4-5 Primary Productivity of Ecosystems

How Fast Can Producers Produce Biomass? A Very Important Rate

The rate at which an ecosystem’s producers convert solar energy into chemical energy as biomass is the ecosystem’s gross primary productivity (GPP).

Net primary productivity (NPP) is the rate at which producers use photosynthesis to store energy minus the rate at which they use some of this stored energy through aerobic respiration.

In agricultural systems, the goal is to increase the NPP and biomass of selected crop plants by adding water (irrigation) and nutrients (mostly nitrates and phosphates in fertilizers). Despite such inputs, the NPP of agricultural land is not very high compared with that of many other ecosystems.

How Does the World’s Net Rate of Biomass Production Limit the Populations of Consumer Species? Nature’s Limits

The planet’s NPP ultimately limits the number of consumers (including humans) that can survive on the Earth.

We might also conclude from Figure 4-23 that we could grow more food for human consumption by clearing highly productive tropical rain forests and planting food crops. According to most ecologists, this is also a bad idea. Here is why. In tropical rain forests, most nutrients are stored in the vegetation rather than in the soil, where they need to be to grow crops. When the trees are removed, frequent rains and growing crops rapidly deplete the nutrient-poor soils.

As for oceans, the problem is that harvesting the widely dispersed, tiny floating producers in the open ocean would take much more fossil fuel and other types of energy than the food energy we would get. In addition, this would disrupt the food webs of the open ocean that provide us and other consumer organisms with important sources of energy and protein from fish and shellfish.

How Much of the World’s Net Rate of Biomass Production Do We Use? Let Them Eat Crumbs

Peter Vitousek, Stuart Roystaczer, and other ecologists estimate that humans now use, waste, or destroy about 27% of the Earth’s total potential NPP and 10%–55% of the NPP of the planet’s terrestrial ecosystems.

These scientists contend that this is the main reason we are crowding out or eliminating the habitats and food supplies of a growing number of other species.