Stage 2 Biology – Ecosystems Notes
E4. Resources
are largely recycled in undisturbed communities
·
Resources can be classified as abiotic or biotic
required to support maintenance and growth of organisms
o
Abiotic = light intensity, rainfall, soil,
temperature, nutrient levels
o
Biotic = living components such as food
·
Productivity
o
Measure of how effective a community is at
converting sun’s energy
o
Nett primary productivity – proportion of energy
trapped in plant tissue available to consumers
o
Measured in dry weight/grams per m2
per year
o
Higher where conditions are more favourable for
plant growth (ie. rainforest compared to desert)
o
Constraints on productivity
§
Energy available
§
Amount and quality of resources
§
Humans use fertilisers and irrigation to boost
productivity – but is detrimental to the environment
§
Dependent on rainfall and amount of trace
elements in the soil
·
Biomass
o
Total weight of living matter in a community
·
Organisms require 40 essential nutrients which
are used to make components of tissues/cells
·
As matter is not created or destroyed, it is
important that these elements are recycled through living organisms and back
into the abiotic environment
Carbon cycle
o
Important for organic molecule structure
o
Carbon dioxide from the atmosphere is
incorporated into tissues by photosynthesis – carbohydrate (glucose) can then
form part of other organic molecules
o
Respired back into the atmosphere in the form of
carbon dioxide
o
When consumers eat plants, they incorporate the organic
material into their tissues – process continues through trophic levels
o
Tissues are broken down by decomposers, which
release carbon dioxide
o
Photosynthesis and respiration generally balance
each other
o
Stores of carbon in fossil fuels – the remains
of organisms that lived millions of years ago
o
Humans have been using large amounts which has
increased the amount of carbon dioxide in the atmosphere
o
Concern about the effects on the environment and
global warming
Nitrogen cycle
o
Nitrogen is an important part of proteins and
nucleic acids
o
Abundance of nitrogen in the atmosphere, but it
is not in a form that can be used by most organisms
o
Some bacteria (nitrogen-fixing bacteria) can
convert nitrogen gas into other nitrogen-containing compounds such as nitrates
that can be taken up by plants
o
Legumes possess nitrogen-fixing bacteria in root
nodules – they are used to increase the nitrogen content of soils in
agriculture
o
Decomposers break down dead and decaying
material into ammonium which is available to plants
o
Some bacteria convert nitrates to nitrogen gas
to complete the cycle
Phosphorus cycle
o
Organisms require much less phosphorus than
other compounds but is still essential
o
Critical component of nucleic acids and ATP
o
It is rare – main store is phosphate rock
o
Erosion of rocks (by rainfall) dissolves
phosphate out of rocks and into the soil
o
Plants can absorb phosphate ions from soil or
dissolved in aquatic ecosystems
o
Some plants have relationships with mycorrhiza –
helps plants grow in phosphate poor soil
o
Animals obtain most phosphate by eating other
organisms
o
Decomposers return it to the ecosystem
o
Organisms are well adapted at extracting it from
solution
o
Excess phosphate from fertilisers enters aquatic
systems as run off, which disturbs populations – ie. leads to algal blooms
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