1. Cycles and Human Impact

2. Nitrogen cycle
Nitrogen enters the cycle from either the atmosphere or fixation
Atmospheric NH4+ (ammonium) and NO3- (nitrate) can be taken up directly by plants
N2 must be first fixed by prokaryotes

4. Fixation results in the creation of NH3 from atmospheric N2
Nitrogenase catalyzes this creation. This reaction is very expensive.
Most soils are naturally acidic so NH4+ readily forms

5. Nitrogen fixation occurs either in free soil bacteria or in bacteria that live symbiotically with plants
Legumes are known to have root nodules for this symbiosis
Plants secrete chemicals to attract bacteria, bacteria secrete a chemical that stimulates root formation

6. Nitrogen in the soil (humus) is converted to ammonium from organic compounds by ammonifying bacteria
Most plants actually utilize NO3-, formed from nitrifying bacteria.
Denitrifying bacteria can return nitrogen to the atmosphere.

7. Crop rotation
A common practice in agriculture is crop rotation
Cash crop > Legume > Cash Crop, rotated every year
Legume years replenish nitrates in the soil.

8. Phosphorus Cycle
Phosphorus cycle is considerably simpler than the carbon or nitrogen cycle because there is no atmospheric step
Phosphates (PO43-) in the soil are absorbed by plants.
Phosphates are added to the soil by erosion.

9. Phosphates cycle through the trophic levels and eventually return to the soil by decomposers.
Geological uplifting can return phosphates to an inaccessible state
Runoff can put phosphates into water
Sedimentation can return phosphate to an inaccessible state.

11. Decomposition rates control nutrient cycling
The rate of decomposition is the most important factor in controlling the rate of nutrient cycling
In areas without decomposers nutrients would remain locked in organic matter, unable to re-enter the cycle in a usable way

12. Ex. Rainforests would not be able to function without high decomposition rates.
Most of the nutrients in rainforests are not found in the soil, in fact very few are
High growth can only be kept up by equally high decomposition

13. Human Impact on Ecosystems

14. Human impact on cycles
Agriculture depends upon proper knowledge of nutrient cycling
Tilling the soil increases rates of decomposition and thus releasing nitrogen and phosphorus that are later removed by the growing plants

15. Over time the nutrients in an area can be depleted and must be enriched with fertilizer

16. Critical Loads and Eutrophication
Too much nitrogen in soil will eventually bleed into the ground water or runoff into freshwater
Nitrogen concentrations in rivers directly correlate with human population size in those areas.
The critical load is the amount of nitrogen that can be absorbed safely by plants

17. Exceeding the critical load can result in nitrate levels being too high for safe consumption or can result in cultural eutrophication

18. Combustion of Fossil Fuels
Combustion of fossil fuels can lead to a variety of issues
One problem caused by burning fossil fuels is acid precipitation
Acid precipitation has a pH of less than 5.6

20. Effects of acid rain
Freshwater systems – Decline in fish populations, rise in populations of acid tolerant fish
Terrestrial ecosystems – Loss of calcium and other nutrients, limiting plant growth

21. Biomagnification
Recall the energy pyramid and biomass pyramid

22. Further up on the pyramid organisms must ingest more biomass to survive
As a result any toxic compounds that are present in the lower trophic levels become more concentrated in higher trophic levels
Compounds may not affect the lowest trophic levels as a result because the concentration is not great enough

23. DDT
Dichlorodiphenyltrichloroethane
Extremely powerful insecticide
Marketed in the 1940s and 50s as completely safe, used to control typhus and malaria

25. DDT is credited with the elimination of malaria in the US and Europe
DDT also nearly eradicated typhus in Europe

26. The Silent Spring
By the 1960s it had become obvious that DDT was not completely harmless to non-insects.
Because of biomagnification organisms higher up on the food chain received much larger concentrations of DDT than expected

28. Introduced Species
Species transplanted from one area to another
Accidental introduction of species have helped us answer one question in biology: What limits ranges?
May have positive or negative effects

29. Tens Rule
About 1 in 10 introduced species manage to establish themselves in an environment
About 1 in 10 newly established species become invasive
Invasive species are those which become prolific enough to become pests

30. The primary problem with invasive species stems from the competitive exclusion principle
They are often better competitors than native species and threaten to drive them to extinction

34. African Honey Bee

38. Positive effects of introduced species
Act as biocontrols of pest species

39. Biocontrols have the advantage of being cheap and easy to maintain
Effective biocontrols should have high specificity to its prey and be capable of living in much the same habitat

40. Biocontrols can include viruses
There has been some success in controlling the Australian rabbit population with the myxoma virus which can cause blindness and secondary bacterial infections

42. Viruses tend to have very high host specificity making them effective biocontrols with minimal chance of affecting other organisms.

43. Dangers of biocontrols
Biocontrols with low host specificity may just become invasive species themselves