12.2. Productivity

A constant input of solar energy is the basic requirement for any ecosystem to function and sustain. Primary production is defined as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is expressed in terms of weight (gm–2) or energy (kcal m–2). The rate of biomass production is called productivity.

This chunk introduces us to the concept of primary production, which is essential for understanding how ecosystems thrive. Primary production refers to the process by which plants convert solar energy into biomass—this means they create organic materials that form the basis of the food web. It's measured per area and time, such as grams of biomass produced in a square meter over a year.

You can think of primary production like the growth of a garden. Just as plants in a garden absorb sunlight to grow over time, ecosystems rely on solar energy to support life. In this analogy, measuring the amount of produce (like fruits and vegetables) from the garden gives a sense of the garden's productivity.

It is expressed in terms of gm–2 yr–1 or (kcal m–2) yr–1 to compare the productivity of different ecosystems. It can be divided into gross primary productivity (GPP) and net primary productivity (NPP).

Here, we learn how to quantify productivity. It can be measured in terms of grams of organic matter produced per square meter over a year or in terms of energy produced in kilocalories. This section also introduces two key concepts in productivity: GPP and NPP. Gross primary productivity (GPP) refers to the total amount of biomass produced via photosynthesis, while net primary productivity (NPP) accounts for the amount of biomass left after subtracting what the plants use for their own respiration.

Imagine a farmer who grows crops. GPP is like the total harvest from the field, while NPP is what the farmer can actually sell after using some of that produce to feed themselves and their family. So, GPP gives a total picture, but NPP is what really counts for available resources.

Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration. Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP). GPP – R = NPP.

This chunk clarifies the relationship between GPP and NPP mathematically. We see that plants produce a lot of organic matter through photosynthesis, but they also use some of that energy for their own maintenance (respiration), which is a necessary process for survival. The remaining biomass after this energy usage is known as net primary productivity and is crucial for energy transfer within ecosystems.

Think of GPP as your total salary at work. You earn a certain amount, but after taxes and other deductions (like bills), what you take home is your net income, which is analogous to NPP. This 'take-home' amount is what you can spend or save, similar to how NPP is what gets passed on to the herbivores in an ecosystem.

Net primary productivity is the available biomass for the consumption to heterotrophs (herbivores and decomposers). Secondary productivity is defined as the rate of formation of new organic matter by consumers.

This section distinguishes between primary and secondary productivity. NPP provides the energy available for heterotrophs (organisms that consume organic matter). Secondary productivity refers to how consumers—like herbivores and decomposers—create their own organic matter from consuming plants. Essentially, it shows how energy flows through an ecosystem, first from producers (plants) to primary consumers (herbivores) and then to higher-level consumers.

You can think of a restaurant as an analogy. The primary productivity is like the ingredients a chef buys to prepare meals (plants growing). The secondary productivity is like the meals that the chef prepares and serves to customers (consuming plants). The customers (herbivores) consume the meals, which represents the transfer of energy and organic matter.

Primary productivity depends on the plant species inhabiting a particular area. It also depends on a variety of environmental factors, availability of nutrients and photosynthetic capacity of plants. Therefore, it varies in different types of ecosystems.

This chunk highlights that not all ecosystems have the same primary productivity due to differing factors. The types of plants, the climate, availability of water, nutrients in the soil, and even sunlight can all influence how productive an ecosystem can be. Some ecosystems, like rainforests, may support high productivity due to ideal conditions, while deserts may have low productivity.

Consider a greenhouse versus a desert. A greenhouse provides optimal conditions for plant growth, much like a tropical rainforest that has plenty of light, nutrients, and moisture. In contrast, a desert lacks these resources, making it difficult for plants to thrive. This difference in conditions leads to significant variability in productivity across different ecosystems.

The annual net primary productivity of the whole biosphere is approximately 170 billion tons (dry weight) of organic matter. Of this, despite occupying about 70 percent of the surface, the productivity of the oceans are only 55 billion tons. Rest of course, is on land. Discuss the main reason for the low productivity of ocean with your teacher.

This chunk gives a global perspective on primary productivity, stating that the total productivity for the Earth's ecosystems is immense. However, it contrasts land productivity with ocean productivity, noting that oceans, while vast, yield less biomass than land areas. This invites discussion about factors that contribute to this difference, such as nutrient availability and light penetration in water.

Think of the oceans as a large cake that looks beautiful but might be missing key ingredients inside. Just like a cake can appear promising yet not taste as good if not properly baked (or with the right ingredients), oceans cover a large area but often lack the necessary nutrients in deeper waters for high productivity compared to the lush, green landscapes of forests and grasslands.

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