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2 The environment
2.1 Planet earth 22
2.1.1 Systems 22
2.1.2 The lithosphere 23
2.1.3 The hydrosphere 24
2.1.4 The atmosphere 25
2.1.5 The biosphere 25
2.2 Thermodynamics 26
2.2.1 Energy, heat and work 26
2.2.2 First law of thermodynamics 26
2.2.3 Thermodynamic systems classification 29
2.2.4 Second law of thermodynamics 30
2.2.5 Plants as open systems 32
2.2.6 Animals as open systems 34
2.3 Ecosystems 37
2.3.1 Energy and nutrient flows 37
2.3.2 Population dynamics 43
2.3.3 System dynamics 50
2.4 Nutrient cycles 56
2.4.1 The carbon cycle 56
2.5 Evolution 59
2.5.1 Biological evolution 59
2.5.2 Coevolution 61
Summary 62
Keywords 62
Appendix: Doubling times with exponential growth 63
Further Reading 64
Websites 65
Exercises 65
In this chapter you will:
 Learn about the ways in which the natural environment functions and
sustains life;
 Look at the first and second laws of thermodynamics;
 Learn about energy and nutrient flows in ecosystems;
 See how the fossil fuels came into existence;
 Study population dynamics;
 Consider the concept of ecosystem resilience;
 Learn about global nutrient cycles;
 Look at evolutionary processes.
Biome (p. 39): a spatially large ecosystem defined by climatic and vegetative conditions.
Carrying capacity (p. 45): the maximum population size that a given environment can
Coevolution (p. 61): the process whereby the environment in which one population is
evolving is itself changing due to the evolution of its constituent populations.
Decomposition (p. 40): the breakdown of dead organic matter into inorganic matter.
Ecosystem (p. 37): a system of living organisms and their non-living environment.
Energy (p. 26): the potential to supply heat or do work.
Entropy (p. 31): energy that is not available for conversion, a measure of disorder.
Equilibrium (p. 48): a population level that if attained will persist in the absence of
Evolution (p. 59): the process of change over time.
Exponential growth (p. 44): growth at a constant proportional rate.
Fossil fuels (p. 42): energy sources of organic origin.
Genotypes (p. 59): an organism’s genetic inheritance.
Keystone species (p. 52): species that carry out functions essential for ecosystem
Logistic growth (p. 46): a particular form of density-dependent growth with the growth rate
declining as the population grows.
Materials balance principle (p. 28): matter can be neither created nor destroyed.
Natural selection (p. 60): genetic adaptation to the environment driven by relative
reproductive success.
Nutrients (p. 56): chemical elements taken up by organisms to maintain their functioning.
Phenotypes (p. 59): an organism’s observable characteristics.
Photosynthesis (p. 32): the process by which plants use solar radiation to convert
inorganic to organic matter.
Population (p. 43): a group of individuals belonging to the same species living in a given
area at a given time.
Primary productivity (p. 33): the rate at which plants create organic matter, usually
measured as energy per unit area per unit time.
Resilience (p. 53): the maintenance by an ecosystem of its functional integrity when
subjected to disturbance.
Speciation (p. 60): the emergence of new species.
Species (p. 43): a set of individuals who are capable of interbreeding.
Stability (p. 48): the tendency of a population size to return to its equilibrium following a
Succession (p. 51): the way in which the species composition of an ecosystem occupying a
particular area changes over time, converging on a climax state.
System (p. 22): a set of interacting components.
Thermodynamics (p. 26): the study of energy transformations.
Trophic pyramid (p. 37): the decline in biomass moving from plants to herbivores to
The Encyclopedia at the ISEE website,, includes two
short entries relevant to this chapter -- one on ‘Entropy’ by S. Baumg¨artner
and one on ‘Resilience defined’ by C. S. Holling and B. Walker. Holling is
originator of the idea of resilience as set out in the chapter here, which is
sometimes referred to as ‘resilience in the sense of Holling’.
NEW - The Earth Portal plans to publish The
Encyclopedia of the Earth, which aims to become the Web’s largest
environmental reference written by environmental scientists, educators, and