2.3.2 Physical stocks and flows
The main concepts governing the supply of biological resources and the maintenance of biodiversity are:-
- the niches, which define the space and food allocated to different species bynatural selection;
- the ecosystems, which are communities of species linked by bonds of food and shelter;
• ecosystems have a structure, defined as groups of particular species and theirspatial distribution;
• ecosystems have a dynamics, defined as;
-changes in the sizes of populations;
-flows of nutrients and energy through food chains,
-renewal cycles of bacteria, which release carbon and nutrients by decomposition.
Wise use of natural resources requires information about their origins, amounts and stability.
The balance between production and utilisation of natural resources may be considered in relation to the physical stocks and flows of the planetary economy. This is defined by changes in physical resources which originated when the solar system and the Earth were created. These are now expressed in the Earth's structural dynamics ( movements of continents, volcanic effects of lunar and solar cycles on the land and oceans. Solar flows result in the balance between heat energy reaching the Earth and its reflection back into space. The interactions between the planetary and solar forces produce long term and short term changes in ocean currents, tides and the weather, which in turn govern rates of erosion of land and sedimentation in the seas. 
However, it is necessary to take into account the wider cosmic picture of the origins of the planetary system in the sense that life on Earth has physical continuity with the origins of the universe.