In natural ecosystems productivity is relatively low.
The additional input to agricultural ecosystems is used to increase the productivity of a crop by reducing the effect of limiting factors on its growth.
The energy used to exclude other species means that the crop has little competition for light, carbon dioxide, water and the minerals needed for photosynthesis.
The ground is therefore covered almost exclusively by the crop.
Fertilizers are added to provide essential ions, and pesticides are used to destroy pests and diseases.
Together these factors mean that productivity is much higher in an agricultural ecosystem than in a natural one.
Food production depends upon photosynthesis.
As the rate of photosynthesis is determined by the factor that is in shortest supply it follows that there is a commercial value in determining which factor is limiting photosynthesis at any one time.
By supplying more of this factor, photosynthesis, and hence food production, can be increased.
It is not feasible to control the environment of crops in natural conditions.
Plants grown in greenhouses mean it is possible to regulate temperature, humidity, light intensity and carbon dioxide concentration.
Different plants have different optimum conditions. Increasing all levels of factors to ensure maximum yield of photosynthesis could end up reducing yield or killing the plant.
For example, high temperatures may increase the yield of one species but denature the enzymes of another.
It is also economic and wasteful to expend energy raising the temperature and other levels beyond what is necessary.
Precise control of the environment is therefore essential.