sujal
03-dic-2008, 10:08
Patricia S. Muir
Professor,
Ph.D., 1984, University of Wisconsin, Madison
muirp@science.oregonstate.edu
http://www.science.oregonstate.edu/bpp/faculty/muir/index.html
TROPHIC ISSUES
Trophic issues address questions about who eats whom. ("Trophic" refers to feeding.) That is, trophic issues deal with information about food chains and food webs. (The grass is eaten by the insect, which is eaten by the bird, which is eaten by the cat, etc.)
Have you ever wondered why there aren't animals that eat the highest order carnivores -- like grizzley bears or wolves? That is, why is there a ceiling to food chains, with most chains containing only three to four links, as in the example given above?
It isn't just because these top carnivores are so big and fierce!
There is a ceiling to food chains, basically because there isn't enough energy in the population of these top carnivores to sustain a viably-sized population of the animals that would prey on them.
This ceiling derives from the following laws of thermodynamics:
1) Energy can neither be created nor destroyed; only converted from one form to another
2) Energy is degraded when it changes from one form to another (entropy is always increasing). No energy transfer is 100% efficient, and most are considerably less efficient than that, such that energy is lost every time it is transferred.
Eating transfers energy from one place and form to another; the energy in the apple you eat is transferred to you, but some is lost along the way.
In any ecosystem, the primary producers (plants) "fix" only so much energy from the sun through photosynthesis. This amount of fixed energy is then available to feed the rest of the living members of the ecosystem. Each level in the food chain (or web) depends for its energy on the level(s) below it. That is, the herbivores (plant eaters) depend for their energy on the plants, and so forth. Remembering that no transfer of energy is 100% efficient, you can see why there is less and less productivity (kcal/m2/year) at each step up a food chain.
This concept about energy limitations in food chains can be visualized using a pyramid, which diminishes in size as it goes up. The bottom level of the pyramid would include the primary producers, which fix a certain amount of the sun's energy into useable form each year (kcal/m2/year). This level also, of course, sustains losses of energy as the plants respire. The herbivore level that depends on the primary producers will contain less energy than the primary producer level, because energy is lost in the process of it being transferred from the primary producers to the herbivores, and so on up the food chain.
Thus, there are not fiercer dragons on Earth simply because the energy supply will not stretch to support super dragons!
What does this have to do with sustainable agriculture?
We could support more people on Earth for a given area of land farmed if we ate lower on the food chain -- if we ate primary producers instead of eating herbivores (corn instead of beef). OR, we could support the same number of people as at present, but with less land degradation because we wouldn't need to have so much land in production. These consequences of a change in our diets result from the basic thermodynamic principles outlined above.
What follows is a general example – the numbers quoted are very approximate, and are given only for the purposes of illustrating the point:
Let's say we have 20,000 kcal of corn. Assume that we feed it to cattle (as we do with about 70% of the grain produced in the U.S. and 37% of world grain production; poorer nations such as India feed only about 2% of their grain production to animals, as doing so is such a luxury – as you'll see!
The cow will produce about 2,000 kcal of useable energy from that 20,000 kcal of corn (assuming 10% efficiency; the efficiency is actually somewhat higher than that, but 10% is easy to work with and illustrates the point reasonably).
That 2,000 kcal of beef would support one person for a day, assuming a 2000 kcal per day diet, which is common in the US.
If instead people ate the 20,000 kcal of corn directly, instead of passing it through the cow, we would be able to support more people for that given unit of land being farmed; not necessarily 10 times more, because people are not as efficient as cattle at using corn energy, but considerably more than the one that could be supported if the corn were passed through the cow first!
(Beef cattle produce about 19 kg of protein per acre per yr while soybeans produce 200 kg/ac/yr. Recognizing that beef protein is a "complete" protein from the human perspective, while soy protein requires complementing with proteins from grain to acquire all necessary amino acids, this is still a huge difference in production!)
Most citizens of the US are big meat eaters. In the US, consumption of meat and poultry averages in the range of 178 - 250 pounds per year, or between 0.5 and 0.68 pounds per day (depending on whose data you use!). Data from the National Cattlemen's Beef Association indicate that, in 1998, the average US family serves beef eight times per week, with highest rates for lower middle income households with children, where the rate is about 14 times per week. (Ten percent of US households do not serve any beef.)
It is estimated that if everyone in world ate as US citizens do (deriving about 25% of our calories from meat or animal products), less than half of the present world population could be fed even on the tremendous grain harvests of 1985 and 1986. If, by contrast, everyone in the world ate a typical South American diet, in which approximately 15% of calories come from meat or animal products, then about 75% of the world's population (about 3.9 billion persons) could be fed. And, finally, if everyone was well-fed but on a strict vegetarian diet, we could feed about 1 billion persons more than the present world population, based on 1986 harvest levels.
Similar data come from an article in Science (5 Dec. 2005): A balanced Chinese diet of the early 1990's containing 20 kg of meat per person per year was produced from an average land area of ~ 1,000 m2/person, which a typical western diet took up four times that land area per person. China's meat consumption is increasing rapidly -- it more than doubled in the past generation. If the world's entire populaton was to converge on a typical western diet of 80 kg of meat per person per year, the global agricultural land required would be about 2.5 billion hectares, or about 2/3 more than we use now (assuming that productivity remained at today's level, rather than either decreasing or increasing!)
So, it is apparently erroneous to argue that every one could eat as we do in the US if only we could improve equitability of food distribution; production is limiting as well!
There is a great deal of variation animal to animal in the efficiency with which they convert grain into food products. In the US (also considerable variation from source to source in these efficiency estimates, I'm sorry to say!):
-- Cattle consume 4.8 pounds of grain + soy for each pound of meat produced (this excludes the grass that most of them eat for at least part of their lives)
-- Pigs consume 7 pounds of corn and soy per pound of meat produced (more than cattle, because cattle eat grass for part of their lives)
--Chickens consume 2.8 pounds of feed per pound of meat produced (this doesn't include their egg production)
--Dairy cows consume 0.1 pound of grain + soy per pound of milk produced (efficient in terms of grain consumption because they are largely grass fed)
So, if we ate lower on the food chain, one or more of the following benefits would be likely:
(1) We wouldn't have to use as much land and other resources raising grain to feed to animals. We could decrease the intensity of agricultural production and all the impacts associated with that production (impacts discussed previously in this course).
(2) Overgrazing on public and private range lands could decrease.
(3) We wouldn't have to farm or graze marginal lands as intensively, and could even leave them alone! Of course, much of the land that is grazed is not suitable for crop production; I am not suggesting that we could farm it instead, but we could certainly decrease pressure on it. In particular, we could decrease the rate at which we convert tropical rainforest (or other natural ecosystems) to crop production, thus decreasing losses of biodiversity on Earth. (As one example, cultivated soy acreage in Brazil doubled over the past decade, with most new acreage resulting from conversion of cerrado (grassland) and rain forest lands to soy production. The soy is grown largely to feed livestock in Brazil, China, India, and elsewhere. [Science 9 Dec. '05]. (I've read that McDonald's announced that it will not buy chicken fed on soy that was raised on former tropical rain forest lands...)
(4) More people in the world could receive an adequate diet now and even somewhat into the future (assuming that inequities in food distribution could be rectified).
(5) Less fossil fuel energy (and associated emissions of CO2) would be required to produce our food. At present, food production accounts for about 10% of US energy use. The typical US diet that is 70% plant-based and 30% based on meat, eggs, dairy and fish generated about 1.5 metric tons of CO2 per person per year more than would a plant-based diet that provided the same number of calories. The emissions difference is analogous to the difference between driving a SUV versus a compact car. (WorldWatch July/Aug '06)
Professor,
Ph.D., 1984, University of Wisconsin, Madison
muirp@science.oregonstate.edu
http://www.science.oregonstate.edu/bpp/faculty/muir/index.html
TROPHIC ISSUES
Trophic issues address questions about who eats whom. ("Trophic" refers to feeding.) That is, trophic issues deal with information about food chains and food webs. (The grass is eaten by the insect, which is eaten by the bird, which is eaten by the cat, etc.)
Have you ever wondered why there aren't animals that eat the highest order carnivores -- like grizzley bears or wolves? That is, why is there a ceiling to food chains, with most chains containing only three to four links, as in the example given above?
It isn't just because these top carnivores are so big and fierce!
There is a ceiling to food chains, basically because there isn't enough energy in the population of these top carnivores to sustain a viably-sized population of the animals that would prey on them.
This ceiling derives from the following laws of thermodynamics:
1) Energy can neither be created nor destroyed; only converted from one form to another
2) Energy is degraded when it changes from one form to another (entropy is always increasing). No energy transfer is 100% efficient, and most are considerably less efficient than that, such that energy is lost every time it is transferred.
Eating transfers energy from one place and form to another; the energy in the apple you eat is transferred to you, but some is lost along the way.
In any ecosystem, the primary producers (plants) "fix" only so much energy from the sun through photosynthesis. This amount of fixed energy is then available to feed the rest of the living members of the ecosystem. Each level in the food chain (or web) depends for its energy on the level(s) below it. That is, the herbivores (plant eaters) depend for their energy on the plants, and so forth. Remembering that no transfer of energy is 100% efficient, you can see why there is less and less productivity (kcal/m2/year) at each step up a food chain.
This concept about energy limitations in food chains can be visualized using a pyramid, which diminishes in size as it goes up. The bottom level of the pyramid would include the primary producers, which fix a certain amount of the sun's energy into useable form each year (kcal/m2/year). This level also, of course, sustains losses of energy as the plants respire. The herbivore level that depends on the primary producers will contain less energy than the primary producer level, because energy is lost in the process of it being transferred from the primary producers to the herbivores, and so on up the food chain.
Thus, there are not fiercer dragons on Earth simply because the energy supply will not stretch to support super dragons!
What does this have to do with sustainable agriculture?
We could support more people on Earth for a given area of land farmed if we ate lower on the food chain -- if we ate primary producers instead of eating herbivores (corn instead of beef). OR, we could support the same number of people as at present, but with less land degradation because we wouldn't need to have so much land in production. These consequences of a change in our diets result from the basic thermodynamic principles outlined above.
What follows is a general example – the numbers quoted are very approximate, and are given only for the purposes of illustrating the point:
Let's say we have 20,000 kcal of corn. Assume that we feed it to cattle (as we do with about 70% of the grain produced in the U.S. and 37% of world grain production; poorer nations such as India feed only about 2% of their grain production to animals, as doing so is such a luxury – as you'll see!
The cow will produce about 2,000 kcal of useable energy from that 20,000 kcal of corn (assuming 10% efficiency; the efficiency is actually somewhat higher than that, but 10% is easy to work with and illustrates the point reasonably).
That 2,000 kcal of beef would support one person for a day, assuming a 2000 kcal per day diet, which is common in the US.
If instead people ate the 20,000 kcal of corn directly, instead of passing it through the cow, we would be able to support more people for that given unit of land being farmed; not necessarily 10 times more, because people are not as efficient as cattle at using corn energy, but considerably more than the one that could be supported if the corn were passed through the cow first!
(Beef cattle produce about 19 kg of protein per acre per yr while soybeans produce 200 kg/ac/yr. Recognizing that beef protein is a "complete" protein from the human perspective, while soy protein requires complementing with proteins from grain to acquire all necessary amino acids, this is still a huge difference in production!)
Most citizens of the US are big meat eaters. In the US, consumption of meat and poultry averages in the range of 178 - 250 pounds per year, or between 0.5 and 0.68 pounds per day (depending on whose data you use!). Data from the National Cattlemen's Beef Association indicate that, in 1998, the average US family serves beef eight times per week, with highest rates for lower middle income households with children, where the rate is about 14 times per week. (Ten percent of US households do not serve any beef.)
It is estimated that if everyone in world ate as US citizens do (deriving about 25% of our calories from meat or animal products), less than half of the present world population could be fed even on the tremendous grain harvests of 1985 and 1986. If, by contrast, everyone in the world ate a typical South American diet, in which approximately 15% of calories come from meat or animal products, then about 75% of the world's population (about 3.9 billion persons) could be fed. And, finally, if everyone was well-fed but on a strict vegetarian diet, we could feed about 1 billion persons more than the present world population, based on 1986 harvest levels.
Similar data come from an article in Science (5 Dec. 2005): A balanced Chinese diet of the early 1990's containing 20 kg of meat per person per year was produced from an average land area of ~ 1,000 m2/person, which a typical western diet took up four times that land area per person. China's meat consumption is increasing rapidly -- it more than doubled in the past generation. If the world's entire populaton was to converge on a typical western diet of 80 kg of meat per person per year, the global agricultural land required would be about 2.5 billion hectares, or about 2/3 more than we use now (assuming that productivity remained at today's level, rather than either decreasing or increasing!)
So, it is apparently erroneous to argue that every one could eat as we do in the US if only we could improve equitability of food distribution; production is limiting as well!
There is a great deal of variation animal to animal in the efficiency with which they convert grain into food products. In the US (also considerable variation from source to source in these efficiency estimates, I'm sorry to say!):
-- Cattle consume 4.8 pounds of grain + soy for each pound of meat produced (this excludes the grass that most of them eat for at least part of their lives)
-- Pigs consume 7 pounds of corn and soy per pound of meat produced (more than cattle, because cattle eat grass for part of their lives)
--Chickens consume 2.8 pounds of feed per pound of meat produced (this doesn't include their egg production)
--Dairy cows consume 0.1 pound of grain + soy per pound of milk produced (efficient in terms of grain consumption because they are largely grass fed)
So, if we ate lower on the food chain, one or more of the following benefits would be likely:
(1) We wouldn't have to use as much land and other resources raising grain to feed to animals. We could decrease the intensity of agricultural production and all the impacts associated with that production (impacts discussed previously in this course).
(2) Overgrazing on public and private range lands could decrease.
(3) We wouldn't have to farm or graze marginal lands as intensively, and could even leave them alone! Of course, much of the land that is grazed is not suitable for crop production; I am not suggesting that we could farm it instead, but we could certainly decrease pressure on it. In particular, we could decrease the rate at which we convert tropical rainforest (or other natural ecosystems) to crop production, thus decreasing losses of biodiversity on Earth. (As one example, cultivated soy acreage in Brazil doubled over the past decade, with most new acreage resulting from conversion of cerrado (grassland) and rain forest lands to soy production. The soy is grown largely to feed livestock in Brazil, China, India, and elsewhere. [Science 9 Dec. '05]. (I've read that McDonald's announced that it will not buy chicken fed on soy that was raised on former tropical rain forest lands...)
(4) More people in the world could receive an adequate diet now and even somewhat into the future (assuming that inequities in food distribution could be rectified).
(5) Less fossil fuel energy (and associated emissions of CO2) would be required to produce our food. At present, food production accounts for about 10% of US energy use. The typical US diet that is 70% plant-based and 30% based on meat, eggs, dairy and fish generated about 1.5 metric tons of CO2 per person per year more than would a plant-based diet that provided the same number of calories. The emissions difference is analogous to the difference between driving a SUV versus a compact car. (WorldWatch July/Aug '06)