Biology – Cellular Control, Biotechnologies, Ecosystems andResponding to the EnvironmentModule 3 – Ecosystems and Sustainability 1) ECOSYSTEMSEcosystem – a unit of living (biotic) and non-living (abiotic) components throughwhich interactions occur, nutrients cycle and energy flows.Habitat – a small part of the ecosystem where organisms live.Microhabitat – a very small part of the ecosystem where organisms live.Niche – how an organism lives as well as where it lives. A niche is its role within theecosystem, including what it feeds on, what it excretes, how it reproduces, etcPopulation – organisms of a species that live in a habitat at the same time andbreed together.Community – populations of different species living in a habitat and interacting withone another.Ecosystems are DynamicAny small changes in abiotic or biotic features of an environment can have an effecton other parts of the ecosystem. Therefore, ecosystems are constantly undergoingchange. Photosynthesis Respiration Light Energy Biotic Component Heat Energy Nutrients Abiotic ComponentEnergy is not recycled, but it flows through the ecosystem as above. Nutrients,however, are recycled via the nitrogen cycle, the carbon cycle, etc.
Plants, and other photosynthetic organisms, are known as producers, because theycan synthesise their own form of chemical energy, which can be supplied to otherorganisms.Organisms that eat other living organisms for energy are called consumers.Primary consumers feed on plants, and are herbivores. Secondary consumersfeed on the primary consumers, and are usually carnivores. Secondary consumersare, in turn, consumed by tertiary consumers.Other organisms, known as decomposers, consume dead or waste material fromorganism. 2) UNDERSTANDING ENERGY TRANSFERWithin an ecosystem, living organisms are usually members of more than one foodchain, and also often feed at different trophic levels in different food chains. Foodwebs help us to see how energy is transferred through an entire ecosystem.At each tropic level, some energy is lost from the food chain, making it unavailablefor the organism at the next tropic level: - All living organisms require energy to carry out life processes, which they get from respiring metabolites from either photosynthesis or consuming other organisms. Respiration produces energy in the form of ATP which the organism uses for cellular activities. - When an organism dies, its energy can no longer be released as heat, and some of it cannot be consumed by predators. Therefore, decomposers become the only organisms the energy is available to.Because of the loss of energy, there is less energy at higher trophic levels to sustainlarge numbers of organism; therefore, organisms higher up the food chain occur inlower frequencies than producers, primary consumers, etc. This is expressed in apyramid of numbers.Organisms may have different masses, so counting the population numbers may notgive an accurate representation of how much living tissue there is. A better measureis to measure the biomass. Dry mass, although destructive to the ecosystem, is abetter measure of mass of tissue because it does not take the water in the organisminto account. It is shown in a pyramid of biomass.Even measuring biomass has problems with accuracy, as different organisms mayrelease different units of energy per unit of mass. Therefore, it would be moreaccurate to measure how much energy is released from the organism, done by
burning the organisms in a calorimeter and working out how much heat energy isreleased. It is shown in a pyramid of energy.Energy is lost through a number of ways; the Sun’s energy is lost because: - Not all sunlight penetrates the atmosphere - Only around 3% of sunlight is absorbed by photosystems in plants/autotrophs - Light reflects off vegetation or passes through, missing photosystems completely - Only certain wavelengths of light are absorbedThe amount of sunlight energy actually converted to chemical energy throughphotosynthesis is called the gross primary productivity. After the plant has usedsome of the chemical energy itself, the net production is taken to the next trophiclevel by the primary consumer. As the net production is transferred to the consumer,some energy is lost to the decomposers, as consumers cannot eat some parts of theplant if it is not digestible or available. 3) INFLUENCING FOOD CHAINSHerbicides – the increase the population by removing competition of weeds andalso by removing plants that harbour pests or diseases. (-) It kills all plants apart from crop, decreasing biodiversity and niches, decreasing the number of small animals, which impacts higher up the food chain.Pesticides – they increase production by removing pests. (-) It removes the lower levels of the food chain which impacts higher up. Also, the amount of pesticide can accumulate higher up the food chain, creating dangerous levels.Fertilisers – nitrogen based fertilisers can help crop to grow taller. (-) Nitrates enter water, encouraging algae growth, blocking sunlight. The algae below the water die, causing decomposers to feed on them and respire, usingoxygen. This causes a lack of oxygen in the water, causing fish and other animals to die, creating a stagnant pool.
4) SUCCESSIONEcosystems can change due to communities of organisms and communities oforganisms can change the ecosystem. Because of this, sometimes there is bareground in an ecosystem. Pioneer plants are the first plants to colonise this bareground.Pioneer plants change the environmental conditions; for example, they can provideshelter for germinating seeds, places for insects to hide, dead leaves, increasedhumus which improves soil quality.Eventually conditions become suitable enough for a wider range of species, so thebiodiversity of the ecosystem increases. Primary succession is the succession ofspecies onto bare ground. Succession of species after the pioneer plants onto thissoil is called secondary succession and the community produced by this successionis the climax community. 5) DECOMPOSERS AND RECYCLINGBacteria and fungi involved in decomposition feed in a different way from animals.They feed saphrotrophically so they are described as saphrotrophs. Theseorganisms secrete enzymes onto dead waste and material, which digest the materialinto small molecules before absorbing the molecules into the organism’s body.From these molecules, the microbes have enough energy for respiration, allowingthem to carry out processes that aid in the cycling of nitrogen and carbon in theecosystem.Living things require nitrogen as part of their nutrition to make proteins, nucleic acids,etc. They get nitrogen through ammonification, nitrogen fixation, nitrification anddenitrification.It is impossible for plants to use nitrogen directly, so they need a “fixed” supply ofnitrogen, i.e. ammonium ions (NH4+) or nitrate ions (NO3-). Nitrogen fixation canoccur when lightning strikes or through the Haber process. However, theseprocesses only account for about 10% of nitrogen fixation.Nitrogen-fixing bacteria, such as Rhizobium, lives inside root nodules of bean plants: - They have a mutualistic relationship with the plant, receiving carbon compounds from the plant in return for providing fixed nitrogen.
Nitrification occurs when chemoautotrophic bacteria in the soil absorb ammoniumions: - Ammonium ions are released by bacteria involved in putrefaction of proteins found in dead/waste organic matter. - Chemoautotrophs obtain their energy by oxidising ammonium ions to nitrites (Nitrosomonas bacteria) or by oxidising nitrates to nitrates (Nitrobacter bacteria). - Because this oxidation requires oxygen, these reactions only occur in well- aerated soils. - Nitrates can be absorbed from the soil by plants and used to make nucleotide bases and amino acids.Denitrification occurs when other bacteria convert nitrates to nitrogen gas. Whenthese bacteria are under anaerobic conditions, they use nitrates as a source ofoxygen for respiration, producing nitrogen bas and nitrous oxide.
6) WHAT AFFECTS POPULATION SIZE? - At first (the lag phase), there are only a few individuals who are still acclimatising to their habitat. The rate of reproduction is slow, meaning population size grows slowly. - After this (the log phase), resources are plentiful and conditions are good, i.e. limiting factors are at a minimum. The rate of reproduction greatly exceeds the mortality rate, meaning population size grows rapidly. - Finally (the stationary phase), the population size has levelled out to its carrying capacity – the habitat itself can’t support a larger population of the species. In this phase, the reproduction rate is approximate to the mortality rate. The population size fluctuates very slightly during different seasons.When the population size reaches the stationary phase, the habitat is unable tosupport a larger population because of limiting factors, i.e. the availability of food,water, light, oxygen, nesting sites, shelter, etc. Predation and competition between orwithin species can also affect the population size as a limiting factor. 7) COMPETITIONCompetition happens when resources (like food or water) are not present inadequate amounts to satisfy the needs of all the individuals who depend on thoseresources. As the intensity of competition increases, the rate of reproductiondecreases.Intraspecific competition happens between members of the same species. Asfactors of the ecosystem become limiting, individuals must compete for them.Individuals who are well adapted and survive the competition breed and pass on
their alleles to the next generation. This slows down population growth and thepopulation enters the stationary phase.The fluctuations in the stationary phase can be explained by intraspecificcompetition: - If the population size drops, competition reduces, and the population size then increases. - If the population size increases, competition increases, and the population size then drops.Interspecific competition happens between individuals of different species, andcan affect both the population size of a species and the distribution of a species in anecosystem.It was concluded by Russian scientist Georgyi Frantsevitch Gause that the moreoverlap between two species’ niches would result in more intense competition. If twospecies have exactly the same niche, one would be outcompeted by the other andwould die out/become extinct in that habitat – competitive exclusion principle.Sometimes, however, interspecific competition can result in one species being muchsmaller than the other without actually dying out, thus resulting in relatively constantpopulation sizes. As well as this, a laboratory study may not take into account othervariables that are present in the world that also affect population size. 8) SUSTAINABLE MANAGEMENTAs the human population is increasing exponentially, the resources we require isalso increasing at this rate. Because of this, we need to exploit our environmentsmore intensively, often resulting in destroying ecosystems, reducing biodiversity, etc.One such example is the harvesting of timber from woodland, however, there aremany sustainable methods to gain the resources required.Coppicing involves cutting a tree trunk close to the ground to encourage newgrowth, and harvesting the timber cut. Once cut, several new shoots grow from thecut surface and eventually mature into stems of quite narrow diameter.To provide a continuous supply of wood, the area is divided into sections, and onesection is cut at a time, giving time for the woodland to recover from the coppicing.This is known as rotational coppicing. Although coppicing is good for biodiversity, itdoes not provide a large amount of timber, so it a relatively small-scale method ofmanagement.
Large-scale production of timber can often involve felling trees in an entire area,known as clear-felling. It can destroy habitats on a large scale, and thus it is rarelyever practised in the UK. Soil erosion can occur which allows soil to run off intowaterways, resulting in desertification.Leaving each area of woodland for 50-100 years to mature before felling allowsbiodiversity to increase. This is not cost-effective, so most modern foresters: - Plant another tree for each that is harvested. - Maintain the woodland’s ecological function regarding biodiversity, climate, mineral and water cycles. - Allow local people to derive benefit from the forest.