Question

Farmer Pete has a corn field. He wishes to increase his corn yield. He hires an ecologist who measures net primary production of the field. The figure is 10,000 kcal/ha of field area. (A) If solar radiation (PAR) is measured at 72,000 kcal/ha/year, what is the photosynthetic efficiency of this field? The ecologist finds that the field primarily contains one species of herbivorous insect. The insect assimilates 76.2 % of what it eats. (B) If this is correct and the insects consume 5000kcal/ha/year, what is the expected net production of insects if the R/A ratio is 70% (C) What is the growth efficiency of the insects?

elaborate how?

Answer 1

A) Photosynthesis efficiency = Net primary production / Amount of light energy absorbed

Plants generally absorb 1-2% of the total solar energy so here the corns abosrb about 720kcal of the total solar radiation.
= 10000kcal/ha / 720kcal/ha
= 13.88 = approx 14 kcal/ha
Photosynthestic efficiency = 14kcal/ha

B) Respiration : Absorption of insects = 70%
R/A = 70/100
Energy from the corn plants = 10% of 10000 = 1000kcal = 70% of energy = production
= 700kcal
Consumption by the insects = 5000kcal/ha/year
Assimilation by the insects = 76.2% = 76.2/100 = 0.762 * 5000 = 3810kcal/ha/year

NPP = GPP - R = production / assimilation
= 700kcal / 3810
= 0.183kcal

C) Growth efficiency = consumer production / prey production
  = (700kcal / 10000) * 100%
= 7%

Because they are the primary consumers as herbivores, they will have higher growth efficiency.