Primary productivity and secondary productivity are the rates of primary and secondary production. They are measured as biomass (or its chemical energy store) in a given area in a given time, for example kJ ha−1 yr−1. Primary productivity refers to producers such as crops; secondary productivity refers to consumers such as livestock.Farming aims to transfer as much as possible of the energy captured by producers into food for humans. In a natural ecosystem, much of the NPP is eaten by other organisms, passes to decomposers or is used in respiration. Farming practices increase the efficiency of energy transfer in two main ways:1. Simplifying food webs to reduce energy losses to non-human food chains, for example by removing weeds and pests.
2. Reducing respiratory losses within a human food chain, for example by limiting the movement of livestock and keeping them warm.Crop productivity can also be raised by removing limiting factors (irrigation, fertilisers, glasshouses) and by selective breeding of high-yielding varieties.Worked example (units): A wheat field has a net primary productivity of 1.2 × 108 kJ ha−1 yr−1. Since 1 ha = 10 000 m2, this is 1.2 × 108 / 10 000 = 12 000 kJ m−2 yr−1.These practices raise yields and lower the cost of food, but they raise issues: intensive rearing raises animal welfare concerns, and pesticides and fertilisers can harm the wider environment. Farmers have to balance productivity against cost, welfare and environmental impact.
2. Reducing respiratory losses within a human food chain, for example by limiting the movement of livestock and keeping them warm.Crop productivity can also be raised by removing limiting factors (irrigation, fertilisers, glasshouses) and by selective breeding of high-yielding varieties.Worked example (units): A wheat field has a net primary productivity of 1.2 × 108 kJ ha−1 yr−1. Since 1 ha = 10 000 m2, this is 1.2 × 108 / 10 000 = 12 000 kJ m−2 yr−1.These practices raise yields and lower the cost of food, but they raise issues: intensive rearing raises animal welfare concerns, and pesticides and fertilisers can harm the wider environment. Farmers have to balance productivity against cost, welfare and environmental impact.