I’ve been looking to find any evidence that might suggest global pressure has changed as suggested by the hypothesis that the end of the ice-age is caused by around a 30% increase in global air pressure (equivalent to an increase in altitude of 3200m) (overview, calculations).
My first thought was that because birds rely on air to fly, that these would show the largest effect. In fact, I find that many birds can fly as high as Mount Everest, that they are actually better adapted to lower air pressures than mammals. (Which might explain why mountains have so many birds of prey). So far from birds having problems during the ice-age, this suggests that birds would have a physiological advantage over mammals in a lower pressure atmosphere. So we might expect more larger birds – even carnivorous birds.
Then I remember the giant carnivorous birds and wondered if their demise coincided with the end of the ice age. But, there’s no evidence for this as they died out much earlier**
**(but that doesn’t stop the BBC who have a page suggesting they died out 15,000BC. This contradicts the evidence above and it likely is another symptom of their view that humans are evil. So I guess this is when the area formerly inhabited by this bird had its first humans).
Finally I started a general search for extinction events associated with ice-ages and came to this paper:
“Determinants of loss of mammal species during, the Late Quaternary ‘megafauna’ extinctions” Johnson(2002)
This is an interesting paper, because if you were to read Wikipedia on the “quaternary extinction event”, you will find that the BBC view (i.e. quaternary extinction is another man-made event) is the PC norm. So this paper is clearly sceptical (aka scientific) in nature when it says:
The results of this analysis reject the hypothesis that these extinctions were caused by selective hunting of large-bodied species. Johnson(2002)
Risk of extinction is clearly related to reproductive rate
More importantly, it clearly links these end of ice-age mass extinction event to reproductive rate:
These analyses demonstrate correlations of both body mass and reproductive rate with extinction. However, of these two variables, only reproductive rate is needed to explain the pattern of extinction.
[T]he most striking feature revealed by this analysis is the consistency of the pattern of loss of species in the regions considered. In each case, the majority of species with reproductive rates below one offspring per female per year went extinct, unless they were arboreal or lived in closed habitats. The impact of reproductive rate on risk of extinction was quantitatively alike in different regions and for different groups of mammals, and the power of habitat and arboreality to alleviate that risk was similar. Johnson(2002)
This is so far the only paper with any general observations of how general physiology might relate to end of ice-age extinction so it is very important.
Metabolic rate & air pressure
Because air pressure directly affects the amount of oxygen available and therefore metabolic rate, I would expect those species that were best adapted to a low-pressure regime would be those with the lowest metabolic rate. In other words, in the same environment (so where species are competing head to head and numbers stable) there would be a competitive advantage for species with a higher metabolic rate if air pressure increases and therefore those with a lower metabolic rate would be more likely to become extinct.
This is important, because the end of the ice-age also saw an increase in temperature which in turn led to other changes to the environment and so it is difficult to find anything that specifically relates to pressure which is not also affected by temperature.
But generally if the only change were temperature, we would expect the same environmental niche to tend to move poleward to offset rising temperature as the world warmed. In this way the same environmental niche would have the same species and so they would continue to compete equally with each other albeit in areas further to the poles.
But, if pressure increased by the equivalent of 3200m, then we would expect the same species tending to generally move down hill by 3200m. However, whereas environments can over time move poleward (barring seas) , it is obviously not possible for an environment to move downhill if it is already below 3200m!
So, all animals should be able to cope with a general slow change in temperature with any particular group obviously being favoured. In contrast if pressure changes, then there is a distinct advantage for those with a higher metabolic rate as pressure increases, and those with a lower metabolic rate as pressure decreases.
Is Metabolic rate linked to Reproductive Rate
Johnson(2002) showed that the end of the ice-age extinction event is linked to species with low reproductive rate. But is reproductive rate linked to metabolic rate and therefore oxygen use so that it would be related to atmospheric pressure?
I found a fairly assertive answer to this question in “TOWARD A METABOLIC THEORY OF ECOLOGY” Brown et al 2004
Population dynamics can be complex and unpredictable, but the potential for exponential growth that underlies these fluctuations has been called the one unequivocal law of population ecology (Turchin 2001). The maximal rate of exponential increase, rmax, is predicted to scale according to Eq. 7. This follows from the fact that reproduction is fueled by metabolism,
[Equation 7 B ∝ M^(-0.25)e^(E/kT)]
Where B is population, M is body mass, E/kT is a value of metabolism.]
Conclusion
This appears to show that it was those animals with the lowest metabolic rate or best adapted to low-oxygen levels which became extinct at the end of the last ice-age. As there is no evidence I know that the level of oxygen changed, this then is strong evidence that those animals that became extinct were those adapted to a low atmospheric pressures and hence that pressure significantly increased at the time of these extinctions.
Sense check
How do I argue against this conclusion. One strand is to say it was actually humans and that low metabolic rate means “slow moving for their size – so easy prey”. But they would be prey to all carnivores equally so this doesn’t work.
Another argument is that metabolic rate is related to temperature. However, I would have thought a higher metabolic rate were needed in the cold (link). I would need evidence that cold-adapted species tend to do so by not only increasing insulation, but also reducing metabolic rate.
However I do find that: “Bergmann’s rule states that within a broadly distributed taxonomic clade, populations and species of larger size are found in colder environments, and species of smaller size are found in warmer regions.” (good explanation here: link). However the initial paper (above) specifically ruled out body size as the main predictor of extinction.
we may state as a tentative generalization that the basal metabolic rate of terrestrial mammals from tropics to arctic is fundamentally determined by a size relation according to the formula Cal./day = 70 kg3/4 and is phylogenetically nonadaptive to external temperature conditions. Equally nonadaptive is the body temperature, and the phylogenetic adaptation to cold therefore rests entirely upon the plasticity of the factors which determine the heat loss, mainly the fur insulation.’ (Scholander et al.’s classic paper ‘Adaptation to cold in Arctic and tropical mammals and birds in relation to body temperature, insulation and basal metabolic rate’, published in The Biological Bulletin in 1950. ) (discussed in detail here)
This seems to reject the counter hypothesis that metabolic rate is linked to temperature and so appears to show that metabolic rate is likely to be be linked to available oxygen and therefore pressure.
More evidence
Note: the following studies are not mammals which thermo-regulate and even so there is no clear relationship with both increase and decreases being shown:
Metabolic cold adaptation (MCA), the hypothesis that species from cold climates have relatively higher metabolic rates than those from warm climates, was first proposed nearly 100 years ago and remains one of the most controversial hypotheses in physiological ecology. In the present study, we test the MCA hypothesis in fishes at the level of whole animal, mitochondria and enzyme. In support of the MCA hypothesis, we find that when normalized to a common temperature, species with ranges that extend to high latitude (cooler climates) have high aerobic enzyme (citrate synthase) activity, high rates of mitochondrial respiration and high standard metabolic rates. Metabolic compensation for the global temperature gradient is not complete however, so when measured at their habitat temperature species from high latitude have lower absolute rates of metabolism than species from low latitudes. Evolutionary adaptation and thermal plasticity are therefore insufficient to completely overcome the acute thermodynamic effects of temperature, at least in fishes.
“Metabolic cold adaptation in fishes occurs at the level of whole animal, mitochondria and enzyme.” White et al (212), Proc Biol Sci. 2012 May 7;279(1734):1740-7
Using a global scale analysis of the standard metabolic rates of 346 species, it is shown that after removing the effects of trial temperature and body mass, environmental temperature significantly influenes interspecific variation in metabolic rate, such that insects from colder environments ten to have higher, whole-organism, metabolic rates. (link)
woodlice from the warmer (i.e. northern) partof the distribution range had markedly higher metabolic rates than those from the cooler(i.e. southern) region, for almost all experimental temperatures. (link)
