Ecological Interactions

There are many kinds of ecological interactions, some of which have been noted above. Trophic, or energy flow, interactions start with primary production. Primary production comes about through the capture of the sun's energy by plant photosynthesis, which manufactures high-energy molecules such as sugars, starches, and oils from carbon dioxide and water. These same plants also capture chemicals other than carbon, hydrogen, and oxygen, notably nitrogen, sulfur, phosphorus, and various mineral elements by uptake from the soil. Compounds containing these elements, and the chemical energy stored by the plants as carbohydrates, are consumed by herbivores, saprophages (organisms feeding on already dead material), or carnivores. Thus, material and energy move up the food chain, but are eventually released through decay and the activities of decomposers. The pathways for atoms entering the Earth's biosphere can be short (in and out) to long and convoluted, and the reentry time may also be short or long.

Symbiosis means "living together." Symbiotic interactions can be thought of as beneficial to one or the other of the partners or, in the case of mutualism, to both partners. Trophic interactions include predation and herbivory, parasitism, and pathogenicity (ability to cause disease). In all these, the prey or host suffers while the predator, parasite, or pathogen benefits. Among vertebrates, the Arctic supports such well-known predators as marine mammals, polar bears, wolves, foxes, weasels, owls, falcons, gulls and terns, shorebirds, perching birds, and most fish. Predators among the arthropods include spiders, many insects, some aquatic crustacea, and many marine invertebrata. Not all are exclusively predatory: foxes eat berries, perching birds consume seeds, and some small insects eat yet smaller invertebrates and spores. Herbivores eat plants or plant parts and may specialize by feeding in particular ways. The large herbivores (caribou and muskoxen) graze; herbivorous insects range from leaf-feeders, to stem borers, gall-makers, seed eaters (semenivores), sap suckers (such as aphids), and anthophiles (deriving food from flowers: nectar and pollen). Fungivorous (mycophagous) insects may feed as grubs in the caps of mushrooms, on the below-ground hyphae, or on shed spores. Parasites affect animals and plants alike. Plant parasites comprise gall-forming and sap-sucking insects, as well as various plant parasitic and pathogenic fungi. Vertebrates are host to numerous endoparasitic organisms (internal), from protozoa to worms and ectoparasitic (external) arthropods. Invertebrates also suffer from parasites. There is a wide diversity of parasitic wasps (Ichneumonidae, Chalcidoidea, etc.) and a few parasitic flies (Tachinidae) that attack insect hosts. These parasites, often referred to as parasitoids, lay eggs that emerge and kill their hosts, eating them from within. Mermithid nematodes are also parasitoids and attack various Arctic insects, and bumblebees carry ectopar-asitic mites. In the High Arctic, one species of bumblebee (Bombus hyperboreus) is a social parasite of the other (B. polaris) whose nests and workers it usurps.

Mutualisms are much less studied. The close relationship between insects and flowers, whereby the plants are pollinated and so can set seed and the insects are provided food (nectar or pollen, or both) and sometimes shelter (see Plant-Animal Interactions), is one of the best understood. The few berry-producing plants have seeds dispersed by various vertebrates, especially birds. Thus, the plants benefit from dispersal and the animals from food. Beneath the ground, the mutualistic interaction between nitrogen-fixing bacteria (Rhizobium spp.) and leguminous plants (e.g., Astragalus and Oxytropis) is known to have some importance in soil fertility. Mycorrhizal interactions (i.e., between fungal hyphae and plant roots) are less common in the Arctic than in southern environments. This mutualism provides carbohydrate and organic nutrition for the fungus that, in turn, provides mineral nutrients for the plant. The most conspicuous of the mutualistic interactions in the Arctic is that of the lichens. Arctic lichens are abundant and diverse. All the different kinds (see Lichen) consist of a blue-green bacterium partnered with a fungus. The former is photosynthetic and the latter gathers mineral nutrients for mutual benefit.

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