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 trees “want” 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 half “as 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 of “N1 equivalents”

- The logistic equations then become:

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