Ecosystems

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


A food chain describes the transfer of food material from producers to various levels of consumer, identifying only one species at each trophic level:


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’)


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


100 kJ of plant

material10 kJ of

vole

Vole faeces

90 kJ

Vole urineVole

respiration

Vole parts not eaten


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

+


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-


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-


plants

Oxid

ati

on

Reduct

ion

0

Upta

ke a

nd

synth

esi

s

Ammonium ions NH4

+


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-


plants

Oxid

ati

on

Reduct

ion

0

Upta

ke a

nd

synth

esi

s

Ammonium ions NH4

+


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-


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


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


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


plants

Ammonium ions NH4

+


animals

Oxid

ati

on

Reduct

ion

0

Endothermic processExothermic processEnergy-neutral process

Upta

ke a

nd

synth

esi

s Decomposition

Food chain

Decomp

Excr


Nitrosomonas

Nitrobacter

Nitrogen fixation

AzotobacterRhizobium

Den

itrific

atio

n

by a

naer

obic

bact

eria

Ecosystems

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