Introduction

Understanding the productivity of Earth’s ecosystems is essential for ecology, conservation, and climate science. Two key measures are global primary production (the total amount of biomass produced worldwide) and net primary productivity (NPP) per unit area (how much biomass is produced in a given space and time). This article explores which ecosystems top these charts and how your options compare to standard global data.

What Is Primary Production?

Primary production is the rate at which plants and other photosynthetic organisms produce organic matter. Net primary productivity (NPP) is the amount of biomass produced after accounting for the energy used by plants for their own respiration. NPP determines how much energy is available for herbivores and higher trophic levels.

Global Primary Production vs. NPP per Unit Area

• Global Primary Production:

  • Open ocean is the largest contributor to global primary production because it covers about 70% of the Earth’s surface. Even though its NPP per unit area is relatively low, its vast size means it produces more total biomass than any other ecosystem56.

• NPP per Unit Area:

  • Swamps and marshes and tropical rainforests are the most productive terrestrial ecosystems per unit area. Swamps and marshes can sometimes surpass tropical rainforests in NPP per unit area, but tropical rainforests are consistently among the top156.

  • Cultivated land and open ocean are not the highest for NPP per unit area, though open ocean is the largest contributor to global primary production.

Analyzing the Options

Let’s examine the options provided and compare them to the standard data:

• Option 1:

  • Tropical rainforest is among the highest for NPP per unit area, but not for global primary production.

• Option 2:

  • Swamp and marsh is sometimes the highest for NPP per unit area (though not always listed separately).

  • Tropical rainforest is also among the highest for NPP per unit area.

• Option 3:

  • Cultivated land and open ocean are not the highest for NPP per unit area.

• Option 4:

  • Open ocean is the highest for global primary production, but not for NPP per unit area.

What Does the Data Say?

• Highest Global Primary Production:

  • Open ocean (due to its vast area)56.

• Highest NPP per Unit Area:

  • Swamps and marshes (sometimes), tropical rainforests (usually among the top)156.

Why Are These Ecosystems So Productive?

• Open Ocean:

  • Vast area and continuous sunlight allow phytoplankton to produce large amounts of biomass globally, even though productivity per square meter is low.

• Swamps and Marshes:

  • Abundant water and nutrients support rapid plant growth, resulting in very high NPP per unit area.

• Tropical Rainforests:

  • Warm temperatures, abundant rainfall, and high biodiversity support rapid plant growth and high NPP per unit area.

Common Misconceptions

A common misconception is that the most productive ecosystems per unit area are also the largest contributors to global primary production. In reality, the open ocean, despite its low NPP per unit area, is the largest contributor to global primary production due to its size. Conversely, swamps, marshes, and tropical rainforests are the most productive per unit area, but their total global contribution is less than the open ocean because they cover a smaller area.

Conclusion

• Highest global primary production: Open ocean

• Highest NPP per unit area: Swamps and marshes (sometimes), tropical rainforests (usually among the top)

Given your options, if you must select one, and “swamp and marsh” is considered the highest for NPP per unit area (as in some data), then:

Correct answer (among options):
(2) Swamp and marsh, and tropical rainforest