Nature of Ecosystems (AQA A Level Geography)

Nature of ecosystems

• These are open systems, as flows of energy and materials cross ecosystem boundaries
• The system is dynamic and works in balance - if one part changes, it changes the characteristics of the whole ecosystem or environment it is in
• They are considered a community of living (biotic - plants and animals) and non-living (abiotic) elements, that interact and work together in the same environment
• The physical environment provides energy, living space and nutrients that flora and fauna need to survive
• Ecosystems can exist at any scale from a biome  to a puddle
• Feedback loops exist within the systems, e.g.
  • Germinating seeds increases the number of plants
  • Biomass increases, which increases the number of seeds etc. 
  • Positive feedback

Inputs and outputs of an open ecosystem

Structure

• Biotic factors include:
  • Producers - autotrophs - organisms that create energy from inorganic matter. Most are phototrophic (they create energy through photosynthesis)
  • Consumers - called heterotrophs as they don't make their own food but rely on producers
    • There are four groups:
      • Primary - these are herbivores (eat only plants) and feed on producers
      • Secondary - primary/small carnivores(eat only meat) and feed on herbivores
      • Tertiary - large carnivores that feed on primary carnivores
      • Quaternary - omnivores eat both meat and plant 
  • Decomposers - break down dead organic matter and release it into the environment
    • Saprophytes - organisms that feed on dead or decaying matter – bacteria, fungi
    • Detritivores – animals which eats detritus – fragments of dead and decaying matter
• Abiotic factors:
  • Climatic
    • Insolation
    • Precipitation
    • Wind
    • Temperature
    • Altitude/latitude
  • Physical 
    • Topography
    • Altitude/Latitude
    • Water quality
    • Soil (edaphic) properties
      • Texture
      • pH of the soil
      • Mineral content
      • Organic matter content
      • Water content/quality

• Ecosystems perform two basic functions:
  • They cycle nutrients
  • They move energy
• These processes are interdependent, as the rate of one limits the other
  • Rate of energy flow limits the rate of nutrient cycling 
• The flow of energy from the sun is fixed by green plants (autotrophs) during photosynthesis
  • Of the energy from the sun, only 2% is used 
  • This energy is stored as biomass and passes through the ecosystem's feeding levels (trophic levels), via food chains (single pathways) or webs (complex networks of linked food chains)
  • At each transfer of the levels, the amount of energy utilised is diminished until eventually it is lost 
  • Typically energy transfer is only 10% efficient

Trophic levels for a simple food chain - the blue arrows show how the chemical energy originally produced by the primary producer (grass) is transferred to other organisms in the community

• The biomass of an organism is:
  • The mass of living material of the organism 
  • The chemical energy that is stored within the organism 
  • Biomass is often measured in kg / m^-2
• During photosynthesis, primary producers (such as plants and algae) convert light energy to chemical energy stored within biological molecules
• Gross primary productivity (GPP) is the total amount of carbon compounds (biomass) fixed during photosynthesis by all producers, in the ecosystem, in a given period of time and measured in kg/m²/year
• Only 2% of light that falls on a plant is used to produce glucose through photosynthesis
• The remaining 98% of light either:
  • Passes through the leaf without hitting chloroplasts
  • Reflected off the leaf
  • Transferred as heat energy
• The quantity of energy now stored in the glucose (from the 2% of energy) is the gross primary production (the total mass of carbon taken out of the atmosphere by plant photosynthesis)

• Net primary production (NPP) refers to the amount of energy available to herbivores in the plant’s biomass after plant respiratory loss
• It is the rate at which an ecosystem accumulates energy or biomass, excluding the energy it uses for the process of respiration 
  • Of the total energy trapped in glucose during photosynthesis, 90% of this energy will be released from the glucose to create fuel for active cellular processes
  • Only a small percentage of the original energy captured will be used to create new plant cells
  • Therefore, little energy will be passed on to herbivores via eating
• The chemical energy that is leftover in a plant after respiratory loss is known as the net primary production (NPP)
• Net primary production can be calculated using the equation below:

• NPP is important because it represents the energy that is available to organisms at higher trophic levels in the ecosystem, such as primary consumers (herbivores and omnivores) and decomposers

Net primary production (NPP) is the chemical energy store in plant biomass after respiratory losses to the environment have been taken into account

Food chains 

• A food chain shows the feeding interactions and transfer of energy from one organism to the next
• The source of energy in a food chain is light energy from the sun

An example of a food chain

Food webs

• The links between the biotic components can also be shown as a food web
• These show the complex variety of plants and animals which feed on each other

Deciduous forest food web

• Within a community, each species depends on other species for food, shelter, pollination, seed dispersal etc
• If one species is removed it can affect the whole community
• This is called interdependence
• A stable community is one where all the species and environmental factors are in balance so that population sizes remain fairly constant

A food web shows the interdependence of organisms

• For example, in the food web above, if the population of earthworms decreased:
  • The population of grass plants would increase as there are now fewer species feeding off them
  • The populations of frogs and mice would decrease significantly as earthworms are their only food source
  • The population of sparrows would decrease slightly as they eat earthworms but also have another food source to rely on (caterpillars)

• Organisms that obtain their food in the same way – herbivores etc. – form a trophic level
• Most ecosystems rarely have more than four or five tropic levels and energy is lost at each stage because:
  
  • Not all of the food is digested
  • Some plants and animals are not eaten by herbivores and carnivores
  • Activities such as chewing, mating and catching prey consumes energy
  • Flora and fauna respiration releases heat energy
• Food chains are very simple models of ecosystems, however, they ignore the fact that:
  • A number of consumers can be in more than one trophic level
  • Many animals are opportunists
  • Some animals are also omnivores, such as the badger, or herbivore when young but become carnivores when adult – tadpoles and frogs 
• Food webs attempt to show more complex feeding patterns, but these don’t show energy loss or relative importance of individuals in the web
• Dead organic matter is produced at every trophic level and broken down by insects and micro-organisms and recycled as nutrients

Energy pyramids

• An energy pyramid, also known as a trophic or ecological pyramid, is a representation of the energy found within the trophic levels of an ecosystem
• The base (largest level) of the pyramid represents the producers and therefore, contains the largest amount of energy
• Energy is lost at each subsequent level through heat, respiration, movement and excretion
  • Only 10% of energy is passed from one level to the next
• Because energy is lost between levels, there are fewer numbers (less biomass) of consumers on each trophic level
  • If all organisms were gathered at each level and weighed, there would be significantly more plants, by weight (or mass) than primary consumers; more primary consumers than secondary consumers and so on
  • This results in fewer organisms within each level
• Decomposers follow alongside the pyramid, as they responsible for breaking down the dead organisms at all trophic levels 
• Each trophic level is responsible for controlling the population of organisms in the level below
• The top trophic level usually holds the 'apex consumer' or 'top predator' and ensures that lower levels don't consume all the producers
  • E.g. too many zebras might eat all the grass
  • Lions control zebra levels
  • Apex predators also help maintain biodiversity and keep an ecosystem in balance
• A keystone species is any organism (animals, plants, bacteria and fungi) that holds a habitat together
• It does not have to be the biggest (in size or quantity), but if it is removed, it sets off a trophic cascade
  • A chain of events that turns the structure and biodiversity of its habitat into something very different
  • E.g. the decline in the bee population has had devastating effects on pollination as bees support the reproduction of as much as 90 percent of the world’s flowering plants
• The impact of removing an organism from an ecosystem depends on the species involved and the complexity of the food web
• Providing herbivores can switch to alternative food sources, the removal of one plant will have little effect 
• But the loss of a top carnivore has a greater impact on the ecosystem 
• Eventually equilibrium will be restored, because once all the vegetation has been eaten, the herbivores and eventually the carnivores will die of starvation, which gives time for the plants to recover

Energy pyramid showing trophic levels 

• The biggest criticism of trophic/energy pyramids is that it ignores the size of the individuals
  • An oak tree carries the same weight as an aphid within the pyramid

Development energy pyramid

• Energy pyramids can also show the extravagant food and energy wastage between HDEs and LDEs
• They represent processing needs within the different diets
• HDEs will utilise more species to feed less people than LDEs
• Therefore, HDEs waste more ecological energy than LDEs

Energy pyramid showing how levels of development consume and waste ecological energy

• The pyramid above shows that the same amount of primary produce feed different amounts of humans based on level of development