Primary and Secondary Productivity — Definition
Definition
Imagine an ecosystem as a bustling factory where energy is constantly being processed and transformed. Productivity, in simple terms, is the rate at which this factory produces new organic material, or biomass. Think of it like the output rate of a manufacturing plant. In ecology, we distinguish between two main types: primary and secondary productivity.
Primary Productivity is all about the initial creation of organic matter. This is the job of the 'producers' in our factory – mainly plants, algae, and some bacteria. These organisms are unique because they can take simple inorganic substances, like carbon dioxide and water, and, using an energy source (usually sunlight through photosynthesis, or chemical energy through chemosynthesis), convert them into complex organic molecules like glucose.
This process is the very foundation of almost all food webs on Earth. The rate at which these producers generate this new organic material is called primary productivity. It's often measured in terms of energy (e.
g., kilocalories per square meter per year) or biomass (e.g., grams of organic matter per square meter per year).
- Gross Primary Productivity (GPP): — This is the total amount of organic matter or energy that the producers synthesize during photosynthesis. It's the 'raw' production before any deductions.
- Net Primary Productivity (NPP): — Not all the organic matter produced by plants is available to other organisms. Plants themselves need energy to live, grow, and maintain their cellular processes (respiration). So, NPP is GPP minus the energy lost by producers during respiration (R). It's the organic matter remaining after the producers have met their own energy needs, and it's this NPP that is available for consumption by herbivores and decomposers. Think of it as the 'net profit' after operational costs.
Secondary Productivity, on the other hand, deals with the production of new organic matter by 'consumers' – the animals and other heterotrophs in our factory. These organisms cannot produce their own food; they must obtain energy by eating other organisms.
When a herbivore eats a plant, or a carnivore eats a herbivore, they assimilate some of the energy from their food and convert it into their own body tissues (growth, reproduction). The rate at which these consumers generate new biomass is called secondary productivity.
Unlike primary productivity, secondary productivity doesn't involve converting inorganic matter into organic matter; it's about converting existing organic matter from one trophic level into new organic matter at a higher trophic level.
It's generally much lower than primary productivity because a significant amount of energy is lost as heat during metabolic processes at each transfer between trophic levels. Just like primary productivity, we can talk about Gross Secondary Productivity (GSP) and Net Secondary Productivity (NSP), where NSP accounts for the energy lost by consumers through their own respiration.