A very indirect human impact on a nutrient cycle is acid rain, caused by pollution from power plants factories. The acidic rain falling on naturally acidic soils reduces the pH so much as to cause aluminium and iron to become soluble and subsequently toxic to the trees. Further more, the trees become stripped of leaves which remove a producer from almost all the food chains in the ecosystem. Primary consumers which use the leaves and trees as habitats are also removed which further messes up the ecosystem.
Deciduous forest ecosystems have a very rich variety of species because of the amount of habitats there are available, subsequently, when the trees of certain woods arte coppiced this has an effect on the woodland nutrient cycle.
Coppicing is practised in Bradfield Wood, Suffolk. Coppicing is done in order to provide a woodland resource and to encourage wild flower species such as oxlip, anemone and wood spurge. These plants provide more food chains with more producers and a greater variety of producers which subsequently means possibly more and different species. Bradfield Wood is very popular because of these wild plants and the wood attracts a lot of visitors. Because of this it has a car park and a visitor centre. It is a popular place to walk a dog and to have a picnic. These two factors affect the nutrient cycle of the wood. The dog faeces and urine will add nutrients to the cycle which would not have been added if the humans hadn’t visited. The same is true for picnic rubbish. Dropped food would be decomposed by decomposers and be added to the nutrient cycle and the litter would either remain intact and go on to possibly kill an animal, causing its body to be decomposed by bacteria and added to the nutrient cycle. Alternatively toxic chemicals would be leached from the material as it is broken down which could pollute ground water, if in high enough quantities, or if they are soluble then poison a tree or subterranean insects or animals, such as worms, which would have adverse effects on the food webs s a whole.
The examples given for Bradfield wood are minor in the impacts on the nutrient cycle. A much greater example would be the tropical rainforests being cut down in South America and Asia. It is estimated that an area of rainforest roughly the size of Belgium is being cut down each day. This, and the subsequent burning is destroying the habitats for untold numbers of organisms, completely obliterating food webs for hundreds of square kilometres. There are a lot of organisms in the world which we do not know about or have not identified. Subsequently we could be destroying these organisms and the webs they are an integral part of without realising they even existed.
Nutrient cycling can best be shown in a Gershmel diagram. Figure 1 shows a Gershmel diagram for a temperate deciduous forest. If any changes occurred in the forest, such as the massed clearing of some trees, then this would have adverse effects on the nutrient cycle and would be shown here.
Figure 1
A Gershmel diagram to show the Nutrient flow for a temperate deciduous forest.
An example of a disrupted nutrient cycle is Highgate Wood in London. The wood has gone through several stages. First it was coppiced to produce resources. Then the London Cooperation took over the management and made it a recreational park by burning all the leaves, clearing the undergrowth and burning fallen wood. This removed the habitat and nutrient source for the secondary decomposers and many consumers. Following this in 1967 the wood was changed and wild flowers such as wood anemones were planted. Also holly, hawthorn and elder were planted. Areas were fenced off to prevent trampling and un-indigenous species were introduced, such as the Corsican pine and red oak. This meant the woods nutrient cycle was again changed, with whole new habitats being introduced. It had been almost a hundred years since the first change to the nutrient cycle had first been drastically changed by humans. Subsequently the ecosystem had just started to adapt to the changes. The nutrient cycle was again changed, and this time unsuitable plants were added, the un-indigenous plants turned the wood into an arboretum for a variety of different trees. The conifers planted couldn’t stand London’s pollution and the heavy clay soil on which London is based. They do not supply birds with enough shelter, do not shed enough leaves to provide a leaf litter and the leaves that are shed will be of such a variety as to form a matting which blocks out the light and prevents competition from growing up, as well as stopping the growth of ground plants and shrubs under the eaves f the tree. The fencing also encouraged trampling in other places in the wood. This would have changed the nutrient cycling in the wood by removing the habitat for many birds, disrupting the food web by forcing them to go elsewhere. The decisions to start planting more native undergrowth would have had a positive affect on the ecosystem. More native invertebrates and subsequently more birds and consumers who fee on the invertebrates will return, this in turn will improve the food chain and nutrient cycle. The leaf litter will return with the planting of more deciduous trees, and with the planting of undergrowth.
Ten years after the changes were implemented, in 1977; a new report questioned the decision and suggested changes to the structure of the park. It advised the planting of holly and hornbeam, oak, cherry and rowan in the forest to replace the un-indigenous trees. The holly and hawthorn have been planted in such a way as to mask the roads from the woods and the pines have so far been replaced with some oaks and hornbeams. Hopefully this will have such an effect on the nutrient cycle that it will recover and grow in the way it should for a temperate deciduous forest. Dry acid precipitation will be a probable in this wood but hopefully this effect will be minimal compared to the returning nesting birds, and invertebrates and butterflies that will populate the dead wood, clearings and undergrowth.
Humans have an impact on almost all nutrient cycles, no matter what the actions taken. The scale of the effect and the significance of the effect are the important points. In this essay I think I have shown that humans actions can vary from complete clearance of leaf litter which will cause a huge adverse on the nutrient cycle and food web, too taking a dog for a walk and it faeces giving a tiny amount of nitrogen into the soil.
