1 Module # 10 Component # 2 Abiotic Structural Components Introduction The abiotic aspects of the ecosystem are often neglected in ecological studies. Therefore, this entire component will be devoted to this topic which discusses some of the more important abiotic factors. The amount of detail on each factor is limited, as only a basic understanding of these factors is required. The origins of the factors and some of the influences of each factor on the ecosystem will be discussed.
2 Climatic Factors Temperature Temperature is influenced by the amount of insolation (incoming solar radiation) that a particular area receives from the sun. This is noted when the average temperatures of the poles and the equator are compared. Temperature plays an important role in stimulating germination in certain plants. As the temperature starts to increase at the beginning of spring, germination is triggered. Temperature will also influence the behaviour of some animals. In deserts, many animals are inclined to burrow into the soil or lie in the shade of rocks during the heat of the day. Low temperatures will cause the process of photosynthesis to cease in plants and the plants will become dormant. The influence of low temperatures also causes animals in cold regions to go into hibernation. Ectothermic (cold blooded) animals such as reptiles cannot regulate their own body temperature and are dependent on the sun s energy. This is seen when a lizard orientates its body to the sun on cold mornings, in order to absorb the maximum amount of the sun s energy. Rainfall Rainfall is based on the climate that occurs in an area. The amount of precipitation that occurs on the earth will vary substantially from region to region. High rainfall in an area will promote leaching of nutrients from the soil as the water enters the soil and passes through it taking nutrients with it. This is simply in response to gravity. The extremes of rainfall (drought and floods) play an important role in the ecosystem. Droughts are a means of population regulation as animal numbers may drop during a drought. Floods assist in the clearing away of deposited sediments, thereby improving the quality of water in a river. Rainfall is essential in the stimulation of growth in plants. With a high rainfall at the beginning of the plants growing season, plants will experience an accelerated growth rate. The structure of vegetation will depend heavily on the quantities of rainfall an area receives. This is noted if the vegetation of a desert and a tropical forest are compared (i.e. a greater biomass in the tropical forest due to a higher rainfall).
3 Air currents and pressure These factors are influenced by the amount of localised heating that occurs in a particular area. With intense heating of the earth s surface, the air adjacent to the surface becomes heated. The air will rise causing a drop in air pressure. Once the heated air rises it eventually cools at a higher altitude and will start to descend. This will cause the air pressure to increase. Where a pressure gradient (or difference in air pressure between two points) exists, the air will move from an area of higher pressure to an area of lower pressure. This moving air is referred to as wind. Wind plays an important role in the movement of moist air which can lead to precipitation. Soil erosion may be caused by wind. Wind is an important means of seed dispersal and as a pollinating vector as found with grasses. As examined in a previous Module (# 2) wind is also required for the transmission of pheromones. Humidity Humidity is a factor which depends on the temperature and moisture content of a body of air. The amount of water in the atmosphere may vary depending on the temperature of the body of air. If air temperature increases, the amount of water that the air can hold will also increase. If the temperature of the air decreases, the amount of water held in the body of air will decrease as well. If the humidity of the air is high (i.e. there is a high concentration of water in the air) this will affect the process of transpiration in plants. If the atmosphere is moist, transpiration will slow down. A high humidity will cause terrestrial animals to become lethargic. Atmospheric gases These are essential for the existence of living organisms on the earth. Oxygen makes up about 21 % of the atmosphere and is used by animals and plants during the process of respiration. Carbon dioxide (less than 0.4% of the atmosphere) is a product of respiration and is consumed by plants during the process of photosynthesis. The bulk of the atmosphere (78%) consists of the gas nitrogen. It is essential in the formation of proteins. For this gas to be used, however, it must be fixed (changed into a useable form) by either nitrogen fixing bacteria or lightning.
4 Solar radiation and light This obviously originates from the sun. The effect of solar radiation on temperature has been discussed above. Light is required for the process of photosynthesis in plants. Only about 0.02% of light reaching the earth is used in photosynthesis. Some plants may prefer shady conditions whilst others prefer well-lit conditions. Photoperiodism is the reaction of plants to a light source. This entails a hormonal reaction by plants to grow towards the light source. Certain nocturnal animals have specific adaptations (e.g. large eyes) which enable the animal to function in low light conditions. Some nocturnal predators have a tapetum in their eyes, which permits the maximum use of limited light sources. It does this by collecting and reflecting the available light around the cornea. The tapetum also causes the reflection of light from their eyes. Most people will have seen this phenomenon when looking into a cat s or dog s eyes at night.
5 Edaphic Factors Geological parent material This forms the basis of the lithosphere. The different rock types found on the earth s surface are derived from the lithosphere. Rocks will influence the shape of the land by, for example giving rise to plains or hills. These rocks will also affect the soil types originating from the lithosphere. The nutrients found in the soils will be directly influenced by the rock type from which it originates. The texture of the soil will be directly linked to the rock type from which it originates. Sandy soil, for example, will be produced from the weathering of granite. Rocky outcrops create habitats for animals such as the rock hyrax.
6 Soils Although all soils may look similar they are very different with different characteristics. These characteristics include amongst others the texture, fertility, depth of soil and humus content. Soil texture, for example, will influence the ability of the soil to retain water for use by plants; sandy soils will retain water less effectively than clay soils. The depth of the soil may, for example, be influenced by a layer of impermeable clay. This will prevent plant roots from extending too deep into the soil and the plants could be dwarfed. The vegetation found growing in soils will also differ from area to area. Saline soils contain quantities of salt and specific species of plants that are adapted to these conditions grow on these soils. Nutrients Nutrients are essential for the growth of plants and animals. The biggest reservoir of nutrients is the soil. Some nutrients are required in large quantities such as nitrogen. Nitrogen is essential to produce proteins in plant and animal bodies. Magnesium is another essential nutrient as it forms an important role as a central atom in the chlorophyll molecule used in the process of photosynthesis. Although minerals are abundant in the soil, plants and animals do not have unlimited access to them. Therefore, these nutrients are cycled in the ecosystem and can be used repeatedly.
7 Water Water is the single most important factor governing all life on Earth. In terms of plant life, the amount and quality of water greatly influences which type of plant will be able to grow in any specific area. Plants that grow in deserts and can live with limited water resources are called xerophytes. These plants can store water and have adaptations that prevent excessive loss of water such as a waxy cuticle on their leaves. Cacti are a good example. Plants that can live in very wet conditions are called hydrophytes. These are found growing in shallow pans. They are adapted to this way of life as they have very long roots for attachment to the bottom sediment and often have flotation devices to give them buoyancy. E.g. Mangroves. Mesophytes are plants that require moderate amounts of water; if exposed to excessive or greatly reduced amounts of water, mesophytes would not survive. Halophytes are plants that have adapted to grow specifically in saline (salty) areas. Estuarine grasses are a good example Those who have completed the Botany Module will recall that leaf size is an indication that a plant is adapted to dry conditions. Many plants have compound leaves thereby reducing the leaf s surface area and the amount of water being transpired from the plant. Deciduous plants shed their leaves before the dry season (in a process known as abscission), thereby preventing loss of moisture when the rains have ceased. However, the best examples of adaptations for reducing water loss are plants that exhibit thorns.
8 Thorns are simply modified leaves. They contain much the same internal structure as normal leaves but may have the ability to close their stomata (holes in the leaves / thorns that allow the passage of water and gases). Thorns are also covered by an impermeable membrane that does not allow for water loss. When discussing animal adaptations for water conservation, there are several large texts on the subject mainly falling under the topic of applied animal physiology. This course does not to go into any detail here except for perhaps one example. Desert rodents can excrete a highly-concentrated urine, which means that harmful waste products are removed from the rodent s body whilst valuable water is conserved. This topic will be explored in an upcoming course.
9 Physiographic Factors Topography This describes the shape or the relief in an area. For example, the topography could be flat plains or undulating hills. The geological parent material and rate of weathering are two factors that contribute to the topography of an area. Topography will influence the depth of soils. On a gently sloping area the deeper soils are likely to be at the base of the slope and the shallow soils will be found along the slope. The possibility of leaching will be increased on sloping ground and may be reduced on level ground. Aspect Aspect is the direction in which a slope is facing. In the southern hemisphere, the aspect of hills that run in an east-west direction will be exposed to totally different conditions. The north facing slopes will receive more direct light and are likely to be warmer. The south facing slopes on the other hand will be in shade longer and will be cooler and receive less light. A difference in plant species growing on these slopes will be seen. On the Highveld Protea species are mostly found on the south facing slopes. The amount of rainfall on these slopes may also vary if there is a rain-shadow caused by a high ridge, where one slope receives more rain than the other slope. Altitude Altitude is the height above sea level. A pronounced temperature difference can be noted as a difference in altitude occurs. The Highveld and Lowveld are good examples of this. The Highveld (Johannesburg / Pretoria region) experiences cold winters and warm summers, whilst the Lowveld (Kruger National Park region) has relatively mild winters and hot summers. This is due to the comparative densities of air at these two altitudes. The denser air at a lower altitude traps the heat being radiated from the earth, making the Lowveld warmer. At a higher altitude, relatively less dense air is found with a reduction in the amount of heat being trapped. Please remember that there are factors other than altitude that also play a role in influencing the temperature differences between these two regions. Latitude This factor will influence the temperatures at different places on the earth s surface. The angle at which the sun s rays strike the earth s surface will be different at the equator and the poles. At the equator, a relatively small area of earth s surface is heated compared to the same amount of heat reaching the poles, where a greater surface area has to be heated by the same amount of the sun s energy. Therefore, there is a marked difference in temperatures at these two regions. This obviously will affect the plant and animal life found there.
10 Fire Fire has been included as an abiotic factor as it plays a pronounced role in the ecosystem. Fire may occur naturally or it may be used by man as a management tool or even originate accidentally. Lightning is the cause of most natural fires. Fire plays a role in nature controlling the densities of some plants and may stimulate the growth of plants. Fire controls the densities of some animal species, such as slow moving reptiles, burrowing animals and ground birds. The effects of fire and specifically the issue of using fire deliberately in a savannah ecosystem are explored in Module # 11 Principles of Habitat Management