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The Terrestial
Ecosystem Or Ecosystems
Material flow: Carbon and nutrient cycling |
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The food or trophic web indicates
the ways in which matter is transferred through the ecosystem. It
looks efficient but, in terms of productivity, it is very inefficient.
The transfer from plant to herbivore to carnivore to carnivore results
in a drop in production of more than 95% - at each step! For example,
of the annual plant production above ground rarely is more than
10-20% eaten by herbivores. Only about half of that is digested.
Most of the digested plant material is then used to maintain activity,
especially in warm-blooded animals, and only a small part is converted
into new production. The predator steps may be slightly more efficient
because the food is more digestible. Invertebrate convert a higher
proportion of digested food into new tissues because they do not
have to maintain their body temperature. |
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Although there is some variation,
the overall pattern remains - each trophic level supports only a
small biomass at the next level of the food chain. Hence, to find
enough food, the herbivores have large ranges, the carnivores even
larger. They also use a wide variety of foods - they tend to be
generalists rather than specialists. They are also well adapted
to conserve as much energy as possible through hibernation and insulation,
particularly because of the low plant production in the Arctic.
It is possible that Arctic food webs, because of their various adaptations
to the particular climatic conditions and sparse food supplies,
are just as efficient as more southerly ecosystems. |
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So, much of the primary or plant production
appears to be wasted. No! The plants transfer a significant part
of their production below ground into storage organs at the end
of the growing season. This is part of the conservation strategy
that enables them to grow rapidly in spring. The actual mass of
plants as roots, rhizomes, tillers etc below ground is far greater
than above ground - a general feature of the North. This below-ground
plant mass is used by some large herbivores which dig it up, and
also by soil animals such as nematodes, aphids and other insects.
Thus the food chain continues in the soil, with the important addition
of dead plant material and faeces as alternative food sources. In
one form or another, most of the plant production ends up in the
soil. There, it supports a much greater species diversity of micro-organisms
and invertebrates, and a much larger production, than occurs above-ground. |
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Dead plant matter is a key to ecosystem
development. It contains much of the small amounts of nutrients,
especially nitrogen, that the plants have managed to absorb. As
it is decomposed by bacteria and fungi and passed through the soil
food web, the nutrients in the plant remains are transferred from
organism to organism, gradually released and reabsorbed by the plant
roots. At the same time the carbon in the plant remains is also
recycled through the organisms, gradually being released through
respiration and returning to the atmosphere. The process of decomposition
in the Arctic is very slow, partly because of the low temperatures
and the cooling effect of the permafrost. Both the lack of moisture
in well drained soils and excess water where drainage is inhibited
reduce decay rates. Most litter falling from plants looses only
about 5-10% of its weight in the first year. The rate decreases
after that as the more resistant material is left and it moves into
colder layers of the soil. The soil organic matter from generations
of plant input gradually accumulates forming more mature soils.
In mires, lack of oxygen and very low temperatures through waterlogging,
cause large accumulations as peat. |
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The circulation of carbon and nutrients
through the ecosystem follows many pathways and processes (**).
It is not a closed system. Both C and nutrients enter from the atmosphere
and circulate within the system. Some of the C and Nutrients is
leached from the system into streams and rivers. Most of the C is
eventually returned to the atmosphere. It is the delicate balance
between input and return which is the focus of much attention in
the question of the role of these northern regions in climate change. |
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Although plant production in the North
is low, rates of decomposition are very low. The result is that
through gradual accumulation, northern soils contain almost 25%
of the World's soil C, mainly in the mires, bogs and muskeg. As
the northern soils are relatively young - a mere 10,000 or so years
old - they have gradually accumulated C in the plant cover and soil
organic matter. Although much of the C has been returned to the
atmosphere through respiration of plants, animals and micro-organisms,
the ecosystems have been 'net sinks' of C. In this way they have
helped to counteract the rise of carbon in the atmosphere which
is causing climate change. |
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Ironically, the current, and predicted,
climate warming is likely to increase the rate of decomposition
and release more of the C stored in the soil. The balance between
uptake through photosynthesis and release through decomposition
will fluctuate. Tundra ecosystems will probably become ' net sources'
rather than 'net sink' of C in the future. There is already evidence
that this change is occurring in Alaska. |
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