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a Terrestrial life forms include plant and animal species as well as bacteria, which are not described in this table. Based on Block (1984). b Uncertain number. c Introduced species.

FIGURE 9.2 Illustration of the Antarctic marine ecosystem, which exists south of the Antarctic Convergence (Figs. 7.8 and 8.3), showing interactions among representative species groups at different trophic levels (Table 9.2; Fig. 9.7): from the phytoplankton in the sea-ice zone; to the zooplankton assemblages with krill (Euphausia superba) as the keystone species (dashed open arrows); to the whales, seals, birds, fish, and squid that consume the krill and other zooplankton species (solid arrows); to the killer whales (Orcinus orca) and leopard seals (Hydrurga leptonyx) that are the apex predators (open arrows). Ultimately, organic production flows from the pelagic food web onto the sea floor, where it is consumed and recycled by diverse assemblages of benthic species (large solid arrows). Humankind interrupts the flow of energy in the Antarctic marine ecosystem by permanently removing biomass from various trophic levels (large open arrow). Modified from Berkman (1992).

FIGURE 9.2 Illustration of the Antarctic marine ecosystem, which exists south of the Antarctic Convergence (Figs. 7.8 and 8.3), showing interactions among representative species groups at different trophic levels (Table 9.2; Fig. 9.7): from the phytoplankton in the sea-ice zone; to the zooplankton assemblages with krill (Euphausia superba) as the keystone species (dashed open arrows); to the whales, seals, birds, fish, and squid that consume the krill and other zooplankton species (solid arrows); to the killer whales (Orcinus orca) and leopard seals (Hydrurga leptonyx) that are the apex predators (open arrows). Ultimately, organic production flows from the pelagic food web onto the sea floor, where it is consumed and recycled by diverse assemblages of benthic species (large solid arrows). Humankind interrupts the flow of energy in the Antarctic marine ecosystem by permanently removing biomass from various trophic levels (large open arrow). Modified from Berkman (1992).

Antarctic terrestrial animals also include protozoans, rotifers, tardigrades, and nematodes, which live with the arthropods in impoverished soils as well as on incidental organic materials, exposed rocks, and ice surfaces. In permanently ice-covered lakes, planktonic animals (zooplankton) are dominated by rotifers with nematodes, tardigrades, and the occasional platyhelminth or annelid worm (Table 9.3). There even are a few planktonic crustaceans, including the copepod Para-broteas sarsi, which is the top predator in Antarctic lakes. In addition, Antarctic lakes contain benthic species, including bacteria, mosses, and green algae, as well

Vostok is that primitive bacteria—with genetic fingerprints from DNA strands— have been discovered from the Vostok ice core (Fig. 7.4) in ''lake ice'' several hundred meters above the lake. Planning is underway to create an uncontaminated new hole with novel technologies, including ''cryobots,'' to enter Lake Vostok and remotely sample this ecosystem as an analog for life in icy environments elsewhere in our solar system.

Antarctic marine and terrestrial ecosystems are classic contrasts. Despite the freezing temperatures and extreme seasonality, Antarctic marine life thrives in one of the richest ecosystems in the ocean (Table 9.2, Fig. 9.2). Conversely, Antarctic terrestrial life is the most impoverished of any continent, with only a handful of species eking out a sparse existence under environmental extremes not unlike those on lunar or Martian landscapes (Table 9.3, Fig. 9.4). Together, these Antarctic biota reveal habitat limitations that generally control the production of biomass in the Earth system.

limiting factors

In circumstances where there are unlimited resources, populations tend to increase in a geometric fashion, with two individuals becoming four, then eight, then 16 and on upward (Fig. 9.5). In fact, as noted by Thomas Robert Malthus (17661834) in his 1798 Essay on the Principle of Population: ''It has been universally remarked that all new colonies settled in healthy countries, where there was plenty of room and food, have constantly increased with astonishing rapidity in their population.'' However, beyond the initial phase of colonization, some factor eventually limits population growth like a weak link in a chain. These limiting factors—which Malthus described for humans in terms of war, pestilence, famine, and ''convulsions of nature''—become progressively more severe as populations approach the carrying capacity of their habitat. In contrast to geometric growth, limited populations tend to level off and form logistic or S-shaped growth curves (Fig. 9.5).

Population growth in all cases depends on the balance among births, deaths, immigrations, and emigrations, which influences recruitment. Conceptually, the per capita recruitment rates (r) can be combined with the initial population sizes (N) and maximal population sizes at the carrying capacity of the habitat (K) to determine how the overall biomass or densities of population will change over time:

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