This Week's Citation Classic

Birch, LC. 1948. The intrinsic rate of natural increase of an insect population. J. Anim. Ecol. 17: 15-26.

In many of my published papers, one can find a simple equation:

r = ln (n_t/n_o)/t

The equation permits the calculation of "little r", otherwise known as the intrinsic rate of increase, the per capita growth rate, the net reproductive rate, or the Malthusian parameter. Roughly "r" is the natural logarithm of the change in animal's abundance over time. I never really stopped and considered its origin, until recently.

The equation was first described in Elements of Physical Biology by AJ Lotka in 1925, however was not applied to animals until Leslie and Ransom published on voles in 1940. But being first is not always best. While Leslie and Ransom's contribution is valuable, I don't think it really captures the broader evolutionary significance of "little r". By contrast, Birch's paper on grain weevil population biology does introduce the reader to the wider implications of the Malthusian parameter. One of the implications that derived from Birch's research was that external processes such as weather and disturbance, were more important to the regulation of animal numbers than internal processes, such as competition or self-regulation. In addition, Birch suspected that it was likely that population demographic analyses could be important clues to the evolution of major life history traits (e.g. generation time, fecundity). This premonition later proved to be spot on.

Not without difficulty, Birch's work persuaded population geneticists of the utility of using "little r" as a measure of absolute fitness. That is, the genotype with the higher "little r" is going to out-compete and eventually supplant the genotype with the lesser "little r". Much significant work in evolutionary biology and ecology has been built on these foundations.

In 1954 Birch went on to publish with HG Andrewartha, "The Distribution and Abundance of Animals", a classic book which introduced an entire generation of evolutionary biologists and ecologists to the study of population biology.

Painting by Emil Zeck, courtesy of NSW Agriculture.
Description: As with other species of Sitophilus, this species of weevil is a pest of stored grain, in this case, primarily of rice. Zeck shows in this painting, however, that the species can also be found feeding in stored corn and wheat. The small size of the beetle is evident from the background image which has adult beetles with rice grains. The detail in the magnified section is superb given the size of the insect and its legless larvae.