ecosystems for as biology. some definitions a population is the set of organisms of one species...
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EcosystemsEcosystems
for AS Biologyfor AS Biology
Some definitions
A population is the set of organisms of one species living in a defined area at a given time (e.g. all the squirrels in Belfairs Wood, all the meadow buttercups on the school field)
A habitat is the physical place a population inhabits (e.g. a wood, a pond, a field)
A community is the set of all the populations in a given habitat
An ecosystem consists of a community, its habitat and physical environment, and all the interactions that occur within and between them
The biosphere is the sum of all the ecosystems on the planet
Food chains and food webs
Producers are autotrophs, able to synthesise organic compounds such as carbohydrates and amino acids from inorganic raw materials such as carbon dioxide and water
Consumers cannot synthesise organic compounds, but must obtain them from producers: typically, consumers are holozoic or parasitic heterotrophs
Decomposers are saprobiontic organisms that feed on the dead remains or products of producers and consumers, re-cycling the chemical elements of which they are made
Food chains and food webs
A food chain describes the transfer of food material from producers to various levels of consumer, identifying only one species at each trophic level:
Food chains and food webs
A food web includes more than one producer or consumer species at each trophic level:
Ecological pyramids
A pyramid of numbers is an elementary way to describe a food chain in quantitative terms:
In a correctly drawn pyramid of numbers, the area of each bar is directly proportional to the number of organisms in that trophic level
Pyramids of numbers do not take into account the size of organisms at different trophic levels.
This makes it difficult to compare pyramids from different ecosystems.
It also gives misleadingly inverted pyramids in some cases.
Ecological pyramids
A pyramid of biomass is a more sophisticated way of describing a food chain in quantitative terms:
In a correctly drawn pyramid of biomass, the area of each bar is again directly proportional to the biomass in that trophic level
Pyramids of biomass make it possible to compare pyramids from different ecosystems, by equating (say) 1 kg of oak tree with 1 kg of phytoplankton.
Ecological pyramids
A pyramid of biomass is a more sophisticated way of describing a food chain in quantitative terms:
This is a correctly drawn pyramid of biomass for deciduous woodland: the area of each bar is directly proportional to the biomass in that trophic level
Ecological pyramids
But even pyramids of biomass can sometimes be inverted:
This is a correctly drawn pyramid of biomass for the surface layer of the ocean: phytoplankton are the sole food source for zooplankton.
How can 4 g of producer biomass give rise to 21 g of consumers?
Woodland ecosystem
Zooplankton (consumers) 21 g m-2
Phytoplankton (producers) 4 g m-2
Ecological pyramids
The problem arises from measuring biomass as standing crop
At any given moment in time an investigator sampling the ecosystem would find 4 g of phytoplankton per m2, and 21 g of zooplankton.
But over a period of time more new phytoplankton biomass is produced than new zooplankton biomass: the phytoplankton has higher productivity.
Woodland ecosystem
Zooplankton (consumers) 21 g m-2
Phytoplankton (producers) 4 g m-2
Productivity
Productivity is usually measured in terms of energy flow per m2 per unit time
Energy enters ecosystems (mostly) as sunlight In most ecosystems photosynthesis is no more
than 1-2% efficient (that is, plants absorb no more than 1-2% of the light energy falling on them)
The quantity of light energy absorbed by plants and ‘fixed’ in photosynthesis is called Gross Primary Production (GPP)
Productivity
The energy fixed by plants in photosynthesis is incorporated into organic chemicals such as carbohydrates, amino acids etc.
Some of this fixed energy is released by the plant in its own respiration
The remaining energy fixed as chemical energy in the plant’s tissues is the quantity available to herbivores: this is called Net Primary Production (NPP)
NPP = GPP – R (where R = quantity released in respiration)
Energy flow in UK pasture
All figures in kJ m-2 yr-1
GPP 23,478
976,522
Reflection, evaporation, ground absorption etc
Respiration
NPP 21,504
Incident solar radiation 106
14,910
1,974
Decomposers
6,594 800
Herbivores
Carnivores
3,500
300
2,294
500
17704
Heat 23,478
Gross ecological efficiency
Gross ecological efficiency is the percentage of the energy received by a trophic level that is passed on to the trophic level above
GEE is typically about 10%
This is the main limitation on the length of food chains, and the declining abundance of organisms as a food chain is ascended (‘why big fierce animals are rare’)
Gross ecological efficiency
Consider a carnivore with a GEE of 10%. The other 90% is lost in
herbivore faeces (plant material consumed by herbivores but not digested)
herbivore excretory products (plant material digested and absorbed but not assimilated)
herbivore respiration (plant material assimilated and then respired)
herbivore material not consumed by carnivores
Gross ecological efficiency
100 kJ of plant
material10 kJ of
vole
Vole faeces
90 kJ
Vole urineVole
respiration
Vole parts not eaten
Gross ecological efficiency
Root SpringsMass.(Teal 1957)
Silver Springs Florida (Odum 1957)
Marine Bay USA(Harvey 1956)
Forest USA (Ovington 1962)
Secondary Forest USA
(Gosz 1978)
Old Field USA (Golley 1960)
Pasture UK (Macfad-yen 1964)
Visible solar radiation
- 1.72x106
1.5x106 2.25 x106
2.5x106 2x106 106
Gross primary production
2,982 87,402 24,528 44,457 44,000 24,486 23,478
Producer respiration 231 50,303 12,264 19,929 24,000 3,680 1,974
Net primary production
Energy available to herbivores
12,201 37,330 12,264 24,528 - - 21,504
Producer energy to decomposers
2,465 23,184 2,920 15,330 - - 14,910
Energy consumed by herbivores
9,736 14,146 9,198 3,066 - - 6,594
Energy consumed by carnivore 1
874 1,608 3,066 307 - - 300
Gross Ecological Efficiency of carnivore 1 (%)
- -
Energy consumed by carnivore 2
- 88 - - - - -
Gross Ecological Efficiency of carnivore 2 (%)
- - - - - -
You will receive a printed copy of this table. Energy values are in kJ m-2 yr-1.
Calculate the missing values and write them into the shaded cells.
2,751 37,099 12,264 24,528 20,000 20,806 21,504
8.98 11.37 33.33 10.01 4.55
5.47
Energy flow summary
Ultimately, all the energy entering an ecosystem is lost into space as radiant heat, by producer respiration, consumer respiration, or decomposer respiration. This lost energy cannot be re-cycled.
Energy flow through an ecosystem is therefore linear.
Energy and nutrient flow
Respiration
Decomposers
Herbivores
Carnivores
Heat radiated into
space
Nutrient pool
Energy flow (linear)
Nutrient flow (cyclic)
Nutrient cycles
In the biological cycling of any element, we must identify
the environmental ‘pool’ of that element from which organisms (generally producers) obtain it
the processes by which it is ‘fixed’ in living cells, and the chemical form in which it is fixed
the processes by which it is passed along food chains and finally returned to the ‘pool’
The water cycle
The carbon cycle
Carbon enters ecosystems as carbon dioxide, assimilated in photosynthesis
Producer, consumer and decomposer respiration return carbon dioxide to the atmosphere
Most of the Earth’s carbon is held in sedimentary rocks (carbonates): marine organisms with calcareous skeletons constantly add to this as they die and sink to the ocean depths
Volcanic action, fossil fuel combustion and cement production return some sedimentary carbon to the atmosphere
The carbon cycle
5.5
1 GtC = 1 gigatonne of carbon = 109 tonnes
The nitrogen cycle The environmental ‘pool’ of nitrogen is mainly nitrate ions and
ammonium ions in soil (or in solution in aquatic ecosystems) Atmospheric nitrogen is not an exploitable source for most
organisms because of its inert nature: only specialised nitrogen fixers (all prokaryotes) can use atmospheric nitrogen
Nitrogen is ‘fixed’ in living cells mainly as amino acids, subsequently as nucleotides and other organic nitrogen compounds
The processes by which it is returned to the ‘pool’ include decomposition to release ammonia, and the subsequent oxidation of ammonium ions to nitrate
Understanding the nitrogen cycle (as opposed to learning it by rote) involves understanding the energy changes involved in the oxidation and reduction of nitrogen
The nitrogen cycle
Nitrate NO3-
Nitrite NO2-
Nitrogen N2
Amino acids & proteins in
plants
Ammonium ions NH4
+
Amino acids & proteins in
animals
Oxid
ati
on
Reduct
ion
0
Endothermic processExothermic processEnergy-neutral process
Upta
ke a
nd
synth
esi
s Decomposition
Food chain
Decomp
Excr
Nitrification by chemoautotrophic bacteria
Nitrosomonas
Nitrobacter
Nitrogen fixation
AzotobacterRhizobium
Den
itrific
atio
n
by a
naer
obic
bact
eria
The nitrogen cycle bit by bit: nitrate utilisation by plants
Nitrate NO3-
Amino acids & proteins in
plants
Oxid
ati
on
Reduct
ion
0
Upta
ke a
nd
synth
esi
s
Flowering plants preferentially absorb nitrate over other nitrogen compounds, but then have to reduce it (from oxidation number +5 to -3).
This is an endothermic process, using energy generated by respiration.
The enzyme nitrate reductase reduces nitrate to nitrite; nitrite is then reduced in chloroplasts to ammonium ions, which are immediately used in amino acid synthesis
The nitrogen cycle bit by bit: return to the environment
Nitrate NO3-
Amino acids & proteins in
plants
Oxid
ati
on
Reduct
ion
0
Upta
ke a
nd
synth
esi
s
Ammonium ions NH4
+
Amino acids & proteins in
animals
Food chain
Decomp
Excr
Plant proteins and other nitrogen compounds are passed along the food chain to consumers
Nitrogenous excretion in animals resulting from the deamination of excess amino acids releases either ammonia, or compounds such as urea or uric acid which decomposers convert into ammonia
When plant and animal remains decay, decomposers (saprobiontic organisms) release the nitrogen in their amino acids and proteins as ammonia
The oxidation state of nitrogen is unchanged in these reactions
The nitrogen cycle bit by bit: nitrification
Nitrate NO3-
Amino acids & proteins in
plants
Oxid
ati
on
Reduct
ion
0
Upta
ke a
nd
synth
esi
s
Ammonium ions NH4
+
Amino acids & proteins in
animals
Food chain
Decomp
Excr
Nitrification is addition of nitrate to soil
Nitrifying bacteria are chemo-autotrophs, obtaining energy for autotrophic nutrition by oxidation of either ammonia to nitrite (Nitrosomonas, Nitrosococcus) or nitrite to nitrate (Nitrobacter)
Nitrite NO2-
Nitrification by chemoautotrophic bacteria
Nitrosomonas
Nitrobacter
The nitrogen cycle bit by bit: denitrification
Nitrate NO3-
Oxid
ati
on
Reduct
ion
0Nitrogen N2
Den
itrific
atio
n
Denitrification is the loss of nitrate from soils resulting from the activity of anaerobic bacteria
It is especially prevalent in waterlogged soils
Facultative anaerobes such as Pseudomonas denitrificans and Thiobacillus denitrificans can use nitrate as an ‘oxygen substitute’ in their respiration:
C6H12O6 + 4NO3- -> 6CO2 + 6H2O + 2N2
The reduction of nitrate to nitrogen gas is endothermic, but this is offset by the net
energy gain from the oxidation of carbohydrate
(respiration)
Glucose
CO2 + H2O
The nitrogen cycle bit by bit: nitrogen fixation
Oxid
ati
on
Reduct
ion
0Nitrogen fixation is direct conversion of nitrogen gas (N2) into ammonium ions (hence amino acids) by combining it with hydrogen removed from sugars during respiration
The enzyme nitrogenase catalyses the reaction: it is found only in a few prokaryote species
Nitrogen fixation is energetically very expensive: this has made it advantageous for some bacteria (the genus Rhizobium) to form a mutualistic relationship with flowering plants of the Family Papilionaceae
Nitrogen N2
Amino acids & proteins in
plants
Ammonium ions NH4
+
Nitrogen fixation
Azotobacter (a free-living nitrogen fixing
bacterium)
Rhizobium
The nitrogen cycle bit by bit: mutualistic nitrogen fixation
Oxid
ati
on
Reduct
ion
0
There is a specific Rhizobium species for each member of the Papilionaceae that can form this relationship
Rhizobium invades root cortex cells and stimulates them to divide and form nodules
Nitrogen N2
Amino acids & proteins in
plants
Nitrogen fixation
Rhizobium
Inside the nodule the bacterial cells form bacteroids, and produce nitrogenase
Nitrogenase is irreversibly denatured by oxygen: the nodule cells protect it by producing leghaemoglobin, which binds oxygen: this gives functioning nodules a pink appearance when cut open
The nitrogen cycle:recap
Nitrate NO3-
Nitrite NO2-
Nitrogen N2
Amino acids & proteins in
plants
Ammonium ions NH4
+
Amino acids & proteins in
animals
Oxid
ati
on
Reduct
ion
0
Endothermic processExothermic processEnergy-neutral process
Upta
ke a
nd
synth
esi
s Decomposition
Food chain
Decomp
Excr
Nitrification by chemoautotrophic bacteria
Nitrosomonas
Nitrobacter
Nitrogen fixation
AzotobacterRhizobium
Den
itrific
atio
n
by a
naer
obic
bact
eria