Lecture 4 Background – Ricklefs Chp. #22, Energy in the Ecosystem
Each year, the earth’s plants, algae, and photosynthetic bacteria harness enough energy from sunlight to make 224 billion tons of biomass (the mass of living organisms in a given ecosystem). 59% (~132 billion tons) of this biomass is produced in terrestrial ecosystems and of terrestrial production, 35% - 40% (~46-53 billion tons) is used by humans.
Pauly and Christensen sought to answer the question of how much of the production of algae in the oceans is required to sustain the fisheries. Assuming that for each step in the food chain about 90% of consumed energy is used to maintain the consumer, only 10% is converted through growth and production into biomass. It's estimated that the number of feeding systems which lead from algae to fish vary from 1.5 (on average) to 3. Thus, for inshore fisheries (which produce most of the seafood consumed by humans) the algal growth required to sustain the harvest amounted to 24-35% of the total production of the ecosystem.
Every organism must feed in some matter, and each may be fed on by some other organism; these feeding relationships link organisms into the single functional entity of the biological community. What an organism digests and absorbs constitutes its assimilated energy; thus ingested energy – egested energy = assimilated energy. The energy channeled into growth and reproduction as a percentage of the total assimilated energy is the net production efficiency.
Alfred J. Lotka believed that the size of a system and the rates of energy and material transformations within it obey certain thermodynamic principles that govern all energy transformations.
Lindeman visualized a pyramid of energy within the ecosystem, with less energy reaching each successively higher trophic level. Energy is lost at each level because of worked performed by organisms at that level and because of the inefficiency of biological energy transformations. Many studies of ecological efficiencies have led to the generalization that 10% of energy is passed from one trophic level to another, but this is not a fixed law. The abundance and activity of higher tropic level organisms depends on the transfer of energy up from the primary producers, and the amount that reaches them depends on how efficiently assimilated energy is converted into biomass that can be consumed by the next tropic level. Herbivores and carnivores are more active than plants and expend correspondingly more of their assimilated energy on maintenance.
Odum depicted ecosystems as a series of simple energy flow diagrams which represent the use and transfer of energy by all the organisms at each trophic level. The follow of energy is difficult to measure directly, but can be measured by the movement of elements among ecosystem components. Understanding how nutrients cycle among components of the ecosystem is crucial to understanding the regulation of ecosystem structure and function.
Primary production is the process by which plants, algae, and some bacteria in photosynthesis capture light energy and transform it into the energy of chemical bonds in carbohydrates. Gross primary production can be split into respiration (energy used to maintain) and net primary production (energy available to consumers). Plants and other photosynthetic autotrophs form the base of most food chains and are therefore referred to as the primary producers of the ecosystem. They form the base of ecological food chains and are the source of all the chemical energy in the ecosystem. The rate of primary production determines the total energy available to the ecosystem.
The rate of photosynthesis depends on the availability of light. Above a certain light intensity, photosynthetic pigments become saturated; that is, they cannot absorb additional light energy or use it efficiently. Irradiance is the amount of light striking a surface, expressed in watts per square meter. Photosynthetic efficiency is the percentage of energy in sunlight that is converted to primary production during the growing season. This measure provides a useful index to rates of primary production under natural conditions. Where water and nutrients do not limit primary production severely, photosynthetic efficiency of an ecosystem varies between 1% and 2%. Agronomists quantify the drought resistance of crop plants in terms of transportation efficiency, also called water use efficiency which is the number of grams of dry matter produced per kilogram of water transpired.
Production in both terrestrial and aquatic environments can be enhanced by the addition of various nutrients, especially nitrogen and phosphorus. Agronomists and ecologists calculate the nutrient use efficiency (NUE) of plants as the ratio of dry matter production to the assimilation of a particular nutrient element, usually expressed as grams per gram. Primary production varies greatly with latitude. Humid tropics result in the highest terrestrial productivity on earth. A given amount of water supports almost three times as much plant production in cooler climates as in the hotter climates within a given latitudinal belt. In the open ocean, the remains of dead organisms tend to sink to the depths, and nutrient regeneration occurs mostly in sediments at the ocean floor.
Net ecosystem production – the balance of carbon gain and carbon loss in an ecosystem. A system cannot persist long with negative net ecosystem production.
“Rising atmospheric CO2 and carbon sequestration inforests” (Beedlow et al 2004)
CO2 is essential for life, but is also threatening to alter temperature and precipitation patterns due to growing concentrations in the atmosphere as a result of human activities. The biosphere plays an important role in absorbing about half the annual amount of anthropogenic carbon emissions, though it is not known if these sequestration rates can be sustained.
CO2 enters plants through the stomata (small pores) in leaves, and some carbon is quickly released back into the atmosphere through respiration. Some carbon is transferred into the soil and sequestered, but decomposition releases most of that carbon back into the atmosphere. Tree trunks, large branches and large roots are the main sources of carbon sequestration, since they do not decompose as quickly as leaves and small roots. That carbon, however, is released back into the atmosphere after branches fall or the tree dies and starts to decompose.
Soil carbon compounds can be classified based on turnover time:
• “active”/”fast” pool turns over in days to a year (easily decomposed litter and fine roots);
• “intermediate” pool turns over in years to decades (mixture of compounds, usually contains the most C);
• “passive”/”slow” pool turns over in more than a century (composed of persistent organic compounds; accumulates slowly).
Increasing atmospheric concentrations of CO2 could have the “fertilizer effect”—an increase in primary productivity and water use efficiency (elevated CO2 will theoretically increase photosynthesis and decrease the need for plants to keep their stomates open widely, thus conserving water). However, environmental factors such as increasing temperatures and drought, and changing climactic patterns affecting net ecosystem productivity, could limit this fertilization effect. Additional factors limiting the fertilization effect include:
• Limited nitrogen availability: N is essential for plant growth, so increased plant growth due to higher CO2 levels would have to be sustained by an abundant supply of N. In areas with deficient N, rising levels of CO2 are therefore not likely to result in higher plant growth and C sequestration.
• Regional air pollution: Forests are negatively impacted by air pollution, esp. higher ozone concentrations, which reduce the amount of C sequestered in plants and trees.
• Carbon reallocation: If rising CO2 levels could increase the amount of C in long-term storage pools in wood or soil, it would increase sequestration even without increased plant growth. However, there has been little evidence that increasing wood density, plant size, or the duration of leaves and roots has actually taken place with elevated carbon.
In conclusion, global net primary production has increased over the past few decades due to changes in climactic conditions, growing season length, precipitation, cloud cover, and temperature, implying that more C is being sequestered by terrestrial systems. Additionally, the regrowth of forests is contributing to the global carbon sink. However, rising atmospheric CO2 levels does not appear to be an important factor in the increased sequestration, and it is therefore important that the C currently in forested ecosystems be preserved.