Arctic and Alpine Biodiversity Its Patterns Causes and Ecosystem Consequences

F. STUART CHAPIN, III AN!) CHRISTIAN KORNER

Ecological changes altering the Earth System and the loss of biotic diversity that have been major sources of ecological concern in recent years. These processes have been pursued independently, with little attention being paid to the environmental causes and the ecosystem consequences of changes in biodiversity. The two processes are clearly interrelated. Changes in ecological systems cause changes in diversity. Unfortunately, we know much less about the converse. What types and magnitudes of change in diversity alter the way in which ecosystems and the Earth System function? What are the processes and circumstances under which this occurs? Arctic and alpine ecosystems are ideal subjects when considering these questions because:

1. high latitudes are predicted to undergo more pronounced warming than other regions of the globe;

2. cold regions are the areas where climatic warming would have the greatest ecological consequences;

3. high altitudes, due to reduced pressure, are regions where CO;. should be particularly limiting and where rising C02 might strongly stimulate plant growth;

4. arctic ecosystems, with their large frozen pools of carbon, may exert strong feedbacks to global climate;

5. owing to their relative simplicity, these ecosystems may show clear effects of species on ecosystem processes and may, therefore, be strongly affected by loss or gain of species.

Hence, arctic and alpine ecosystems provide unique insights into the causcs and consequences of diversity in general. Furthermore, arctic and alpine ecosystems are the only biome with a global distribution, making them ideal for global monitoring of environmental change.

Functional Roles of Biodiversity: A Global Perspective ,___^

Edited by H.A. Mooriey, J.H. Cushtnan, E. Medina, O.E. Sala and E.-D. Schulze tjKfi © 1996 SCOPE Published in 1996 by John Wiley & Sons Ltd ^

This chapter summarizes the conclusions of a workshop on biodiversity and ecosystem processes in arctic and alpine ecosystems (Chapin and Körner 1995), and extends the discussion of the role of biodiversity in the persistence and functioning of arctic and alpine ecosystems.

2.! THE ARCTIC AND ALPINE BIOTA

The land area covered by arctic and alpine vegetation is roughly 11 million km1, or 8% (5% arctic, 3% alpine) of the terrestrial surface of the globe, stretching from 80L'N to 67:'S and reaching elevations of more than 6000 m in the subtropics (Figure 2.1). This area is similar to that covered by boreal forests or crops and supports about 4% of the global flora (10 000 alpine and 1500 arctic lowland species; Körner 1995; Walker 1995). The fauna of these cold environments also comprises about 3-4% of the world's animal species (Chernov 1995). The local floras of individual mountains (except for isolated volcanic peaks) throughout the world support between 200 and 300 species - a surprisingly constant number. The floras of whole mountain ranges may have over 1000 species in diversity hot-spots such as the Caucasus and the mountains of Central Asia (Agachanjanz and Breckle 1995) or parts of the subtropical Andes. In most areas of the arctic and alpine, fewer than 30 species of higher plants make up more than 90% of the vascular-plant biomass.

The magnitude of genetic diversity within species does not change with latitude or altitude within either the arctic or the alpine floras (McGraw 1995; Murray 1995). Genetic differences among populations result in ecotypic differentiation along both large- and small-scale environmental gradients. Within populations, genetic diversity is created in some plant taxa by frequent hybridization and polyploidy (particularly in deglaciated regions where formerly isolated species come into secondary contact), and in other taxa by recruitment of genetically distinct individuals from the buried seed pool (McGraw et al. 1991).

Species richness generally declines with increasing latitude and altitude because low temperatures and the short growing season are a severe environmental filter that excludes species from progressively more severe climates (Chernov 1995; Körner 1995; Meyer and Thaler 1995; Walker 1995). Under the most extreme conditions, major functional groups of organisms (e.g. tall shrubs, plants or animals with annual life cycles, amphibians and reptiles) are absent. There are also predictable changes with latitude and altitude in specific groups of animals. For example, the Coieoptera decline more strongly with decreasing temperature than do other groups, resulting in a relative increase in the abundance of the Colembola (alpine) and Diptera (arctic). In all animal groups, the proportion of species that are carnivorous increases with latitude (Chernov 1995).

Figure 2.1 Geographie distribution of arctic and alpine regions of the world. Reproduced by permission from Körner, 1995

In cold-dominated ecosystems the balance between the formation of a soil organic mat and disturbance results in an inverse relationship between soil carbon and species diversity. Thus, arctic ecosystems have three times more soil carbon (55 Pg) than alpine ecosystems (20 Pg) but only 13% of the number of plant species. This pattern reflects the active accumulation of soil organic matter and a low degree of disturbance in low-arctic compared to high-altitude ecosystems. In alpine ecosystems, gravity (1) prevents water accumulation that would reduce decomposition and cause organic accumulation, and (2) disrupts the soil organic mat as freeze thaw action displaces the soil surface down-slope, opening space for many colonizing species. Such slope effects are found in both arctic and alpine areas, so that within each region the greatest diversity is found on slopes steep enough to minimize soil organic accumulation (Körner 1995; Walker 1995). Most of the arctic landscape has a thick organic mat and very low species diversity, whereas areas of topographic relief such as pingos (ice-cored mounds) and mountain slopes are hot-spots of diversity. Conversely, in alpine regions, where vertical relief is more pronounced, many areas have a high diversity within each square meter (even higher than in tropical rain forests), and flat, peat-covered areas of low diversity are less common. In both arctic and humid alpine areas a substantial part of the regional flora and fauna can be found within 1 km' (often within 10 nr) of cach other, and very few additional species are added at the mountain-range or regional scale (Körner 1995; Walker 1995).

On a regional and continental scale, arctic and alpine organismic diversity is determined by the ancestral (mostly tertiary) stock of species, longdistance migration during the Holocene, and the evolution of new taxa (Agachanjanz and Brcckle 1995; Ammann 1995; Chernov 1995; Murray 1995). In the Central Asiatic mountains the rate of tectonic uplift of mountain systems is similar to the rate of speciation, so that climatic changes caused by uplift are an important selective influence. Following glacial disturbances and extinctions, migration becomes crucial for the rearrangement and diversity of arctic and alpine flora and fauna. Whereas the floristic composition of the arctic tends to intergrade continuously from 4-5 centers of fioristic richness, the alpine biota are often more discrete, owing to the absence of large contiguous areas of suitable habitat which contribute to local speciation and endemism (Agachanjanz and Breckle 1995; Orabhcrr et al. 1995; Murray 1995). Thus, the dominant species of the most widespread ecosystems in any region have a circumpolar distribution and are common throughout the arctic, whereas each mountain range has a different group of alpine dominants. For example, the alpine flora of New Zealand, with 650 species, shares hardly any species with other mountain areas of the globe.

2.2 PAST, PRESENT AND FUTURE CHANGES IN BIODIVERSITY

Climatic changes since the Pleistocene altered the geographic distribution of arctic ecosystems and caused vertical migration of alpine vegetation belts (Agachanjanz and Breckie 1995; Ammann 1995; Brubaker et al. 1995). However, each species typically showed a unique pattern of migration in response to climatic change because of individualistic responses to the environment. Consequently, past communities often had quite different spccics composition from those of today. Extinction of large grazers by human hunting may have contributed strongly to these community changes because of the large effect of herbivores on ecosystem processes (Zimov et al. 1995).

The paleorecord suggests that it will be difficult to predict future patterns of migration. A given spccies often migrated into quite different ecological communities, indicating that there was no predictable pattern of succession, nor was any "preparation" (e.g. presence of nitrogen fixers) necessary to allow the invasion of new taxa (Brubaker el al. 1995). Migrating populations often reached peak pollen abundance in sediment cores soon after they first appeared, suggesting either rapid migration or rapid reproduction of non-flowering clones that were previously not represented in the pollen profile. Very different dominant species (e.g. birch and spruce) were more similar in their ecosystem impacts (e.g. effccts on watershed chemistry) than were communities that differed in dominant life form (e.g. herbaceous vs. forest communities) (Brubaker et al. 1995).

Climatic warming during the past century (0.7CC) has already caused upward migration of alpine species {Grabherr et al. 1994). If climatic warming continues, taxa restricted to narrow alpine zones at the summits of mountains may disappear. However, this migration is half the rate that would be expected if species had maintained an equilibrium relationship with temperature. Thus, both the rate of individual migration and the movement of ecosystems are slower than would be predicted from change in temperature. This is consistent with recent findings that altitudinal ecotones of forest species move slowly in response to climatic shifts, since their position is strongly determined by species interactions, particularly in the understory (Körner 1995).

Experimental studies provide a strong basis for predicting how arctic and alpine communities may respond to climatic change. At high latitudes experimental increases in air temperature cause large changes in growth, reproductive output and clonal expansion, whereas in the mid- and low-arctic, changcs in other factors, such as nutrient supply, are more important (Callaghan and Jonasson 1995). In the high arctic, temperature seems to operate directly on the vegetation rather than through soils processes, at least over the first five years of experimentation. CO; enrichment has little dFecl on plant growth in the arctic or alpine regions in the short term, perhaps because other factors more strongly restrict growth (Tissue and Oechel 1987; Grulke et et!. 1990; Callaghan and Jonasson 1995; Körner et at. 1995)

In both the arctic and alpine regions, human impact will be the greatest source of environmental change in the coming decades (Young and Chapin 1995). Although there have been substantial direct impacts associated with resource extraction in the arctic, changes associated with arctic haze, nitrogen deposition, and altered fire and grazing regimes may have greater impact on arctic biodiversity and ecosystem processes. For example, air pollution from industrial Europe has dramatic effects on the species composition and ecosystem effects of arctic mosses and on lake acidification. In the alpine regions, tourist, agricultural, forestry and hydroelectric developments have causcd the most severe impacts. Human impacts depend strongly on economic and social forces outside the arctic and alpine areas, and therefore feedback loops involving people are relatively insensitive to changes within these ecosystems. People directly influence biodiversity by harvesting targeted species of plants and animals. In some areas this harvest threatens species because of changes in local social institutions and exogenous forces such as demand for animal products.

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