Life cycles: An old idea from natural history with great integrating power in ecology and evolutionary biology
Life cycles and life histories determine how organisms reproduce and how they interact with their environment. They are the core of an organism's natural history. In the past 100 years, over 10,000 titles of papers published in the journals of the Ecological Society of America include the terms "life cycles" or "life history". Beginning with Aristotle, biologists and ecologists have traditionally considered an organism's life cycle or life history strategy in terms of its effects on survival and reproductive output. In recent decades, however, the concepts of life cycles and life histories have been greatly expanded beyond these traditional considerations. I will review how the concepts of life cycles and life histories have changed during the past century and show several ways in which ecologists still find these concepts to be fruitful sources of research ideas.
In the past several decades, we have learned that the ability of organisms to invade and change communities and food webs, to control the fluxes of materials and energy through ecosystems, and to coevolve with other species differs at different stages of a life cycle and with different life history strategies. These new developments require us to expand the concepts of life cycles and life histories to include aspects other than survival and reproductive output. In some cases, it may be advantageous to expand the scope of a life cycle to include the dead carcasses, litter, or waste products of organisms, where these are deposited, and their subsequent fate on the organism's progeny as well as the progeny's competitors, symbionts, or predators. Other examples of expanded views of life cycles and life histories will also be considered.