tag:blogger.com,1999:blog-6666208467022745303.post1708888294535106280..comments2024-03-13T02:17:39.644-07:00Comments on Restating the Obvious: Book Review 355: AirHarry Eagarhttp://www.blogger.com/profile/04196202758858876402noreply@blogger.comBlogger6125tag:blogger.com,1999:blog-6666208467022745303.post-68109765467636175562015-10-23T19:32:58.285-07:002015-10-23T19:32:58.285-07:00I wonder if life modulated oxygen, but then it'...I wonder if life modulated oxygen, but then it's hard to explain how it got as high as it did. Presumably there's an upper limit at which multicellular life cannot tolerate free oxygen, just as there is a lower limit for carbon dioxide at which vascular plants cannot survive.<br /><br />I think you are right about contingency; it does not appear there is some natural value to which free oxygen reverts.<br /><br />I have seen suggestions that at some point fire starts to become a problem.<br /><br />I cannot find it, but some entomologists on my FB feed sometimes post pictures of insects eating birds: praying mantises v. hummingbirds mostly.Harry Eagarhttps://www.blogger.com/profile/04196202758858876402noreply@blogger.comtag:blogger.com,1999:blog-6666208467022745303.post-64864891309287055152015-10-23T14:57:34.207-07:002015-10-23T14:57:34.207-07:00True.
But, to me, it points to contingency. What...True.<br /><br />But, to me, it points to contingency. What if higher O2 levels lasted a bit longer? <br /><br />Maybe bugs would be eating birds.Hey Skipperhttps://www.blogger.com/profile/10798930502187234974noreply@blogger.comtag:blogger.com,1999:blog-6666208467022745303.post-85005098926949526282015-10-23T10:12:25.645-07:002015-10-23T10:12:25.645-07:00I knew that. Logan discusses the reason insects ar...I knew that. Logan discusses the reason insects are small today; since they don't have lungs, oxygen gets to each cell by diffusing through spiracles. Only so much can diffuse that way.<br /><br />But that was in a different section of the book. You'd think that in a book about air, that would have come up. <br /><br />More interesting to me is the fluctuation of oxygen. Most of it is in rocks.<br /><br />Harry Eagarhttps://www.blogger.com/profile/04196202758858876402noreply@blogger.comtag:blogger.com,1999:blog-6666208467022745303.post-68880371605671334282015-10-23T02:07:36.862-07:002015-10-23T02:07:36.862-07:00Here is the abstract from the source paper:
Giant...Here is the abstract from the source paper:<br /><br /><i>Giant insects, with wingspans as large as 70 cm, ruled the Carboniferous and Permian skies. Gigantism has been linked to hyperoxic conditions because oxygen concentration is a key physiological control on body size, particularly in groups like flying insects that have high metabolic oxygen demands. Here we show, using a dataset of more than 10,500 fossil insect wing lengths, that size tracked atmospheric oxygen concentrations only for the first 150 Myr of insect evolution. The data are best explained by a model relating maximum size to atmospheric environmental oxygen concentration (pO2) until the end of the Jurassic, and then at constant sizes, independent of oxygen fluctuations, during the Cretaceous and, at a smaller size, the Cenozoic. Maximum insect size decreased even as atmospheric pO2 rose in the Early Cretaceous following the evolution and radiation of early birds, particularly as birds acquired adaptations that allowed more agile flight. A further decrease in maximum size during the Cenozoic may relate to the evolution of bats, the Cretaceous mass extinction, or further specialization of flying birds. The decoupling of insect size and atmospheric pO2 coincident with the radiation of birds suggests that biotic interactions, such as predation and competition, superseded oxygen as the most important constraint on maximum body size of the largest insects.</i><br /><br />Interesting.Hey Skipperhttps://www.blogger.com/profile/10798930502187234974noreply@blogger.comtag:blogger.com,1999:blog-6666208467022745303.post-59790218621924018192015-10-23T01:58:55.144-07:002015-10-23T01:58:55.144-07:00This comment has been removed by the author.Hey Skipperhttps://www.blogger.com/profile/10798930502187234974noreply@blogger.comtag:blogger.com,1999:blog-6666208467022745303.post-4831075343574708062015-10-23T01:57:35.135-07:002015-10-23T01:57:35.135-07:00He mentions they were very low before the evolutio...<i>He mentions they were very low before the evolution of photosynthesis, but there’s no hint that they seem later to have been very high — perhaps 50% above current levels.</i><br /><br /><a href="http://blogs.discovermagazine.com/notrocketscience/2012/06/04/giant-insects-disappeared-thanks-to-falling-oxygen-levels-and-agile-birds/#.Vin1F64rKHo" rel="nofollow">At one time, there were insects absolutely gigantic by todays standards</a>. <br /><br />The theory being that due to their respiration, the upper limit of insect size was determined by O2 concentration: 30% during allowed dragonflys to have five foot wing spans.<br /><br />As O2 concentration went down, so did insect size. <br /><br />Until birds showed up -- now they are the limiting factor.<br /><br />Anyway, insect size is a hint that O2 levels were at one time much higher than now.Hey Skipperhttps://www.blogger.com/profile/10798930502187234974noreply@blogger.com