Protozoans Phyla In Antarctica And Arctica

When population sizes are nearly zero, for example after a volcanic eruption or other ''convulsion of nature,'' resources are effectively unutilized and there is an opportunity for geometric population growth. Conversely, as resources become limiting toward the carrying capacity of the habitat (when N approaches K), population growth rates become zero. If there are more individuals than the habitat can

FIGURE 9.4 Diagram of the principal species groups and their trophic linkages in the impoverished terrestrial ecosystems of Antarctica (Table 9.3; Fig. 9.7). Organic materials and nutrients are added through seal and bird rookeries in some Antarctic coastal areas. Antarctic terrestrial taxa shown are with their species name, phylum and adult size in millimeters (mm), as available: a, Hypsibius asper (Tardigrada, 0.3 mm); b, Teratocephalus tilbrooki (Nematoda); c, Astromoeba radiosa (Protozoa, 0.02 mm); d, Belgica antarctica (Arthropoda, 1.5-2.5 mm); e, Maudheimia petronia (Arthro-poda, 0.6 mm); f, Stereotydeus delicatus (Arthropoda, 0.4 mm); g, Calothrix parietina (Cyanophy-ceae, 0.01 mm); h, Chlorophyta (green algae); i, Cyanophyceae (blue-green algae/bacteria); and j, Cyanophyceae (blue-green algae/bacteria). Modified from Greene et al. (1969) with species from Block (1984) and Vincent (1988).

FIGURE 9.4 Diagram of the principal species groups and their trophic linkages in the impoverished terrestrial ecosystems of Antarctica (Table 9.3; Fig. 9.7). Organic materials and nutrients are added through seal and bird rookeries in some Antarctic coastal areas. Antarctic terrestrial taxa shown are with their species name, phylum and adult size in millimeters (mm), as available: a, Hypsibius asper (Tardigrada, 0.3 mm); b, Teratocephalus tilbrooki (Nematoda); c, Astromoeba radiosa (Protozoa, 0.02 mm); d, Belgica antarctica (Arthropoda, 1.5-2.5 mm); e, Maudheimia petronia (Arthro-poda, 0.6 mm); f, Stereotydeus delicatus (Arthropoda, 0.4 mm); g, Calothrix parietina (Cyanophy-ceae, 0.01 mm); h, Chlorophyta (green algae); i, Cyanophyceae (blue-green algae/bacteria); and j, Cyanophyceae (blue-green algae/bacteria). Modified from Greene et al. (1969) with species from Block (1984) and Vincent (1988).

in its liquid phase because of the extreme pressures at depths below 3000 meters, there are nearly 70 lakes. The largest of these ancient and yet unexplored ecosystems (each of which may be several million years old) is Lake Vostok with a surface area around 14,000 square kilometers and water depths greater than 600 meters—placing it among the 15 largest lakes on Earth. The intrigue of Lake

FIGURE 9.5 Population growth with unlimited resources is geometric and solely dependent on the per capita recruitment rate of new individuals into the population [Eq. (9.1)]. Maximum population growth rates occur at 50% of their carrying capacity. Resources in most habitats, however, are limited, with population growth becoming logistic or S-shaped as the carrying capacity of the habitat is approached. Ultimately, populations will decline or fluctuate as they approach the carrying capacity or over utilize the resources in their habitat beyond sustainable limits.

FIGURE 9.5 Population growth with unlimited resources is geometric and solely dependent on the per capita recruitment rate of new individuals into the population [Eq. (9.1)]. Maximum population growth rates occur at 50% of their carrying capacity. Resources in most habitats, however, are limited, with population growth becoming logistic or S-shaped as the carrying capacity of the habitat is approached. Ultimately, populations will decline or fluctuate as they approach the carrying capacity or over utilize the resources in their habitat beyond sustainable limits.

support (when N exceeds K), populations decline. In addition, for logistic growth, the maximum population growth rate occurs at a population size that equals 50% of its carrying capacity.

In the extremely seasonal polar ecosystems, light is a limiting resource for photosynthesis (Fig. 7.1). In general, planktonic plants (phytoplankton) float in the water at depths where there is sufficient sunlight for photosynthesis. This light corridor near the surface of aquatic ecosystems, called the euphotic zone, can extend several hundred meters in clear oceanic areas and only centimeters in turbid coastal areas (Fig. 8.7). In most water bodies, the lower limit of the euphotic zone is considered to be the depth where 1% of the incident solar radiation penetrates from the surface.

In the ice-covered marine and terrestrial ecosystems around Antarctica, however, the euphotic zone extends to depths where less than 0.1% of the incident radiation penetrates because aquatic algae have become adapted with specialized chlorophyll and complexes of accessory pigments. The distinct absorption spectra (e.g., Fig. 8.8b) of these pigments also enable Antarctic algae to utilize more deeply penetrating colors of light (Fig. 8.7). When these qualities and quantities of light become limiting—either at depths below the euphotic zone or as winter approaches—primary production ceases in polar ecosystems.

In many aquatic ecosystems, even with unlimited light, algal growth is limited by nutrients. Among the major nutrients, nitrogen and phosphorus contribute most to the utilization of carbon by photosynthetic species. Generally, the ratio of carbon, nitrogen, and phosphorus in the sea (C:N:P = 106:16:1)—known as the ''Redfield ratio'' after the 20th-century American scientist Alfred Redfield (1890-1983)—provides a useful baseline for determining when these nutrients become limiting. Silicon is another important nutrient because it is an essential ingredient for the pillbox-shaped frustules of diatoms, which are the most abundant algal forms in the Antarctic marine ecosystem. There also are micronutrients, notably iron, that may limit primary production even when the major nutrients are abundant.

Habitats with scarce nutrients, where plants grow poorly, are oligotrophic (scant nourishment), whereas habitats with abundant nutrients that stimulate plant production are considered to be eutrophic (good nourishment). For example, permanently ice-covered lakes on Antarctica tend to be oligotrophic because of limited nutrient inflows, unlike eutrophic areas in the Antarctic marine ecosystem near the Antarctic Divergence (Fig. 7.8) where nutrients are continually upwell-ing. Consequently, the amount of food available for larger animals is limited in Antarctic lakes compared to the adjacent marine ecosystem (Figs. 9.3 and 9.4). Outside of these aquatic environments, water becomes a limiting resource.

Resources also can become limiting as individuals compete with other organisms, either within their own species (intraspecific) or between other species (interspecific). In either case, the evolutionary outcome is to reduce competition so that organisms will have more resources available for their own production. As concisely stated by Hardin (1960): ''Complete competitors cannot coexist.'' This statement implies that when competition is complete, organisms will be unable to coexist because some will dominate at the exclusion of others. Conversely, when organisms coexist, competition is minimal because the necessary resource is either unlimited or partitioned in some way among the organisms.

How do organisms partition limited resources within and between species as well as over time and space?

An example of resource partitioning exists among competing marine mammals feeding in the Southern Ocean (Figs. 9.3 and 9.6). These mammals include seven species of baleen whales (suborder Mysticeti), such as the blue whale, which have large parallel comblike arrays in their mouth for sieving their prey from the seawater. There also are eight species of toothed whales (suborder Odon-toceti) that occur south of the Antarctic Convergence. Toothed species include the killer whale (Orcinus orca) and the ''great white leviathan'' of Herman Melville's Moby-Dick—the sperm whale (Physeter macrocephalus), which has lengths exceeding 18 meters.

There also are six seal species in the Antarctic marine ecosystem from among the 36 on Earth (Fig. 9.2). The harem-forming southern elephant seal (Mirounga leonina) and Antarctic fur seal (Arctocephalus gazella) both colonize exposed coastal areas around the continental margin. In contrast, the crabeater seal (Lobo-don carcinophaga), the predatory leopard seal (Hydrurga leptonyx), and the rare

Distribution

Minke

Blue

Humpback tn

Pigmy blue

Right

Sperm

Weddell Leopard Crabeater Ross Elephant Fur. gazella tropkalis

Food 10-20mm

20-30mm

Euphausia vallentini

Continent

Shelf

Distribution

Food 10-20mm

20-30mm

Minke

Blue

Humpback

Pigmy blue

Right

Sperm

Euphausia vallentini

North

Shelf

North

fish

FIGURE 9.6 Resource partitioning among the eight whale and six seal species south of the Antarctic Convergence (Figs. 8.3), in the Antarctic marine ecosystem (Fig. 9.2). These marine mammals adjust their zonal distributions and migration patterns in relation to the seasonal sea-ice cycle to reduce their competitive overlap (Fig. 8.1). These species also reduce their competition for food by consuming different size classes of krill (Euphausia superba) and other common prey species. Modified from Laws (1977).

fish

Ross seal (Ommatophoca rossii) are found among pack-ice flows and intervening open-ocean areas. The Weddell seal (Leptonychotes weddelli) is uniquely adapted to coastal habitats that are completely covered by sea ice, where they submerge for periods longer than an hour, dive to depths below 500 meters, and then return by echolocating breathing holes (Plate 7).

Most of these marine mammals feed principally on a small shrimplike herbivore called krill (Euphausia superba), which drifts in the plankton. The importance of krill in the Antarctic marine ecosystem derives from its circumpolar distribution and dense aggregations, or swarms, which can contain more than 50,000 individuals in a cubic meter of water with biomasses exceeding 15 kilograms. Viewed in three dimensions, each swarm has a relatively narrow edge, which enhances oxygen diffusion into the mass. The mechanisms behind this schooling behavior are not fully understood; however, it is known that individuals follow pressure changes associated with the wake of preceding individuals. The other species of Euphausia in the Antarctic marine ecosystem do not form dense aggregations, and in the absence of krill, the dominant zooplankton are copepods (primarily Calananoides acutus, Calanuspropinquus, and Rhincalanus gigas).

Baleen whales minimize their interspecific competition for krill by occupying different zones south of the Antarctic Convergence and selecting different sizes of krill prey (Fig. 9.6). The baleen whales also stagger their southward migrations, with the blue whales arriving first, followed by the fin (Balaenoptera physalus), humpback (Megaptera novaeangliae), and sei (Balaenoptera borealis) whales. In addition, there are intraspecific differences in whale migrations by sex, age, and reproductive status. This timing and extent of annual whale migrations into the Antarctic marine ecosystem corresponds with the stored energy in their blubber, which ranges from 27% of the body mass in the blue whale to 17% in the sei whale.

The crabeater seal, with its specially adapted sievelike arrangement of teeth for filtering zooplankton, also is a principal krill predator. However, because crabeater seals feed on krill in the pack ice, they have minimal overlap with the baleen whales, which are feeding northward of the retreating ice edge (Fig. 9.6). These adaptations and the extreme krill abundances explain why the crabeater seal has a population size estimated between 15 and 30 million, accounting for about half of the global seal population and more than two-thirds of its biomass.

Squid also are important prey for mammals in the Antarctic marine ecosystem, especially among sperm whales and elephant seals. There are approximately 70 squid species that inhabit the Southern Ocean, generally below the euphotic zone and commonly deeper than 2000 meters. Many of these cephalopod mollusc species have mantle sacs that are less than 20 centimeters in length, but a few grow to very large sizes. In particular, Mesonychoteuthis hamiltoni can have total lengths exceeding 4 meters with weights greater than 150 kilograms. Competition for these squid as prey is minimized by elephant seals having their highest feeding activities during the austral winter, whereas the sperm whales mainly feed during the summer around Antarctica.

In addition, Antarctic marine mammals consume fish. Among the 270 Antarctic fish species, nearly 75% are bottom-dwelling. This benthic fish fauna generally occurs in coastal areas, whereas pelagic forms tend to occur at depths below 1000 meters. Moreover, the deep pelagic forms are dominated by 33 lanternfish species in the family Myctophidae, which is widely known for echoing human-generated sound pulses that distinguish ''deep scattering layers'' throughout the ocean.

Along with the marine mammals, penguins also compete for the krill, squid, and fish in the Antarctic marine ecosystem (Fig. 9.2). Among these flightless sea-birds—which evolved exclusively in the Southern Hemisphere since the early Ce-nozoic—7 of the 18 extant species breed south of the Antarctic Convergence. King (Aptenodytes patagonicus) and rockhopper (Eudyptes crestatus) penguins breed on the peripheral islands surrounding Antarctica. Chinstrap (Pygoscelisant-arctica), gentoo (Pygoscelis papua), and macaroni (Eudyptes chrysolophus) penguins breed in the maritime zone along the Antarctic Peninsula as well as on the peripheral islands. The Adelie (Pygoscelis adeliae) and emperor (Aptenodytes for-steri) penguins breed in the Antarctic Peninsula region as well as in the continental zone south of 65° south (Fig. 9.1, Plate 7).

An additional 31 species of flying seabirds also breed and feed south of the Antarctic Convergence. These include 4 albatross and 20 smaller petrel species as well as 7 species among the gulls, skuas, terns, and cormorants. Many of the smaller seabirds dwell in burrows that they excavate in loose rocks and moss banks. The larger species, which are better insulated and less affected by predation from other seabirds, have surface nesting sites. Most of these seabirds travel annually into the Antarctic region during the summer sea-ice retreat, some of which migrate great distances, as with Wilson's storm petrel (Oceanities oceani-cus), which flies between the Arctic and Antarctic.

The top predators in the Antarctic marine ecosystem are the leopard seal and killer whale (Fig. 9.2). Leopard seals have pointed front teeth for grasping and tearing their prey and can grow to nearly 4 meters in length with weights over half a ton. Killer whales are much larger with lengths over 9 meters and weights over 7 tons. Both of these species prey on other large vertebrates, such as penguins and seals as well as fish and squid. Killer whales, which often hunt cooperatively in pods, also are known to feed on other whales. Overlap among these two top predators, however, is minimal since the killer whale is effectively unchallenged as the dominant predator at the apex of the Antarctic marine ecosystem (Fig. 9.2).

pyramid of life

Predators and prey transfer biomass through the Earth system in a cascade of biological activities. At the base of this organic (carbon-based) pyramid are the autotrophic (self-nourishment) organisms, which start the flow of food energy. These primary producers include plants, which photosynthesize basic sugars with light energy, as well as bacteria, which chemosynthesize sugars using chemical energy from sources such as the 350°C hydrothermal vents in the deep sea [Eq. (1.1)]. Above the primary producers, at all subsequent trophic levels, are the heterotrophic (different nourishment) organisms, which utilize organic matter synthesized by other organisms, up to the apex predator at the top of the trophic pyramid (Fig. 9.7). The shape of this pyramid indicates that available food energy, either in terms of biomass or the number of organisms, progressively decreases toward the higher trophic levels.

Why are production and biomass fundamental concepts in interpreting ecological interactions?

At each trophic level above the primary producers (Fig. 9.7), food is consumed and allocated by animals for various life-sustaining functions (Fig. 9.8). Initially, much of the food energy is used simply for metabolism and other maintenance activities of the animal. Food energy also is lost through excretion with only a small amount of energy available for animals to increase in size or number. For example, we may only gain several ounces in weight after eating a 1-pound hamburger. Some of this assimilated food energy then can be allocated toward growth and reproduction, which together contribute to the biomass of the population.

The proportion of food that is converted into biomass reflects the ecological efficiency of transferring energy between trophic levels:

Ecological efficiency = Production*°phic level n (9.2)

Productiontrophic level n-1

Apex Predator

Apex Predator

FIGURE 9.7 Trophic pyramid showing the feeding relationships among species in ecosystems (Figs. 9.3 and 9.4). At the base of the pyramid are the primary producers that synthesize carbohydrate energy [Eq. (1.1)], which is subsequently consumed by herbivores and various levels of carnivores up to the apex predator. In this example the ecological efficiency [Eq. (9.2)] for calculating the production at different trophic levels [Eq. (9.3)] is 10%.

FIGURE 9.7 Trophic pyramid showing the feeding relationships among species in ecosystems (Figs. 9.3 and 9.4). At the base of the pyramid are the primary producers that synthesize carbohydrate energy [Eq. (1.1)], which is subsequently consumed by herbivores and various levels of carnivores up to the apex predator. In this example the ecological efficiency [Eq. (9.2)] for calculating the production at different trophic levels [Eq. (9.3)] is 10%.

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