Revised 26 September 2021



  • Ecology can be defined as that branch of science which studies the relationship of living organisms with each other within their physical environment.

  • Organisms and their physical environment form ecosystems. Human settlements also form ecosystems within larger ecosystems.

Producers & Consumers

  • Within an ecosystem there are food producers in the form of plants which use sunlight to produce food and food consumers in the form of animals which use this food.

  • Both producers and consumers maintain an ecosystem by recycling the by-products given off by each other.

  • Plants develop living biomass from the process of photosynthesis. Upon dying, this biomass falls to the ground as wood and forest litter. Some of this biomass is eaten by consumers before dying, or is consumed by insects and microbes after dying. These processes form a food chain.

  • A food chain is linear and follows the generalized form of Plant - Herbivore – Carnivore

  • The quality of energy is upgraded at each step of the chain, but with an inevitable accompanying loss of energy to heat due to the Second Law of Thermodynamics.

  • In nature, the food and feeding relationships of plants and animals are rarely in the form of a simple linear food chain, but instead interconnects with a large number of other food chains to form a food web.

  • A food web can become extremely complex and tampering with a food web can have some unexpected and undesirable effects.

  • Animals at the base of a food chain are relatively abundant, while those at the other end are relatively few in numbers.

  • When comparing the population of other carnivores and herbivores with that of humankind, humankind does not have the same symbiotic relationship within the ecosystem.

  • Humankind has been able to tap resources of energy which other parts of the ecosystem cannot use. By utilising this energy, humankind has been able to sustain a larger population than that of other similar sized animals.

Growth & Unlimited Energy Source

  • Whether any system can continue to grow or not depends on whether the system can tap a source of energy which can maintain further growth.

Maximum Power Principle

  • The Maximum-Power Principle can be stated as follows: The systems that survive in competition are those that develop more power inflow and use it best to meet the needs of survival.

Growth & Limited Energy Source

  • Over the past 100 years, economic growth as measured by GDP in the developed countries has grown at a faster rate than growth in population.
  • A limited energy source is where the source itself controls the energy flow. An example is sunlight falling upon Earth. Photosynthesis depends on this incoming flow of energy. Once this incoming energy has been used to the fullest, growth either declines - a situation in which outflows exceeds inflows and storages are decreasing - or the system maintains a steady state where the inflows of energy just keep up with depreciation and losses.


  • All ecosystems have developmental stages corresponding to that of a single organism - birth, early rapid growth, followed by maturity. Each developmental stage brings and ecosystem closer to steady state, a state of homeostasis in which there is a dynamic equilibrium interaction between the ecosystem and its physical environment.
  • Succession is a natural process where organisms within the same ecosystem succeed one another by maximising their energy inflow until a highly stable climax ecosystem develops.
  • A climax ecosystem is stable and in a condition of internal self-regulation where feed-back mechanisms enable the ecosystem to return to equilibrium following any stress of change in climate, energy, and nutrient resources.
  • There is a relationship between diversity or complexity and the stability of an ecosystem. Increasing the complexity of an ecosystem may or may not increase the stability of the system.

  • During the succession stage there is a low diversity of species. At climax there tends to be a high level of species diversity. A careful evaluation of an ecosystem should be made before any change in diversity is considered and implemented. 
  • Agriculture is an attempt to increase the P/R ratio for human consumption by preventing the natural process of succession and decreasing diversity.
  • Humankind has been able to crop a high yield from an unstable agricultural ecosystem by feeding in energy in the form of fertilizers and insecticides.
  • Monoculture systems of growing crops, building dams and roads, and pollution threaten the homeostasis of ecosystems.
  • Environmental impact assessments of proposed developments should use an eco-energetic approach to establish whether proposed development will result in runaway instabilities in the surrounding ecosystem.   

Carrying Capacity

  • The carrying capacity of an environment is the maximum population of a species that can be supported in that environment.
  • The limiting factor that prevents further population growth is the availability of nutrients.
  • The carrying capacity of each organism in a given environment is limited by the stock of any indispensable necessity of life that is in shortest supply.
  • The ultimate limiting factor of total biomass (combined carrying capacities of all organisms) in an ecosystem is the process of photosynthesis carried out by plants, algae, and certain bacteria.

Gross Primary Production (GPP)

  • As much as 95% to 99% of the available energy from the sun is lost from the plants in the form of sensible heat and heat of evaporation. The remaining 1% to 5% of energy is used in photosynthesis and is transformed into the chemical energy of plant tissues (phytomass).
  • The phytomass of the tissues of a plant represents the Gross Primary Production (GPP) of the plant.

Net Primary Production (NPP)

  • The Net Primary Production (NPP) which is continuously available to herbivores is the sum of the Gross Primary Production of plants less the respiration (R) of plants.
  • The photosynthetic efficiency of plants which is the ratio of Net Primary Production to the amount of solar energy received while the plant is in leaf ranges from 1% to 5%.
  • Over one year an average Net Primary Production is approximately 0.25% of the incident solar energy for land plants. Under favourable conditions this may reach 2% over the growing season.
  • It is more useful to consider productivity rather than photosynthetic efficiency as a comparison indicator of different plants. 
  • Herbivores feed on the available Net Primary Production. On average they convert approximately 10% of their food intake to growth.
  • The individual Gross Growth Efficiency – (Calories of Growth)/(Calories Consumed) - varies from 6% to 37% among the species.
  • A high Gross Growth Efficiency indicates an efficient assimilation of food energy for growth with little being voided as faeces or used in respiration. 
  • The Gross Growth Efficiency of an organism reduces as the organism grows larger
  • This efficiency ratio accounts for why the total phytomass of plants is greater than the biomass of herbivores which is greater than the biomass of carnivores.
  • Although the biomass of bacteria and fungi accounts for only 0.58% of the community biomass, they account for approximately 17% of the energy flow because smaller organisms have a higher metabolic rate in general and reproduce more rapidly than larger ones.

Net Community Production (NCP)

  • The Net Community Production (NCP) available to humankind as a food and energy source is that share of the Net Primary Production or net phytomass which humankind shares alongside with other herbivores
  • Humankind is concerned with maximising food and energy resources by maximising the NPP/GPP and the NCP/GPP ratios. 
  • If humans wish to maximise food and energy resources, then humans should remain exclusively herbivores and feed directly off plants and use sustainable phytomass as fuel alongside with hydro-electricity and other solar based energy sources. In doing so, humankind would be sharing the available net phytomass alongside other herbivores.
  • For humankind to attempt its own monoculture of species would be to upset the balance of nutrient and energy cycles resulting in succession of humankind by lower order species. Humankind needs to live in harmony with plants, animals, insects, birds, fish, and bacteria in order to survive. 
  • In striking a balance between food consumption and energy consumption from phytomass together with other forms of solar energy collection, we need to understand more fully the patterns of energy flows within ecosystems and, in particular, our own energy flows and their impact on the environment.

Summary Table of the Attributes of a Growth versus Climax Ecosystem