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The Arctic is an Ecosystem
by Bill Heal
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The Terrestial Ecosystem Or Ecosystems
The web of life and death
  From a distance, who eats who seems clear when you take a snapshot in time:

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Lichen ---> Reindeer ---> Wolf
Grass ---> Lemming ---> Snowy owl
Birch ---> Autumnal moth ---> Warbler ---> Falcon
Seeds ---> Bunting ---> Merlin

  The reality is a complex food web (see figure 8) in which the distinction between herbivores and predators is blurred; most species take a wide variety of food, depending on what is available. The so-called 'top predators', the big ones, take a wide variety of prey and many resort to feeding on insects and plants when their main food is in short supply.
  Large animals cannot be eaten by smaller ones - but that is not true when you see foxes feeding on remains of reindeer that died of starvation, or on undefended new-born calves.
  Lemmings and other small mammals form a key food for many predators. The large fluctuations in lemming populations may seem a problem for predators which depend on them to meet the demands of their young during the their breeding season. A particular adaptation of some predators to lemming cycles is to adjust the number of young to be fed. When lemming populations are high after winter breeding under the snow, large clutches or broods are produced by snowy owls, skuas, stoats and other predators, some of which migrate to areas of lemming 'highs'. Even reindeer eat lemmings at this stage. Predation, disease and overgrazing cause the lemming populations to crash. In the following year(s) predators produce fewer or even no young or emigrate. The lemming population then recovers.
  This is the classic lemming cycle with classic predator-prey dynamics - the prey population expands, followed by expanding predation, which drives the prey population down, and the predator population then declines. The truth is rarely so simple, but it illustrates some of the food web dynamics and the key place of lemmings in the Arctic system. Hidden within this dynamic is another dimension of the system - the decomposer cycle.
  Lemmings feed on the leaf bases of grasses and sedges. At a lemming 'high' a tundra meadow can resemble a hay field after cutting but before harvesting. The fresh leaves and the lemming faeces are consumed by bacteria, fungi, soil invertebrates and insect larvae, which are consumed by other invertebrates. The decomposer cycle leads to a flush of emerging adult insects preyed on by surface beetles and spiders. The early summer flush of craneflies (tipulids), midges and mosquitoes are a key food source for another group of the larger, more obvious predators - the insectivores - pipits, larks, buntings, and in wetter areas the waders. These now join the above-ground food web as prey for falcons, skuas and owls. Thus, again, different parts of the ecosystem are joined together.
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The Arctic is an Ecosystem, by Bill Heal.
Copyright Stefansson Arctic Institute and individual authors ©2000
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