Marcus Clauss explains Codron et al. 2012: Ontogenetic niche shifts in dinosaurs influenced size, diversity and extinction in terrestrial vertebrates

This is a guest post by Marcus Clauss of the Clinic for Zoo Animals, Exotic Pets and Wildlife of the University of Zurich, Switzerland. Marcus is a member of and leading voice in the Research Unit 533 ‘Sauropod Biology’ of the German Science Foundation. When Biology Letters published Codron et al. (2012), a paper Marcus is a co-author on, a lot of Internet articles and other places, including messages to the Dinosaur Mailing List, commented on the paper. Some of these were accurate and to the point, some clearly missed the point. I suggested to Marcus that he write a guest post for dinosaurpalaeo, and here it now is.

Please note that Marcus will not reply to DML posts, because he does not subscribe to it!

There has already been some heated discussion on our Biology Letters paper that went online this week (Codron et al., 2012), and I want to accept the invitation of Heinrich Mallison, colleague in our research group, to make some statements on what we think we show with our work. First of all, I want to mention that first author Daryl Codron did most of the work and the writing, and that the brilliant details in the model are his, with input from co-author Chris Carbone, Dennis Müller, and me.

The major statement is: we did not demonstrate a ‘disadvantage’ of dinosaurs, but explained a fundamental difference in the species abundance pattern between populations of oviparous and viviparous (terrestrial) organisms. This difference in itself is interesting – our model explains why a viviparous population will have a typical right-skewed pattern of species abundance, whereas in an oviparous population, a lack of species in the middle size range will occur. This does not mean that there are no individuals in the middle size range. There are. But these individuals are mostly not representatives of ‘own’ species, but juveniles of larger species.

This phenomenon is explained by a difference in the scaling of adult mass to neonate/hatchling mass between viviparous and oviparous animals, which means that oviparous animals have to pass through a larger range of body sizes until they reach adulthood. When assuming that a body size class corresponds to a niche, then this means that the juveniles of oviparous species pass through, and hence occupy, a larger number of niches than the juveniles of viviparous species.

One assumption we then make is that competition occurs between similar-sized animals. Note that the model does not deny the existence of niche differentiation within a body size class (this is implicit in the model because it uses typical body size-species abundance allometries as a basis, which means that there are more different niche spaces within a body size class for smaller body size classes than for large ones, a fundamental assumption in ecology). And we also do not claim that in competition, a juvenile of a ‘large’ species always outcompetes a similar-sized adult of a ‘smaller’ species. Our model assumes equal chances. But because loss of a juvenile affects the population much less than the loss of a reproductively active adult, the ‘larger’ species wins out in the end. This might also be a scenario that explains a selective advantage of larger body size, i.e. an evolutionary driver.

The reduced abundance of middle-sized species is thus considered a ‘characteristic’ of oviparous populations, not a ‘disadvantage’. The media make a ‘disadvantage’ out of it, because that is their typical reaction to ANYTHING linked to dinosaurs – the default reaction of journalists is “we want to explain why dinosaurs are an evolutionary failure”, and you can tell them again and again that we should consider, according to the current score, dinosaurs as the most successful dominators of terrestrial ecosystems. We also mention that in our paper (first page, second column, “dominance of terrestrial life for ca 180 Myr”). Countering the view of dinosaurs as evolutionary failures is a tiring task for anyone who talks to reporters (by the way, a similar tiring task is to explain that birds are actually dinosaurs, which therefore have not really died out – I get the impression that many reporters treat that as surprising, exciting news).

Now, the reduced number of species in the middle body size range has consequences in the case of a global catastrophe that shuts down niches for large-sized animals for a time period longer than a generation. Calling the characteristic a ‘disadvantage’ would be like saying that because a concrete block dropped on the head of my uncle 20 years ago and crashed his skull, he is a typical example for an organism that has the disadvantage of a weak cranial skeleton. You could only make this claim if dropping concrete blocks were a common feature of ecosystems and occurred in such frequency that they might induce selection for adaptations against them. There is no way that asteroid impacts (or unusual increased volcanic activity) will induce evolutionary adaptation. I personally think we should just call them tough luck.

Allowing for competition between oviparous and viviparous animals leads to some other results described in the paper that correspond to current concepts or to findings (e.g. the absence of large mammals in the presence of dinosaurs). Finally, if you have a look at the supplementary material you will see the enormous work Daryl did in simulating different scenarios in terms of life histories, fecundities, mortalities etc. to check if the effect of a ‘gap’ in the species abundance graph is robust against changes in these parameters. It looks like it is.

All this is a theory. For me, one important concept when dealing with large non-avian dinosaurs is that one has to keep in mind that there must have been large numbers of juveniles around (this fact is not new, but maybe there are still consequences to detect that this has that we are not aware of). Another example for this concept was the nice idea that my colleague Jürgen Hummel had, that it might explain a higher diversity of carnivorous species in dinosaur than in mammal ecosystems, because the reproductive output of large herbivores is available for carnivores in a higher proportion in a dinosaur than in a mammal ecosystem (Hummel and Clauss, 2008).

References

Codron, D., Carbone, C., Müller, D.W.H. and Clauss, M. 2012. Ontogenetic niche shifts in dinosaurs influenced size, diversity and extinction in terrestrial vertebrates. Biology Letters (online) doi:10.1098/rsbl.2012.0240
(as of 21.04.2012, 23:41 MEZ the doi does not wok yet)

Hummel, J. and Clauss, M. 2008. Megaherbivores as pacemakers of carnivore diversity and biomass: distributing or sinking trophic energy. Evolutionary Ecology Research 10:925–930