Ecosystems

1. Ecosystems for AS Biology

2. 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

3. 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

4. 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:

5. Food chains and food webs • A food web includes more than one producer or consumer species at each trophic level:

6. 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.

7. 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.

8. 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

9. Woodland ecosystem Ecological pyramids • But even pyramids of biomass can sometimes be inverted: Zooplankton (consumers) 21 g m-2 Phytoplankton (producers) 4 g m-2 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?

10. Woodland ecosystem Ecological pyramids • The problem arises from measuring biomass as standing crop Zooplankton (consumers) 21 g m-2 Phytoplankton (producers) 4 g m-2 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.

11. 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)

12. 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)

13. All figures in kJ m-2 yr-1 Energy flow in UK pasture Reflection, evaporation, ground absorption etc Heat 23,478 976,522 Respiration Incident solar radiation 106 3,500 300 Herbivores 1,974 800 6,594 GPP 23,478 NPP 21,504 2,294 14,910 Carnivores 500 17704 Decomposers

14. 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’)

15. 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

16. Gross ecological efficiency 100 kJ of plant material 10 kJ of vole 90 kJ Vole faeces Vole parts not eaten Vole urine Vole respiration

17. Gross ecological efficiency 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

18. 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.

19. Energy and nutrient flow Heat radiated into space Energy flow (linear) Nutrient flow (cyclic) Respiration Herbivores Carnivores Nutrient pool Decomposers

20. 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’

21. The water cycle

22. 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

23. The carbon cycle

24. 5.5 1 GtC = 1 gigatonne of carbon = 109 tonnes

25. 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

26. Endothermic process The nitrogen cycle Exothermic process Energy-neutral process Decomposition Decomp Amino acids & proteins in animals Amino acids & proteins in plants Food chain Ammonium ions NH4+ Excr Reduction Rhizobium Azotobacter Nitrogen fixation 0 Nitrosomonas Nitrogen N2 Uptake and synthesis Denitrification by anaerobic bacteria Nitrite NO2- Oxidation Nitrobacter Nitrification by chemoautotrophic bacteria Nitrate NO3-

27. The nitrogen cycle bit by bit: nitrate utilisation by plants Amino acids & proteins in plants 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 Reduction 0 Uptake and synthesis Oxidation Nitrate NO3-

28. The nitrogen cycle bit by bit: return to the environment Decomp Amino acids & proteins in animals Amino acids & proteins in plants Food chain Ammonium ions NH4+ Excr Reduction 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 0 Uptake and synthesis Oxidation Nitrate NO3-

29. The nitrogen cycle bit by bit: nitrification Decomp Amino acids & proteins in animals Amino acids & proteins in plants Food chain Ammonium ions NH4+ Excr Reduction 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) 0 Nitrosomonas Uptake and synthesis Nitrite NO2- Oxidation Nitrobacter Nitrification by chemoautotrophic bacteria Nitrate NO3-

30. The nitrogen cycle bit by bit: denitrification 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: Reduction C6H12O6 + 4NO3- -> 6CO2 + 6H2O + 2N2 0 Nitrogen N2 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 Denitrification Oxidation CO2 + H2O Nitrate NO3-

31. The nitrogen cycle bit by bit: nitrogen fixation Amino acids & proteins in plants Ammonium ions NH4+ Reduction Azotobacter (a free-living nitrogen fixing bacterium) Rhizobium Nitrogen fixation Nitrogen N2 0 Nitrogen 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 Oxidation

32. The nitrogen cycle bit by bit: mutualistic nitrogen fixation Amino acids & proteins in plants Reduction Rhizobium Nitrogen fixation Nitrogen N2 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 Oxidation 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

33. Endothermic process The nitrogen cycle:recap Exothermic process Energy-neutral process Decomposition Decomp Amino acids & proteins in animals Amino acids & proteins in plants Food chain Ammonium ions NH4+ Excr Reduction Rhizobium Azotobacter Nitrogen fixation 0 Nitrosomonas Nitrogen N2 Uptake and synthesis Denitrification by anaerobic bacteria Nitrite NO2- Oxidation Nitrobacter Nitrification by chemoautotrophic bacteria Nitrate NO3-