Arctic Biodiversity Assessment. Status and trends in Arctic biodiversity. Terrestrial Ecosystems. – Chapter 12

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.

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 ofherbivore 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.