Supporting article U: All about a-biotic components and their roles in the functioning of the ecosystem
The way in which plants and animals grow and carry out their different activities is a result of several abiotic factors. These factors are light, temperature, water, atmospheric gases, wind as well as soil (edaphic) and physiographic (nature of land surface) factors.
Light energy (sunlight) is the primary source of energy in nearly all ecosystems. It is the energy that is used by green plants (which contain chlorophyll) during the process of photosynthesis; a process during which plants manufacture organic substances by combining inorganic substances. Visible light is of the greatest importance to plants because it is necessary for photosynthesis. Factors such as quality of light, intensity of light and the length of the light period (day length) play an important part in an ecosystem.
• Quality of light (wavelength or colour):
Plants absorb blue and red light during photosynthesis. In terrestrial ecosystems the quality of light does not change much. In aquatic ecosystems, the quality of light can be a limiting factor. Both blue and red light are absorbed and as a result do not penetrate deeply into the water. To compensate for this, some algae have additional pigments which are able to absorb other colours as well.
• Light intensity (“strength” of light)
The intensity of the light that reaches the earth varies according to the latitude and season of the year. The southern hemisphere receives less than 12 hours of sunlight during the period between the 21st March and the 23rd of September, but receives more than 12 hours of sunlight during the following six months.
• Day length (length of the light period):
Certain plants flower only during certain times of the year. One of the reasons for this is that these plants are able to “measure” the length of the night (dark periods). However, it was thought that it is the day length (light periods) to which plants reacted and this phenomenon was termed photoperiodism. Photoperiodism can be defined as the relative lengths of daylight and darkness that effect the physiology and behaviour of an organism.
o Short-day Plants
These plants flower only if they experience nights which are longer than a certain critical length. The chrysanthemum (Chrysanthemum sp.), the poinsettia ( Euphorbia pulcherrima) and the thorn-apple (Datura stramonium) are examples of short day plants.
o Long-day plants
These plants flower if they experience nights which are shorter than a certain critical length. Spinach, wheat, barley, clover and radish are examples of long plants.
o Day-neutral plants
The flowering of day-neutral plants is not influenced by night length. The tomato (Lycopersicon esculeutum) and the maize plant (Zea mays) are examples of day-neutral plants.
The following definitions are also important:
Phototropism is the directional growth of plants in response to light where the direction of the stimulus determines the direction of movement; stems demonstrate positive phototropism i.e. they came towards the light when they grow.
Phototaxis is the movement of the whole organism in response to a unilateral light source, where the stimulus determines the direction of movement.
Variation in intensity of locomotory activity of animals which is dependent on the intensity of light stimulation, and not the direction, is called photokinesis.
Photonasty is the movement of parts of a plant in response to a light source, but the direction of the stimulus does not determine the direction of the movement of the plant.
Light requirements of plants differ and as a result distinct layers, or stratification, can be observed in an ecosystem. Plants which grow well in bright sunlight are called heliophytes (Greek helios, sun) and plants which grow well in shady conditions are known as sciophytes (Greek skia, shade ).
The distribution of plants and animals is greatly influenced by extremes in temperature for instance the warm season. The occurrence or non-occurrence of frost is a particularly important determinant of plant distribution since many plants cannot prevent their tissues from freezing or survive the freezing and thawing processes. The following are examples of temperature effects with ecosystems:
• the opening of the flowers of various plants during the day and night is often due to temperature difference between the day and night;
• the seed of some plants (biennials) normally germinate in the spring or summer; this phenomenon is well observed in carrots and is called vernalization;
• some fruit trees such as the peach require a cold period each year so that it can blossom in the spring;
• deciduous trees lose their leaves in winter and enter into a state of dormancy, where the buds are covered for protection against the cold;
• the seeds of many plants, e.g. peach and plum, must be exposed to a cold period before they germinate; this chilling ensures that seeds don’t germinate during autumn, but after winter, when the seedlings have better chances to survive;
• in animals, a distinction is made between ectothermic (“cold-blooded” or poikilothermic) animals and endothermic (“warm-blooded” or homothermic) animals although the difference is not clear cut;
• in desert conditions are a greater temperature variation between day and night and organisms have distinct periods of activity, for e.g. many cacti flower at night and are pollinated by nocturnal insects;
• seasonal changes have also a great influence on animal life in an ecosystem; torpor in winter is common in reptiles and some mammals in South Africa, but a winter sleep occurs in bears of the northern hemisphere; some animals collect fat or other resources during favourable periods (often summer and autumn) and become dormant (this is called hibernation), there are also animals that are dormant during warm and dry conditions and this is known as aestivation; examples of such animals are snails and the African lung-fish;
• seasonal movements occur in some animals; this phenomenon is called seasonal migration, examples of such animals are migratory locusts, butterflies and various marine animals like whales, penguins and marine turtles.
Plant and animal habitats vary from entirely aquatic environments to very dry deserts. Water is essential for life and all organisms depend on it to survive in especially desert areas.
• Water requirements of plants
Plants can be classified into 3 groups according to their water requirements:
Hydrophytes are plants which grow in water e.g. water-lilies and rushes.
Mesophytes are plants with average water requirements e.g. roses,sweetpeas.
Xerophytes are plants which grow in dry environments where they often experience a shortage of water e.g. cacti and often succulents.
Adaptations of plants to survive without water include reversed stomatal rhythms, sunken stomata, thick cuticles, small leaves (or the absence of leaves) and the presence of water-storage tissues.
• Water requirements of animals
Terrestrial animals are also exposed to desiccation and just a few interesting adaptations are mentioned here:
o the body covering limits water loss e.g. the chitinous body covering of insects, the scales of reptiles, the feathers of birds and the hair of mammals;
o some mammals have few or no sweat glands and use other cooling devices, less dependant or independent of evaporative cooling;
o the tissues of animals may be tolerant to water loss e.g. a camel can live without water for long periods because its body tissues have this adaptation;
o there are also known cases where insects are able to absorb water in the form of water vapour directly from the atmosphere for example the dew from the coastal fog is an important source of moisture for insects of the Namib.
• The Water Cycle in Nature
Water cycles through the biosphere in the manner is described in the figure below. Freshwater is distilled from salt water. The sun’s rays cause fresh water to evaporate from sea water and the salts are left behind. Vapourized fresh water rises into the atmosphere, cools and falls as rain over the oceans and the land. A smaller amount of water also evaporates from bodies of fresh water. Since land dies above sea level, gravity eventually returns all fresh water to the sea, but in the meantime, it is contained within the standing waterbodies such as lakes and ponds, flowing water (streams and rivers) and groundwater.
When rain falls, some of the water sinks or percolates into the ground and saturates the earth to a certain level. The top of the saturation level is called the groundwater table or simply the water table. Ground water is also sometimes located in a porous layer, called an aquifer, that lies between two sloping layers of impervious rock. Wells can be used to extract some of this water for human consumption.
The most important gases used by plants and animals are oxygen, carbon dioxide and nitrogen.
Oxygen is used by all living organisms during respiration.
• Carbon Dioxide
Carbon dioxide is used by green plants during photosynthesis.
Nitrogen is made available to plants by certain bacteria and through the action of lightning.
Winds or air currents arise on a world-wide scale as a result of a complex interaction between hot air expanding and rising (convection) in the mid latitudes. This has various effects on the rotation of the earth and results in a centrifugal force which tends to lift the air at the equator. This force is known as the Coriolis force and tends to deflect winds to their left of the southern hemisphere and to the right in the northern hemisphere. Winds carry water vapour which may condense and fall in the form of rain, snow or hail. Wind plays a role in pollination and seed dispersal of some plants, as well as the dispersal of some animals, such as insects. Wind erosion can remove and redistribute topsoil, especially where vegetation has been reduced. Warm bergwinds results in desiccation which creates a fire hazard. If plants are exposed to strong prevailing winds are they usually smaller than those in less windy conditions.
Soil (edaphic factors)
These factors include soil texture, soil air, soil temperature, soil water, soil solution and pH, together with soil organisms and decaying matter.
• Soil texture
The size of soil particles varies from microscopic particles called clay to larger particles called sand. Loam soil is a mixture of sand and clay particles. Sandy soils are suitable for growing plants because they are well aerated, excess water drains away quickly, they warm up quickly during the day and is easy to cultivate. Sandy soils is unsuitable because they do not retain much water and soon dry out and contain few soil nutrients required for plant growth. Clay soils are suitable for plant growth because they hold large quantities of water and are rich in mineral nutrients. They are unsuitable in that they are badly aerated, soon becomes waterlogged and is difficult to cultivate; it also cold during winter. Loam soils possess desirable properties of both sand and clay – it has a high water retaining capacity, good aeration, good nutrient content and is easily cultivated.
• Soil air
Soil air is found in those spaces between the soil particles that are not filled with soil water. The amount of air in a soil depends on how firmly the soil is compacted. In well-aerated soil at least 20% of its volume is made up of air.
• Soil temperature
Soil temperature is an important ecological factor. It has been found that the temperature of soil below a depth of about 30cm is almost constant during the day but seasonal temperature differences do occur. At low temperature there is little decay by decay-causing micro-organisms.
• Soil water
Soil water can be classified into three types, namely hygroscopic, capillary and gravitational water. Hygroscopic water occurs as a thin film of water around each soil particle. Capillary water is that water held in the small spaces between the soil particles and gravitational water is the water which drains downwards through the soil.
• Soil solution
Soil solution is the decaying remains of plants and animals, together with animal excretory products and faeces, form humus. This increases the fertility of the soil.
Acidity or alkalinity of soil (the pH of the soil) influences the biological activity in soil and the availability of certain minerals. Thus the pH of soil has a greater influence on the growth and development of plants. Some plants e.g.azaleas, ericas, ferns and many protea species grow best in acid soils ( soils with a pH below 7), while lucerne and many xerophytes grow better in alkaline soils (soils with a pH above 7).
These factors are those associated with the physical nature of the area, such as altitude, slope of land and the position of the area in relation to the sun or rain-bearing winds. Altitude plays a role in vegetations zones. Slopes are important when considering the temperature of the soil surface on land with a northern slope, on level and on land with south facing slopes. In South Africa the south-eastern slopes face the rain-bearing winds and in some areas are covered with forest, whilst the slopes on the leeward side are in a rain-shadow and thorn scrub is often found growing on these slopes. A very good example of this is the South Eastern Wind blowing in Cape Town.