Suppose one individaul of species 1 reduces the population growth rate of species 2 by a factor of , where the subscript 21 is read as the effect of species 1 on species 2. The entire population of species 1 reduces the population growth rate of species 2 by a factor . What we need to do, however, is scale this effect relative to the negative effect that species 2 exerts on itself via intraspecific competition. If you'd like to explore further, the Questions give some suggestions. Details has a second presentation of the information above.
References cites the text.