Energy Flow and

Nutrient Cycle

THE EFFICIENCY OF TROPHIC LEVELS IN AN

ECOLOGICAL NICHE

Ecosystem, Habitat & Ecological Niche

An ecosystem is a definable area containing a relatively self-sustained community of organisms interacting with their non-living surroundings.

A Habitat is the area in which an individual lives.

An ecological niche consists of a community of interacting populations in an ecosystem. Each population is a collection of individuals belonging to the same species that play a particular role in the community. This role of the population called the ecological niche.

Energy Flow in an Ecosystem

In an ecosystem energy flow is all ways in one direction. This one-way flow of energy can be represented in several ways including food chains, food webs, and pyramids of numbers, biomass, and energy.

  A food chain is single sequence of energy flow

usually starting with a producer and ending with a top consumer, in which each organism is the food of the next one in the chain.

A food web shows the interconnection of food chains in an ecosystem

Efficiency of Energy

Ecological efficiency describes the efficiency with which energy is transferred from one trophic level to the next. It is determined by a combination of efficiencies relating to organismic resource acquisition and assimilation in an ecosystem

Energy transfer between trophic levels is generally inefficient, such that net production at one trophic level is generally only 10% of the net production at the preceding trophic level. This means that 90% of the energy available at one trophic level never transfers to the next. Due to non-predatory death, egestion, and respiration, a significant amount of energy is lost to the environment instead of being absorbed for production by consumers, as depicted in the figure below. The figure approximates the fraction of energy available after each stage of energy loss in a typical ecosystem, although these fractions vary greatly from ecosystem to ecosystem and from trophic level to trophic level.

The loss of energy by a factor of 1/2 from each of the steps of non-predatory death, defecation, and respiration is typical of many living systems. Thus, the net production at one trophic level is 1 / 2 * 1 / 2 * 1 / 2 = 1 / 8 or approximately 10% that of the trophic level before it.

Example: Assume 500 units of energy are produced by trophic level 1. One half of that is lost to non-predatory death, while the other half (250 units) is ingested by trophic level 2. One half of the amount ingested is expelled through defecation, leaving the other half (125 units) to be assimilated by the organism. Finally one half of the remaining energy is lost through respiration while the rest (63 units) is used for growth and reproduction. This energy expended for growth and reproduction constitutes to the net production of trophic level 1, which is equal to 500 * 1 / 2 * 1 / 2 * 1 / 2 = 63 units

Energy Flow in an Ecosystem

Quantifying ecological efficiency

Ecological efficiency is a combination of several related efficiences that describe resource utilization and the extent to which resources are converted into biomass.

Exploitation efficiency is the amount of food ingested divided by the amount of prey production (I / Pn)

Assimilation efficiency is the amount of assimilation divided by the amount of food ingestion (A / I)

Net Production efficiency is the amount of consumer production divided by the amount of assimilation (Pn +

1 / A) Gross Production efficiency is the assimilation

efficiency multiplied by the net production efficiency, which is equivalent to the amount of consumer production divided by amount of ingestion (Pn + 1 / I)

Ecological efficiency is the exploitation efficiency multiplied by the assimilation efficiency multiplied by the net production efficiency, which is equivalent to the amount of consumer production divided by the amount of prey production (Pn + 1 / Pn)

Theoretically, it is easy to calculate ecological efficiency using the mathematical relationships above. It is often difficult, however, to obtain accurate measurements of the values involved in the calculation. Assessing ingestion, for example, requires knowledge of the gross amount of food consumed in an ecosystem as well as its caloric content. Such a measurement is rarely better than an educated estimate, particularly with relation to ecosystems that are largely inaccessible to ecologists and tools of measurement. The ecological efficiency of an ecosystem is as a result often no better than an approximation. On the other hand, an approximation may be enough for most ecosystems, where it is important not to get an exact measure of efficiency, but rather a general idea of how energy is moving through its trophic levels.

Energy Efficency

Consumption efficiency = 200/1000

Assimilation efficiency 70/200

Production efficiency = 14/70

Amt produced by trophic level n-1

Amt ingested by trophic level n

Amt egested as feces (waste) by trophic level n

Amt assimilated (i.e. absorbed into body) by trophic level n

Amt respired by trophic level n

Secondary production by trophic level n

Efficiency of energy transfer

Energy loss between trophic levels

Efficiency of production

Energy Budget

traces energy flow through ecosystems

• Primary Production -- Gross Primary Production = gpp: amount of light energy converted to chemical energy by PS per unit time in a given area. Net Primary Production = npp: gpp minus amount energy used by primary produces for respiration.  • J/m 2/ yr or as weight g/m 2 /yr = biomass  • NOT TOTAL BIOMASS but new BIOMASS added to ecosystem in given amount of time  • Measured as dry weight of organic material  • Standing crop - total biomass of photosynthetic autotrophs at given time

A pyramids of numbers is a graphical method of showing the quantitative relationships between organisms at each trophic level.

While a pyramid of biomass shows the dry mass at each trophic level.

A pyramid of energy shows the energy entering each trophic levelin an ecosystem over one year.

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. 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. For the oceans, the bottom level would be quite large, due to the enormous number of small algae. For other ecosystems, the pyramid of numbers might be inverted: for instance, if a forest's plant community was composed of only a handful of very large trees, and yet there were many millions of insect grazers which ate the plant material.

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. 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). You should now ask "how can that be?" If the amount of energy in biomass at one level sets the limit of energy in biomass at the next level, as was the case with the hares and foxes, how can you have less energy at the lower trophic level? This is a good question, and can be answered by considering, as we discussed in the last lecture, the all important aspect of "time". 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 4c). 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.

 

Ecosystem energetics - terminology

Standing crop biomass – amount of accumulated organic matter found in an area at a given time [g/m2]

Productivity – rate at which organic matter is created by photosynthesis [g/m2/yr]

Primary productivity – autotrophs Secondary - heterotrophs Gross versus net primary productivity