Excerpts from Jim Conrad's
Naturalist Newsletter

from the July 18,, 2010 Newsletter issued from Hacienda Chichen Resort beside Chichén Itzá Ruins, central Yucatán, MÉXICO

These nestling experiences have gotten me thinking about the r/K Selection Theory which, loosely interpreted, relates to the trade-off between producing lots of offspring with a low probability of surviving to adulthood ("r-selected species"), as opposed to a few ones who have a better chance at staying alive ("K-selected species").

r-selected species tend to live in unstable environments, produce many offspring that disperse widely, mature early, have a small body size, and live relatively briefly -- weeds are a good example. K-selected species tend to occupy stable environments, produce few offspring, be relatively large, and live relatively longer -- Sequoia trees in protected mountain coves are good examples, and humans. Each offspring of a K-selected species, whether it is a seed, nestling or a higher primate baby, requires more "preparation" by the parents before they are set into the world.

The concept's variables are expressed algebraically in the Verhulst Equation of Population Dynamics shown below:

Verhulst Equation of Population Dynamics

In that equation, r is the growth rate, K is the environment's carrying capacity and N is the population size. These terms are generally recognized by those acquainted with standard ecological algebra.

This is not a case of something in Nature being summed up completely and elegantly in a tiny equation. There's a continuous spectrum of species between the r and K extremes, so by no means do all species fall into one or the other categories. Also, some species display both r- and K-traits. For example, sea turtles are large and have long lifespans but they produce many unnurtured offspring.

What's interesting in the context of precocial and altricial bird species is to consider whether altricial birds, whose parents invest a lot in their upbringing, fit the description of K-selected species, while precocial bird species, whose parents are much less involved in providing care, fit the mold of r-selected species.

Consensus seems to be that there's a certain correlation between the two concepts, but so many exceptions can be thought of that it's debatable whether it's useful to link them.