Cycles and Human Impact

Cycles and Human ImpactNitrogen cycleNitrogen enters the cycle from either the atmosphere or fixationAtmospheric NH4+ (ammonium) and NO3- (nitrate) can be taken up directly by plantsN2 must be first fixed by prokaryotes

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

Nitrogen fixation occurs either in free soil bacteria or in bacteria that live symbiotically with plantsLegumes are known to have root nodules for this symbiosisPlants secrete chemicals to attract bacteria, bacteria secrete a chemical that stimulates root formationNitrogen in the soil (humus) is converted to ammonium from organic compounds by ammonifying bacteriaMost plants actually utilize NO3-, formed from nitrifying bacteria.Denitrifying bacteria can return nitrogen to the atmosphere. Crop rotationA common practice in agriculture is crop rotationCash crop > Legume > Cash Crop, rotated every yearLegume years replenish nitrates in the soil.Phosphorus CyclePhosphorus cycle is considerably simpler than the carbon or nitrogen cycle because there is no atmospheric stepPhosphates (PO43-) in the soil are absorbed by plants.Phosphates are added to the soil by erosion.Phosphates cycle through the trophic levels and eventually return to the soil by decomposers.Geological uplifting can return phosphates to an inaccessible stateRunoff can put phosphates into waterSedimentation can return phosphate to an inaccessible state.

Decomposition rates control nutrient cyclingThe rate of decomposition is the most important factor in controlling the rate of nutrient cyclingIn areas without decomposers nutrients would remain locked in organic matter, unable to re-enter the cycle in a usable way11Ex. 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 areHigh growth can only be kept up by equally high decompositionHuman Impact on Ecosystems Human impact on cyclesAgriculture depends upon proper knowledge of nutrient cyclingTilling the soil increases rates of decomposition and thus releasing nitrogen and phosphorus that are later removed by the growing plantsOver time the nutrients in an area can be depleted and must be enriched with fertilizer Critical Loads and EutrophicationToo much nitrogen in soil will eventually bleed into the ground water or runoff into freshwaterNitrogen 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 plantsExceeding the critical load can result in nitrate levels being too high for safe consumption or can result in cultural eutrophication Combustion of Fossil FuelsCombustion of fossil fuels can lead to a variety of issuesOne problem caused by burning fossil fuels is acid precipitationAcid precipitation has a pH of less than 5.6

Effects of acid rainFreshwater systems Decline in fish populations, rise in populations of acid tolerant fishTerrestrial ecosystems Loss of calcium and other nutrients, limiting plant growth BiomagnificationRecall the energy pyramid and biomass pyramid

Further up on the pyramid organisms must ingest more biomass to surviveAs a result any toxic compounds that are present in the lower trophic levels become more concentrated in higher trophic levelsCompounds may not affect the lowest trophic levels as a result because the concentration is not great enoughDDTDichlorodiphenyltrichloroethane Extremely powerful insecticideMarketed in the 1940s and 50s as completely safe, used to control typhus and malaria

DDT is credited with the elimination of malaria in the US and Europe DDT also nearly eradicated typhus in EuropeThe Silent SpringBy 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

Introduced Species Species transplanted from one area to anotherAccidental introduction of species have helped us answer one question in biology: What limits ranges?May have positive or negative effectsTens RuleAbout 1 in 10 introduced species manage to establish themselves in an environmentAbout 1 in 10 newly established species become invasive Invasive species are those which become prolific enough to become pests The primary problem with invasive species stems from the competitive exclusion principleThey are often better competitors than native species and threaten to drive them to extinction

African Honey Bee

Positive effects of introduced speciesAct as biocontrols of pest species

Biocontrols have the advantage of being cheap and easy to maintainEffective biocontrols should have high specificity to its prey and be capable of living in much the same habitatBiocontrols can include virusesThere has been some success in controlling the Australian rabbit population with the myxoma virus which can cause blindness and secondary bacterial infections

Viruses tend to have very high host specificity making them effective biocontrols with minimal chance of affecting other organisms.Dangers of biocontrolsBiocontrols with low host specificity may just become invasive species themselves