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The Terrestial
Ecosystem Or Ecosystems
Changes over time |
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As the glaciers and ice caps retreated
10-20,000 years ago, the bare ground, shattered rock debris and
boulder clay formed the initial 'soil'. On hard granite rocks weathering
by the climate was slow, producing few coarse soil particles and
little soluble nutrients. On limestone weathering was much faster
and generated alkaline soils, but the particles dissolved quickly,
drainage channels opened and nutrients tended to leach out of the
system. On other sedimentary rocks, sand and clay particles were
more abundant and essential nutrients such as phosphorus and potassium
retained. The geology set the course of ecosystem development. Nitrogen
was the missing element needed to start the plant succession. Input
in rain was minimal, but blue-green algae with the ability to 'fix'
atmospheric nitrogen grew where water was available. Some algae,
using reflected light, even live under stones which efficiently
absorb heat from the sun. Lichens grew slowly on rock surfaces.
Made up of an alga and a fungus, lichens have the capacity to produce,
decompose and recycle - a self-sufficient internal ecosystem. |
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It is these early colonisers, with
the capacity to capture essential nitrogen and nutrients from the
racks, which initiate succession in places where there was no organic
matter from previous systems - accumulation of nutrient capital
is essential. Bacteria that can extract elements from the rocks
- the chemolithotrophs - aid the process. As small amounts of organic
matter accumulate, other plants begin to colonise by seed, the tiny
amounts of organic material providing focal points for successful
germination. In other areas where some organic matter remains from
vegetation that preceded the ice cover or has been redistributed
on river banks. Here, colonisation is more rapid, but is still often
initiated by plants with associated nitrogen fixing bacteria. |
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Early colonising plants tend to have
a strategy of rapid growth and reproduction to take advantage of
the sites that have colonised. Gradually other species come into
the site, taking advantage of the protection of existing plants
and the accumulated organic matter. Gradually the cover increases,
moss cover increases and the ground becomes more insulated from
the low temperatures. Ironically, this can cause the permafrost
to thaw less, the active layer to become thinner giving more difficult
rooting conditions, often waterlogging, and reduced recycling of
nutrients. The plants that conserve their resources and recycle
them internally to give them a kick-start at the beginning of summer,
now tend to have the advantage. These are often the better competitors
- what they have they hold. The gradual changes in plants and soil
are followed by the fauna. The 'hotspots' of early colonising plants
attract migrant grazers. Small insects are blown by the wind, the
fortunate ones landing on the patches of vegetation. In the later
stages, many of the plants are long-lived, woody and often produce
'defense' compounds that protect them against animals. The fauna
then becomes more specialised and grazing is reduced to times when
more palatable food is not available. |
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The time scale of these dynamics,
the primary succession, is measured in centuries and the sequence
varies greatly from place to place, as do the initial conditions
causing succession. Ground disturbance through freeze-thaw cycles,
erosion, lake drainage, fire and Human activity such as trampling
or industrial development, can all restart succession. Adjacent
land with its vegetation and fauna, provides sources for colonisation.
Thus the time scale of such secondary succession is often measured
in decades. |
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Fundamentally, the whole region has
been subjected to variations in climate over thousands of years,
sometimes warmer, sometimes cooler. The animals and plants have
been selected to survive these changing, yet continually harsh conditions.
They have adapted. Sophisticated adaptations, such as the use of
anti-freeze chemicals, will enable them to respond and survive under
the expected changes in climate caused by the carbon pumped into
the atmosphere by Human activity. The distribution of species will
change; some will move into new areas, others will withdraw; patterns
will change - just as they always have done. The ebb and flow of
varieties of reindeer around Greenland over the last 10,000 years
illustrates these long-term dynamics. In this case caused by
changes in land form, ice connections and glacial barriers, combined
with climate, overgrazing, and predation by Man and wolves. |
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