The Flow of Energy: Primary Production to Higher Trophic Levels

The Flow of Energy: Primary Production to Higher Trophic Levels

1. How much of the sun’s energy is available to and assimilated by plants, and how is that measured?

The sun produces heat energy which reaches the earth through radiation. The amount of radiation reaching the earth’s surface is dependent on the location. The majority of the sun’s energy is received at the equator, and the energy decreases in the Polar Regions. The majority of solar radiation that hits the earth’s surface is reflected into space by deserts, oceans, snow, and ice. The energy is also absorbed into the earth’s atmosphere by gases. Plants use solar radiation for photosynthesis. Half of the light coming from the used by plants in photosynthesis since it has a wavelength of between (~400-700 nm) (Page 3, 1^st, 2nd, and 3^rd paragraph).

2. How are gross production, net production, and ecosystem production-related?

Gross primary production refers to the overall amount of carbon dioxide produced by plants during photosynthesis. Net primary production, on the other hand, refers to the net amount during primary production after including the effects of respiration. Respiration refers to the amount of carbon dioxide that is lost by plants, animals, and other organisms. Net ecosystem production is the total amount of primary production after including respiration from all organisms.  Gross and net production are required to determine the amount of carbon dioxide in the various ecosystems (Page 4, 2^nd, 3rd, 4^th, 5^th, and 6^th paragraph).

3. How are standing crop, turnover rate, and net primary production-related?

Standing crop is the amount of biomass of the system at any one particular time. It is measured in either gram per m² or calories. The relationship between standing crop and production is time. For example, what would be the focus of a forester who wants a huge harvest from a particular plot. If the forester is focused on the short term, then the priority has a high standing crop. In the long term, the main focus is assessing the rate at which the trees are producing new biomass (Page 7, 2^nd paragraph).

On the other hand, the turnover rate is the ratio of standing crop to production.  The turnover rate is acquired by dividing the standing crop by production. The turnover rate is normally in terms of years. Therefore time is a crucial aspect to consider when analyzing organisms in various ecosystems. Decisions made by the forester will require an understanding of forest production, standing crop, and turnover (Page 7, 3^rd paragraph).

4. What types of ecosystems have the highest rates of production, and which make the biggest contributions to worldwide primary production?

The world is divided into several ecosystems, all of which have various rates of production. As outlined in figure 4, ecosystems with the highest rates of production include estuaries, swamps, and marshes and tropical and temperate rainforests. The net primary productivity is acquired by multiplying the figures provided in figure 4 by the area occupied by these ecosystems. By doing so, ecosystems with the high rates of production include oceans, tropical rainforests, savannas, and tropical seasonal forest (Page 8, 1^st paragraph).

Production in ecosystems is affected by nutrients as well as climate within the ecosystem. Therefore, areas with warm and wet climate have high production rates. Land production is limited due to the huge area occupied by oceans and deserts. Therefore plant production is dependent on the use of water and fertilizers (Page 9, 1^st paragraph).

5. What factors limit the amount of primary production locally and worldwide?

Production is affected by the precipitation and temperature of a particular ecosystem. Aspect such as turnover rate and the availability of nutrients affect production in local systems. For example, during a particular season, grasslands can experience high production rates. However, the region may not have a perfect standing crop. The result is that the region has a high turnover rate. Local production is affected by the availability of nutrients, while worldwide production is affected by precipitation (Page10, 1^st paragraph).

6. What is the efficiency with which energy is converted from trophic level to trophic level?

The trophic level consists of primary producers and consumers and secondary consumers and tertiary consumers. Therefore energy is produced and stored and moves from one trophic level to another. For example, a hare which is a herbivore feeds on plants through ingestion. The ingested substance is assimilated into the body, and whatever that is not assimilated is excreted from the body. Efficiency is the rate of assimilation, and this varies among animals. Assimilated energy is used as a source of energy for body functions. Secondary production is where the assimilated energy is converted into new tissue. The net production efficiency is assimilation in animals divided by production. Ecological efficiencies determine the energy supply between the trophic levels.  Ecological efficiencies are acquired by dividing the energy supply in trophic level N+1 by energy consumed in trophic level N (Page 13, 2^nd, and 4^th paragraph).

7. What are the differences between assimilation efficiency, net production efficiency, and ecological efficiency?

Assimilation efficiency is the energy difference between ingestion and excretion and is calculated by the following formula (Ingestion-Excretion). Net production efficiency is the production divided by assimilation. The formula for calculating this efficiency is (production/assimilation). Ecological efficiencies are a combination of net production efficiency and assimilation efficiency. It calculated by dividing the efficiencies of one organism with another organism within the same trophic level — for example, hare production divided by fox production (Page 13, 4^th paragraph).

8. How do ecosystems differ in the amount of biomass or number of organisms present at any point in time, and generated over time, at each trophic level?

A pyramid of biomass represents the amount of energy present in biomass at different trophic levels. The amount of biomass is dependent on the amount of energy that is stored by the trophic below. The reason is that there is energy loss when energy is transferred from one trophic level to another. Therefore higher trophic levels have minimal biomass due to low energy (Page 14, 1^st paragraph).

Ecosystems also differ in the number of organisms. A pyramid of numbers would be crucial in identifying the number of organisms in a particular ecosystem. In the case of oceans, the bottom level of the pyramid would be huge because of the huge presence of algae. In other ecosystems, the pyramids would be inverted. For example, in a forest ecosystem with few large trees can have several insect grazers feeding on the plant material (Page 14, 2^nd paragraph).

The construction of a pyramid of energy can also take place, and this will indicate the production rates instead of standing crop. Pyramid of energy cannot be inverted since it focuses on the turnover rate of organisms. Turnover rate is focused on time, but in the pyramid of energy, the focus is on residence time. Residence time is the energy in biomass divided by net productivity and is calculated using the formula: Rt = (energy in biomass / net productivity). If Rt of primary producers in various ecosystems is calculated, then time in oceans is 10-15days, 3-5 years in grasslands, and 20-25 years in forests (Page 15, 1^st paragraph).

9. How much energy is available to humans, and how much do we use?

Humans are consumers, and the energy they use is acquired from producers. Products from terrestrial plant activities are the source of energy for humans. The energy used by humans is based on net production efficiency. The reason is that only energy leftover from plant metabolism is used to sustain consumers and decomposers. As outlined in Table 1, the total NPP of the earth is 224.5 Pg. Humans use 19% of the earth’s NPP, which is equivalent to 42.6 Pg. (Page 16 &17, Table 1 and 2).