The Antarctic Ecosystem: Food Web & Food Chains
- Biogeographic Background & the Physical Environment of the Antarctic Ecosystem
- Southern Ocean Marine Ecosystem: The Food Web of the Antarctic Ocean
- The Terrestrial Ecosystem of Antarctica
- Sub-Antarctic Terrestrial Ecosystem
- Climate Change Impacts on the Antarctic Ecosystem Food Chain
Cruise-goers to Antarctica experience some of the most astonishing and pristine land- and seascapes in the world—and some of the most incredible wildlife spectacles anywhere! That superlative physical environment and its inhabitant species—abundant in some areas and seasons, yet few and far between on its high, ice-locked Polar Plateau—come linked together as a unique ecosystem at the bottom of the world.
It’s worth considering (very briefly) the evolutionary context of today’s Antarctic ecosystem. Long linked to the other southern continents in the huge conjoined landmass of Gondwana—including during Gondwana’s roughly 150-million-year residency as part of the supercontinent Pangaea—Antarctica was fully separated via the restlessness of the planet’s plate tectonics between about 20 and 50 million years ago or so.
Heavily forested in Gondwana days, Antarctica became a harsher and harsher environment as it drifted southward into higher and higher latitudes, gradually accumulating its vast ice sheet as it developed a polar climate. Flowering trees, as represented by the southern beech (Nothofagus), may have lingered until as “recently” as about three million years ago.
By 15 or 20 million years ago, as Antarctica sailed poleward, the Southern Ocean developed its great circumpolar current, the West Wind Drift: the greatest ocean current in the world, driven by the fierce westerly winds of these latitudes and, like them, unimpeded by major land barriers. Its powerful surge, as well as the Antarctic Convergence (or Polar Front) marking the boundary between icy polar waters and the warmer ocean northward, helped isolate the White Continent as well as its associated marine realm.
Today, Antarctica is the coldest, the driest, the windiest, and the highest (in terms of average elevation) continent in the world. It’s also the greatest reservoir of terrestrial ice on the planet, with roughly 98 percent of its bedrock slathered in the stuff.
Severe as the Antarctic’s terra firma might be, its oceanic realm is one of the most productive on the planet. When it comes to the Antarctic’s biological vibrancy and diversity, the Southern Ocean encircling the White Continent is where it’s at.
Major zones of upwelling—where cold, nutrient-packed waters rise to the surface—are one fundamental reason for this productivity, as they help fuel huge concentrations of phytoplankton: the autotrophic, or energy-making, primary producers of the marine food web. The nearly daylong sunshine of the Antarctic summer is another, providing superabundant fuel for phytoplanktonic photosynthesis.
The vast blooms of phytoplankton in the Southern Ocean provide food for the tiny animals composing the zooplankton guild. These include such creatures as copepods and a number of species of krill. In terms of the latter, the most important is the Antarctic krill: an exceedingly numerous zooplanktonic crustacean that filters out free-floating diatoms and grazes on the algal film coating the underside of sea ice.
This krill grows to about the size of the human pinkie—small, sure, but good-sized compared to most zooplankton. The biomass of Antarctic krill probably exceeds that of the global human population.
To have such abundant, decently sized zooplankton available ensures that a wide variety of good-sized secondary consumers, from toothfish, seabirds, and penguins to seals and whales, can glean the productive energy of the Antarctic marine ecosystem quite close to its source. (Energy is dramatically diminished as it’s transferred up each link of the food chain.)
At the top of the marine Antarctic food chain? That would be two very impressive predators, the orca (or killer) whale and the leopard seal. Orcas exist in the Southern Ocean in several forms with different morphologies and dietary preferences. The largest is the Type A or Antarctic orca, known for preying on minke whales (a smallish baleen whale common in these waters); other types target seals, penguins, or fish.
Leopard seals are large, formidably armed seals that, in addition to krill, squid, and fish, will actively hunt penguins (which they often snatch along the edge of the ice) and the pups and subadults of other seals. They, in turn, need fear only the orca.
The annual expansion and contraction of Antarctic sea ice in the Southern Ocean—a seasonal pattern that essentially sees the White Continent about double in effective size during the winter—is also a fundamental part of Antarctica’s marine environment.
The “pack-ice seals”—leopard, crabeater, Ross, and Weddell—partly rely on this sea ice as habitat for breeding, molting, and resting. The great emperor penguin famously breeds on pack-ice. An important winter and spring food for Antarctic krill, as already mentioned, is the algae that grows on the underside of sea ice. Melting pack ice (and icebergs) impact adjacent waters by releasing nutrients and lowering salinity.
The incredibly productive ecosystem of the Southern Ocean transports and transforms nutrients and chemicals along myriad pathways. Certain organisms play outsized roles in these biogeochemical cycles, including Antarctic krill themselves. Carbon dioxide absorbed into surface oceanic waters is taken up by phytoplankton, which use it to convert solar energy into nutrient energy via photosynthesis. Organic carbon is thus produced, which is then absorbed by krill feeding on the phytoplankton. The downward drift of krill poop, molted exoskeletons, and dead krill helps transfer large amounts of organic carbon down to the seafloor.
Down there on the seabed, Antarctic clams—impressively long-lived mollusks—are another important player in Southern Ocean biogeochemical cycling, absorbing carbon and otherwise helping shuttle nutrients between the water column and seafloor sediments.
The great whales that feast on krill in large numbers are also important components of these cycles. The so-called “whale pump” describes the process by which baleen whales absorb iron locked away in ingested krill, then excrete this essential element back into the ocean, helping to fuel phytoplankton growth for full-circle biological productivity.
By comparison to the Antarctic marine ecosystem, the terrestrial Antarctic food web is a much simpler and scantier construction. Very little ice-free land exists, so most organisms are excluded from inhabiting the vast majority of the White Continent. The combination of frigid cold and very low moisture—for most of the Antarctic continent is classified as a polar desert—limits biological diversity, growth, and productivity, and also means what relatively little exposed rock can be found weathers only very slowly into lean soil.
The demanding environment and limited energy available at the food web of Antarctica’s landmass make for only a few trophic (energy) levels and only small organisms, not least microbes.
Plants are represented by low-lying mosses and liverworts, with lichen—a symbiotic relationship between algae and fungi—being the most widespread lifeform. The largest land animals are invertebrates, which max out at the size of a midge. Lakes in ice-free areas in Antarctica support microbes, crustaceans, and other limited life.
(Certainly larger animals are found on the coastal margin of the continent, namely in the form of birds and pinnipeds, but these creatures are part of the marine food web, not the terrestrial one.)
Milder temperatures, a longer growing season, and a moister climate foster more productive terrestrial ecosystems in the sub-Antarctic realm around and to the near north of the Antarctic Convergence. Plantlife tends to be more diverse and larger, as in the tall tussock grasses that grow on such sub-Antarctic islands as South Georgia and Macquarie. Numerous sub-Antarctic islands support carpets of so-called “megaherbs,” which are comparatively big, often extravagantly flowered forbs that form a lush groundcover.
As the British Antarctic Survey notes, the greater productivity of sub-Antarctic islands is reflected by the success on some of them of such non-native land mammals as rats, cats, goats, and reindeer introduced (intentionally or not) by human beings.
While we don’t have the room to go into the subject in detail here, suffice it to say that climate change is already affecting the Antarctic ecosystem, and threatens to do so more radically in the coming years and decades.
Temperature and ice trends are not uniform in Antarctica, with major differences seen, for example, between parts of West Antarctica and East Antarctica. But the western Antarctic Peninsula, which has seen both air and sea temperatures on a steep increase, ranks among the most rapidly warming corners of the globe. Loss of sea ice and warming and acidifying waters are thought to be contributing to a decline in krill, and replacement in some areas of more ice-dependent penguin species (such as Adélie) by open-water counterparts (such as chinstraps).
Warming (and consequently less oxygen-rich) ocean water may also significantly impact the Antarctic clam, that sea-bottom mollusk so important in nutrient/energy cycling. While younger, smaller clams can move when they detect warmer and/or lower-oxygen waters, older, larger clams—the ones that breed—appear to be more sedentary and fixed in place, more likely to try to wait out such undesirable conditions. If said conditions become the norm with a warming climate, this may pose a real risk to the species.
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