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Life History

Life history is an ecological term that refers to the significant features of the life cycle of organisms and their relationships with environmental conditions. Life cycle refers to the sequence of discrete developmental stages of an organism from their origin as gametes to their eventual death. Life cycle also refers to the stages through which generations of organisms pass from their own origin through to their production of gametes toward establishment of the succeeding generation.

Most ecological studies of life history focus on strategies that influence survival and reproduction at the levels of individuals, populations, or species. Studies of this sort are relevant to the notion of adaptation or the complex of biological and ecological traits that enhance the persistence and reproductive success of organisms. In this sense, life histories represent unique biological solutions to the opportunities and difficulties provided by the ecosystems and environments in which organisms live. Each solution involves a complex of life-history traits involving allocations of limited resources of energy, biomass, and time among various competing attributes that may have significant effects on survival and reproduction.

Many adaptational tradeoffs are associated with alternative life-history possibilities. For example, for any given expenditure of energy on reproduction, organisms could potentially produce large numbers of relatively small offspring or smaller numbers of larger offspring. Depending on the ecological circumstances, each of these life-history alternatives has potential benefits and potential detriments.

For instance, the production of large numbers of offspring is beneficial to the relatively short-lived species of plants that are common in recently disturbed habitats. These so-called ruderal plants are only successful for a few years following disturbance, after which they are eliminated from the vegetation by more competitive species. Consequently, the ruderals have to colonize newly disturbed sites on the landscape for the species to survive over the longer term. Because colonization is a very risky business, the chances of evolutionary success of individuals and of persistence at the metapopulation level, that is, of various populations on the landscape, are enhanced if mature plants produce large numbers of small seeds having physical characteristics that enhance dispersal. These include an aerodynamic shape as in dandelions and willows, a tendency to stick to the fur of mammals like burs and ticks, and an ability to pass unharmed through the gut of a large animal after being eaten with the fleshy fruit like cherries and elderberries. In contrast, plants that inhabit relatively mature and stable ecosystems such as forests may be better served by producing relatively few but large, well-provisioned seeds such as acorns and walnuts that are better suited to perennating the species population in predictable, local habitats.

A major sector of activity in ecology is composed of diverse studies directed toward quantifying the benefits and tradeoffs of life-history characteristics and understanding the survival, reproductive, and evolutionary costs and benefits to individuals, populations, and the species as a whole. This type of ecological research has its own intrinsic interest and importance. However, these life-history studies also allow ecologists to develop deeper insights into the relationships between species and their environments. This knowledge may eventually be important in allowing humans to develop sensible methods of utilizing the ecological resources that sustain them and their societies.

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