Week 5 Allometry & Body Size
Reading Feb. 14 2017- Intrinsic Rate of Natural Increase: The Relationship with Body Size (1974)
Information about the commentary author Richard M Sibly- Sibly video
Information about the commentary author Richard M Sibly- Sibly video
Professor Sibly is currently one of England's leading
macroecologists. Could not find any information about where he got his phD or
undergrad work but he's currently a professor at University of Reading and
studies population ecology, energetics/ metabolic ecology, body size... the
list is endless. The man has published 190 articles! His substantial work on
earthworm populations makes him living proof that you can do work on anything
as long as you give it enough attention and thought, even if other people don't
see the significance of the work just yet. The subjects of his research are
varied and all in all, the man is one of the most prolific scientists alive
today.
Information about the article's author Tom Fenchel-
Tom is a marine biologist who did his ph.D at the University of
Copenhagen. He is known for this paper most of all but he has 300+ articles
with 18,000 citations on research gate. This paper has given him his most
famous claim to fame, known as “Fenchel’s law” which gives the relationship
between maximum possible growth rate and body mass.
Body of the article-
The population curve of any population in nature is an
exponential curve that reaches some asymptote that is usually called carrying
capacity. It’s just the maximum number a population can be sustained by the
environment. The equation at the beginning of the paper, dN/dt=rm*N simply
states that the population growth is just the derivative of the population
graph.
He uses rm as a maximum possible growth rate that has
“the absence of density dependent limitations.” I take that to basically mean
growth in a utopia where there is unlimited energy, food, resources, space, no
death/ predators/ disease. How fast can the critters make babies? That’s your
rm.
Fenchel mentions that there are two distinguished
categories of organisms- the “colonizers” and the “equilibrium” species. The
colonizers would be animals that fit his definition of rm. When a new land mass
is empty there are none of these “density dependent limitations” and with
resources and space aplenty, there is selective pressure to reproduce as
quickly as possible, but once space and resources become more limited, there is
selective pressure for organisms that compete effectively for limited
resources. Natural selection takes place and when that occurs, animals can’t
afford to spend all their energy on reproduction but must use energy on growth,
combat for mates, protection of offspring. Most of the time, conditions that
allow for the actual growth rate of a population are not at all the conditions
that are allowed for rm, but not all the time.
The commentary author mentions
that rm is the mechanism that is responsible for how populations rebuild after
catastrophe where there are suddenly few survivors left and a lot of available
resources and space to be exploited. A population with a larger rm bounces
back, and it’s no surprise that the animals that we tend to think are pests
that we can’t quite deal that knock-out blow, and not for lack of effort, have
large rms.
Fenchel found that rm is correlated with body size but
the life-history trait that effects rm the most is preproductive period, or the
length of time of an animal’s juvenile stage. Animals that mature longer happen
to have less fecundity. I assume this means that if offspring takes more energy
and time to raise, obviously you have to have less of them
.
Questions: Most of the data that Tom Fenchel worked
with are on bacteria, with relatively few mammals and a few species like
Daphnia (water flea) and a phage thrown in. Do you feel that this is
unacceptable or is this what macroecology is all about? Should the patterns
have any meaning if they are made by comparing such disparate organisms?
Are there selective pressures to reproduce at certain
rates pushing body size along or is body size controlled mostly by other
factors such as energy acquisition? How important is rm in speciation?
As Mark pointed out, all the data obtained in this paper is based on experimental laboratory populations only (metazoans mainly). This allows to control for environmental factors (food, temp, humidity…) that otherwise would affect rm values but it can also be biased since these organisms have high reproductive potentials. At the end of the paper, Fenchel talks about how Merz (1971) and Leslie (1966) noted that “species with low reproductive potentials fall below the predictions represented in Figure 1”. Are these differences due to the fact that these authors were talking about K-strategist species versus the r-strategists used in Fenchel’s paper?
ReplyDeleteFenchel also interprets rm as “the productivity of exponential growth” that correlates with metabolic rate. How exactly it correlates with metabolic rate? Could we assume then that Kleiber’s allometry law and Fenchel’s law would be somewhat comparable? How?
This is great. I had a biology teacher at CNM, Rich Calabro - in fact, I think Mark and I both took the same class from him - and Mr. Calabro taught us this equation and some of the basic principals you could derive from it. Now I'm reading it and thinking, 'hey, I remember this, I remember r, the intrinsic rate of increase!' I don't remember the little subscript m though, what's that? Is that subscripted m something to do with Malthus? Fenchel mentions something about the 'Malthusian parameter" but he doesn't go into it. Also, I think the way I was taught it, it was dN/(dt*r)=N, but I guess that's the same thing he has written a different way. Or am I misremembering it? I'm sure Mark can help me remember, he was there after all!
ReplyDeleteOh look, little hearts! Must be valentines day!
I like Mark's comment on whether the results are skewed based on all the bacteria used, or if this is just how macroecology is done. Although I see how it's useful to compare such differences in size to get more dramatic data, Laura pointed out that perhaps it was easier to use bacteria because they have high reproductive rates and are easy to observe. Also, perhaps Fenchel came across the same problem as Rosenzweig did last week, and there just wasn't a whole lot of data to work with at the time.
ReplyDeleteI've been thinking about Mark's question on rm's effect on speciation. Initially I thought that a larger population size would allow more chances for speciation, but now I'm thinking that if rm were smaller (lots more limitations in the environment), maybe it would put more pressure on part of the population to speciate? Not sure about this.
I agree with Mark and Agathe about the lack of mammals/larger organisms being somewhat of an issue. Although, the Duncan et. al. 2007 paper cited in the commentary blurb seems to lead to the same conclusion, albeit with seemingly slightly more variance than Fenchel's original regression.
ReplyDeleteI was also thinking about Mark's point on species' rebound to disturbance, and how body size might influence that. For example, we have easily extricated/eradicated wolves from many parts of the US, while no matter what we do, coyotes simply come back year after year (despite the average mass of a wolf only being about twice that of a coyote, which is nothing in log space). This is really peculiar despite them being so closely related.