Arctic species diversity
Arctic species diversity is generally low, and decreases from the boreal forests to the polar deserts of the extreme north. Only about three percent (approximately 5900 species) of the global flora (excluding algae ) occur in the Arctic north of the treeline. However, primitive plant species (mosses and lichens) are particularly abundant. Although the number of plant species in the Arctic is low in general, individual communities of small arctic plants have a diversity similar to or higher than those of boreal and temperate zones: there can be up to 25 species per square decimeter. Latitudinal gradients suggest that arctic plant diversity is sensitive to climate , and species number is least sensitive to temperature near the southern margin of the tundra. The temperature gradient that has such a strong influence on species diversity occurs over much shorter distances in the Arctic than in other biomes.
The diversity of arctic animals beyond the latitudinal treeline (approximately 6000 species) is nearly twice as great as that of vascular plants and bryophytes. As with plants, the arctic fauna accounts for about 3% of the global total, and in general, primitive groups (e.g., springtails) are better represented in the Arctic than are advanced groups such as beetles. In general, the decline in animal species with increasing latitude is more pronounced than that of plants (frequently greater than 2.5-fold). An important consequence of the decline in numbers of species with increasing latitude is an increase in dominance. "Super-dominant" plant and animal species occupy a wide range of habitats, and generally have major effects on ecosystem processes.
Microbial organisms are more difficult to enumerate. Arctic soils contain large reserves of microbial biomass, although diversity of all groups of soil microorganisms is lower in the Arctic than further south. Many common bacteria and fungi are rare or absent in tundra areas. As with plants and animals, there are large reductions in numbers of microbial species with increasing latitude, and increasing dominance of the species that occur.
|Table 1. Biodiversity estimates in terms of species richness (number of species) for the Arctic north of the latitudinal treeline and percentage of world biota.|
|Group||Number of species||% of world biota||Group||Number of species||% of world biota||Group||Number of species||% of world biota|
|^Amphibians and reptiles (7 species), centipedes (10 species), terrestrial mollusks (3 species), oligochaetes (earthworms and enchytraeids) (70 species), and nematodes (~500 species).|
About 3% (~5,900 species) of the global flora occurs in the Arctic as defined in this chapter (0.7% of the angiosperms (flowering plants), 1.6% of the gymnosperms (cone-bearing plants), 6.6% of the bryophytes, and 11% of the lichens) (Table 1). There are more species of primitive taxa (cryptogams), that is, mosses, liverworts, lichens, and algae, in the Arctic than of vascular plants. Less than half (about 1,800) of arctic plant species are vascular plants. There are about 1,500 species of vascular plants common to both Eurasia and North America. Similar numbers of non-vascular plants probably occur in the Arctic on both continents, although their diversity has been less thoroughly documented. In the Russian Arctic, for example, 735 bryophyte species (530 mosses and 205 liverworts) and 1,078 lichen species have been recorded. In general, the North American and Eurasian Arctic are similar to one another in their numbers of vascular and non-vascular plant species, of which a large proportion (about 80%) of vascular plant species occurs on at least two continents. An even larger proportion (90%) of bryophyte species occurs in both the North American and Eurasian Arctic.
About 40% of arctic vascular plants (and a much higher percentage of mosses and lichens) are basically boreal species that now barely penetrate the Arctic. They currently occur close to the treeline or along large rivers that connect the subarctic with the Arctic. These boreal species within the Arctic will probably be the primary boreal colonizers of the Arctic in the event of continued warming. Polyzonal (distributed in several zones), arctoboreal (in taiga and tundra zones), and hypoarctic (in the northern taiga and southern part of the tundra zone) species have even greater potential to widen their distribution and increase their abundance in a changing climate. The majority of cryptogams have wide distributions throughout the Arctic. Such species are likely to survive a changing climate, although their abundance is likely to be reduced.
In contrast to the low diversity of the arctic flora at the continental and regional scales, individual communities (100 square-meter [m2] plots) within the Arctic have a diversity similar to or higher than those of boreal and temperate zones. These diversities are highest in continental parts of the Arctic such as the Taymir Peninsula of Russia, where there are about 150 species of plants (vascular plants, lichens, and mosses) per 100 m2 plot, 40 to 50 species per square meter, and up to 25 species per square decimeter.
The diversity of arctic terrestrial animals beyond the latitudinal treeline (6,000 species) is nearly twice as great as that of vascular plants and bryophytes (Table 1). As with plants, the arctic fauna accounts for about 2% of the global total, and, in general, primitive groups (e.g., springtails, 6% of the global total) are better represented in the Arctic than are advanced groups such as beetles (0.1% of the global total).There are about 322 species of vertebrates in the Arctic, including approximately 75 species of mammals, 240 species of birds, 2 species of reptiles, and 5 species of amphibians. Insects are the most diverse group of arctic animals; of approximately 3,300 arctic species, about 50% are Diptera, 10% are beetles (Coleoptera), 10% are butterflies (Lepidoptera), and 10% are Hymenoptera. The Arctic has about 300 species of spiders (Arachnida), 700 species of mites (Acarina), 400 species of springtails (Collembola), 500 species of nematodes, 70 species of oligochaetes (of which most are Enchytraeidae), only a few mollusks, and an unknown number of protozoan species.
In the Arctic as defined by Conservation of Arctic Flora and Fauna (CAFF) (which includes forested areas), some 450 species of birds have been recorded breeding. Some species extend their more southern breeding areas only marginally into the Arctic. Others are not migratory and stay in the Arctic throughout the year. About 280 species have their main breeding distribution in the Arctic and migrate regularly. An estimate of the total number of individuals involved is not possible, as too little is known about the population size of most species or their arctic proportion. However, a rough first approximation suggests that there are at least several hundred million birds. Population sizes of water birds are better known and the Arctic is of particular importance for most water birds, such as divers, geese, and waders. Twelve goose species breed in the Arctic, eleven almost entirely and eight exclusively. These comprise about 8.3 million birds. The total number of arctic-breeding sandpipers (24 species) exceeds 17.5 million birds. The total number of water birds, including other wader species, divers, swans, ducks, and gulls is estimated to be between 85 and 100 million birds.
Microbial organisms are critically important for the functioning of ecosystems, but are difficult to study and are poorly understood compared with other species. However, the International Biological Programme (IBP; 1964–1974) significantly advanced understanding of arctic microorganisms, compared with those of other biomes, when an inventory of microbial communities was undertaken in the tundra. At the beginning of the 21st century, the knowledge of microbial diversity in the tundra remains little better than 30 to 40 years ago, and recent outstanding progress in molecular microbial ecology has rarely been applied to arctic terrestrial studies.
Presently there are 5,000 to 6,000 named bacterial species globally and about the same number of fungi, compared with more than one million named plant and animal species. Some scientists have interpreted this difference to mean that the bacteria are not particularly diverse. However, there are several reasons to believe that the apparent limited diversity of microbes is an artifact.
Recent progress in molecular biology and genetics has revolutionized bacterial classification and the understanding of microbial phylogeny ("family trees") and biodiversity in general. The DNA sequencing technique has reorganized bacterial classification and brought order to microbial taxonomy. Moreover, the microbial inventory can now be done without isolation and cultivation of the dominant microorganisms, because it is enough to extract the total community DNA from the soil and amplify, clone, and sequence the individual genes. This culture-independent approach has been applied occasionally for analysis of microbial communities in subarctic and arctic soils, most often to study relatively simple communities in hot springs, subsoils, and contaminated aquifers. Analysis of Siberian subsurface permafrost samples resulted in the formation of a clone library of 150 clones which has been separated into three main groups of Eubacteria. From 150 clones so far analyzed, the authors have identified several known species (Arthrobacter, Clostridium, and Pseudomonas), while the most abundant phylotypes were represented by completely unknown species closely affiliated with iron-oxidizing bacteria.
Another area of intensive application of molecular tools was northern wetlands (cold, oligotrophic (nutrient poor), and usually acidic habitats) related to the methane cycle. The most challenging tasks were to determine what particular microbial organisms are responsible for the generation and uptake of methane (so-called methanogens and methanotrophs) in these northern ecosystems and what their reaction might be to warming of arctic soils. It was found that most of the boreal and subarctic wetlands contain a wide diversity of methanogens and methanotrophs, most of them distantly related to known species. Only recently, some of these obscure microbes were obtained in pure culture or stable consortia. The novel microbes of methane cycles are extreme oligotrophic species that evolved to function in media with very low concentrations of mineral nutrients. Taxonomically, these methanogens form new species, genera, and even families within the Archaea domain . The acidophilic methanotrophs form two new genera: Methylocapsa and Methylocella, the latter affiliating with heterotrophic Beijerinckia indica.
A DNA-based technique allows determination of the upper limit for variation of microbial diversity in the Arctic as compared with other natural ecosystems, that is, how many species (both cultured and unculturable) soils contain. This technique is called DNA reassociation (how quickly the hybrid double helix is formed from denatured single-stranded DNA).
Arctic polar desert and tundra contain considerable microbial diversity, comparable with boreal forest soils and much higher than arable soils. Although extreme environmental conditions restrain the metabolic activity of arctic microbes, they preserve huge potential that is likely to display the same activity as boreal analogues during climate warming.
There is a much higher degree of genomic diversity in prokaryotic communities (prokaryotes such as cyanobacteria have a simple arrangement of genetic material whereas eukaryotes such as microalgae have genetic material arranged in a more advanced way, in that the DNA is linear and forms a number of distinct chromosomes) of heterogeneous habitats (virgin soils, pristine sediments) as compared with more homogeneous samples: the DNA diversity found in 30 to 100 cc of heterogeneous samples corresponds to about 104 different genomes, while in pond water and arable soils the number of genomes decreases to 100 to 102. Based on extrapolation and taking into account that listings of species can significantly overlap for microbial communities of different soils, a rough estimate is that there could be from 104 to 109 prokaryotic species globally.
The conventional inventory approach based on cultivation suggests that at present in the Arctic, it is possible to identify in any particular soil no more than 100 prokaryotic species from the potential of 1,000 to 3,000 "genome equivalents" (Table 2) and no more than 2,000 species of eukaryotes. In the broadly defined Russian Arctic, 1,750 named fungi species (not including yeast and soil fungi) have been identified. About 350 of these are macromycetes. However, the number of fungi species in the Arctic proper is 20 to 30% less, but these data are far from complete. The Arctic has fewer species of bacteria, fungi, and algae than other major biomes; actinomycetes are rare or absent in most tundra sites. While most major phyla of microflora are represented in tundra ecosystems, many species and genera that are common elsewhere, even in subarctic ecosystems, are rare or absent in tundra. Gram-positive bacteria, including gram-positive spore forms, are absent or rare in most tundra sites. Arthrobacter and Bacillus can rarely be isolated and then only from drier areas. Azotobacter, the freeliving nitrogen-fixing bacterium, is extremely rare in tundra, and the moderate rate of nitrogen fixation observed in situ is mainly due to the activity of cyanobacteria. Sulfur-oxidizing bacteria are also reported to be rare or absent. Even using enrichment techniques, Bunnell et al. rarely found chemoautotrophic sulfur-oxidizing bacteria. Photosynthetic sulfur bacteria were not found in any IBP tundra biome sites and were reported in only one subarctic site, although they are common in coastal areas of the west and south coasts of Hudson Bay. Sulfur-reducing bacteria, while not abundant in tundra sites, have been reported in sites in the Arctic and Antarctic. Iron-oxidizing bacteria are very rare in tundra sites. Despite ample iron substrate in tundra ponds and soils, chemoautotrophic ferrous iron oxidizers were not found in IBP tundra sites. In contrast, methanotrophic and methanogenic bacteria appear to be widespread in tundra areas.
As with bacteria, many generally common fungi are conspicuous by their rare occurrence or absence in tundra areas. Aspergillus, Alternaria, Botrytis, Fusarium, and Rhizopus simply do not occur and even Penicillium are rare. Yeasts can be isolated readily but there is very low species diversity in culture media. Only three different species were reported for Point Barrow tundra. Aquatic fungi show high diversity, especially Chytridiales and Saprolegniales. However they may not be endemic, and reflect the annual migration into the Arctic of many avian species, especially waterfowl. The so-called higher fungi, Basidiomycetes and Ascomycetes, also have low diversity. They are reduced in the Arctic to 17 families, 30 genera, and about 100 species. In comparison, subarctic and temperate regions contain at least 50 families, not less than 300 genera, and anywhere up to 1200 species. Mycorrhizal symbionts on tundra plants are common. Arbuscular, ecto-, ericoid, arbutoid, and orchid mycorrhizal fungi are associated with plants in arctic ecosystems. The ectomycorrhizal symbionts are important as they form mycorrhizal associations with Betula, Larix, Pinus, Salix, Dryas, Cassiope, Polygonum, and Kobresia. Based on fungal fruitbodies, Borgen et al. estimate that there are 238 ectomycorrhizal fungal species in Greenland, which may increase to around 250 out of a total of 855 basidiomyceteous fungi in Greenland when some large fungal genera such as Cortinarius and Inocybe have been revised. With the exception of Eriophorum spp., Flanagan found endotrophic Arbuscula-like mycorrhizae on all ten graminoid plants examined. The number of fungal species involved in other mycorrhizal symbioses is not clear.
|Table 2. Microbial genome size in the Arctic as compared with other habitats.|
|DNA source||Number of cells per cm3||Community genome complexity (base pairs)||Genome equivalents|
|Arctic desert (Svalbard)||7.5 x 109||5–10 x 109||1,200–2,500|
|Tundra soil (Norway)||37 x 109||5 x 109||1,200|
|Boreal forest soil||4.8 x 109||25 x 109||6,000|
|Forest soil, cultivated prokaryotes||0.014 x 109||0.14 x 109||35|
|Pasture soil||18 x 109||15–35 x 109||3,500–8,800|
|Arable soil||21 x 109||0.57–1.4 x 109||140–350|
|Salt-crystallizing pond, 22% salinity||0.06 x 109||0.029 x 109||7|
|^the number of nucleotides in each strand in the DNA molecule; ^measure of diversity specified at a molecular level.|
Tundra algae exhibit the same degree of reduction in species diversity seen among the fungi and bacteria, which documents a diversity much reduced from that of the microflora of temperate regions. Cyanobacteria and microalgae are among the oldest (in evolutionary terms) and simplest forms of life on the planet that can photosynthesize. Unicellular and filamentous photosynthetic cyanobacteria and microalgae are among the main primary colonizers adapted to conditions of the arctic terrestrial environment. They are widespread in all terrestrial and shallow wetland habitats and frequently produce visible biomass. Terrestrial photosynthetic microorganisms colonize mainly the surface and subsurface of the soil and create the crust. Algal communities in shallow flowing or static wetlands produce mats or mucilaginous clusters that float in the water but are attached to rocks underneath. Terrestrial and wetland habitats represent a unique mosaic of cyanobacteria and algae communities that occur up to the highest and lowest possible latitudes and altitudes as long as water (liquid or vapor) is available for some time during the year. The arctic soil and wetland microflora is composed mainly of species from Cyanobacteria, Chrysophyceae, Xanthophyceae, Bacillariophyceae, Chlorophyceae, Charophyceae, Ulvophyceae, and Zygnemaphyceae. A wide range of species diversity (between 53 and 150 to 160 species) has been reported from various arctic sites.