6.2 Trade-Offs in Life History Traits
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The Principle of Allocation states that an organism has a finite (or limited) quantity of resources (energy, nutrients, etc) to invest in the various processes that sustain life. Resources that an organism invests in one process cannot also be invested in other processes, which produces trade offs in the decisions organisms make about resource investment. This section will discuss some of the major trade offs associated with various life history traits and strategies and how these trade offs produce sets of traits that are commonly found together.
As outlined in the previous section, a fundamental life history trait is the number of offspring produced per mating. This number varies widely across organisms. Bison (Bison bison) and other large mammals produce an average of one offspring per mating, as illustrated in Fig 6.2.1A where each female escorts a single calf. Other organisms produce many offspring at a time, such as the European grass frog (Rana temporaria, Fig 6.2.1B) which can lay up to 2,000 eggs in a single reproductive event (Fig 6.2.1C).
A critical trade-off in offspring number relates to the relative size of the offspring. Generally, an individual has a finite amount of resources to invest in producing offspring, which means that a larger number of offspring will be, on average, smaller in size than if only few offspring were produced. This pattern is seen both within and across species. For example, when comparing plant species in the family Asteraceae, species that produce relatively larger seeds also produce fewer seeds per reproductive event, while species that produce relatively small seeds also produce more seeds per reproductive event. Even within a species, individuals who produce more offspring per reproductive event will, on average, produce smaller offspring. Consider, for example, the size and number of human offspring. Though multiple births (twins and triplets) do occasionally occur naturally, humans generally produce a single offspring per mating and at birth, single-birth babies weigh on average ~7.5 pounds (3.5 kg). Twins, however, weigh on average ~5.5 pounds (2.5 kg) and triplets ~4 pounds (1.8 kg). With advances in medical technology such as in-vitro fertilization, it is possible for women today to carry many more offspring than would normally occur. Famous examples of sextuplets and octuplets have occurred in recent years, which follow the expected pattern of birth weight and number. Kate Gosselin’s sextuplets born in 2004 weighed on average 2.7 pounds (1.2 kg) at birth and Nadya Suleman’s octuplets born in 2009 weighed on average 2.5 pounds (1.11 kg) at birth. In species like humans where offspring are carried inside the parent’s body as they develop, the trade-off in number and size of offspring is not just about energetic or nutritional resources, but also about space. Even with medical advances in fertilization techniques, the human body can only hold so many babies.
The trade-off between offspring size and number influences the survival of offspring as they mature. This is an example of trade-offs that occur across life cycle stages, meaning that the benefits and costs to a particular decision are experienced by the organism at different stages in the life cycle. For example, consider the plants in the Asteraceae family mentioned in the previous paragraph. Larger seeds contain more nutritional content for the developing embryo plant. Consequently, species that produce larger seeds will likely have higher seed and seedling survival, as the juvenile plant has more resources available early in development. If small seeds are less likely to survive to seedlings, why would any plant species produce small seeds? First, recall that smaller seeds are also more numerous, and so, even though the likelihood of any one seed surviving is lower, there are more total seeds, and so more ‘chances’ for offspring to survive the juvenile stage. Second, in these species, smaller seeds have a different advantage over large seeds: dispersal. Smaller seeds are better able to travel on the wind than larger (and heavier) seeds, and so have an advantage at a different life history trait than large seeds. Large seeds may have another advantage, however: competition. With their higher nutritional resources, large seeds are more likely to successfully compete with other plants, especially at the juvenile stage when the seedlings primarily rely on nutritional resources within the seed before being able to fully access resources from the air and ground.
The descriptions above illustrate the complexities of trade-offs in life history traits both within and across life cycle stages. The initial trade-off between seed number and seed size leads to different benefits or drawbacks in future life history traits such as seed dispersal, seedling survival, and seedling competitive success. Large seeds exhibit a higher likelihood of survival and stronger competitive advantage, but are few in number and unable to disperse long distances. Small seeds are numerous and able to disperse further than large seeds, but are less likely to survive as seedlings and struggle more in competition with other species.
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