- Describe the energy budgets of, and the life history strategies used in, reproduction
Life history patterns and energy budgets
Energy is required by all living organisms for their growth, maintenance, and reproduction. At the same time, energy is often a major limiting factor in determining an organism’s survival. Plants, for example, acquire energy from the sun via photosynthesis, but must expend this energy to grow, maintain health, and produce energy-rich seeds to produce the next generation. Animals also have the additional burden of using some of their energy reserves to acquire food. In addition, some animals must expend energy caring for their offspring. Thus, all species have an energy budget in which they must balance energy intake with their use of energy for metabolism, reproduction, parental care, and energy storage, as when bears build up body fat for winter hibernation.
Parental care and fecundity
Fecundity is the potential reproductive capacity of an individual within a population. In other words, it describes how many offspring could ideally be produced if an individual has as many offspring as possible, repeating the reproductive cycle as soon as possible after the birth of the offspring. In animals, fecundity is inversely related to the amount of parental care given to an individual offspring. Species that produce a large number of offspring, such as many marine invertebrates, usually provide little if any care for those offspring, as they would not have the energy or the ability to do so. Most of their energy budget is used to produce many tiny offspring. Animals with this strategy are often self-sufficient at a very early age. This is because of the energy trade-off these organisms have made to maximize their evolutionary fitness. Since their energy is used for producing offspring instead of parental care, it makes sense that these offspring have some ability to be able to move within their environment to find food and perhaps shelter. Even with these abilities, their small size makes them extremely vulnerable to predation, so the production of many offspring allows enough of them to survive to maintain the species.
Animal species that have few offspring during a reproductive event usually give extensive parental care, devoting much of their energy budget to these activities, sometimes at the expense of their own health. This is the case with many mammals, such as humans, kangaroos, and pandas. The offspring of these species are relatively helpless at birth, needing to develop before they achieve self-sufficiency.
Plants with low fecundity produce few energy-rich seeds (such as coconuts and chestnuts) that have a good chance to germinate into a new organism. Plants with high fecundity usually have many small, energy-poor seeds (as do orchids) that have a relatively-poor chance of surviving. Although it may seem that coconuts and chestnuts have a better chance of surviving, the energy trade-off of the orchid is also very effective. It is a matter of where the energy is used: for large numbers of seeds or for fewer seeds with more energy.
Early versus late reproduction
The timing of reproduction in a life history also affects species survival. Organisms that reproduce at an early age have a greater chance of producing offspring, but this is usually at the expense of their growth and the maintenance of their health. Conversely, organisms that start reproducing later in life often have greater fecundity or are better able to provide parental care, but they risk not surviving to reproductive age. Examples of this can be seen in fish. Small fish, such as guppies, use their energy to reproduce rapidly, but never attain the size that would give them defense against some predators. Larger fish, such as bluefin tuna and mako sharks, use their energy to attain a large size, but do so with the risk that they will die before they can reproduce or reproduce to their maximum. These different energy strategies and trade-offs are key to understanding the evolution of each species as it maximizes its fitness and fills its niche.
Single versus multiple reproductive events
Some life history traits, such as fecundity, timing of reproduction, and parental care, can be grouped together into general strategies that are used by multiple species. Semelparous species are those that only reproduce once during their lifetime and then die. Such species use most of their resource budget during a single reproductive event, sacrificing their health to the point that they do not survive. Examples of semelparity are bamboo, which flowers once and then dies, and the Chinook salmon, which uses most of its energy reserves to migrate from the ocean to its freshwater nesting area, where it reproduces and then dies. In contrast, iteroparous species reproduce repeatedly during their lives. Some animals are able to mate only once per year, but survive multiple mating seasons. Primates, including humans and chimpanzees, are examples of animals that display iteroparity.
- The amount of parental care given to an individual offspring is inversely related to the reproductive capacity of an animal species.
- Animal species that produce many small, vulnerable offspring tend to provide little or no care for them due to their energy budget constraints; just enough offspring survive to maintain the species.
- Animal species that have few offspring expend large amounts of their energy budgets on caring for helpless offspring that need to develop before being on their own.
- Plants with low fecundity produce few energy-rich seeds with high germination rates, while plants with high fecundity usually have many small, energy-poor seeds with poor survival rates.
- Species that reproduce early ensure a greater chance of having surviving offspring than do those that must survive to a later reproductive age.
- Semelparous species use all of their reproductive budgets on one single reproductive event, while iteroparous species spend it on multiple mating seasons.
- iteroparous: reproducing more than once in a lifetime
- semelparous: reproducing only once in a lifetime
- fecundity: number, rate, or capacity of offspring production