Patterns of diversity differ between arctic and alpine ecosystems for both historical and current ecological reasons. Low temperature is an effective filter that limits the number of species that can colonize arctic and alpine environments. Greater isolation and niche differentiation promoted specia-tion and restricted species migrations in the alpine regions, resulting in a higher species richness in alpine than in arctic ecosystems. Moreover, the greater vertical relief in alpine areas is unfavorable to the formation of a soil organic mat and causes more disturbance, leading to high ecological diversity at the landscape level and the concentration of arctic biodiversity in localized sites of high vertical relief. Because the most widespread communities in arctic areas (and in alpine areas of low relief) have very few species, the loss of even a few species would dramatically alter species diversity.

Biodiversity in arctic and alpine ecosystems is currently threatened most strongly by diffuse impacts of human activity. COi-induced climatic warming is causing upward migration of alpine species, with the possible loss of some alpine ecosystems from low-altitude summits. Input of pollutants from low latitudes and altitudes has a low-level chronic impact on key functional groups. This, combined with climatic warming, could alter conditions for establishment and cause an advance of the treeline, leading to changes in the role of arctic and alpine ecosystems in the global carbon and energy balance. Because these human impacts are caused by forces outside the arctic and alpine regions, there is no clear mechanism by which ecological impacts will feed back to alter the human activities responsible for the problems.

Steady-state biogeochemical pools and fluxes are the ecosystem traits and processes that are least sensitive to changes in biodiversity. Generalized plant strategies reflecting differences in size and RGR are important in explaining differences in biogeochemical cycling among sites, and in determining which species dominate steady-state rates of productivity and nutrient cycling within sites. However, even large changes in specics abundance and diversity usually have only moderate direct effects on pools and fluxes of carbon and nutrients in closed communities because changes in the abundance of one species cause compensatory changes in other species, with minimal effects being observable at the ecosystem level. Only if there arc major changes in abundance of functional groups of plants, animals or microorganisms will biogeochemical processes be strongly altered.

Species within arctic and alpine communities differ strikingly in the location (height or depth), timing, and, in the case of N and P, the form of resource captured. In some cases (e.g. specialization by canopy height) this may have relatively little impact on ecosystem energy budgets. However, in other cases (e.g. specialization in the form of N or P absorbed) this specialization among species may increase the overall rate of resource capture by plants, resulting in substantial effects on ecosystem processes. The importance of phenologica! specialization probably depends on the extent to which this enables vegetation to capture resources that would otherwise be lost or immobilized. Gain or loss of species that differ strongly from other species in the community with respect to the timing, location or form of resource capture might substantially alter ecosystem processes, although there is currently little evidence from which to draw conclusions.

Relatively few species regulate the annual input and loss of nitrogen from arctic and alpine ecosystems. Similarly, animal species which transport nutrients into low-fertility ecosystems can greatly alter the rates and heterogeneity of ecosystem processes. Changes in the abundance of these species could profoundly alter the resource base that governs rates of biogcochem-ical processes. Similarly, the invasion of arctic or alpine communities by frees or shrubs that are tall enough to mask the snow could substantially increase the annual energy gain and the heat available for biological processes and atmospheric exchanges.

Maintenance of ecosystem integrity over a complete cycle of common disturbance events is critical to the long-term persistence of ecosystem processes. Pioneer species, which may be uncommon in undisturbed patches of vegetation, may be critical to the re-establishment of closed nutrient cycles following disturbance. Finally, species diversity is functionally important because it provides insurance against large changes in ecosystem processes. The more species there arc in a functional group, the less likely it is that any extinction event or series of such events will have serious ecosystem consequences.

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