Patterns of energy flow:
Patterns of energy flow can be compactly described by 3 transfer energy efficiency ratios:

Why does it appear that so much is lost?

The basics of this diagram can be abstracted as follows:

Notice that there are actually 2 trophic chains, the grazers and the decomposers. Eventually everything must be taken in by these small but critical

 

Fine tuning the parameters: Consumption efficiency

1. Consumption efficiency, or the proportion of food out there that is actually consumed, is dependent on the ecosystem and its' primary plant form. Compare the different percentages of productivity consumed. Trees because of their high lignin content are less delectable than grasses, and certainly less edible than aquatic phytoplankton. However this impervious primary productivity comes at a cost to the organism. Thus turnover must be low in forested ecosystems and inevitably high in less protected planktonic communities to make lignin production a worthwhile investment.


Sometimes these differential preferences can have long term impacts on the system as a whole...In North American tallgrass prairies, diversity and productivity are controlled to a large extent by nitrogen availability. Historically, nitrogen availability in prairies was driven by interactions between frequency of fires and grazing by large herbivores. In general, spring fires enhance growth of certain grasses, and herbivores such as bison preferentially graze these grasses, keeping a system of checks and balances working properly, and allowing many plant species to flourish.

2. With higher trophic levels, the amount of animals & insects consumed may be dependent on the energy it takes to capture these organisms. When you consider the factors involved in foraging- the distribution of the food item and the energy it takes to catch, process/handle each item, there is a limit to how much can be consumed given how much energy & time it takes to capture each food item. Although there may be plenty of mice in the field, if they are widely dispersed or capture rate is low due to avoidance techniques few may actually be taken in the end.

Assimilation efficiency: An/ In x 100

assimilated across the gut wall --> available for growth (either new tissue or new offspring) and work (metabolism).

Herbivores, detritivores & microbiovores ( values range from 20 - 50%; seed & fruits = 60-70%; leaves 50% but this may be much lower for species with high biochemical defense; wood = 15%)

Carnivores because of the high quality of the food ingested average 80% assimilation efficiency. When a bird eats an insect, the exoskeleton cannot be digested, and thus is worthless. Meat eaters however have easily digested, high quality ( in terms of protein content) food source. Thus most of what they consume can be directly converted into new tissue once losses for respiration are dealt with.

Production efficiency: Pn / An x 100%

Of that portion which has been assimilated, the amount left over for new growth after respiration. Look at the chart below. Why do invertebrates have so much energy left for new production? Part of the answer lies in thermoregulation. If energy is not spent in maintaining high body temperatures, it can be used for new growth instead.

At the ecosystem level, we can make rapid evaluations of who in the system actually processes most of the energy. As you can see from the chart below, it is the decomposers who really use most of the energy of any ecosystem. What percentage of the calories are actually processed by large herbivores? Contrast the numbers across and down.

When we compare ecosystems, some critical differences emerge. Note the differences in flow between NPP and grazer in the forest vs the grassland vs.plankton vs. stream How can you explain these differences?

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Essay1: Compare the energetics flow in a grassland to a forest. How would it be different? what difference would it make to the animals that could be supported? where would the biomass be locateded? would" quality" of calories be a different in the two ecosystems? explain. Would fire cause a loss of energy available to the system? How could this loss be compensated for?

If you don't know much about the grasslands see below or look it up in your text: http://wwwfac.mcdaniel.edu/Biology/wildamerica/grasslands/graslandoutline.html

Essay2 . The discussion above emphasizes transfer of kilocalories between trophic level, as if caloric value is the most critical factor. Isn't quality rather than quantity of food more important... as with meat eating vs.plant eating animals? read the abstract below

High CO2 produces less nutritious plants,.

Increased levels of atmospheric carbon dioxide, due the burning of fossil fuels and global deforestation, are likely to produce plants that contain more mass but are less nutritious, according to plant research.

Research already conducted in Maricopa, Ariz., found that both cotton and wheat thrived when carbon dioxide levels in the field were enriched by about 200 parts per million above the usual level.

However, the plants may be less nutritious, said Thomas Thompson from the University of Arizona Department of Soil, Water and Environmental Sciences. Thompson found after two years of work with wheat that grain protein dropped under enriched-CO2 conditions. "That would have an impact on the food value of the grain and the baking quality of the grain," he said.

 The researchers have recently set up an elaborate experiment that will keep atmospheric concentrations of carbon dioxide at the level expected in 50 to 100 years, about 560 parts per million, on an experimental sorghum plantation. By mimicking "real world" conditions, the researchers are testing the hypothesis that higher CO2 levels will produce more massive but less nutritious food.