University of Technology Environmental Studies Lecture Notes PDF

1 UNIVERSITY OF TECHNOLOGY FACULTY OF SCIENCE AND SPORT Module: Environmental Studies Lecture #5 ECOSYSTEMS AND LIVING ORGANISMS The environment in which an organism lives will be de...

1 UNIVERSITY OF TECHNOLOGY FACULTY OF SCIENCE AND SPORT Module: Environmental Studies Lecture #5 ECOSYSTEMS AND LIVING ORGANISMS The environment in which an organism lives will be determined by a range of physical or abiotic factors such as light, heat and moisture as well as by the influence of other living organisms (biotic factors). BIOTIC FACTORS: INTERACTIONS AMONG LIVING ORGANISMS The biotic factors which affect the survival and distribution of an organism include: Intraspecific interactions – those that occur between members of the same species such as mating, caring for offspring as well as competition for food, water, light and territory. Interspecific interactions – those that occur between members of different species such as predator-prey interactions, host-parasite interactions and competition. Humans – we have become the predominant biotic influence on the distribution and success of other species. The main types of associations among living organisms are: predation, symbiosis and competition. Predation Predation is the consumption of one living or recently killed organism, the prey, by another, the predator. It includes both animals eating other animals, animals eating plants (cows grazing on grass) and plants eating animals. Symbiosis Symbiosis is the living together, in close association, of two or more organisms of different species. Nutrition is usually involved. There are three common types of symbiotic relationships. Mutualism This is a symbiotic relationship in which both partners benefit. The association between nitrogen fixing bacteria of the genus Rhizobium and legumes is an example of symbiosis. Rhizobium supplies the plant with all the nitrogen it needs and the legumes supply sugar to their bacterial symbionts. Another example of mutualism involves the clown fish which dwells within the tentacles of the sea anemone on the coral reef. These tentacles possess stinging cells. The clown fish themselves are not affected by the tentacles. They, however, lure predatory fish into the tentacles. These are then captured by the anemone’s tentacles, stung, stunted and ingested by the anemone. In turn, by living among the anemone’s tentacles the clown fish is protected from predators.  2 clownfish-sea-anemone.jpg (600×385) (thefishdoctor.co.uk) For more on intriguing symbiotic relationships found in the marine environment check out the following link: http://www.ms-starship.com/sciencenew/symbiosis.htm Commensalism Commensalism is a type of symbiosis in which one organism benefits (the commensal) and the other is neither harmed nor helped. An example is the relationship between a tropical tree and the epiphytes (mosses, orchids and ferns) that live attached to the bark of the tree. The epiphyte anchors itself to the tree but obtains neither water nor nutrients directly from the tree. Its location on the tree enables it to obtain adequate light, water (as rainfall dripping down the branches) and required minerals (washed out of the tree’s leaves by rainfall). Thus, the epiphyte benefits from the association, while the tree is apparently unaffected. Parasitism Parasitism is a symbiotic relationship in which one organism (the parasite) benefits and the other (the host) is adversely affected. The parasite obtains nourishment from the host, but although a parasite may weaken its host, it rarely kills it. Parasites which live on the outer surface of a host are termed ectoparasites e.g. ticks, fleas, leeches. Those that live within a host are endoparasites e.g. tapeworms. COMPETITION When organisms strive for the same resources in the same place, this is known as competition. Intraspecific competition occurs among organisms of the same species and interspecific competition occurs among organisms of different species. Ecological Niche An organism’s ecological niche comprises the habitat in which the organism lives, the role that it plays in the environment, it’s activity pattern and the resources it obtains from the habitat. The potential ecological niche of an organism is its fundamental niche, but various factors such as competition with other species may exclude it from part of its fundamental niche. Thus, the lifestyle that an organism actually pursues and the resources that it actually uses make up its realised niche. Competitive Exclusion When two species are very similar, their fundamental niches may overlap. Many ecologists believe that no two species can indefinitely occupy the same niche in the same community, because competitive exclusion  3 eventually occurs. In competitive exclusion, one species is excluded from a niche by another as a result of competition between species i.e. interspecific competition. Although it is possible for different species to compete for some necessary resource without being total competitors, two species with absolutely identical ecological niches cannot coexist. Coexistence can occur if the overlap in the two species’ niche is reduced. Resource Partitioning To reduce competition organisms often engage in resource partitioning. This allows organisms with very similar niches to exist in the same habitat by exhibiting a unique activity pattern (e.g. the time of day when they are active) or occupying specific areas of the habitat (e.g. above or below the high water mark). EVOLUTION AND SUCCESSION Evolution is the theory that various types of animals and plants have their origin in other pre-existing types, and that the distinguishable differences between them are due to modifications in successive generations. The 19th century English naturalist Charles Darwin argued that organisms came about by evolution, and he provided a scientific explanation of how evolution occurs, and why it is that organisms have features – such as wings, eyes, and kidneys – clearly structured to serve specific functions. Natural selection was the fundamental concept in his explanation. Darwin’s theory was heavily influenced by the teachings of Thomas Malthus. Malthus believed that as populations grew demands on natural resources would increase leading to problems such as famine, disease and poverty, helping to check population growth (http://geography.about.com/od/populationgeography/a/malthus.htm; http://www.ucmp.berkeley.edu/history/malthus.html). Darwin’s Theory of Natural Selection Fact #1 – Without constraints, populations will grow exponentially, producing a rapidly growing number of organisms. Fact #2 – In spite of this prediction, the numbers of individuals in a population remains near equilibrium, fluctuating above or below some mean value. Fact #3 – Resources are limited. From this fact, Malthus concluded that there was a struggle for existence. Darwin combined this with two additional facts: Fact #4 – Individuals are unique. There is individual variation within a population. Fact #5 – Much (but not all) of the individual variation is hereditable. This observation also came from animal breeders. (Some of observed variations are environmental, some genetic.) These facts led Darwin to conclude that some individuals are better equipped to survive and reproduce (Natural Selection). Evolution results from the gradual accumulation of inherited adaptations that are then passed down to successive generations over time. A clear example of natural selection was discovered among “peppered” moths living near industrial cities in England. These insects have varieties that differ in wing and body colouration from light to dark. During the 19th century, sooty smoke from coal burning furnaces killed the lichen on trees and darkened the bark. When moths landed on these trees, the dark coloured ones were harder to spot by birds, and subsequently, they more often lived long enough to reproduce. Over generations, the environment continued  4 to favour darker moths. As a result, they progressively became more common. By 1900, approximately 98% of the moths in the vicinity of English cities like Manchester were black. Since the 1950’s, air pollution controls have significantly reduced the amount of pollutants reaching the trees. As a result, lichen has grown back, making trees lighter in colour. Now, natural selection favours lighter moth varieties so they have become the most common. SUCCESSION The orderly replacement of one ecosystem by another is a process known as ecosystem development or ecological succession. Succession occurs when a sterile area such as a lava flow, is first colonised by living things, or when an existing ecosystem is disrupted, as when a forest is destroyed by fire. Types of succession Two different types of succession, primary and secondary, have been distinguished. Primary succession occurs in essentially lifeless areas – regions in which the soil is incapable of sustaining life as a result of such factors as lava flows, newly formed sand dunes, or rocks left from a retreating glacier. Secondary succession occurs in areas where a community that previously existed has been removed; it is typified by smaller-scale disturbances that do not eliminate all life and nutrients from the environment. Events such as a fire that sweeps across a grassland, or a storm that uproots trees within a forest create patches of habitat that are colonised by early successional species. Depending on the extent of the disturbance, species may recolonise the area from nearby habitats, and others may actually be released from a dormant condition by the disturbance. For example, many plant species in fire-prone environments have seeds that remain dormant within the soil until the heat of a fire stimulates them to germinate. For more on succession, you may view the following link: http://uk.encarta.msn.com/encyclopedia_781534152/Succession_(ecology).html TUTORIAL QUESTIONS 1. Use examples to explain what is meant by commensalism, mutualism and parasitism. 2. Explain the benefits that legumes and nitrogen fixing bacteria obtain from their association. 3. What is competition? What are the possible effects of competition in an ecosystem? 4. What is meant by the competitive exclusion principle? Discuss how limiting factors restrict an organism’s niche. 5. Differentiate between the following: a) habitat and niche b) fundamental niche and realized niche 6. State and discuss the four observations of Darwin’s mechanism of evolution by natural selection as the way in which populations change over time. 7. a) What is ecological succession? b) To what extent is it important to an ecosystem? c) Provide differences between the processes of ecological succession and evolution?  5 8. a) Differentiate between primary and secondary succession using examples of each in your response. b) Define and give characteristics of climax and pioneer communities? c) How do the pioneer communities in primary and secondary succession differ? Originally Compiled By: Raymond Martin First Revision By: Nikki Bramwell, Raymond Martin and Damian Nesbeth Second Revision By: Adonna Jardine-Comrie, May 2012 Updated By: Adonna Jardine-Comrie Aug 2021