1978). These terms refer to the parameters of the widely-used logistic
growth curve for populations, where "r" is the slope (growth rate of the
population), and "K" the upper asymptote (sometimes referred to as the
carrying capacity of the environment). We prefer the terms altricial and
precocial life history style to "r-selected" and "K-selected", respectively, for
reasons we explained earlier (Balon 197913) and will elaborate below. First,
some further implications of this general dichotomy in life histories are
provided.
Species are said to be "r-selected" if they have a life history pattern that
includes a short growth interval and early maturation, high fecundity,
reduced parental care and short life (generation) span. These features
characterize animals in environments where rapid colonization is favored,
where catastrophic mortality is liable to act in a density-independent manner,
or where environmental disruptions are liable to be serious and unpredictable.
Species are said to have a "K-selected" strategy if they have a prolonged
growth interval and deferred maturation, reduced fecundity, increased
parental care and an extended life (generation) span. They are favored in
environments where conditions are more stable, where competition is
liable to be a more serious factor, where mortality is likely to be density-
dependent, and where conditions are likely to be stable over long periods of
time (May 1976; Horn 1978; Krebs 1978 ;Green 1980). In a relatively stable
(numerically) population, with limiting resources and severe intraspecific
competition, variation in juvenile survival is likely to be greater than variation
in adult survival. In a population going through a series of colonizing episodes,
the reverse is likely to be the case (Steams 1977).
This is a simplification of what is clearly an idealization. But it is useful
because it brings our attention to these aspects of life histories, and couches
our considerations in evolutionary terms. It brings the important questions
to our attention, even if it does not immediately provide the answers. And
the consideration of "r- and K-selection" has its obvious limitations (Steams
1976, 1977; Horn 1978; Green 1980). Certainly, the life history of any
species, when considered in these terms may well be a compromise (i.e., a
mixed rather than a pure strategy), and it may not even be correct to consider
"r" and "K" as being on the same continuum (Horn 1978; Green 1980). As
this concept has obvious limitations, we will refer to it largely because
it has been so widely used, and carries at least some general connotations
(Fryer and Iles 1972; Lowe-McConnell1975, this volume).
Guilds and Ontogeny: Hierarchical SystemsOur consideration of life histories will focus on the ontogeny of individ-
uals, especially early ontogeny. The reasons for believing that the early life
history will be of critical importance and have the primary determining
influence on the entire life history have been given elsewhere (Balon 1964,
l971,1978,1979a, 1979b, 1980,1981a; Noakes 1978a, 1978b, 1981), and
so need not be elaborated here. In particular, we will stress eco-ethological
guilds (reproductive guilds), and an approach based upon an hierarchical
model of ontogeny with a uniform system of nomenclature for development
intervals (Balon 1975a, 1975b, 1981b).