# lecture note 20 for BGYB50

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19 Jul 2010
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LECTURE 20:
- Competition affects per capita survivorship, susceptibility to disease, and, very
predictably, body size and weight (Yoda’s Law: individual body weight and density are
correlated inversely; in a log-log plot of these parameters, a linear relationship is
frequently found, with a slope of -3/2; example: forest stands: trees are smaller when
there are many of them compared to only a few; conversely, if treeswant to grow
larger, they will have to reduce their own density through self-thinning)
- In order to describe the competitive interactions between populations of two species, we
need to modify the logistic equations for those populations by incorporating the per
capita competitive effect (!) each has upon the other
- ! gives the relative “muscle power” of the two species populations in relation to each
other
- Subscripts to ! denote the directionality of the competition effect (!12 = the per capita
competitive effect on species 1 of species 2; !21 = the per capita competitive effect on
species 2 of species 1); e.g. !12 = 0.5, if the average individual in the population of
species 2 is halfas strong” as the average individual in the population of species 1; in
other words, multiplying N2 by !12 converts any given number of individuals of
population 2 to a number ofN1 equivalents
- The logistic equations then become:
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