Sunday, August 3, 2014

My Year of Darwin: 8/3/2014 What regulates populations?

 Charles Darwin

"These are the facts, which make one understand the working or oeconomy of nature. There is one subject, on which I am very curious, & which perhaps you may throw some light on, if you have ever thought on it, -namely what are the checks & what are the periods of life, by which the increase of any given species is limited. Just calculate the increase of any bird, if you assume that only half the young are reared & these breed; within the natural ie, if free from accidents life of the parents, the number of individuals will become enormous, & I have been much surprised to think, how great destruction must be annually or occasionally be falling on on every species, yet the means & period of such destruction scarcely perceived by us." Charles Darwin, Letter to Leonard Jenyns 12 October 1844

Populations grow exponentially without any regulation - and this is obvious to that brew beer or deal with microbes. Less obvious with the life around us, which I find to be ironic. We rarely see death in action yet it must be common. In Darwin's example, he's being exceedingly generous with 50% survivorship. For most songbirds it is probably closer to 98%. But death is probabilistic and some individuals (as with humans) may have great success and some may never breed at all. 

Lately, I've been thinking about the teacher that criticized me for releasing a bird that was then quickly captured by a Peregrine Falcon. I've been meaning to calculate the population of falcons and other bird eating hawks and do a rough estimate of how many hundreds of thousands of birds are captured and eaten each year. 

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