Ecosystems

• Population is a group of organisms of the same species.
• Population numbers fluctuate depending on births, which increase population size, deaths, which decrease population size, immigration, which increases population size, and emigration, which decreases population size.
• In a stable population, births plus immigration equal deaths plus emigration.
• When writing about bacteria or yeast, use 'cell reproduction' in place of the term 'birth'.
• Factors that affect the death rate in populations can be density independent or density dependent.
• Density independent factors are abiotic (non-living) and affect a greater proportion of the population if the population is larger (and more dense).
• Density dependent factors are biotic (living) and affect a greater proportion of the population if the population is larger (and more dense).
• A population growth curve for a bacterium or yeast grown in a nutrient broth has four phases: lag, exponential, stationary, and death/decline.
• Carrying capacity is the maximum number of individuals of a species that the environment can support indefinitely.
• If a more complex organism, such as a grey squirrel, colonises a new area, the population in the lag phase is limited by low numbers to reproduce.
• Planetary boundaries define the 'safe operating space' for all of humanity, including all governments, organisations and communities, as a precondition for sustainable development.
• In the exponential phase, a doubling of numbers occurs with abundant resources and low predation.
• If the impact of human activities exceeds these thresholds, there is a risk of abrupt and irreversible environmental change.
• The stationary phase shows fluctuation; as the population increases, competition for resources (and probably predation) increases to a point where death rate is higher than birth rate.
• The boundaries are usually represented as a circular graph, coloured green, amber and red.
• The population decreases, which reduces competition and the population can increase again.
• The ocean acidification boundary is avoidable due to increased carbon dioxide dissolving into oceans forming carbonic acid.
• Energy flow through ecosystems can be represented diagrammatically.
• Organisms with calcium carbonate shells, such as corals and molluscs, can’t make shells in acid water, disrupting food chains and webs and potentially drastically reducing fish stocks.
• Energy flows along food chains in an ecosystem, from sunlight → producers (autotrophs) → primary consumers (herbivores) → secondary consumers (carnivores).
• Photoautotrophs (mainly green plants) use light energy from the sun to fix carbon dioxide into organic molecules by photosynthesis.
• The climate change boundary has been crossed due to the level of greenhouse gases (e.g. carbon dioxide) rising dramatically and continuing to do so.
• The rate at which this happens is the gross primary production (GPP).
• Scientists believe that the loss of polar sea ice could be irreversible, which could push global temperatures and sea levels up.
• Photoautotrophs use some organic molecules up in respiration; what is left is the net primary production (NPP).
• Destruction of rainforest and weakened carbon sinks in tundra and ocean could accelerate the Earth’s warming and climate change.
• Not all sunlight energy is used in photosynthesis; some is the wrong wavelength, transmitted through the leaf or reflected.
• The aerosol boundary is unquantified due to the complex effect of air pollution by particulates on the atmosphere.
• All organisms respire; therefore, energy is lost as heat at every trophic level.
• The fresh water boundary is avoidable due to globally scarce fresh water caused by modification of water bodies and land use change.
• The land use boundary describes land converted for human use, such as for agriculture.
• The ozone boundary is avoided due to the Montreal Protocol preventing a hole in the stratospheric ozone layer from widening.
• Organic molecules are passed along food chains.
• Plants and animals respire and excrete CO2.
• Decomposers use organic molecules and convert them to inorganic compounds.
• Decomposition rate is affected by temperature, pH, O2 availability, and water-logged soils.
• Nitrogen-fixing bacteria convert nitrogen gas to ammonium ions and make amino acids which they export to the legume.
• CO2 is a 'greenhouse gas' that traps heat and causes high global temperatures, causing global warming.
• Plants absorb nitrates and ammonium ions and use them to synthesise nitrogen containing compounds.
• Decomposers respire excreting CO2.