What is the relationship between primary productivity and food webs

what is the relationship between primary productivity and food webs

An ecosystem's gross primary productivity (GPP) is the total amount of organic way to describe the flux of energy through ecosystems is as a food chain in which without factoring in more complex relationships between individual species. This idea is typically tested by looking for relationships between primary productivity and food-chain length (although as others have noted, variation in. assemblage levels. At the species level, the relationship between . association between primary productivity and food chain length, the first such report for a.

In almost all ecosystems, photosynthesizers are the only "gateway" for energy to flow into food webs networks of organisms that eat one another. If photosynthesizers were removed, the flow of energy would be cut off, and the other organisms would run out of food. In this way, photosynthesizers lay the foundation for every light-receiving ecosystem.

Energy flow & primary productivity

Producers are the energy gateway Plants, algae, and photosynthetic bacteria act as producers. Producers are autotrophs, or "self-feeding" organisms, that make their own organic molecules from carbon dioxide.

what is the relationship between primary productivity and food webs

Photoautotrophs like plants use light energy to build sugars out of carbon dioxide. The energy is stored in the chemical bonds of the molecules, which are used as fuel and building material by the plant. The energy stored in organic molecules can be passed to other organisms in the ecosystem when those organisms eat plants or eat other organisms that have previously eaten plants.

In this way, all the consumers, or heterotrophs "other-feeding" organisms of an ecosystem, including herbivores, carnivores, and decomposers, rely on the ecosystem's producers for energy.

Production also is a rate, measured per time unit, while standing crop biomass is the amount of plant matter at a given point in time. The ratio of standing crop to production is called turnover. The turnover time of a system is important in determining how a system functions. Production rates can be quantified by a simple method by which oxygen or carbon production is measured. Production can also be quantified by measuring the rate of new biomass accumulation over time.

The distinction between gross primary production GPPnet primary production NPPand net ecosystem production NEP is critical for understanding the energy balance in plants and in whole ecosystems. Production varies among ecosystems, as well as over time within ecosystems. Rates of production are determined by such factors as climate and nutrient supply. Precipitation is the dominant control worldwide, but nutrient availability often limits primary production in any particular, local system.

The Flow of Energy to Higher Trophic Levels In the section above we examined the creation of organic matter by primary producers. Without autotrophs, there would be no energy available to all other organisms that lack the capability of fixing light energy. However, the continual loss of energy due to metabolic activity puts limits on how much energy is available to higher trophic levels this is explained by the Second Law of Thermodynamics. Today we will look at how and where this energy moves through an ecosystem once it is incorporated into organic matter.

Most of you are now familiar with the concept of the trophic level see Figure 1. It is simply a feeding level, as often represented in a food chain or food web. Primary producers comprise the bottom trophic level, followed by primary consumers herbivoresthen secondary consumers carnivores feeding on herbivoresand so on.

When we talk of moving "up" the food chain, we are speaking figuratively and mean that we move from plants to herbivores to carnivores. This does not take into account decomposers and detritivores organisms that feed on dead organic matterwhich make up their own, highly important trophic pathways.

What happens to the NPP that is produced and then stored as plant biomass at the lowest trophic level? On average, it is consumed or decomposed. You already know the equation for aerobic respiration: If NPP was not consumed, it would pile up somewhere.

Usually this doesn't happen, but during periods of Earth's history such as the Carboniferous and Pennsylvanian, enormous amounts of NPP in excess of the degradation of organic matter accumulated in swamps. It was buried and compressed to form the coal and oil deposits that we mine today. When we burn these deposits same chemical reaction as above except that there is greater energy produced we release the energy to drive the machines of industry, and of course the CO2 goes into the atmosphere as a greenhouse gas.

This is the situation that we have today, where the excess CO2 from burning these deposits past excess NPP is going into the atmosphere and building up over time, dramatically changing our climate. But let's get back to an ecosystem that is balanced, or in "steady state" "equilibrium" where annual total respiration balances annual total GPP.

As energy passes from trophic level to trophic level, the following rules apply: Only a fraction of the energy available at one trophic level is transferred to the next trophic level. Typically the numbers and biomass of organisms decrease as one ascends the food chain. The Fox and the Hare To understand these rules, we must examine what happens to energy within a food chain. Suppose we have some amount of plant matter consumed by hares, and the hares are in turn consumed by foxes.

The following diagram Figure 2 illustrates how this works in terms of the energy losses at each level. A hare or a population of hares ingests plant matter; we'll call this ingestion. Part of this material is processed by the digestive system and used to make new cells or tissues, and this part is called assimilation.

What cannot be assimilated, for example maybe some parts of the plant stems or roots, exits the hare's body and this is called excretion. Thus we can make the following definition: The hare uses a significant fraction of the assimilated energy just being a hare -- maintaining a high, constant body temperature, synthesizing proteins, and hopping about. This energy used lost is attributed to cellular respiration. The remainder goes into making more hare biomass by growth and reproduction that is, increasing the overall biomass of hares by creating offspring.

The conversion of assimilated energy into new tissue is termed secondary production in consumers, and it is conceptually the same as the primary production or NPP of plants. In our example, the secondary production of the hare is the energy available to foxes who eat the hares for their needs. Clearly, because of all of the energy costs of hares engaged in normal metabolic activities, the energy available to foxes is much less than the energy available to hares.

Just as we calculated the assimilation efficiency above, we can also calculate the net production efficiency for any organism. This efficiency is equal to the production divided by the assimilation for animals, or the NPP divided by the GPP for plants.

The "production" here refers to growth plus reproduction. These ratios measure the efficiency with which an organism converts assimilated energy into primary or secondary production.

Energy flow & primary productivity (article) | Khan Academy

These efficiencies vary among organisms, largely due to widely differing metabolic requirements. The reason that some organisms have such low net production efficiencies is that they are homeotherms, or animals that maintain a constant internal body temperature mammals and birds. This requires much more energy than is used by poikilotherms, which are also known as "cold-blooded" organisms all invertebrates, some vertebrates, and all plants, even though plants don't have "blood" that do not regulate their temperatures internally.

Just as we can build our understanding of a system from the individual to the population to the community, we can now examine whole trophic levels by calculating ecological efficiencies. You might think of it as the efficiency of hares at converting plants into fox food. Note that the ecological efficiency is a "combined" measure that takes into account both the assimilation and net production efficiencies.

You can also combine different species of plants and animals into a single trophic level, and then examine the ecological efficiency of for example all of the plants in a field being fed on my all of the different grazers from insects to cows. Thinking about the overall ecological efficiency in a system brings us back to our first rule for the transfer of energy through trophic levels and up the food chain. For example, If hares consumed kcal of plant energy, they might only be able to form kcal of new hare tissue.

For the hare population to be in steady state neither increasing nor decreasingeach year's consumption of hares by foxes should roughly equal each year's production of new hare biomass.

Primary production - Wikipedia

So the foxes consume about kcal of hare biomass, and convert perhaps 10 kcal into new fox biomass. The overall loss of energy from lower to higher trophic levels is important in setting the absolute number of trophic levels that any ecosystem can contain. From this understanding, it should be obvious that the mass of foxes should be less than the mass of hares, and the mass of hares less than the mass of plants.

Generally this is true, and we can represent this concept visually by constructing a pyramid of biomass for any ecosystem see Figure 3. A pyramid of biomass showing producers and consumers in a marine ecosystem. Pyramids of Biomass, Energy, and Numbers A pyramid of biomass is a representation of the amount of energy contained in biomass, at different trophic levels for a given point in time Figure 3, above, Figure 4-middle below.

The amount of energy available to one trophic level is limited by the amount stored by the level below. Because energy is lost in the transfer from one level to the next, there is successively less total energy as you move up trophic levels. In general, we would expect that higher trophic levels would have less total biomass than those below, because less energy is available to them. We could also construct a pyramid of numbers, which as its name implies represents the number of organisms in each trophic level see Figure 4-top.

For the grassland shown in Figure 4-top, the bottom level would be quite large, due to the enormous number of small plants grasses. For other ecosystems such as the temperate forest, the pyramid of numbers might be inverted: Just as with the inverted pyramid of numbers, in some rare exceptions, there could be an inverted pyramid of biomass, where the biomass of the lower trophic level is less than the biomass of the next higher trophic level.

what is the relationship between primary productivity and food webs

The oceans are such an exception because at any point in time the total amount of biomass in microscopic algae is small. Thus a pyramid of biomass for the oceans can appear inverted see Figure 4b-middle. You should now ask "how can that be? This is a good question, and can be answered by considering, as we discussed above, the all important aspect of "time".

what is the relationship between primary productivity and food webs

Even though the biomass may be small, the RATE at which new biomass is produced may be very large. Thus over time it is the amount of new biomass that is produced, from whatever the standing stock of biomass might be, that is important for the next trophic level. We can examine this further by constructing a pyramid of energy, which shows rates of production rather than standing crop. Once done, the figure for the ocean would have the characteristic pyramid shape see Figure 4-bottom.

Algal populations can double in a few days, whereas the zooplankton that feed on them reproduce more slowly and might double in numbers in a few months, and the fish feeding on zooplankton might only reproduce once a year.

Thus, a pyramid of energy takes into account the turnover rate of the organisms, and can never be inverted. Note that this dependence of one trophic level on a lower trophic level for energy is why, as you learned in the lectures on predation, the prey and predator population numbers are linked and why they vary together through time with an offset.

Pyramids of numbers, biomass, and energy for various ecosystems. The Residence Time of Energy. We see that thinking about pyramids of energy and turnover time is similar to our discussions of residence time of elements.

Larger autotrophs, such as the seagrasses and macroalgae seaweeds are generally confined to the littoral zone and adjacent shallow waters, where they can attach to the underlying substrate but still be within the photic zone. There are exceptions, such as Sargassumbut the vast majority of free-floating production takes place within microscopic organisms.

Differences in relative photosynthesis between plankton species under different irradiance The factors limiting primary production in the ocean are also very different from those on land. The availability of water, obviously, is not an issue though its salinity can be.

Similarly, temperature, while affecting metabolic rates see Q10ranges less widely in the ocean than on land because the heat capacity of seawater buffers temperature changes, and the formation of sea ice insulates it at lower temperatures. However, the availability of light, the source of energy for photosynthesis, and mineral nutrientsthe building blocks for new growth, play crucial roles in regulating primary production in the ocean.

This is a relatively thin layer 10— m near the ocean's surface where there is sufficient light for photosynthesis to occur. Light is attenuated down the water column by its absorption or scattering by the water itself, and by dissolved or particulate material within it including phytoplankton. Net photosynthesis in the water column is determined by the interaction between the photic zone and the mixed layer.

Turbulent mixing by wind energy at the ocean's surface homogenises the water column vertically until the turbulence dissipates creating the aforementioned mixed layer. The deeper the mixed layer, the lower the average amount of light intercepted by phytoplankton within it. The mixed layer can vary from being shallower than the photic zone, to being much deeper than the photic zone.

When it is much deeper than the photic zone, this results in phytoplankton spending too much time in the dark for net growth to occur. The maximum depth of the mixed layer in which net growth can occur is called the critical depth. As long as there are adequate nutrients available, net primary production occurs whenever the mixed layer is shallower than the critical depth.

Both the magnitude of wind mixing and the availability of light at the ocean's surface are affected across a range of space- and time-scales.

The most characteristic of these is the seasonal cycle caused by the consequences of the Earth's axial tiltalthough wind magnitudes additionally have strong spatial components. Consequently, primary production in temperate regions such as the North Atlantic is highly seasonal, varying with both incident light at the water's surface reduced in winter and the degree of mixing increased in winter.

In tropical regions, such as the gyres in the middle of the major basinslight may only vary slightly across the year, and mixing may only occur episodically, such as during large storms or hurricanes.

Annual mean sea surface nitrate for the World Ocean. Data from the World Ocean Atlas Mixing also plays an important role in the limitation of primary production by nutrients. Inorganic nutrients, such as nitratephosphate and silicic acid are necessary for phytoplankton to synthesise their cells and cellular machinery. Because of gravitational sinking of particulate material such as planktondead or fecal materialnutrients are constantly lost from the photic zone, and are only replenished by mixing or upwelling of deeper water.

This is exacerbated where summertime solar heating and reduced winds increases vertical stratification and leads to a strong thermoclinesince this makes it more difficult for wind mixing to entrain deeper water. Consequently, between mixing events, primary production and the resulting processes that leads to sinking particulate material constantly acts to consume nutrients in the mixed layer, and in many regions this leads to nutrient exhaustion and decreased mixed layer production in the summer even in the presence of abundant light.

However, as long as the photic zone is deep enough, primary production may continue below the mixed layer where light-limited growth rates mean that nutrients are often more abundant. Iron[ edit ] Another factor relatively recently discovered to play a significant role in oceanic primary production is the micronutrient iron. A major source of iron to the oceans is dust from the Earth's desertspicked up and delivered by the wind as aeolian dust.

In regions of the ocean that are distant from deserts or that are not reached by dust-carrying winds for example, the Southern and North Pacific oceansthe lack of iron can severely limit the amount of primary production that can occur.

These areas are sometimes known as HNLC High-Nutrient, Low-Chlorophyll regions, because the scarcity of iron both limits phytoplankton growth and leaves a surplus of other nutrients. Some scientists have suggested introducing iron to these areas as a means of increasing primary productivity and sequestering carbon dioxide from the atmosphere.

Gross production is almost always harder to measure than net, because of respiration, which is a continuous and ongoing process that consumes some of the products of primary production i. Also, terrestrial ecosystems are generally more difficult because a substantial proportion of total productivity is shunted to below-ground organs and tissues, where it is logistically difficult to measure.