tropical vertebrate populations, the average population of Arctic species rose by 16% between 1970 and 2004. This pattern is very similar to the temperate LPI [4] and is consistent in both the North American and Eurasian Arctic. The overall increasing trend in the Arctic is thought to be partly driven by the recovery of some vertebrate populations (e.g., marine mammals) from historical over-harvesting [5] as well as from recent changes in environmental conditions both inside (e.g. Bering sea pollock, Boreogadus saida [6]) and outside of the Arctic (e.g., lesser snow geese, Chen c. Caerulescens [7]) resulting in dramatic increases in some species’ populations. This increasing trend, however, is not consistent across biomes, regions, or groups of species.

2Populations in the high, low, and sub-Arctic boundaries (Figure 8.1), for instance, show markedly different trends. High Arctic vertebrate abundance has experienced an average decline of 26%. Despite an initial growth period until the mid-1980s, sub-Arctic populations (mostly terrestrial and freshwater populations) have, on average, remained relatively stable (–3% decline) whereas low Arctic populations, largely dominated by marine species, show an increasing trend (+46%). This pattern may reflect, to some extent, varying and predicted responses [1, 2] to changing pressures such as climate change and harvest patterns, but may also reflect natural, cyclic patterns for some species and populations. However, caution is needed in interpreting these results.

The high Arctic has experienced the greatest increases in temperature to date and even greater temperature increases are expected resulting in further loss of sea ice extent and range contraction of high Arctic ecosystems and species [1, 8]. However, 34 years is too limited a time series to attribute these changes to declining trends in high Arctic vertebrates. For example, wild barren-ground caribou and reindeer herds are known to naturally cycle over long time periods and recent, largely synchronous declines across the Arctic are thought to be natural and, in part, responsible for the declining high Arctic index. Declines in other species populations, such as lemmings, in Greenland, Russia, and Canada, however, may be, in part, the beginning of a negative response to a dramatically changing system. In contrast, increasing trends in low Arctic populations are biased by dramatically increasing fish populations in response to changing marine conditions [6] and recovering marine mammal populations [5] in the eastern Bering Sea. More data is needed in other Arctic marine systems before an accurate picture regarding Arctic marine vertebrate population trends can be developed.

3Divergent patterns are also observed between the different biomes (marine, freshwater, terrestrial). Whereas the freshwater and marine indices increase over the time period (52% and 53% respectively), the terrestrial index shows an overall decline of 10% despite increasing in the late 1970s to the mid-1980s. The data behind the freshwater index is currently too sparse (51 species, 132 populations) to fully reflect the circumpolar freshwater situation, and although the marine index is robust in terms of species and populations (107 species, 390 populations), it is not spatially robust being largely driven by an overweighting of population data from the eastern Bering Sea. The moderate decline in the terrestrial index (–10%) is largely a reflection of declines (–28%) in terrestrial high Arctic populations (mostly herbivores, such as caribou, Rangifer tarandus, lemmings, and the High Arctic brent goose, Branta bernicla) (Figure 8.2). Terrestrial low Arctic population increases (+7%) are driven, in part, by dramatically increasing goose populations, but may also reflect an ecological response to climatic changes whereby species with more southerly distributions are responding favorably to these climatic changes [2]. This northward movement of southern species (e.g., red fox, Vulpes vulpes [9]) coupled with increasing incidence of severe weather events in the high Arctic [2, 10] and changing tundra vegetation [11–13] may explain, in part, the declines in terrestrial high Arctic populations and the possible negative impact on herbivorous species.

The major Arctic taxa (birds, mammals, and fish) also exhibit divergent trends. Birds, which comprise 52% of the ASTI populations are revealing a very flat trend overall (–2%), whereas mammal populations increased fairly steadily (+33%) over the same time period. The fish index experienced the greatest increase (+96%), however the data behind the fish index is not currently representative enough to provide meaningful results. Within the bird taxa, freshwater birds have increased dramatically (+43%), largely a reflection of increases in some waterbird populations, and likely in response to stricter hunting regulations and land-use changes on their wintering grounds [14]. The terrestrial bird index, despite a doubling in the numbers of geese, has experienced a slight decline (–10%) over the past 34 years, whereas marine birds, although fluctuating, have remained steady (–4%). An analysis of migrant versus nonmigrant birds showed an increasing trend for non-migrants (+20%) and a flat trend (–6%) for migrants although there was no significant differences between the two groups. However, the slight decline in migrant birds would have likely become a more significant decline if the increasing geese populations were not included and we were able to include shorebird population trend data derived from non-Arctic survey sources3. Declines in migrant shorebirds to date are mostly regarded as a response to pressures (landuse changes, etc.) found on wintering and stop-over sites [15–17], but expected changes to Arctic breeding habitat as a response to climate change may also become a factor in the long-term as most high Arctic species and populations would be at risk [2, 18].

While the ASTI offers some initial insight into recent trends in Arctic vertebrate populations, and notwithstanding the over-representative sample of Arctic vertebrate species, careful interpretation of the ASTI is required as it does not yet adequately represent all populations, taxa, biomes, and regions. While rapid, human-induced changes in Arctic ecosystems are already likely resulting in winners and losers among Arctic species and populations [2], more data coverage and longer-time series are needed to give an accurate, unbiased picture. Despite the limited time series for the index, the large and diverse collection of data in the index, representing a multitude of taxa across regions, biomes and longitudes does provide some insight into potential responses to human-induced pressures, outside of natural variation. This index will improve with the scale, number and breadth of contributions and future analyses will be more robust in their results.

Concerns for the future

A number of pressures, many global in nature, are acting cumulatively to exert growing pressure on Arctic biodiversity [2]. Climate change is of paramount concern and recent evidence suggests that our current projections are too conservative, with much higher rates of change already being experienced [19, 20]. These increasing pressures and rates of change are expected to fundamentally change Arctic ecosystems [1, 2]. With changing extent and quality of Arctic habitats, potential ecological bottlenecks emerging due to extreme events and other pressures, limited functional redundancy, and increasing competition from northward shifting species, in conjunction with either natural downward trends or other human-induced pressures such as development or contaminants, loss of some Arctic species and ecosystems is expected [2]. In particular, high Arctic and marine ecosystems and the species they currently support are expected to undergo the greatest changes [1] reducing the potential for these species and ecosystems to persist.

These expected rapid changes will challenge both Arctic residents directly dependent on the Arctic’s ecosystems and the global community as a changing Arctic is expected to upset the Earth’s physical, chemical, and biological balance. Enhanced, integrated, and coordinated research, monitoring, conservation, and adaptation efforts are needed to meet these growing challenges.