Life history consists of the adaptations of an organism that influence aspects of their biology such as the number of offspring it produces, its survival, and its size and age at reproductive maturity. This chapter presents concept discussions bearing on some of the central concepts of life history ecology.Because all organisms have access to limited energy and other resources, there is a trade-off between the number and size of offspring; those that produce larger offspring are constrained to produce fewer, while those that produce smaller offspring may produce larger numbers. Turner and Trexler found that larger darter species produce larger numbers of eggs. Their results also support the generalization that there is a trade-off between offspring size and number. On average, darters that produce larger eggs produce few eggs. They found a strong positive relationship between gene flow among darter populations and the number of eggs produced by females and a negative relationship between egg size and gene flow. Plant ecologists have also found a negative relationship between sizes of seeds produced by plants and the number of seeds they produce. Westoby, Leishman, and Lord found that plants of different growth form and different seed dispersal mechanisms tend to produce seeds of different sizes. Larger seeds, on average, produce larger seedlings that have a higher probability of successfully recruiting, particularly in the face of environmental challenges such as shade and competition.Where adult survival is lower, organisms begin reproducing at an earlier age and invest a greater proportion of their energy budget into reproduction; where adult survival is higher, organisms defer reproduction to a later age and allocate a smaller proportion of their resources to reproduction. Shine and Charnov found that as survival of adult lizards and snakes increases, their age at maturity also increases. Gunderson found analogous patterns among fish. In addition, fish with higher rates of mortality allocate a greater proportion of their biomass to reproduction. In other words, they show higher reproductive effort. These generalizations are supported by comparisons both between and within species. For instance, pumpkinseed sunfish allocate greater energy, or biomass, to reproductive effort where adult pumpkinseed survival is lower.The great diversity of life histories may be classified on the basis of a few population characteristics. Examples include fecundity or number of offspring, mx, survival, lx, relative offspring size, and age at reproductive maturity, α. One of the earliest attempts to organize information on the great variety of life histories that occur among species was under the heading of r selection and K selection. r selection refers to the per capita rate of increase, r, and is thought to favor higher population growth rate. r selection is predicted to be strongest in disturbed habitats. K selection refers to the carrying capacity in the logistic growth equation and is envisioned as a form of natural selection favoring more efficient utilization of resources such as food and nutrients. Grime described plant strategies, or life histories, that match the requirements of three environments: (1) low disturbance–low stress, (2) low disturbance–high stress, (3) high disturbance–low stress. His plant strategies matching these environmental conditions were competitive, stress-tolerant, and ruderal. Based on life history patterns among fish, Kirk Winemiller and Kenneth Rose proposed a classification of life histories based on survivorship especially among juveniles, lx, fecundity or number of offspring produced, mx, and generation time or age at maturity, α. By basing their classification system on some of the most basic aspects of population ecology, lx, mx, and α, Winemiller and Rose established a common currency for representing and analyzing life history information for any organism.
Eric Charnov developed a new approach to life history classification free of the influences of size and time that facilitates the exploration of life history variation within and among groups of closely related taxa. Charnov’s classification, based on relative offspring size, I/m, relative reproductive life span, E/α, and reproductive effort per unit adult mortality, C•E, suggests that mammals, fish, and altricial birds have life histories that are substantially different.
Life history information is playing a key role in the conservation of riparian forests across western North America. As ecologists contribute to the management of endangered populations, they also increase our understanding of basic population ecology.
