Impacts on environments: Natural
Natural environmental changes affect the kinds and numbers of organisms
in an area at any given time.
Short-term
Some environmental factors experience regular cyclic
changes causing short-term variations in conditions and populations.
- Tides cause changes in water availability and temperature, salt
concentration, and food.
- Day/ night variation affects temperatures, and light intensity
(eclipses occasionally disrupt this!). Changes in day length stimulate
flowering in some plants.
- Seasons alter weather and often trigger hibernation, or migration
(to follow food sources), or life cycles.
Medium-term
Floods, drought and fire are natural factors impacting on environments
but which, over a longer time frame, are still part of the natural,
stable ecosystem.
- Floods
Extreme variations in rainfall causing floods usually occur
in late winter, increasing in spring with snow-melt. This inundates
wetlands promoting aquatic life to breed and provide food for
nesting waterbirds. Loss of topsoil and erosion can harm burrowing
animals and wash away plants. Ultimately, floods can change
river courses isolating some areas, and silt is deposited further
downstream, e.g. the fertile delta of the Nile.
- Drought
Conversely, prolonged absence of rain causes plants to die,
leaving no food or water for animals. Without the roots' anchoring
effects, topsoil is lost, and the Earth becomes cracked and
wind-eroded. Desertification increases (e.g. spreading sand-dunes
of the Sahara). Many seeds and eggs can remain dormant for years
until water is once again available and germination can begin.
Analysis of global annual rainfall shows patterns of extreme
drought in some areas while others suffer storms and floods.
This cycle which occurs every seven years or so is linked to
the warming of currents in the southern oceans (El Nino).
- Fire
Fires are natural ecological occurrences, often started by lightening
during thunderstorms. Some plants (e.g. wattles) are adapted
to germinate only after being burnt in a fire. When the undergrowth
has been burnt, it no longer competes with the germinating seedlings
for light, space and water. Most eucalypts also have dormant
buds under the fire resistant thick bark. They sprout after
the other leaves are burnt by fire.
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Long-term
Some natural impacts are extreme and have very long-lasting effects.
- Volcanoes
Volcanic action is devastating to natural ecosystems, with all living
things in the path of the lava buried and killed by heat and suffocation.
In time, however, life will return to the area as simple life forms
grow and alter the environment making it more suitable for other organisms.
This is called succession. (see Ecosystems
and foodwebs)
- Meteor impact
Evidence suggests that, at various times in the Earth's history, large
meteors have collided with Earth creating huge dust-storms. One such
event is suggested to have been the cause of the dinosaurs' extinction
through low light intensity and subsequent reduced plant growth.
- Continental Drift
The gradual shifting of the world's continents over hundreds
of millions of years (due to upwelling of magma) is believed
to have greatly influenced the distribution of plants and animals.
Fossils of temperate zone organisms now found in the Antarctic
suggest that this area was once much further north than at present
as part of the great landmass of Gondwanaland.
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