Having dealt with the processes occurring in the natural habitat and in horticulture, it remains to mention some of the factors working against a diverse habitat. The main stresses to ecosystems in Britain and other parts of Western Europe are acidity, excess nutrients, high water tables and heavy metals.
Research suggests that in the last 25 years in Holland, for example, there has been a 25 per cent decrease in woodland species attributable to the increased acidity in the air. The same survey indicated species losses of 50 per cent, 6 per cent and 5 per cent in lakes due to excess nutrients, high water table and air acidity respectively.
Plants are resilient organisms, but stresses imposed on habitats such as those near large towns, those in the wind-path of polluted air, those watered by rivers receiving industrial effluent, and agricultural fertilizers and pesticides are likely to lose indigenous species. The rapid increase in annual temperatures attributed to 'the greenhouse effect' is likely to change wild plant communities in as important a way as the environmental stresses mentioned above.
The eff ects of specific abiotic factors (pollutants) on plants
Acidity: Continuing increase in soil acidity reduces vital mycorrhizal activity, causes leaching of nutrients such as magnesium and calcium, and leaves phosphate in an insoluble form. In soils formed over limestone and chalk, the effects of acid rain are much less damaging.
Excess nutrient levels in water and soils (especially from fertilizers and farm silage) encourage increase in algae and a corresponding loss of dissolved oxygen. This process (called eutrophication) has a serious effect on plant and animal survival. It is seen most strikingly when fish in rivers and lakes are killed in this way.
Heavy metals may be released into the air or into rivers as by-products of chemical industries and the burning of fossil fuels. Cadmium, lead and mercury are three commonly discharged elements. While plants are more tolerant of these substances than animals, there is a slow increase within the plant cells, and more importantly the levels of chemicals increase dramatically as the plants are eaten and the chemicals move up the food chains (see also DDT).
Pesticides: Recent legislation has led to a greater awareness of pesticide effects on the environment. However, herbicide and insecticide leaching through sandy soils into watercourses continues to be a threat if application of the chemicals occurs near watercourses. A herbicide such as MCPA can kill algae, aquatic plants and fish.
A high water table can have a marked effect on a habitat if the effect is prolonged. The anaerobic conditions produced often lead to root death in all but the aquatic species present in the plant community.
Monitoring abiotic factors
There is constant monitoring by Government agencies for the factors mentioned above. This is especially so in National Parks, National Nature Reserves (NNRs) , and Sites of Special Scientific Interest (SSSIs). Environmental scientists and laboratories have a range of techniques for assessing levels of these factors. Chemical tests for common nutrient substances and for pH can be performed in the field. More sophisticated analysis is required for heavy metals and pesticides. A common five day test for water quality called 'Biological Oxygen Demand' (BOD) enables a confident assessment of a water sample’s pollution level. An unpolluted sample would register at about 3 mg of oxygen per unit volume per day whereas a sample polluted by fertilizers could be about 50 mg of oxygen per unit volume per day.
Plant modifications to extreme conditions
A survey of plants worldwide shows what impressive structural and physiological modifications they possess to survive in demanding habitats. A few examples of British species are described here. Marram grass (Ammophila arenaria) living on sand dunes controls water-loss by means of leaf lamina which in cross-section is shown to be rolled up. It also possesses extremely long roots. The yellow water lily (Nupar lutea) shows the following modifications: leaves with a thin cuticle (but with numerous stomata), large flat leaves and a stem with air sacs. The coastal habitats provide many examples of species which must conserve water in salty, windy coastal conditions. Glasswort (Salicornia stricta) is a succulent with greatly reduced leaves which have a thick cuticle, and its stomata remain closed most of the day. The species is able to extract water from the seawater and is tolerant to internal salt concentrations that would kill most plant species (see plasmolysis). An interesting halophyte is the sugar beet plant which was bred in France from a native coastal species. It is the only crop grown in Europe that receives salt (sodium chloride) as part of its fertilizer requirements.
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