Structural characteristics of a low Arctic tundra ecosystem and the retreat of the Arctic fox
Introduction
The Arctic is currently subject to changes threatening the integrity of tundra ecosystems (CAFF, 2003). Global warming, which is expected to be most pronounced in polar regions, is particularly highlighted (Callaghan et al., 2004b, Foley, 2005, Hinzman et al., 2005, Chapin et al., 2005). However, climate change takes place in conjunction with several other anthropogenic stressors, such as intensified land use (CAFF, 2003, ACIA, 2004). Although the entire Arctic region may be subject to change, the low Arctic tundra zone (Bliss et al., 1973) may be particularly prone to early and rapid change, because it balances against more southern and vastly different ecosystem states (Epstein et al., 2004).
It has been proposed that upper trophic levels (i.e. predators) may be particularly sensitive to fundamental ecosystem alterations (Schmitz et al., 2003, Sergio et al., 2005, Voigt et al., 2003). Tundra ecosystems have typically a tri-trophic, plant-based food web topped by a guild of predators preying mainly on herbivorous small mammals (lemmings and voles) (Elton, 1942, Wiklund et al., 1999, Krebs et al., 2003, Ims and Fuglei, 2005). Some of these small mammal predators, such as the Arctic fox Alopex lagopus (Angerbjörn et al., 2004, Fuglei, 2005), are found exclusively in the Arctic region and should for this reason be particularly vulnerable to global warming (CAFF, 2003, ACIA, 2004, Callaghan et al., 2004a).
In a seminal paper Hersteinsson and MacDonald (1992) analyzed harvest statistics and demonstrated that the Arctic fox during the last century had exhibited a warming related retreat of from the southern edge of the Arctic. They proposed an underlying process scenario that involved a bottom-up trophic cascade by which a warming-induced increase in primary productivity (i.e. plant biomass) in turn gave way to a higher secondary productivity (i.e. prey biomass) available to the northward expanding red fox expelling the competitive subordinate Arctic fox. Hersteinsson and MacDonald (1992) wrote, however, that they were “mindful” with regard to the exact mechanisms in the paucity of data on variables other than fox harvest rates and climatology. Indeed, in Fennoscandia where the Arctic fox is on the verge of regional extinction, several other explanations for the “Arctic fox problem” have been forwarded (for reviews see Hersteinsson et al., 1989, Angerbjörn et al., 1995, Linnell et al., 1999a). As a result management actions (such as population enhancement by introduction of captive bred Arctic foxes in Norway) are now undertaken that implicitly assume that no fundamental change of the ecosystem underlies the Arctic fox decline.
In the present study, we aim to provide the first direct evaluation of Hersteinsson and McDonald’s proposition that the retreat of the Arctic fox from the southern edge of the Arctic is ultimately due to a climate-induced change in ecosystem structure. We do this by analyzing relevant ecosystem state variables measured during a large-scale, targeted field campaign encompassing regions in eastern Finnmark, northern Norway with current presence or absence of the Arctic fox. According to Hersteinsson and MacDonald (1992) we predicted that tundra areas where arctic fox breeding had ceased should show increased biomass of plants likely have increased under warmer climate and, moreover equivalent responses in variables reflecting higher trophic levels consistent with a bottom-up trophic cascade.
Section snippets
General characteristics of the study area
The region of eastern Finnmark forming the northeastern tip of Norway at 70–71°N, is bio-climatically classified as low Arctic tundra (Walker et al., 2005). The Finnmark tundra, which is most clearly defined on the low-altitude peninsulas bordering the Barents sea (Fig. 1), constitutes the westernmost fringe of the vast Euro-Asian tundra (Virtanen et al., 1999). The study area is low-altitude (<400 m a.s.l.), relatively coast-near tundra. The climate is characterized by relatively mild winters
Plants
For the contrasts between regions, the reference blocks in VARANGER (i.e. the region with most recent Arctic fox breeding) had more bilberry and acrocarp mosses than all the other regions (Fig. 3). Except for a significant higher abundance of dwarf birch and crowberry and less prostrate Salix in VARANGER than in NORDKINN, there were no other clear differences between the reference blocks at the regional level.
Within both regions with known fox dens (i.e. VARANGER and LAKSEFJORD) landscape
Assessing the premises of the Hersteinsson–MacDonald hypothesis
In this study, we assessed whether the geographic presence-absence pattern of Arctic fox in low arctic tundra were associated with differences in ecosystem state variables compatible with the climate-induced, trophic cascade proposed by Hersteinsson and MacDonald (1992). The study area in eastern Finnmark offered a good case for such an assessment being a relatively homogenous area in terms of topography and geography and located at the climatic periphery of the circumpolar tundra biome (
Conclusion
By the present study we have provided a first assessment of whether changes in ecosystem structure according to a proposed climate change scenario proposed by Hersteinsson and MacDonald (1992) could underlie the retreat of the Arctic fox from low arctic tundra. Our analysis relied on data from a spatially extensive, but short-term, campaign-based study. Of course such an approach provides only a snap-shot picture of a highly dynamic system (see Krebs et al., 2003 for a discussion). On the other
Acknowledgements
This project was supported financially by the Norwegian Directorate of Nature Management and the Research Council of Norway. Field assistance was provided by Pieter Beck, Tina Dahl, Ingrid Jensvold, Bjørn Hugo Kristoffersen, Stein Tore Pedersen, Arne Petter Sarre, Guro Saurdal, Cecilie Steffen and Alfred Ørjebu. Comments on the manuscript by Eva Fuglei and two anonymous referees are much appreciated.
References (91)
Ecology and use of Arctic fox Alopex lagopus dens in Norway: tradition overtaken by interspecific competition?
Biological Conservation
(2003)- et al.
Rodent dynamics as community processes
Trends in Ecology and Evolution
(1988) - et al.
The arctic fox in Fennoscandia and Iceland: management problems
Biological Conservation
(1989) Predation may link the cycles of lemmings and birds
Trends in Ecology and Evolution
(1988)- et al.
Intrinsic and climatic determinants of population demography: the winter dynamics of tundra voles
Ecology
(2002) Impacts of a warming Arctic – Arctic climate impact assessment
(2004)- et al.
Compositional analysis of habitat use from radio-tracking data
Ecology
(1993) - et al.
The homologies of the Fennoscandian mountain and coastal birch forests in Eurasia and North America
Vegetatio
(1969) - et al.
Fjellrev i Norge 2003 – Overvåkingsrapport
(2004) Fjällrevens existens er hotad