Week 6 Allometry & Body Size
Felisa A. Smith – Commentary author - https://biology.unm.edu/fasmith/
Felisa Smith is a professor in the University of New Mexico
interested in body size (mostly mammal body size). She is interested in how
paleoecology and modern ecology can help us understand the ecological and
evolutionary consequences of different body sizes.
After being a High School Science Teacher for 5 years,
Felisa decided to go back for a PhD at the University of California at Irvine. After
that, she got a postdoc position with James H. Brown in the University of New
Mexico where she has evolved as a highly successful scientist until today (81
articles, 13 conference papers and a total of 2760 citations – ResearchGate).
James H. Brown – Author – PowerPoint
James Brown is considered one of the ‘godfathers’ of modern
ecology (65469 citations, this paper 272; over 350 published articles – he doesn’t
like ResearchGate…). His three main research interest are: (1) The study at a population
and community level of rodents in the Chihuahuan desert; (2) The elaboration of
the Metabolic Theory which gave him
the Robert H. MacArthur Award in 2005; and (3) The abundance, geographic range
and body size of species which made him the father of Macroecology. He got the
Distinguished Professor Award at the University of New Mexico in 2011.
Paul F. Nicoletto – Author
Paul is a behavioral ecologist interested in animal
communication and mating systems. He is currently the Department Chair of
Biology in Lamar University (Beaumont, TX) and he is main research interest is “the
physiological costs of ornamentation and courtship behavior and their
relationship to male physical condition”. He has published 29 paper until 2013.
Cliff Notes of the paper:
James Brown and Paul Nicoletto study in this paper how body
size distribution varies within the number of mammal species across three different
spatial scales: continental (North America); regional (biomes) and local habitat
patches (homogenous).
They believe that if the body size distribution changes with
spatial scale is because there is a different replacement of certain sizes
(small) between regional and local scale. They think that two main reasons that
can cause that are: (1) because the geographic range border for that size-group
has been crossed; (2) because specialize species for a particular habitat may
not occur in all their geographical range. To do that, they use the
Interquartile range (IQR) as a measure of variability (skewness).
The results show that when the median and the interquartile
values increase, the skewness decreases from continent to biome and to local
habitat. Therefore, continental distribution shows a strong right skewed patter
and local distribution versus local distribution where all are log-uniform.
Regional distributions show and intermediate patter between the other two.
Brown and Nicoletto hypothesize that these differential
pattern distributions among spatial scales is due to a higher replacement of
species of modal sizes that tend to not coexist in a local habitat. Smaller
sizes species live in fewer biomes than larger size species [Fig. 3]. Also,
smaller size species live in broader geographic ranges than large size species which
are concentrated mainly in big geographic areas [Fig. 5].
Continental
mammal fauna distribution is not the sum of smaller spatial scale because there
are not random subsamples of the species pool on large spatial scales. Based on
that, they formulated three main hypotheses to explain that differential
distribution: (1) Competitive exclusion among similar sized species; (2) Large
species with low geographic ranges have higher extinction probability; (3)
Specialization of modal-sized of modal-sized species – allometric constrains.
As Felisa pointed out in the commentary, distribution patterns
are more ‘complex’ than what Brown and NIcoletto present because local distributions
also depend on the environmental characteristics. Several papers have been part
of this debate since the paper was published (Reuman et al.2008; Hunt et al.
2010) but the definitive patter hasn’t been found yet.
Some
questions: Do you think that this distribution patter could be applied to other
groups of organisms? Do you believe that smaller species within a group tend to
specialize more than larger species? Is that only applicable to mammals?
Main question: what is a modal sized species? I confuse am. Anyways...
ReplyDeleteFinally I've found and read this paper which Dr. Smith must have referenced me to at least fifty or so times. I felt uniquely prepared while reading it. Dr. Smith has already familiarized me with most of the conclusions and themes. Did we talk about niche conservation yet? The first hypothesis on page 258 seems to relate to the idea. I know there's been work done on closely related species having (or not having) similar niches, and therefore tending to exclude (or not) each other. Has any work been done on determining if species size is conserved between closely related organisms? Intuitively it seems like it would be, and I think I've been taught that size does go along with phylogeny. If its true, (I think its probably true) it would seem to support his first hypothesis on page 258. Then again, just because two species are the same size, is there any reason for them to competitively exclude one another? I doubt a mosquito does much to compete with a moth, or a wasp with a beetle. Is size really a good indicator of ecological disposition? Perhaps this only applies to mammals?
I think Dr. Smith has told me at one point or another that hypothesis 2 - the idea that extinction influences large species differentially thereby causing a preponderance of small species - is, with rare exception, not true. Was that just recently in class? I can't remember... anyway, it would make sense if it was true, given the distribution, but I guess there's more than one way to make a budge in a curve. The paragraph on page 259 which starts "Large species with small geographic ranges have high extinction probabilities" seems a bit odd. Sure, large animals with small ranges are more likely to go extinct, but aren't all species with small ranges more likely to go extinct? In fact, don't small species tend to have smaller ranges generally? If anything, you might expect small species to be more likely to go extinct, since they tend to have smaller ranges. I feel like maybe I misunderstood the point. The paper he references is Martin and Klein's 1984 paper on Quaternary Extinctions, which I've been given to understand are the exceptions to the rule.
Do large mammals actually have lower speciation rates? That totally makes sense to me, intuitively speaking, but I'd hate to assume anything so important. I mean, grossly speaking it seems inevitable: any tiny organism which can have a thousand generations in the time it takes for another giant organism to live one lifetime must be able to evolve faster, but does faster evolution equate to faster speciation? IDK.
I was looking at the graphs on page 255 (the ones about number of biomes vs. body size. vs. diversity) as well as the graphs on page 256 (the body size vs. range class vs. body size ones) and I was wondering: is there any way to represent this data contiguously without binning size groups? I've been thinking about this general problem a lot recently for other sorts of bins. Is there a good way to represent diversity through time continuously, without binning things into 5 or 10 million year intervals? Anyway, that's not really very topical, I was just wondering.
Lastly, I'm confused by the graphs on page 257 - the geographic ranges thing. What's the difference between a broader geographic range and a big geographic area? How can large species be concentrated mainly in big geographic areas? I'm afraid I understand neither the redundancy thing nor how these graphs relate to range size. But I'm sure someone'll explain it to me.
I've been thinking about this pattern, and I wonder if it applies to birds as well. It would probably be harder to test given that birds are mostly carnivorous/insectivorous (and nectivorous in the case of hummingbirds), and they certainly don't have the body size range in log space that mammals do. With regard to smaller species being able to species faster, I wonder if it has anything to do with niche partitioning, which could lead to speciation. We've talked about herbivory in this class and in EoP about how smaller herbivores are able to only eat the good parts of the leaf while larger herbivores are essentially forced to consume the whole leaf (even the bad parts). I was wondering if this concept applies to other diet types between large and small species.
ReplyDeleteI appreciate Lucius bringing up speciation in larger mammals. This part of the discussion grabbed my attention the most, and I'm not so sure how true this is either, but it does seem like a very important generalization. Also, this is a TOTAL guess, but based on context clues in the paper, I'd think modal-sized meant the size we see the most of in the distribution? (Again - total guess). But would this mean that their third hypothesis, "Specialization of modal-sized species," is just generally talking about species we see the most of in a distribution, or more specifically smaller critters in the distribution?
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