Energy Flow In Ecosystem

Energy Flow in Ecosystem

Primary Productivity

Approximately 1 to 5 % of the solar energy falling under plants is converted into organic material. Primary production or primary productivity are terms used to define the amount of organic matter generated by solar energy over a given period of time in a given area. Gross primary productivity is the total produced organic matter, including that used for respiration by the photosynthetic organism. Net primary productivity is a measure of the amount of organic matter produced for heterotrophs in a community in a given time. It is equal to the primary gross productivity minus the amount of energy spent by the photosynthetic organisms ' metabolic activities. As a result of its net production, the net weight of all organisms living in an ecosystem, its biomass, increases.

Secondary Productivity

The rate of production by heterotrophs is called secondary productivity. Because herbivores and carnivores cannot carry out photosynthesis, they do not manufacture biomolecules directly from CO2. Instead, they obtain them by eating plants or other heterotrophs. Herbivores ' secondary productivity is about less than the primary productivity on which it is based. Much of the biomass is not consumed by herbivores but supports the community of decomposers. In addition, some energy is not assimilated by the body of the herbivore but transmitted to the decomposers as feces. 

The Energy in Food Chains

There are 2 or 4 steps in food chains. At each step, a significant amount of energy is lost and very little usable energy remains in the system after being incorporated into the organism bodies at four successive trophic levels.

Ecological Pyramids

A plant fixes about 1% of the energy of the sun falling on its green parts. In turn, the successive members of a food chain process about 10% of the energy available in the organisms they feed into their own bodies. Therefore, at the lower trophic levels of an ecosystem, there are generally far more individuals than at higher levels. Similarly, the primary producers ' biomass present in a particular ecosystem is greater than the primary consumers ' biomass, with successive trophic levels having lower and lower biomass and correspondingly far less potential energy. Such relationships appear as pyramids if they are shown diagrammatically.

Inverted Pyramids

Certain aquatic ecosystems have inverted pyramids of biomass. For example, in a planktonic ecosystem — overtaken by small living organisms floating in the water— the lowest-level turnover of photosynthetic phytoplankton is very fast, with zooplankton consuming phytoplankton so fast that phytoplankton (the producers at the food chain's base) can never develop a large population size. This is so because even though phytoplankton reproduces very quickly, the community can support a heterotrophic population that is larger in biomass and more numerous than phytoplankton.

Top Carnivores

It can be supported by a community. As we've seen, only about one-thousandth of photosynthesis energy passes through a three-stage food chain to a tertiary consumer like a snake or a hawk. This explains why there are no predators that survive on lions or eagles — these animals ' biomass is simply insufficient to support another level of the trophic level. Top-level predators tend to be quite large animals in the pyramid of numbers. The small residual biomass at the top of the pyramid is therefore concentrated in a relatively small number of people.

Interactions among Different Trophic Levels

The existence of food webs creates the possibility at different trophic levels of interactions between species. Predators will affect not only the species they prey on, but also, indirectly, the plants they eat. In contrast, higher primary productivity will not only provide more food for herbivores but will also indirectly lead to more food for carnivores.

Trophic Cascades

We see a profusion of plant life when we look at the world around us. Why is it? Why don't the populations of herbivores increase to the extent that they consume all available vegetation? The answer is, of course, that predators keep the populations of herbivores under control so that plant populations can thrive. This phenomenon is called a trophic cascade, in which the effect of one trophic level flows down to lower levels. The existence of trophic cascades was confirmed by experimental studies. For instance, sections of a stream were isolated with a mesh in one study in New Zealand that prevented fish from entering. Brown trout have been added in some of the enclosures, while other enclosures have been left without large fish. After 10 days, in the control, the number of invertebrates in the trout enclosures was half that. In turn, in the trout enclosures, the biomass of algae that feeds on invertebrates was five times greater than in the controls.

Trophic cascades logic leads to the prediction that a fourth trophic level, carnivores preying on other carnivores, would also result in cascading effects. In this case, lower-level predator populations would be kept in check by the top predators, which would result in a profusion of herbivores and a lack of vegetation. Enclosures have been created in free-flowing streams in northern California in an experiment similar to the one just described. In this case, some enclosures were filled with large predatory fish, not others. In the large fish enclosures, the number of smaller predators, such as damage nymphs, has been greatly reduced, leading to an increase in their prey, including algae - fed insects, which in turn lead to a decrease in algae biomass.

Bottom-Up Effects

Factors at the bottom of food webs, on the other hand, can have consequences that ramify to higher trophic levels, leading to so-called bottom-up effects. The basic idea is to be too small to support any predators when an ecosystem's productivity is low. Increased productivity will be completely consumed by herbivores, whose populations will actually increase in size. At some point, populations of herbivores will be large enough to support predators. Consequently, further increases in productivity will not lead to higher populations of herbivores, but rather to higher populations of predators.

Once again, top predators that can prey on lower - level predators will be established at some level. With lower predator populations under control, the populations of herbivores will increase again with increased productivity.

An elegant study on the Eel River in northern California provided experimental evidence for the role of bottom-up effects. Enclosures that excluded large fish were built. There was a roof above every enclosure. Some roofs have been clear and light has passed through, while others have produced light or deep shade. As a result, the enclosures differed in how much sunlight they reached. The primary productivity differed, as one might expect, and was the largest in the unshaded enclosures. This increased productivity resulted in both more vegetation and more predators, but the trophic level sandwiched between them, the herbivores did not increase exactly as predicted by the bottom-up hypothesis.

Relative Importance of Trophic Cascades and Bottom-Up Effects

There is no inevitability of trophic cascades or bottom-up effects. For instance, if two species of herbivores exist in an ecosystem and compete strongly, and one species is much more vulnerable to predation than the other, top-down effects will not spread to the next lower trophic level. Increased predation will simply decrease the vulnerable species population while increasing its competitor's population, with no potential net change in vegetation at the next lower trophic level.

Similarly, increases in productivity may not rise across all trophic levels. For example, in some cases, prey populations are increasing so rapidly that they cannot be controlled by their predators. In such cases, productivity increases would not move the food chain upwards.

In other cases, each other may be strengthened by trophic cascades and bottom-up effects. Large fish were removed from one lake in one experiment, leaving only minnows that ate most of the zooplankton eating algae. In contrast, there were few minnows and a lot of zooplankton in the other lake. Nutrients were then added to both lakes by the researchers. There was little zooplankton in the minnow lake, so the resulting algal productivity increase did not propagate the food chain and large algae mats formed. By comparison, increased productivity moved up the food chain in the large fish lake and controlled algae populations.

Top-down, as well as, bottom-up processes were operating in this case. Nature is not always that simple, of course. In some cases, species can operate at multiple trophic levels simultaneously, such as the jaguar eating smaller carnivores and herbivores, or the bear eating both fish and berries. Nature is often considerably more complicated than a simple, linear food chain. Currently, ecologists are working to apply food chain interaction theories to these more complicated situations.