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| Figure 1. Showing the theory, that organisms undergo a 'trade-off' between survival and reproduction. Organisms must sacrifice individual investment in-order to reproduce. Credit to: (Stearns, 1992; Roff, 1992; Flatt and Heyland, 2011). |
Migration behaviours differ within the life histories of many birds worldwide where environmental pressures are, at some point of the year, worth avoiding. These environmental pressures could be biotic (related to living) or abiotic (related to non-living). Biotic pressures could include a decrease in food sources, predator avoidance, innate hormonal urges and inter-species competition. Abiotic pressures can heavily impact the biotic influences where seasonal changes in regional climates can affect ambient temperatures and daylight hours that encourage migration behaviours in many organisms.
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| Figure 2. Sequence of life history stages in a typical migratory bird. Some behaviours are observed in multiple stages of a migratory cycle, however, the hormonal mechanisms regulating these behaviours are likely to be different. Credit to: (Wingfield, Lynn & Soma, 2001). |
Necessary behaviours evolved to avoid abiotic and biotic stresses in-order to take advantage of the change in seasons and follow the trend of increasing primary production. Primary production is a mechanism performed by all photosynthesizing organisms (autotrophs), turning sunlight, air and soil nutrients into more complex organic molecules (figure 3).
A cooperation of solar radiation input, the world's atmosphere, lithosphere and biosphere contributes to net primary productivity (NPP) that dictates a vast majority of the life histories of the world's biosphere.
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| Figure 3. Showing a typically basic ecosystem and the relevant energy transfers between the trophic levels. This exchange of energy as NPP between trophic levels is a major driver of the life histories of all living organisms. |
References:
Figure3. http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/ec/m3/s2/assets/images/ecm3s2_1.jpg
Flatt, T. & Heyland A. (2011). eds. Mechanisms of Life History Evolution. The Genetics and Physiology of Life History Traits and Trade-Offs. Oxford: Oxford University Press.
Roff, D. A. (1992). The Evolution of Life Histories. Theory and Analysis. New York: Chapman and Hall.
Stearns, S. C. (1992). The evolution of life histories. Oxford: Oxford University Press.
Wingfield, J.C., Lynn, S.E. & Soma, K.K. (2001). Avoiding the 'costs' of testosterone: ecological basis of hormone-behaviour interactions. Brain, Behaviour and Evolution. 57: 239-251.