A few resources control the production of living organisms. All living organisms are built mainly from six elements – CHNOPS (carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur). If any of these basic elements is in short supply, growth and reproduction will be slowed. Generally, it seems that the two most important elements are nitrogen and phosphorus – for both plants and animals. Hence, we can use the availability of nitrogen and phosphorus as one way to judge how suitable a habitat is for living creatures. The general word we use to describe this suitability is fertility. Let us start with plants.

Fertility and plants

Fertility controls primary productivity (biomass), with nitrogen and phosphorus supplies often limiting plant growth. Sites with low levels of these nutrients are termed infertile. While biomass is directly correlated with fertility, species richness is inversely correlated with fertility. The importance of particular nutrients can be determined through field experiments in which they are supplemented and the effects on plants documented.

Nitrogen and phosphorus often limit plant and animal growth

The availability of nitrogen and phosphorus determines how fast plants can grow. More fertile areas have, among other things, bigger plants, faster-growing plants, more biomass, and hence more production to support more animals. You can easily do the simple experiment of adding nutrients to plots in a marsh, and find, not surprisingly, that the plants get bigger. So what more, you might ask, do we need to know?