Species Diversity

For the most part lakes in the Arctic (1) are relatively depauperate with respect to biodiversity compared to temperate and tropical zones, (2) are populated mainly by cold adapted or cold tolerant species, and (3) have low productivity and simplified food webs. This relative impoverishment increases moving further north or to higher elevation, and it increases moving from lower to higher trophic levels in the food web (Table 1). That is, the ratio of arctic diversity to temperate or tropical diversity is much lower for fish than for algae (early indications suggest that this pattern holds for bacterial diversity as well). Additionally, the species-area curves in arctic regions reach an asymptote very quickly compared to regions further south. To illustrate these points consider the well-studied lakes near the NSF-LTER site of Toolik Lake on the North Slope of Alaska (low arctic, 68.38° N). There are ^150 species of phytoplankton in Toolik and surrounding lakes, and perhaps 50-100% more could be found in a similar number of lakes in one area of the temperate zone. Strikingly, most of the phytoplankton species are extremely small (nanoplankton <20 mm in size), with very few larger species that dominate diversity in lakes of regions further south. For the zooplankton, surveys of 100s of lakes near Toolik and extending northward to the Arctic Ocean (70.3° N) found a total of 13 macrozooplankton and ~15 rotifer species, whereas 40 and 60 species, respectively, were found in just 39 similar lakes in tropical Cameroon. The greatest contrast occurs at the top of the food chain where only 6 fish species exist in and around Toolik (and perhaps only 10 genera and 15 freshwater species in lakes throughout the Arctic), and yet the U.S. state of Michigan alone has 157 species of fish. Of course there is some variability in these patterns (e.g., 33 rotifer species occur in only 5 small ponds on Ellesmere Island, 78.37° N), and few detailed taxonomic surveys of arctic lakes exist, especially for phytoplankton. Overall the generalities of patterns in diversity are quite strong, and are consistent with the explanation that while autotrophs or decomposers may generate sufficient energy to fill many available niches in lakes, the minimal energy available to subsequent trophic levels severely truncates the species richness.

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