Energy Dynamics

The Food Chain

Energy is one of the most important abiotic factors in an ecosystem and organisms in an ecosystem are connected by the flow of energy and matter among one another. Since energy can be neither created nor destroyed, it can only change form or be transferred to the next organism in a food chain. For example, every time a cow grazes on grass or an osprey hunts and consumes fish, energy is transferred from the consumed organism to the consumer. Each of these interactions in a food chain is called a trophic level. Energy gained from these food sources is used to build the tissues of these consumers which, in turn, become sources for the next organisms in the food chain. Understanding the dynamics of energy flow in an ecosystem provides a clearer picture of the delicate balance of our natural world.

At the base of the ecosystem, primary producers unlock the energy for the rest of the organisms in the environment. Primary producers are autotrophic or self-feeding organisms because they can synthesize organic molecules from inorganic material. Examples of producers include chemosynthetic bacteria and photosynthetic plants. These organisms simply trap the energy from sources such as gases from hydrothermal vents or sunlight into organic molecules to sustain themselves. They then become a resource for consumers, which are heterotrophic organisms that cannot create their own organic materials and obtain them from other organisms. The organisms that get their energy from autotrophs are called primary consumers. Next on the food chain are secondary consumers that can feed on primary consumers. Similarly, consumers that can feed on secondary consumers are called tertiary consumers.

Energy flow in a food chain starts with the primary producers, thus the size of the community depends on the amount of energy captured into organic material by the primary producers. Organic material stored in an organism is called the biomass and excludes the water contained by the organism. Therefore, to calculate biomass, the weight of water of an organism is subtracted from its total weight. Biomass within a food chain is partially conserved and generally only a fraction of biomass is transferred to the next trophic level. Therefore, the amounts of biomass passed down in a food chain resemble a pyramid, greatest at the bottom, gradually shrinking towards the top. In such a trophic or energy pyramid, the amount of biomass decreases gradually due to the loss of energy as metabolic heat. Hence, the large fraction of energy consumed, but not turned into biomass, indicates how organisms must work to maintain themselves. Respiration is an exothermic reaction that works to power each individual cell by breaking down nutrients to capture the energy into adenosine triphosphate (ATP), which powers the synthesis and transfer of structures and proteins within the cell. Waste products and heat are produced at the same time, resulting in a smaller amount of biomass in the upper-level organism.

Productivity of an Ecosystem

Changes in biomass in a system is related to the productivity of a particular ecosystem, where productivity is the rate at which organisms gain biomass from received energy. Productivity of primary producers is called primary productivity and that of others is called secondary productivity. This can be further categorized into two forms: gross and net. For instance, gross primary productivity is the rate at which photosynthesis or chemosynthesis occurs, while net primary productivity is the rate at which energy is stored as biomass in these organisms. One can think of net primary productivity as the gross primary productivity subtracted by the energy lost via metabolic processes and daily activities of the organism. Allocation of these biomass resources varies across organisms and indicates the limits of energy supply.

Human Impacts on Energy Flow

The trophic pyramid model of energy flow underscores the importance of the primary producers to the health of the ecosystem: If they are removed from a system, the consumers that rely on them must be forced to turn to another food source. If the consumers are unable to find another source of nutrition, secondary extinctions can occur¹. This is especially important in the near future as human-induced changes will cause unprecedented variations in numerous ecosystems around the world¹. Therefore, understanding energy dynamics in food chains that are under threat can help mitigate negative effects of environmental changes and prevent secondary extinctions.

Human-induced changes can also impact health of organisms at the top of the food chain, including humans. One well-known example is bioaccumulation and biomagnification of mercury in aquatic food chains. Bioaccumulation of mercury begins at the first level, when mercury is absorbed by organisms at the bottom of the food chain. Biomagnification occurs when mercury is passed down to consumers with its concentration increasing at each trophic level. Finally, various large fish species that are higher on the food chain may contain high levels of mercury². Therefore, health professionals advise against consumption of large quantities of certain fish species.

While loss of a primary producer can be detrimental, elimination of certain consumers called “keystone predators” can also have a negative impact. In an experiment where ochre sea stars were removed from a rock, the mussels that these sea stars preyed on overpopulated and exacerbated environmental resources and free space³. As keystone predators, the ochre sea stars kept the mussel population in check and helped keep diversity thriving on the rock. When energy accumulated as biomass in the form of mussels, the environment was tipped out of balance. Thus, understanding how energy is transferred in an ecosystem can aid scientists in learning which populations keep others from overgrowing.

References

1. Eklof, A and Ebenman, B. Species loss and secondary extinctions in simple and complex model communities. J Animal Ecology. 2006, Vol. 75, 1 (239-46).
2. Gray, JS. Biomagnification in marine systems: the perspective of an ecologist. Marine Pollution Bulletin. 2002, Vol. 45, 46-52.
3. Paine, RT. Food Web Complexity and Species Diversity. The American Naturalist. 1966, Vol. 100, 910 (65-75).

The first law of thermodynamics, energy cannot be created or destroyed, only transformed, can be demonstrated within a classic food web. Here light energy from the sun is first harnessed as radiant energy by plants and is then converted into chemical energy stored as complex carbohydrates. Some vegetation is eventually consumed by animals. In their process of breaking down the sugars, energy is either released as heat, stored in macromolecules as chemical energy reserves to be used later, or passed along to a predator. Each of these stages in the food chain is referred to as the tropic level. Plants are the producers. The squirrel would be the primary consumer, and the predatory fox would be the secondary consumer. The organic matter that is getting transferred from tropic level to the next is called the biomass - usually measured in energy units, calories or kilocalories.

However, the transfer of biomass is not linear. After primary producers receive energy from the sun and make food, due to cellular respiration a small amount is transformed into unusable heat energy which is released along with carbon dioxide into the environment. The total light energy harnessed is called the gross primary productivity or GPP. If you subtract the energy lost to respiration from GPP the result is net primary productivity or NPP, which is the energy rate at which biomass is stored. Similarly, while primary consumers are harvesting chemical energy from plants, they also release a small amount of heat energy along with carbon dioxide during metabolism. And only a part of the consumed biomass is restored into their tissues. Finally, at the predator level, only a fraction of the original energy harnessed from the sun is available to use. Therefore, the biomass transferred from the producer to the primary consumer is not equal to the biomass transferred from the producer to the secondary consumer. These changes in biomass at each level of a food chain tell us about the productivity of a particular ecosystem as a whole.

In this laboratory you will investigate these principles of energy dynamics and productivity by measuring the transfer of biomass and energy from a producer, the cabbage, to a primary consumer, the cabbage worm.