Ecosystem Energetics (Chapter 6) A little physics Primary production

1. Ecosystem Energetics (Chapter 6) • A little physics • Primary production • Transfer of energy across trophic levels

3. 2 Laws of Thermodynamics • Energy can neither be created nor destroyed, only transformed from one type to another • conservation of energy • In any transfer of energy, some energy is lost • entropy

4. 1st law: all energy in an ecosystem ultimately comes from… • the sun • chemical compounds (hydrogen sulfide) • 2nd law: energy is constantly being lost to the environment as heat

5. Primary production – capturing light energy and storing it in chemical bonds of carbon compounds • primary productivity – the rate of primary production • primary producers – photosynthetic autotrophs

6. Energy + 6CO2 + 6H2O → C6H12O6 + 6O2 • Each gram of C assimilated = 39 kilojoules (kJ) of energy stored • Joules = unit of energy • Biomass is dominated by carbon • biomass and energy are equivalent

7. Plants use the products of photosynthesis in 2 ways • new proteins, tissues, cells, structures – growth & reproduction • fuel for the above processes

8. Assimilated Carbon structures fuel for biological processes

10. Gross primary production (GPP) – total amount of energy assimilated by photosynthesis • Net primary production (NPP) – energy actually stored as biomass • GPP – NPP = respiration

11. GPP NPP

12. Gross primary production (GPP) Respiration, maintenance Net primary production (NPP)

13. Measuring NPP in nature • Units: energy per unit area per year • kJ per m2 per yr, or W per m2 • 1 g C assimilated = 39 kJ energy • can use plant biomass or CO2 uptake as an estimate of energy • Ignoring roots – annual aboveground net productivity (AANP)

14. Measuring NPP in nature • Measure amount of CO2 absorbed by leaves • extrapolate from a very small area • Use radioactive isotopes to measure total uptake of C • In aquatic systems, measure changes in O2 concentration

15. LightDark

16. Limits on Primary Production • Light • in shade, forest understory • photosynthetic efficiency – percentage of the energy in sunlight converted to NPP • averages just 1 to 2% • Temperature • most plants have an optimum temperature • respiration increases with temperature

17. Water • transpiration (or water use) efficiency – amount of plant tissue produced per kilogram of water transpired • 2-4 g tissue/kg water • increasing precipitation -> increased NPP • some water is lost to runoff

18. Nutrients • Liebig’s law of the minimum… • most important in deserts, open ocean, agriculture

21. Transfer of energy across trophic levels • All energy used by higher trophic levels originates with primary producers • With each step in the food chain, 80-95% of energy is lost

22. Ecological efficiency – proportion of the biomass of one trophic levels transformed into biomass at the next higher trophic level

23. For heterotrophs, ecological efficiencies average 5-20% • Why? • indigestible tissues • hair, feathers, insect exoskeletons, cartilage, bone • cellulose, lignin • maintenance costs • loss of energy as heat (entropy)

24. Exploitation efficiency – proportion of production on one trophic level consumed by the next higher level • usually less than 100% • Not all food consumed by heterotrophs is transformed into biomass

25. Gross primary production (GPP) Respiration, maintenance Net primary production (NPP) Ingestion by herbivores Exploitation efficiency = Ingestion/NPP

26. Amount of energy actually absorbed from food is assimilated energy • Assimilation efficiency – proportion of ingested energy actually absorbed by the body • seeds – 80% • young vegetation – 60-70% • grazing/browsing – 30-40% • wood – 15% • animals – 60-90%

27. Gross primary production (GPP) Respiration, maintenance Net primary production (NPP) Ingestion by herbivores Indigestible Assimilation efficiency = Assimilation/Ingestion Assimilation

28. Growth and reproduction in heterotrophs adds biomass • Net production efficiency = (biomass production)/(assimilated energy) • the proportion of energy not used for maintenance and not lost as heat • birds: 1% • small mammals: 6% • cold-blooded animals: 75%

29. For plants, net production efficiency = NPP/GPP • fast-growing temperate plants – 75-85% • tropical species – 40-60%

30. Gross primary production (GPP) Respiration, maintenance Net primary production (NPP) Ingestion by herbivores Indigestible Net Production Efficiency = Growth/Assimilation Respiration, maintenance Assimilation Growth

31. Gross production efficiency = (biomass production)/(ingested energy) • 1-5% for warm-blooded animals • 5-15% for insects • up to 30% for aquatic animals

32. Gross primary production (GPP) Respiration, maintenance Net primary production (NPP) Ingestion by herbivores Indigestible Gross production efficiency= Growth/Ingestion Respiration, maintenance Assimilation Growth

33. Detritus (dead stuff) • Assimilation efficiency of herbivores is only 30-70% • most plant tissue is not digested by animals and ends up as detritus • Two independent food chains • herbivores • most important in plankton communities • detritivores • terrestrial communities

34. Gross primary production (GPP) Respiration, maintenance Decomposition Net primary production (NPP) Ingestion by herbivores Indigestible Respiration, maintenance Assimilation Growth

35. Gross primary production (GPP) Respiration, maintenance Decomposition Net primary production (NPP) Ingestion by herbivores Indigestible Ecological Efficiency = Biomass (higher level)/ Biomass (lower level) Respiration, maintenance Assimilation Growth

36. Gross primary production (GPP) Respiration, maintenance Decomposition Net primary production (NPP) Ingestion by herbivores Indigestible Respiration, maintenance Assimilation Ingestion by predators Growth

37. Residence time – average time that energy spends on one trophic level = (energy stored in biomass)/(net productivity) • Biomass accumulation ratio – residence time based on biomass rather than energy = (biomass)/(rate of biomass production)