Friday, January 27, 2006

Big botany

Two papers on plants caught my eye this week. The first is from Nature on the scaling of plant metabolic rate. Peter Reich and his colleagues say that plant respiration increases linearly with plant size — that is, a plant has a metabolic rate twice that of one half its size.

This seems to contradict the West, Brown and Enquist (WBE) model of metabolic rate, which says that metabolic rate in all organisms is proportional to the 3/4 power of body mass — so a plant would have a metabolic rate only about 1.7 times that of one half its size.

Whether this is true, and whether the WBE model is the right way to explain metabolic rate, is controversial. I think that there is good evidence for what's known as quarter-power scaling over a wide range of plants and animals, and I also think that the WBE model makes a lot of sense (but then I've written a book about all this, so I would say that). But these things take decades to sort out, so there will be a lot of to-ing and fro-ing between both sides before any hard consensus emerges. Don't expect the quarter-power people to give up their ideas in the face of this new evidence.

The other paper deals with another big issue in ecology — what maintains the high diversity in tropical forests? Writing in Science, a massive team of ecologists led by Christopher Wills present evidence from forest plots in Panama and Malaysia showing that rare species are more likely to survive than common ones — an advantage that buffers against extinction.

The team don't seem to plump for a reason as to why this should be. One possibility is the Janzen-Connell hypothesis, which says that mature trees attract pathogens, herbivores and so on that make it harder for seeds of the same species to germinate in the vicinity. Another possibility stems from the fact that all species compete most strongly with other members of their own kind — because their needs are so similar — than with other species, and that this competition is weaker for rare species.

I am reading Jonathon Silvertown's new book on plant ecology, Demons in Eden at the mo. He has an excellent discussion of these issues that comes down in favour of the J-C view of things. I don't think things are as clear cut as all that, but there does seem to be good evidence that the process is important in forests.

1 comment:

Anonymous said...

Quarter power scaling, as presented by WBE, is almost complete nonsense. If metabolic rate scales to the 3/4 power of body mass, without considerations of metabolic efficiency [which reduces the value of the exponent], that is, if metabolic efficiency is automatically taken to be 100%, then things smaller than one gram [insects, for example] have extremely small metabolic rates, not high ones, when compared to larger creatures of more than 1 gram mass. Do the math. This means that low metabolic rates mean short lives, something I agree with - that is - that metabolic rate and longevity are directly related. The problem is that there are too many exceptions to this picture. More massive rats live shorter lives than less massive rats, though one might conclude their metabolic rates should be greater, and so they should live longer.
WBE even go so far as to claim that the metabolic rate modeled by the quarter power equation is basal metabolic rate, despite that basal metabolic rate cannot possibly countenance or account for motor activity. These guys are going off half-cocked. Their confusion is best displayed in the claim that quarter power scaling of metabolism might hold the key to the secrets of aging. While saying this they all aver that a rat and a pigeon have the same mass, and so, metabolic rate, but the pigeon lives ten times longer. If this is so then the equation has no significance for aging - unless the pigeon has a metabolic rate 10 times that of the rat. And it might, if metabolic rate is field rather than basal, and the metabolic efficiency of the rat is 17% while that of the pigeon is 26%. But this is a refinement that WBE are not capable of making given their dedication to the idea that the metabolic rate is basal.
Finally, if bacteria have a mass of 10 to the -9 grams, and if metabolic rate is directly proportional to life span, then bacteria should live less than 1 second. Yet it is known that some bacteria are virtually immortal, or are capable of life spans in the centuries. How could this be according to quarter power scaling? Well, they could have such life spans if their metabolic efficiency were less than 25%, more like 2 or 3 percent, but, again, this is a refinement that WBE are not capable of making since they are stuck on basal metabolic rates which, they insist, are mass specific, and therefore must be divided by mass as if the structure of the mass had no effect on the efficiency of energy capture and distribution, and could be ignored. WBE should go back to school rather than parade around as profound thinkers.