Ecosystems

• An ecosystem is a natural unit of living organisms, their interactions with each other and the non-living components of the habitat
• Ecosystems consist of different communities

  • a community is composed of populations of different species occupying the same habitat at the same time
  • communities are found in a particular habitat and are based upon dynamic feeding relationships between different species
• A population is a group of organisms of the same species occupying the same habitat at the same time
• A species are organisms with similar characteristics which can interbreed to produce fertile offspring. they have similar DNA and share the same ecological niche
• • Within an ecosystem an organism occupies a particular niche governed by its adaptation to the food supply, the habitat it occupies and abiotic factors that are present
  • the niche isn't shared with other species
• Ecosystem biotic factors: producers

  • all autotrophic
  • plants convert light energy into chemical energy during the process of photosynthesis
  • produce organic compounds and form the basis of all food chains
• Ecosystem biotic factors: consumers

  • heterotrophic
  • primary consumers are herbivores whereas secondary, tertiary consumers are carnivores
• Ecosystem biotic factors: saprobionts

  • consists of bacteria and fungi which break down dead organisms and are essential for the recycling of nutrients in the environment
  • also known as decomposers
• Ecosystem biotic factors: detritivores

  • feed on detritus
  • break down decaying matter into smaller pieces increasing the surface area for decomposition by microbes
  • aids the recycling of nutrients in the ecosystem
• Ecosystem abiotic factors: light

  • essential for photosynthesis
  • light availability affects the number and diversity of plant species and consequently the number and type of consumers in an ecosystem
• Ecosystem abiotic factors: pH

  • similar effect with acidic and alkaline soils favoring the growth of specific plant species so determining the fauna of an ecosystem
  • enzymes have a narrow pH range and deviation from the optimum can lead to denaturation
• Ecosystem abiotic factors: temperature

  • essential for enzymic reactions to proceed
  • affects flora and fauna shown by the limited variation in diversity of species in very hot and cold habitats
  • in aquatic habitats an increase in temperature reduced the amount of dissolved oxygen available to living organisms
• Food chains

  • feeding relationship from producers to consumers
  • first trophic level = primary producers
  • the number of trophic levels in a food chain is limited to below five due to the loss of energy between each trophic level
• Pyramid of numbers

  • generally the number of organisms at each trophic level decreases along a food chain
  • when the producer is large in size and inverted pyramid can be produced
  • the presence of parasites in a food chain also alter the shape of the pyramids due to parasites often occurring in large numbers as a result of their small size
• Pyramid of biomass

  • gives the total mass of organisms at each trophic level in a food chain
  • biomass can be measured in terms of mass of carbon or dry mass of tissue per given area per given time, kgm-2 yr-1
  • inverted pyramids of numbers usually produce a typical pyramid of biomass
• Method of measuring the biomass of plant material

  1. heat all the plant material at 100C for 15 minutes and weigh mass. a lower temperature for a longer time period can be used.
  2. reheat at the same temperature for the same period and reweigh
  3. repeat until a constant mass is obtained which indicates all the water has been removed
  4. divide the dry mass by the area from which the plant material was removed
  5. to determine overtime repeat the procedure removing plant material form equal areas of land over specified time-periods
• Calorimeter

  • chemical energy store in dry biomass can be estimated

  1. a known mass of a sample of biomass is completely burnt so that all the energy released is used to heat a known volume of water
  2. the increase in temperature of the water is then used to calculate the chemical energy of the biomass
• Pyramids of energy

  • each bar of a pyramid of energy represents the amount of energy per unit area or volume that flows through that trophic level in a given time period
  • energy flow is measured in units of energy kJ/m2
• Reasons why the utilisation of sunlight to chemical energy is inefficient:

  • light misses the chloroplasts/ chlorophyll/ photosynthetic tissue
  • some light which hits the chlorophyll is reflected
  • only certain wavelengths of light are used in photosynthesis
• Reasons why energy transfer between trophic levels are inefficient:

  • most energy is transferred to the environment in the form of heat released during respiration
  • not all parts of an organisms are consumed or they are indigestible and aren't absorbed
  • loss occurs via excretory products
• Percentage energy transfer between each trophic level

  • energy transfer= energy available after the transfer/ energy available before the energy transfer x100
• Gross primary productivity (GPP)= gross photosynthesis is the total amount of light energy converted to chemical energy in photosynthesis
• Net primary productivity(NPP)= net photosynthesis is the amount of energy from photosynthesis which remains available to the primary consumers
• Net primary productivity = gross primary product - respiratory loss
• Productivity in animals

  • N = I - (F+R)
  • N represents the net production of consumers and is usually measured in kJ m-2 yr-1
  • I represents the chemical energy store in ingested food
  • F represents the chemical energy lost to the environment in faeces and urine
  • R represents the respiratory losses to the environment
• Efficiency of energy transfer

  • energy available after the transfer/ energy available before the transfer x100
• Farming practices increases the efficiency of energy transfer

  • growing plant crops may be more efficient than cattle farming
  • growing crops involved only one trophic level whereas cattle farming involve two with a much larger percentage of the energy trapped in photosynthesis being lost as heat energy from respiration
• Intensive rearing and energy conversion

  • intensive farming of domestic livestock is designed to maximise yield from animals by reducing energy losses
  • it attempts to convert the smallest possible amount of food into the maximum yield of meat or products
• Keeping animals confined increases the efficiency of energy conversion because:

  • movement restriction means less energy use in muscle contraction
  • the environment is warmer so animals lose less heat and therefore respire less in order to maintain body temperature
  • food is highly controlled so that optimum nutrition is provided with no wastage. food has a high energy content and high digestibility so more of the food is used to increase biomass
• Other means of improving yield in domestic livestock:

  • selective breeding of animals to produce high yield varieties
  • using growth hormones to increase growth rates and biomass
• Stability of populations: density-dependent factors

  • higher proportion of the population is affected when the density of the population is high
  • due to increased competition, increased predation or parasitism
• Stability of populations: density-independent factors

  • same proportion of the population is affected whatever the population size
  • size of the populations in an ecosystem can vary as a result of the effect of abiotic factors, interactions between organisms, inter- and intra-specific competition, predation
• Stability of populations: interactions between organisms

  • some organisms form mutualistic relationships where both organisms benefit form this association
  • most interactions involve feeding relationships
  • very harsh abiotic factors mean that few species survive so only a small community exists; a change that affects one species affects the whole ecosystem
  • a greater diversity of species can make an ecosystem more stable
• Stability of populations: interspecific competition

  • competition between species
  • different species of plant compete for light, soil minerals and water
  • different species of animal may compete for the same prey, water supply or nesting site
  • competitive exclusion principle states that no two species can occupy precisely the same niches so don't compete for the exact same resources
• Stability of populations: intraspecific competition

  • competition within a species
  • this type of competition is often more intense as the organisms are competing for identical resources
• Stability of populations: predation

  • interspecific interaction
  • important in controlling population size
  • the larger the prey population the larger the predator population becomes
  • removal of a particular prey from an ecosystem can disrupt feeding relationships as the predator often feeds more on another prey species
• The nitrogen cycle

  • nitrogen in the atmosphere is made available to living organisms by nitrogen fixation carried out in the soil by bacteria
  • nitrogen is essential for the production of amino acids and nucleotides
• Nitrogen fixation

  • nitrogen fixing bacteria can fix nitrogen into ammonia compounds which can be used to form amino acids/ proteins
  • N2 + 3H2 --> 2NH3
  • nitrogen fixing bacteria are essential for maintaining soil fertility
  • rhizobium forms mutualistic associations with leguminous plant where their roots posses characteristics called nodules by nitorgen fixing bacteria
  • plant gains fixed nitrogen in ammonia from the bacteria
  • bacteria gain carbohydrates and vitamins produced by plants during photosynthesis
• Nitrogen cycle: feeding

  • animals obtain their source of nitrogen by feeding on plant or other animals
• Ammonification

  • the nitrogen in organic compounds in living organism is made available again when organisms die or following nitrogenous excretion
  • ammonification is the production of ammonia from these organic compounds
  • saprobiotic microorganisms cause decay and break down animal and plant proteins into ammonia which forms ammonium ions in the soil