Week 5 Allometry & Body Size
Reading Feb 16 2017- Population Density and Body Size in Mammals (1981)
Information about the commentary author Alistair Evans- Evans video
Alistair Evans studies body size and paleontology at Monash University, the largest university in Australia. He specializes in teeth. He wins in life by having both a bad ass name and a bad ass twitter handle, @DrTeethAl. I would imagine he and professor Smith have collaborated in the past multiple times. Their interests overlap quite a bit and I probably couldn't do him justice. They are both authors of the paper Size Matters: Examining Patterns of Maximum Body Size of Mammals Over Time and Space, which I recall Dr. Smith mentioning the meetings that went into this paper. I'll turn it over to our professor.
Information about the author J. Damuth
John Damuth is another pioneer of macroecology. He currently researches at the Marine Science Institute at UCSB. His expertise is body size and this paper introduced the concept of the energy equivalence rule, which states that energy use for a population as a whole is not a function of body size. Populations of small animals as a whole use as much energy as populations of larger animals. This is actually a very surprising result to me, as I would have imagined that populations of small animals with larger individual metabolic rates would use more energy, and thus acquire more energy than larger ones.
Body of the paper-
The author uses the term "secondary productivity." This is the energy that is used for growth, or assimilation into an animal's tissues, but not including energy that is used for maintenance or activities.The paper states that Klieber's law states that secondary productivity is independent of body size in mammals, which is not the Klieber's law I found floating on the internet, which is that metabolic rate scales with body mass as a factor of .75, which is stated in this paper. I really don't understand how Damuth could have drawn that conclusion from Klieber's law, the one that I looked up (metabolic rate scales with body mass as factor of .75). Energy that is not being metabolized is energy that is stored for growth and activity and has nothing to do with the relationship between metabolic rate and body mass. Any clarification for my understanding would be great.
The density of a population with body size scales as a factor of -.75, with the equation log D = -.75 (log W) + 4.23. What Damuth did was take the relationship between body mass and density with Klieber's law, the relationship between body mass and metabolic rate, and combined them to get the relationship between the energy requirements of a population in terms of the body mass of the individuals that make up the population. How could he do that?
Density is individuals per unit area, so by multiplying this by the metabolic rate of a single individual, we now have the energy required among a population per unit area.
When multiplying exponents together when the bases are the same(in this case, body mass), we have to add them. In doing so, Damuth realized that the scaling factor between the energy required among a population per unit area and body mass came out to 0, meaning that a population of small animals do not use more energy than a large population of animals, per unit area. This is not what I would have expected at all.
The commentary author has said that the conclusion drawn from Damuth is widely challenged still to this day. The commentator says that there is quite a large amount of variation around this regression, almost an order of magnitude to each side. For instance, James Brown & Maurer, in their paper in the beginning of our foundations book, uses a scaling factor of .67 when he defines energy as a function of body mass. This is the ratio that biologists had used before Klieber's law which describes the 2/3 relationship between the area and volume of an object. It is telling that Brown decides not to use Klieber's scaling exponent. There's no doubt that James Brown is well familiar both with Klieber's law which Damuth used to derive his conclusions and he must have been familiar with this paper from Damuth as well, as it was 8 years old when Brown & Maurer published Macroecology: The Division of Food and Space among Species on Continents (1989).
Information about the commentary author Alistair Evans- Evans video
Alistair Evans studies body size and paleontology at Monash University, the largest university in Australia. He specializes in teeth. He wins in life by having both a bad ass name and a bad ass twitter handle, @DrTeethAl. I would imagine he and professor Smith have collaborated in the past multiple times. Their interests overlap quite a bit and I probably couldn't do him justice. They are both authors of the paper Size Matters: Examining Patterns of Maximum Body Size of Mammals Over Time and Space, which I recall Dr. Smith mentioning the meetings that went into this paper. I'll turn it over to our professor.
Information about the author J. Damuth
John Damuth is another pioneer of macroecology. He currently researches at the Marine Science Institute at UCSB. His expertise is body size and this paper introduced the concept of the energy equivalence rule, which states that energy use for a population as a whole is not a function of body size. Populations of small animals as a whole use as much energy as populations of larger animals. This is actually a very surprising result to me, as I would have imagined that populations of small animals with larger individual metabolic rates would use more energy, and thus acquire more energy than larger ones.
Body of the paper-
The author uses the term "secondary productivity." This is the energy that is used for growth, or assimilation into an animal's tissues, but not including energy that is used for maintenance or activities.The paper states that Klieber's law states that secondary productivity is independent of body size in mammals, which is not the Klieber's law I found floating on the internet, which is that metabolic rate scales with body mass as a factor of .75, which is stated in this paper. I really don't understand how Damuth could have drawn that conclusion from Klieber's law, the one that I looked up (metabolic rate scales with body mass as factor of .75). Energy that is not being metabolized is energy that is stored for growth and activity and has nothing to do with the relationship between metabolic rate and body mass. Any clarification for my understanding would be great.
The density of a population with body size scales as a factor of -.75, with the equation log D = -.75 (log W) + 4.23. What Damuth did was take the relationship between body mass and density with Klieber's law, the relationship between body mass and metabolic rate, and combined them to get the relationship between the energy requirements of a population in terms of the body mass of the individuals that make up the population. How could he do that?
Density is individuals per unit area, so by multiplying this by the metabolic rate of a single individual, we now have the energy required among a population per unit area.
When multiplying exponents together when the bases are the same(in this case, body mass), we have to add them. In doing so, Damuth realized that the scaling factor between the energy required among a population per unit area and body mass came out to 0, meaning that a population of small animals do not use more energy than a large population of animals, per unit area. This is not what I would have expected at all.
The commentary author has said that the conclusion drawn from Damuth is widely challenged still to this day. The commentator says that there is quite a large amount of variation around this regression, almost an order of magnitude to each side. For instance, James Brown & Maurer, in their paper in the beginning of our foundations book, uses a scaling factor of .67 when he defines energy as a function of body mass. This is the ratio that biologists had used before Klieber's law which describes the 2/3 relationship between the area and volume of an object. It is telling that Brown decides not to use Klieber's scaling exponent. There's no doubt that James Brown is well familiar both with Klieber's law which Damuth used to derive his conclusions and he must have been familiar with this paper from Damuth as well, as it was 8 years old when Brown & Maurer published Macroecology: The Division of Food and Space among Species on Continents (1989).
I get that Evans and Damuth are saying that the scaling coefficients cancel each other, but I sure don't get it. How you go from 'Density is related to individual metabolic requirements' to 'body size has nothing to do with the energy used by a population as a whole' is beyond me. I'm missing something, I'm sure.
ReplyDeleteRegardless of how Damuth defines Kleiber's law, it still says that metabolic rate scales 3/4 to mass. I understand the cancelling each other out, but as Lucius (and Mark) said, I don't understand how this would work. I'm sure we'll talk all about it in class. Short papers can be awesome, but this one seems so abridged that it feels like we're not getting the whole story. Also, why use primary consumers for a study like this? Is it because they're all of the same trophic level and simpler in terms of their energy use (is this even true?)?
ReplyDeleteI'm with everybody else here. I don't understand how he can say that “the amount of energy that a species population uses in the community is independent of its body size” because W cancels out. Also, I’m a little bit confuse when he gets statistical significance from his ‘Habitat types and constructed communities’ (Table 1) when he has some communities/habitat types with n=3; and at the bottom of the table says that “the individual values for these regressions are not very reliable”. And, like Agathe said, why mammalian primary consumers’ herbivores? And why he things this pattern could be applied to broader taxa when he is using such an specific group?
ReplyDeleteI pretty much agree with what everyone else is saying. The math seems to make sense, but I kid of feel like the paper relies on the idea that "correlation equals causation." Also, what does the data do if you include predators/omnivores/scavengers? Does that make the (already large variance wider, or does it muddle the entire regression line.
ReplyDelete