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The Terrestial
Ecosystem Or Ecosystems
The web of life and death |
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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
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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. |
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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. |
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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. |
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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. |
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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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