Download Terrestrial Ecosystems chapter chapter 12
TERRESTRIAL ECOSYSTEMS (Chapter 12)
Lead Authors: Rolf A. Ims and Dorothee Ehrich
Key Contributing Authors: Bruce C. Forbes, Brian Huntley, Donald A. Walker and Philip A. Wookey
Contributing Authors Dominique Berteaux, Uma S. Bhatt, Kari A. Bråthen, Mary E. Edwards, Howard E. Epstein, Mads C. Forchhammer, Eva Fuglei, Gilles Gauthier, Scott Gilbert, Maria Leung, Irina E. Menyushina, Nikita Ovsyanikov, Eric Post, Martha K. Raynolds, Donald G. Reid, Niels M. Schmidt, Audun Stien, Olga I. Sumina and Rene van der Wal
The Arctic tundra biome is geographically restricted to a strip around the margins of the Arctic Ocean. A key force determining the tundra biome’s zonal structure is the bottom-up effect of decreased vegetation productivity and complexity with increasing latitude. Accordingly, there are trends of decreasing diversity within and among trophic guilds of consumers with increasing latitudes. Low food web complexity in the northern parts of the biome is also due to island biogeographic features, as large parts of the high Arctic are located on islands. Similarly, a substantial proportion of the high biodiversity of low Arctic zones stems from ‘spillover effects’ from sub-Arctic ecosystems. Historic processes have also contributed to shaping the current large-scale Regional provinces in terms of Arctic species communities. At sub-regional scales the terrestrial Arctic harbors diverse mosaics of communities that are structured by gradients and disturbances in climate, substrate, hydrology and cryosphere that form unique patterns of within – and among – community diversity. Hot spots of high regional diversity are currently found in some old, topographically and geologically complex regions.
It has progressively become warmer. I recall that only in our traditional area did the trees occur, but when I returned there via plane last year, a lot more of the tundra was inundated with trees, small mind you, but they have moved north and east. The area we used to inhabit has been overgrown with vegetation, mainly shrubs and small trees. It has become almost like a mini-forest where we used to have our main camp. We visited the site in 2000 and it was almost unrecognizable due to all of the growth that occurred during our absence. I think this is due to a shorter spring, a longer summer and longer frost free falls. Utok; Elders Conference on Climate Change 2001.
The architecture of tundra food webs is modulated by inter-specific interactions within and between trophic levels. Herbivores can regionally exert strong top-down controls on tundra vegetation, whereas predators often control small mammal herbivores and the reproductive success of ground nesting birds. Multi-annual, cascading bottom-up and top-down interaction cycles mediated by lemming populations are crucial for the maintenance of terrestrial Arctic biodiversity in many tundra ecosystems. Functional traits of plants in interactions with below-ground microbial communities and herbivores maintain essential roles in the regulation of the global climate system through controls on fluxes of greenhouse gasses (GHG) and heat fluxes between the earth surface and the atmosphere. Changes to the composition of terrestrial biodiversity may determine whether the Arctic will become a source or a sink for GHGs in a warming climate.
Climate is historically and currently the most important driver of change of Arctic terrestrial ecosystems, through alteration of coastal sea ice, glaciers, snow and permafrost, changed seasonality and extreme events. At present, a second emerging driver is an increased footprint of human presence within the Arctic. Currently, the most profound ecosystem impacts include (1) increased plant biomass due to growth of tall woody plants that cause lower albedo and possibly enhance GHG emissions and thereby accentuating the Arctic amplification of climate change, (2) collapsed cycles of lemmings and emergent Outbreaks of insect herbivores and plant pathogens with cascading impacts on food webs and ecosystem functioning, and (3) increasing abundance of boreal and human commensal species impacting Arctic endemics as predators or competitors. Recommended actions to conserve Arctic terrestrial ecosystems under the impacts of climatic change and other anthropogenic stressors include conservation of topographically diverse areas with landscape-scale ‘buffer-capacity’ to maintain cold refuges in a warmer climate and of remote high Arctic islands that are the most physically protected from species invasions from the south and human presence. Prudent management of Arctic herbivores such as reindeer Rangifer tarandus, using their capacity for shaping vegetation on landscape scales, may be considered for counteracting encroachment of tall woody vegetation that otherwise will eliminate tundra habitats, while avoiding the negative impacts of herbivore overabundance that have been documented in some regions.
A key message from the present assessment is that essential attributes of terrestrial Arctic biodiversity, some of which have global repercussions, are ultimately dependent on how interactions within ecological communities and trophic webs are impacted by rapidly changing external drivers. Consequently, research, monitoring and management ought to be properly ecosystem-based. Because ecosystems are structurally and functionally heterogeneous across the tundra biome and may also be subjected to external drivers of different strengths, new ecosystem-based observatories that include state-of-the art research, often combined with adaptive management, should be widely distributed across the circumpolar Arctic. Model-based predictions about how Arctic species and ecosystems will respond to the substantial climate change currently projected for the Arctic have limited powers to accommodate surprises in terms of novel climates and ecosystems that may rapidly emerge. New efforts urgently need to be deployed to enable well designed real-time observations as a basis for empirically based documentation and understanding of cause-effect relationships of future ecosystem changes in the terrestrial Arctic.
The Arctic tundra biome is characterized by low-growing vegetation composed of low shrubs, sedges, grasses, forbs, lichens and mosses (bryophytes) that grow beyond the northern climatic limit of trees (see Section 2 in Meltofte et al., Introduction for this assessment’s definition of the Arctic). A polar view of the biome from space reveals that the continental portion of the Arctic tundra occupies a thin strip of land between the Arctic Ocean and the boreal forest (Fig. 12.1). Eighty percent of the lowland portion of the Arctic lies within 100 km of seasonally ice-covered seas. The biome essentially owes its existence to cold sea breezes that keep the temperatures during the growing season below that required for tree growth. One fifth of the total coastline of the world, or about 177,000 km, occurs in the Arctic, a biome that comprises only about 5% of the Earth’s terrestrial surface. Three main aspects of the extensive Arctic coastlines make the tundra biome extremely vulnerable to climate warming: (1) the strong climatic influence of the nearby sea ice, (2) narrow bioclimate zonation associated with these coastlines, and (3) extensive lowland plains near most of the Arctic coast (CAVM Team 2003).
In terms of climate, the Arctic tundra can be viewed as a strongly oceanic-influenced biome, but one that varies considerably in the degree of maritime expressions of cloudiness, fog, humidity and equitable temperatures, because the Arctic Ocean is covered by ice to a varying extent during the winter and summer. The longevity of the ice near the coast in summer strongly affects summer land temperatures and local continentality of the climate as well as the diversity of organisms and total productivity of the land (Bhatt et al. 2010). Steep temperature gradients occur inland from these coastlines resulting in extraordinarily long and narrow ecological transition zones with several bioclimate subzones compressed near the coast. Permafrost strongly affects the ecosystems of most of the biome, but is not a condition that defines the biome, as permafrost also extends far into the boreal forest in continental areas of Siberia and North America. On the other hand, there are portions of coastal tundra with no or only discontinuous permafrost (Callaghan et al. 2004a, AMAP 2011).
The integrity of terrestrial Arctic ecosystems, as shaped by biotic and abiotic processes, is ultimately conditional on low primary productivity resulting from short and cool summers that restrict plant growth and metabolic activity of other poikilothermic1 organisms, such as bacteria, fungi and invertebrates. The low productivity at the base of trophic chains restricts secondary productivity and the complexity of food webs and decomposer webs. Tundra food webs are usually composed of only three major trophic levels: plants, herbivores and predators (Krebs et al. 2003, Ims & Fuglei 2005). The structure of decomposer webs, in which cryptic microbial communities and soil faunas play a central role, is considerably less known (Callaghan et al. 2004b), but may be more complex than the more conspicuous food webs composed of green plants and macroscopic animals (see Hodkinson, Chapter 7). Terrestrial food webs also include fewer trophic levels than, for instance, aquatic ecosystems in the Arctic (Wrona & Reist, Chapter 13, Michel, Chapter 14), although high Arctic limnic systems may be as simple as their terrestrial counterparts (van der Wal & Hessen 2009, Wrona & Reist, Chapter 13).
Although Arctic tundra ecosystems have a simple trophic structure, often with relatively low species richness within each trophic level, other structural features of biodiversity can be remarkably complex. Spatial variability in temperature, winds, precipitation, hydrology, cryosphere and soil chemistry creates gradients and complex mosaics of abiotic conditions that shape the composition of species assemblages (i.e. ecological communities) at multiple spatial scales. For this reason, a spatially hierarchical approach to characterize biodiversity patterns in terms of differences in species assemblages as functions of abiotic controlling factors from local to circumpolar scales appears to be particularly applicable to Arctic tundra. In terms of ecosystem functions, and the biotic and abiotic processes that shape these functions, tundra ecosystems are no less diverse than other ecosystems. Some of the ecosystem functions are crucial for the livelihood of local people, such as locally produced food, while others have essential roles in the global climate system, such as controls of exchange of heat and GHG.
In this chapter we start with a review of present knowledge of how natural abiotic and biotic factors shape biodiversity in terms of ecosystem structure, processes and functions within the tundra biome (Section 12.2). This provides the background for assessing past and present trends in terrestrial Arctic Biodiversity, and the drivers of such trends (Section 12.3). Towards the end of the chapter we provide a synthesis of the assessment’s key findings (Section 12.4) before we conclude with a set of recommendations on how policy makers, managers and ecosystem scientists could act on these findings (Section 12.5).
CONCLUSIONS AND RECOMMENDATIONS
Status and trends: Implications for the future
The Arctic tundra biome is a bio-climatically defined zone, the integrity of which is ultimately conditional on cold climates. Based on an extensive peer-reviewed literature, the present assessment testifies to the fact that all aspects of tundra ecosystems and their embedded biodiversity are shaped by past and current climates, although in conjunction with other environmental factors. This also means that future climate warming – in combination with other drivers of change – will fundamentally alter Arctic biodiversity. Indeed, our review of contemporary trends demonstrates that the tundra ecosystems have already changed as a result of recent climate warming as well as by intensified human land-use, including industrial development in certain areas.
Concerning the impacts of drivers of change in general and those related to climate warming in particular, the present assessment arrives at the following conclusions:
- Impacts of change are often indirect, both in the abiotic and biotic domains of tundra ecosystems.
- In the abiotic domain, climate warming exerts some of its most profound impacts through second-order disturbances in the cryosphere, such as ground surface icing (ROS) and permafrost thaw, or through drought-related increase of tundra fires.
- In the biotic domain, pervasive driver-impacts are mediated both by bottom-up and top-down cascades in trophic webs. Both types of cascades have recently been found ultimately to harm species endemic to the Arctic such as lemming-dependent predators and grazing-sensitive cryptogams.
Concerning the functioning of tundra ecosystems, new insights have emerged about the essential but complex roles of terrestrial Arctic biota in the evolution of regional-global climates:
- Ecosystem structure in terms of the composition of species guilds, communities and trophic webs may determine whether the terrestrial Arctic will become a future sink or source for GHGs, and whether it will strengthen or weaken the Arctic amplification of climate warming.
- The set of species traits that dominate in an ecological community is important for overall ecosystem functionality, implying that the processes involved in the global C cycle are not independent of the species (and functional traits) involved.
- An important overall message is that ‘the Devil is in the details’ regarding how terrestrial Arctic biodiversity interacts with climate change, which is indeed an argument for emphasizing Arctic biodiversity in climate research.
The tundra biome’s geographic configuration alone, as an irregular and in places very narrow strip of low-lands squeezed in between boreal forest and the Arctic marine environment, implies that the whole biome is vulnerable to climate change-related ‘edge effects’; i.e. species invasions from sub-Arctic Ecosystems (e.g. northward expansion of forests) and marine encroachment (erosion of coastlines and rising sea levels). Considering paleoecology, the whole biome can already be considered a refugium. Moreover, certain tundra subzones and regions may be particularly sensitive and vulnerable:
- The high arctic subzone A should be considered to be endangered. It is currently restricted to a very small area, about 2% of the non-glaciated terrestrial Arctic, mostly islands surrounded by perennial sea ice. An increase in July mean temperature of only 1-2 °C will permit the introduction of prostrate shrubs, sedges and other temperature-limited species. Disappearing sea ice may also change the levels of marine nutrient and production subsidies to the otherwise extremely nutrient/production limited high Arctic terrestrial food webs.
- The low Arctic subzones (D and E) are particularly vulnerable to increased pressures from range-expanding species with current strongholds in the sub-Arctic. Reported cases include boreal shrubs and trees, outbreaks of insect defoliators and meso-predators. ‘Human commensal’ meso-predators may also be synergistically enhanced by intensified land-use and expanding infrastructure/industries.
- Steppe-tundras that currently are confined to a few regions with continental climate and calcareous substrate are expected to be strongly affected by increased humidification of the climate and acidification of the substrate.
Conservation and management actions
The Arctic tundra biome is still characterized by relatively pristine ecosystems over large areas compared with other biomes on Earth. However, the impact of ongoing and future climate change is expected to be huge and represents the single most severe threat to terrestrial Arctic ecosystems. Moreover, there is significant spatial overlap with other stressors indicating that we must pay special attention to potential synergies. Area protection (reserves and national parks) will be an important means for preserving Arctic biodiversity in the era of climate change, especially since it will act to diminish synergistic impacts of local anthropogenic stressors and climatic warming. With regards to climate warming, there are certain biogeographical features that will make some areas particularly valuable for protection:
- Topographically diverse areas with mountain ranges that include landscape-scale climatic gradients may have ‘buffer-capacity’ to maintain cold refuges in a warmer climate.
- Remote high Arctic islands that are far north of southern bioclimate subzones and boreal ecosystems, and where Arctic marine waters will serve at least as a partial barrier (‘filter’) to invasions from the south.
However, regardless of how remote and well-protected, no Arctic reserves or national parks will be immune to the impact of climate change. To conserve Arctic biodiversity it may be necessary to implement active management actions especially within protected areas:
- Encroachment of tall shrubs and trees into tundra can be counteracted, with the added benefit that plant community diversity can be maintained under future warming, by management of large herbivores as shown by recent research in Fennoscandia and Greenland. Such management needs to consider both the positive and negative effects of increasing grazing pressures, other ecological effects of high herbivore densities (e.g. subsidies to meso-predators) and the economies of local people (see Huntington, Chapter 18).
- Certain boreal species expanding their range northwards and anthropogenically introduced invasive species may be controlled locally in the manner currently attempted with meso-predators in northern Fennoscandia.
- Increasing populations of human commensal species should be counteracted, for instance by effective waste management associated with human settlements or encouragement of hunting.
Indeed, in a much warmer climate, a network of ‘Arctic parks’ which are actively managed to maintain ecosystem processes that are representative of the main geographic regions and sub-zones of the tundra biome may be the only way to conserve terrestrial Arctic biodiversity in the future.
Needs for area- and ecosystem representative measurements
Over most of the Arctic, it will continue to be easier (and cheaper) to detect changes from space than on the ground. Thus, remote sensing and technological advances to improve it will undoubtedly be important for monitoring the terrestrial Arctic, and Arctic ecologists ought to be in the forefront of the application of such technologies. However, although we may be able to detect changes in gross ecosystem properties from space, we need to be on the ground to explain and manage those changes. Moreover, most of the biodiversity and many of the factors that drive its dynamics will remain unseen from space regardless of future improvements in remote sensing technologies.
Ground-based measurements currently have very poor geographical coverage considering the vast spatial extent of the tundra biome and the large spatial heterogeneity in its habitats and biota. This heterogeneity must be accounted for, if we are to obtain robust estimates of status and trends, for instance by means of meta-analysis (e.g. Elmendorf et al. 2012). To do this, research and monitoring efforts need to become much more area representative than is now the case. This means that many more long-term sites ought to be established, with the demand that sampling design, measurement methods and criteria for classifications are harmonized among sites.
Those processes that exceed the spatial scale of small plots or include ecosystem components dominated by microbial communities and invertebrates are currently underrepresented in terrestrial Arctic reSearch and monitoring. Both of them are, however, critically important for understanding the important biogeochemical and biophysical processes coupling the tundra ecosystem to the climate system. These problems of lack of area and ecosystem-representativeness are acute challenges that need to be addressed in the upcoming CBMP (see Box 1.4 in Meltofte et al., Chapter 1).
Needs for ecosystem-based approaches
A key message emerging from this assessment is that essential attributes of Arctic biodiversity, some of which have global repercussions, are ultimately dependent on how interactions within ecological communities and trophic webs are impacted by external drivers. This provides a compelling argument for research, monitoring and management of Arctic terrestrial biodiversity to adopt ecosystem-based approaches. At present, however, there are very few sites in the Arctic where long-term projects are explicitly ecosystem-based. This state of affairs must be improved, and CBMP ought to play a key role by helping to orchestrate an area-representative, circumpolar network of ecosystem-based monitoring sites.
The planning of a future network of ecosystem-based programs should strive to harmonize monitoring design and measurement protocols and to accommodate a common set of ‘essential biodiversity variables’ (Pereira et al. 2013). However, the fact that the ecosystems are structurally and functionally heterogeneous across sub-zones and regions of the tundra biome, as well as partly subjected to different external drivers of change, implies also a need for site-specific efforts to focus on site-specific processes and components of the ecosystem. Ecosystem-based monitoring should be guided by the best empirical knowledge and most plausible hypotheses regarding key drivers, processes and trends in the focal ecosystem (Lindenmayer & Likens 2009). In order to be relevant to stakeholders, managers and policy makers, those drivers and components of the ecosystem that actually can be amenable to actions in ecosystem-based management ought to be given particular attention in monitoring programs (Westgate et al. 2013).
The magnitude of climate warming in the Arctic during the present century may become as extreme as 10 °C. However, the projected temperatures and precipitation patterns vary so much between different models and geographic regions (Overland et al. 2011, Xu et al. 2013) that one may question the value of the many attempts now taken to derive explicit model-based predictions about how Arctic species and ecosystems will respond. Moreover, the combination of unprecedented rates of climate change, abnormal levels of other stressors, evolution of novel climates Williams et al. 2007) and ecosystem structures (Macias-Fauria et al. 2012) accentuate the possibility that present knowledge about past changes, contemporary ecosystems states and trends may have little bearing on what will become the future states of terrestrial high latitude ecosystems (Post 2013b). In such a dire situation it becomes crucial to establish flexible observation systems to enable real-time detection, documentation and understanding of cause-effect relations (Ims et al. 2013). The framework of adaptive monitoring as proposed by Lindenmayer et al. (2010) may be particularly suitable in the context of ecosystems as likely to be prone to uncertainties and surprises as those currently located in the terrestrial Arctic.