(1) When fishing, it is best to control the population density at point B, where the population growth rate is the largest, which is K2 of the S-shaped curve. If the fish population at point B in Figure A is 1000, the k value of the fish population in the lake is 2000, the birth rate at point D is equal to the death rate, and the population growth rate is 0, so the age structure is stable.

(2)m 1 represents the total energy flowing into the ecosystem, which is fixed by solar energy or producers, and m2 represents the energy lost by the ecosystem, that is, the heat energy or the heat lost by respiration.

(3) Energy decreases step by step in the process of transmission, so the higher the trophic level, the less energy can be used, so the number of individuals is often less.

(4) Figure B shows that the total energy flowing through the ecosystem is the producer's fixed total solar energy. The fixed total solar energy = decomposer+respiratory consumption+flow to the next trophic level+unused = 3.5+52.5+20.0+43.5 =119.5 million kilojoules, and the energy flow shows the characteristics of one-way flow and step-by-step flow. The energy transfer efficiency from B to C is 4÷20× 100%=20%, and the percentage of energy consumed by respiration in trophic level C is 2.8 ÷ 4× 100% = 70%.

So the answer is:

(1)B2000 stable type?

(2) Solar energy (or absorption of fixed energy) thermal energy (or energy lost by breathing)

(3) High trophic level and less available energy?

(4) 1 19.5? One-way flow, step by step downhill? 20%