Ecosystem Productivity
Ecosystem productivity or Biomass Productivity refers to the rate at which an ecosystem can convert sunlight and nutrients into plant biomass. It is a key attribute that determines the abundance and distribution of organisms within that ecosystem.
Types of Ecosystem Productivity
There are different ways to measure and classify ecosystem productivity.
Primary Productivity
Primary production is the synthesis of new organic material from inorganic molecules such as water and CO2 via photosynthesis or chemosynthesis. The rate at which radiant energy is stored by photosynthetic and chemosynthetic activity of producers is called primary productivity. It can be divided into two parts.
- Gross Primary Productivity (GPP): The total energy fixed by plants via photosynthesis is called Gross Primary Productivity. In other words, it is the total rate of photosynthetic production of plant biomass per unit area over a period. It estimates the amount of solar energy converted into chemical energy by plants. Tropical rainforests have highest GPP.
- Net Primary Productivity (NPP): A small fraction of energy fixed via GPP is used in the respiration of plants, which gives them necessary energy for various physiological and morphological functions. When this respiratory utilization is reduced from Gross Primary Productivity, what we get is Net Primary Productivity. It represents the plant material available for consumption by herbivores and omnivores. Estuaries have very high NPP.
The primary productivity is also known as energy storage at producer level.
Secondary Productivity
Productivity of heterotrophs such as animals is called secondary productivity. It is also known as energy storage at consumer level. Secondary productivity is done by consumers via assimilation of the food they take. Since not all the primary biomass is consumed; and since not all the consumed is digested; secondary production is only a small fraction of primary production. Intensively managed pasturelands can have high secondary productivity.
Units of Measurement of Ecosystem Productivity
Ecosystem Productivity is expressed in units of energy (example: joules per meter² per day) or in units of dry organic matter (example: kg per meter² per year). Generally, Primary Productivity is measured in units of energy fixed per unit area per unit time. Common units for this purpose are:
- Kilocalories per square meter per year (kcal/m2/yr)
- Joules per square meter per year (J/m2/yr)
Secondary Productivity is measured in units of dry weight biomass accumulated per unit area per unit time. Common units are:
- Grams per square meter per year (g/m2/yr)
- Kilograms per square meter per year (kg/m2/yr)
These are the standard units used to quantify both biomass productivity as well as ecosystem productivity. The choice of units depends on whether we are measuring the rate of energy storage by producers (primary productivity) or the rate of biomass accumulation by consumers (secondary productivity).
But in all cases, the rate is standardized based on per unit area (m2) per unit time (yr) to allow comparison across ecosystems.
Classification of Ecosystems on the basis of Productivity
On the base of ecosystem ecosystems can be categorized from least productive to most productive in the following classes:
Unproductive Ecosystems
Extreme ecosystems like deserts and tundra have limited plant growth and primary productivity due to lack of moisture and nutrients. Per unit area NPP is less than <200 g (carbon)/ m2/yr. Herbivore consumers also very low.
Moderate Ecosystems
Coniferous forests, rainforests and most grasslands have modest net primary productivity between 300-1500 g (carbon) /m2/yr. Support medium density wildlife and livestock populations.
Highly Productive Ecosystems
Intertidal wetlands, estuaries, coral reefs, temperate deciduous forests and fertilized croplands have very high primary or secondary productivity exceeding 1500-2000 g/m2/yr. Support high density of animal and plant biomass.
Highest primary productivity is found in Tropical Forests, Estuaries and Swamps/ Marshes. Tropical Rainforests have high primary productivity because of availability of plenty of solar light and water. A typical estuary has high primary productivity because it is shallow (gets plenty of sunlight) and has turbulent water (which brings the nutrient rich material from sea bed). Swamps and Marshes have high primary productivity because they have lots of nutrients and sunlight.
After Tropical Rainforests, Estuaries and Swamps / Marshes, the highest primary productivity is found in coral reefs, algal beds and temperate forests. Least primary productivity is found in cold and hot deserts including tundra.
Table of productivity of various Ecosystems
| Type of Ecosystem | Primary Productivity (g/m²/yr) | Description |
| Tropical Rainforest | 2200 – 2500 | High biodiversity, dense vegetation, significant rainfall. |
| Temperate Forest | 1200 – 2000 | Deciduous and coniferous trees, distinct seasonal changes. |
| Boreal Forest (Taiga) | 800 – 1500 | Coniferous forests, cold climate, shorter growing seasons. |
| Temperate Grassland | 600 – 1200 | Dominated by grasses, seasonal temperature variation, moderate rainfall. |
| Savanna | 900 – 1500 | Grassland ecosystem with scattered trees, warm temperatures, seasonal rainfall. |
| Desert | < 250 | Low rainfall, extreme temperatures, sparse vegetation. |
| Tundra | < 400 | Cold, treeless, with low-growing vegetation. Short growing seasons. |
| Wetlands | 1500 – 2500 | Marshes, swamps, and bogs; high water saturation, diverse plant and animal life. |
| Freshwater Lakes and Rivers | 500 – 1500 | Freshwater bodies, supporting aquatic plants and animals. |
| Estuaries | 1500 – 2500 | Where freshwater meets the sea, high nutrient levels, supports diverse species. |
| Coral Reefs | 2000 – 2500 | Marine ecosystems with coral structures, high biodiversity, warm shallow waters. |
| Open Ocean | 100 – 350 | Largest ecosystem, lower productivity due to nutrient availability, supports diverse marine life. |
| Coastal Ocean | 300 – 500 | Higher productivity than open ocean, influenced by land runoff, supports kelp forests and seagrass beds. |
| Mangroves | 1000 – 2000 | Coastal ecosystems with salt-tolerant trees and shrubs, high productivity, and biodiversity. |
| Polar Ice and Seas | < 200 | Extreme cold, low productivity, supports specialized wildlife like polar bears and penguins. |
Threats to Ecosystem Productivity
Several human activities pose threats to ecosystem productivity:
- Land Use Changes: Conversion of forests, wetlands and natural ecosystems into concrete urban centers sharply reduces primary productivity and negatively impacts food chains.
- Overexploitation: Excess hunting, fishing, intensive agriculture lead to loss of species and nutrient exhaustion, declining productivity over time.
- Pollution: Air and water pollution reduce photosynthesis rates and plant growth causing lower NPP. Oil spills smother intertidal estuarine productivity.
- Invasive Species: Non-native species introductions outcompete native flora/fauna changing community structure and productivity.
Protecting ecosystem productivity is vital for sustainable food production for the growing human population worldwide. Achieving a balance between conservation practices and responsible use of these natural biological systems is essential.
