Air pollution affects biodiversity on a great scale. The atmosphere, lithosphere, and hydrosphere are negatively affected by pollution . Air pollution affects lower life forms more than higher life forms. Plants are generally more affected than animals on land, but not in fresh water. A decline in most species due to pollution is evident except for a minority that increase. I will be focussing on plants and how they are affected by air pollution.
Plants constantly take up atmospheric gases i.e. air everyday to sustain their biological processes. Vegetation growing under optimum conditions is most susceptible to air pollution . As air pollution is for the most part man-made, we are the main source of this phenomenon. Pollution can be derived from two kinds of sources namely, stationary and multiple point sources. Stationary point sources include backyard fires (on a small scale) and the burning of a thousand tons of coal each day in coal-fired electrical power plants (on a large scale). Multiple point sources are usually mobile and include automobiles and other vehicles. The vehicles are the most important source of atmospheric pollutants as they release carbon monoxide. This is followed by industrials sources which release sulphur oxides, steam and electric power plants, space heating and lastly refuse burning. Agricultural chemicals also form part of air pollution .
The uptake of pollutants depends on the concentration gradient between the ambient air and the absorptive sites within the leaf. It also depends on the conductance of the stomata. The toxic effect of a pollutant may thus be almost directly related to the functioning of the stomata. Stomata openings are related to the physiological activity of the plant in that they regulate gas exchange; correlation exists between the extent of air pollution effects and the degree of opening of the stomata . Pollutant flow may be restricted by the physical structures of leaves or scavenging by competing chemical reactions. However, as conditions change the ambient dose to which plants are exposed does not necessarily reflect actual cellular exposure. The initial flux of gases to the surface is controlled by boundary layer resistance i.e. the amount of gas able to contact the surface. This includes epidermal characteristics and air movement across the leaf.
At slower wind speeds (less that 2m/s), boundary layer thickness decreases as wind speed increases. Thus more pollutant enters the leaf when air is in motion. Pubescence is also important in that leaf hairs provide major areas of impact. Cuticle wax is also important in limiting uptake even if the cuticle is thin. Stomatal resistance is the most critical. Resistance is determined by stomatal number, size, anatomical characteristics for example the degree to which stomata is sunken and the size of the stomatal apperature .
The effect of pollution on the plants is usually visible in one form or the other. Pollution injury can be classed as acute, chronic (chlorotic) or hidden. In acute conditions intercostal leaf areas first take on a water-soaked appearance. The leaves then become dry and bleached to an ivory colour in some species while in others they become brown to brownish red. In the case of chronic injury the leaves become yellow and bleach until most of the chlorophyll and carotenoids are destroyed. This is caused by absorption of gas, insufficient to cause acute injury or absorption of sub lethal amounts over an extended time .
As the pollutants are taken up a “damage process” is followed. The epidermis is the first target as air pollution passes through the stomata and acts on this opening. The intercellular spaces are next affected as the pollutants dissolves in the surface water of the leaf cells changing the pH of the cells. In the second step the walls of the mesophyll cells are affected. As the walls contain cellulose, the cell membranes are most likely affected, notably their protein components.
As the pollutants react within the plant it is not necessarily in its original form. The pollutants pass into solution and form free radicals with electric charges. These radicals are more reactive and toxic. In the third step the cell organelles are affected for example, the chloroplast and mitochondria. In the case of the chloroplast the inner thylakoid membrane is the most sensitive. The enzymes of thylakoid and protein components of membranes are most likely to be targets. The precise protein will vary with the pollutant. Enzymes essential to carbon dioxide fixation is especially sensitive. In the mitochondria respiration, carbohydrate and lipid metabolism is adversely affected by air pollution. Changes in the ultra structure of the organelles are the first symptom of injury. The symptoms vary with the pollutant . Some particular processes of sexual reproduction in plants are known to be very sensitive to toxic gases . This therefore causes long-term changes to population ecology.
From the above information it is obvious to see that air pollution has severe adverse effects on the ultra structure and biological processes of plants. As plants form the bases of all food chains and also supplies us with oxygen, we should value and treasure them. Many of our forest ecosystems will be destroyed or at least be disturbed, resulting in considerable changes in plant communities and losses of plant resources and ecosystems. We should therefore increase our awareness of pollution in general and see what we are able to do to decrease pollution levels.
Air pollution also changes the distribution of many plants species and plant communities. It reduces biodiversity and does not respect boundaries set by conservation areas and nature reserves. Air pollution therefore contributes to the decline of biodiversity on a global scale. This global impact is also evident with climatic changes i.e. increase in temperature caused by gases polluting the atmosphere .
Something needs to be done to reduce pollution at the source. This can be done by reducing energy demands, conserving energy, switching of fuel and having technical pollution controls. The sixth major extinction is being triggered by humans’ inconsideration for our planet. Deforestation and fossil-fuel combustion have caused an increase in carbon dioxide by 30% in the past three centuries. We have already caused the extinction of 5-20% of the species in many groups of organisms. How much more disaster are we going to cause and what will it take to bring about a reformation? Air pollution is only one factor that influences biodiversity but controlling it can make the world of difference.
 McNeely, J.A., Gadgil, M., Lévêque, C., Padoch, C. & Redford, K. (1995). Human influences on biodiversity. In: Global biodiversity assessment, V.H. Heywood (ed.), Cambridge University Press, Cambridge, pp. 711—821. 0-521-56481-6 ISBN
 Stern, A.C., Wohlers, H.C., Boubel, R.W., Lowry, W.P. (1973). Fundamentals of Air Pollution. Academic Press, New York.
 Kozlowski, T.T., Mudd, J.B. (1975). Responses of Plants to Air Pollution. Academic Press Inc., New York.
 Anderson, F.K., Threshow, M. (1991). Plant stress from Air Pollution. John Wiley and sons, New York.
 Scholz, F., Gregorious, H.R., Rudin, D. (1987). Genetic Effects of Air Pollution in Forest Tree Populations. Springer-Verlag, New York.
 Leemans, R. (1996) Biodiversity and global change. In: Biodiversity, a biology of numbers and difference, K.J. Gaston (ed.), Blackwell Science, Oxford, pp. 367—387. 0-86542-804-2 ISBN