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Ecology: economy of nature
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Life history traits represent the schedule of an organism’s lifeThe life history of an organism includes the traits includes the traits connected to the birth
or hatching of offspring. These include the time required to reach sexual maturity, fecundity,
parity, parental investment, and longevity. In essence, life history traits describe an
organism’s strategy for obtaining evolutionary fitness throughout its lifetime. In addition,
life history traits represent the combined effect of many morphological, behavioural and
physiological adaptations of organisms, all of which interact with environmental conditions
to affect survival, growth, and reproduction.
The slow-to-fast life history continuum
Life history traits vary widely among species and among populations within a species. An
organism’s life history represents a solution to the problem of allocating limited time and
resources to achieve maximum reproductive success. A remarkable fact about reproductive
success is that the results is always nearly the same. On average, only one of the offspring
than an individual produces lives to reproduce. In short, each individual replaces only itself.
Is this were not the case, populations would either dwindle to extinction because individuals
fail to replace themselves or populations would continually expand. Two points can be made
about variation in life histories. First, life history traits often vary consistently with respect to
life form, habitat, or conditions in the environment. Second, variation in one life history trait
is often correlated with variation in other life history traits.
Combinations of life history traits in plants
J. Philip grime conceptualised the relationship between the life history traits and
environmental conditions as a triangle, with each of the three points representing an
extreme environmental condition: abiotic stress, competition, and the frequency of
disturbances. Grime proposed that plants functioning at the extremes of these three axes
possessed combinations of traits that could be categorised as stress tolerators, competitors,
or ruderals.
Stress tolerators can survive and reproduce under extreme environmental conditions, such
as very low water availability, very cold temperatures, or high salt concentrations. Because
growing from a seed is very difficult in such stressful environments, stress tolerators put
little of their energy into seeds. Instead, they rely on vegetative reproduction, form of
asexual reproduction in which new plants developed from the roots and stems of existing
plants. Where conditions for the plant growth are less stressful, plants can evolve life history
traits that fall along a continuum from competitors to ruderals. Without the abiotic stresses
of extreme temperatures or an extreme lack of water and without frequent disturbances,
plants can grow rapidly for long periods of time, this creates more competition among
plants for soil nutrients and light. Competitors can grow relatively quickly, achieve sexual
maturity early in life, and still devote only a small proportion of their energy to seed
production as they often spread by vegetative reproduction. Competitors also tend to grow
to larger sizes and exhibit long lifespans. At the other point of the triangle, with low stress
and high-frequency disturbance, we find ruderals. These plants colonise disturbed patches
of habitat, exhibit fast growth, early maturation, and use a high portion of the energy to
, make seeds. Ruderals typically have seeds that are easily dispersed and that can persist in
the environment for many years as they wait for favourable environmental conditions. This
collection of traits allows ruderals to reproduce quickly and to disperse their seeds to other
disturbed sites.
Life history traits are shaped by trade-offs
If we consider the many types of life history traits, it would seem that an organism could
have very high fitness if it could grow fast, achieve sexual maturity at an early age, and have
a long lifespan. However, no organism has the best of all such life history traits, which
highlights the fact that organisms face trade-offs. When one life history trait is favoured, it
often prevents the adoption of another advantageous life history trait.
The principle of allocation
Organisms often have limited time, energy, and nutrients at their disposal. Natural selection
will favour those individuals that allocate their resources in a way that achieves maximum
fitness. Selection on life history traits can be complex because when one trade is altered, it
often influences several components of survival and reproduction. As a result, the evolution
of a particular life history trait can be understood only by considering the entire set of
consequences. From an evolutionary point of view, individuals exist to produce as many
successful progeny as possible. Doing so, however, involves many allocation problems,
including the timing of sexual maturity, the number of offspring to have any one time, and
the amount of parental to bestow on the offspring. an optimised life history is one that
resolves conflicts between the competing demands of survival and reproduction to the best
advantage of the individual in terms of its fitness. Although it is believed that trade-offs
constrain the specific combination of life history traits that a species can evolve,
demonstrating that this has proved difficult. In some cases, trade-offs can only be exposed
by using experimental manipulations.
Offspring number versus offspring size
Most organisms face a trade-off between the number of offspring they can produce and the
size of those offspring in a single reproductive event, this can also be limited by energy an
nutrients. There is a negative correlation between offspring number and size. The size of the
offspring can be relatively constant, suggesting that selection often favours a uniform,
perhaps even optimal, offspring size and that an individual able to acquire additional energy
can only use it to make greater numbers of offspring.
Offspring number versus parental care
The number of offspring produced in a single reproductive event can also cause a trade-off
with the amount of parental care that can be provided. As the number of offspring
increases, the efforts of the parents to provide food and protection will be increasingly
spread thin. Life history traits not only contribute to reproductive success but also influence
evolutionary fitness. Life histories vary constantly with respect to factors in the
environment, this observation suggests that life history traits are moulded by natural
selection. The number of offspring parents can successfully rear is limited by food supply.
Fecundity and parental care versus parental survival
Adding eggs to the nests of birds increases the effort for parents to supply food.
Consequently, parents with small or intermediate size broods have the greatest fitness.
Sometimes, however, having more mouths to feed stimulates parents to hunt harder for
food for their chicks. In this case, an artificially enlarged brood might result in higher
reproductive success in the short term. However, the additional parental effort can impose
a cost on the parent that affects their subsequent fitness.
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