Understanding Ecosystem Productivity: Primary vs. Secondary

Ecosystems function based on the flow of energy and the cycling of matter. Productivity is a key concept in understanding how much energy is captured and how much organic matter is created at different levels within an ecosystem.

Let's break down the different types of productivity mentioned in the options:

Primary Productivity Explained

• What it is: Primary productivity is the rate at which producers (like plants, algae, and some bacteria) convert light energy (or chemical energy in some cases) into organic compounds. This process is mainly photosynthesis.
• Who does it: Autotrophs (organisms that make their own food).
• Output: The total amount of organic matter produced by producers is called Gross Primary Productivity (GPP). When you subtract the energy producers use for respiration, you get Net Primary Productivity (NPP = GPP - Respiration). NPP is the organic matter available to consumers.

Secondary Productivity Explained

• What it is: Secondary productivity is the rate at which consumers assimilate energy and convert it into their own biomass (new organic matter). This happens when consumers eat producers or other consumers.
• Who does it: Heterotrophs (organisms that consume other organisms for energy). This includes herbivores, carnivores, and omnivores.
• Output: It measures the rate of biomass increase at the consumer level. Similar to primary productivity, there's Gross Secondary Productivity (GSP, total energy assimilated) and Net Secondary Productivity (NSP = GSP - Respiration), which is the energy converted into consumer biomass.

Other Types of Productivity

• Tertiary Productivity: While not a standard term in the same way as primary and secondary, it can be used to refer to the rate of biomass formation by tertiary consumers (carnivores that eat other carnivores). It's essentially a type of secondary productivity occurring at a higher trophic level.
• Net Productivity: This is a general term referring to the productivity remaining after accounting for respiration losses. It can be Net Primary Productivity (NPP) or Net Secondary Productivity (NSP). The question specifically asks about the rate of formation of new organic matter *by consumers*, which is covered by secondary productivity.

Identifying the Rate of Organic Matter Formation by Consumers

The question asks for the rate of formation of new organic matter specifically by consumers. Based on our definitions:

• Primary productivity is by producers.
• Secondary productivity is by consumers.
• Net productivity is a general term (NPP or NSP) but the core process by consumers is secondary productivity.
• Tertiary productivity is a specific type of secondary productivity at a higher level.

Therefore, the term that directly describes the rate of formation of new organic matter by consumers is secondary productivity.

Revision Table: Key Productivity Terms

Additional Information on Ecosystem Productivity

Understanding productivity is crucial for studying energy flow and biomass accumulation in different ecosystems. Productivity rates vary greatly depending on factors like climate, nutrient availability, and the types of organisms present.

• Ecosystems with high primary productivity, like tropical rainforests or algal beds, can often support higher levels of secondary productivity.
• Energy transfer between trophic levels is inefficient; typically, only about 10% of the energy from one trophic level is converted into biomass at the next level (the 10% law). This limits the amount of secondary productivity relative to primary productivity.
• Measuring secondary productivity can be more complex than primary productivity, as it involves tracking consumption rates, assimilation efficiency, and respiration losses of various consumer populations.