(a) Two examples of man-made ecosystems are: Gardens and parks, crop fields and fishery tanks.
(b) Salient features of man-made ecosystems:
(i) These are simple and highly efficient.
(ii) These are generally monoculture (e.g. field crops) with low biodiversity.
(iii) These are generally unstable
(iv) These are more vulnerable to sudden changes because of low biodiversity, stability and self-regulation.

(a) Removal of all producer reduce primary production in the ecosystem. Hence, no biomass will be available to the successive/higher trophic level or heterotrophic organisms.
(b) Elimination of all organisms of herbivore level results into an increase in primary productivity and biomass of producer and the carnivorous animal will not survive due to inavailability of food due to the elimination of herbivores.
(c) Removal of top carnivores also disturbs the ecosystem as it will result in huge increase in a number of herbivores which will finish plants creating desertification.

Primary productivity can be defined as the rate at which primary producers (e.g. green plants) traps and store solar radiation in the form of biomass. This is measured in terms of weight (gm) and in terms of energy (kcal m²) per year in a given time.
So, primary productivity varies from ecosystem to ecosystem. The ecosystem which possesses more producers will be more productive in terms of primary productivity. The young forest is still developing it may contain fewer trees compared to the old forests. The young forests therefore will not quench the solar radiation as much as old forests. Hence, old forest is more productive as compared to foung one. The shallow polluted lake will be less productive because of less number of producers and high amount of dead organic matter.

In terms of ecosystem dynamics, flow of energy takes place from one trophic level to the next trophic level in unidirectional way. About 50% of solar energy incident over earth is present in Photosynthetic Active Radiation (PAR) and 2-10% of this PAR is utilised by green plants to form chemical energy(Gross Primary Productivity) (GPP). Out of 90% of gross primary productivity is lost maximum during respiration and other vital activities. Gross Primary Productivity of an ecosystem is the rate of production of organic matter during photosynthesis. So nature tend to increase gross primary productivity through supporting the large number of plant (producers) in an ecosystem. Net Primary Productivity is the available biomass for the consumption to heterotrophs (human and animals) man tries to increase net primary productivity by cultivating food and other crops which the depend on to fulfil their needs.

Three above statements do not mean one and the same thing because:
(i) Statement (a) indicates total productivity of wheat but does not indicate the extent of area.
(ii) Statement (b) indicates productivity of wheat/acre on a specific day.
(iii) Statement (c) indicates productivity of wheat/acre/specific period.