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Summary chapter 2
chapter 2 ecology ; economy of nature
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Adaptations to variable environmentsSome temporal variation is predictable, including alternation of day and night and seasonal
changes in temperature and precipitations. Superimposed on these predictable cycles are
irregular and unpredictable variations, including weather and climate. Some types of
temporal variation can cause large impacts on ecosystems but occur infrequently in a
particular place. Other sources of temporal variation occur very slowly over decades and
centuries. How organisms and populations respond to temporal variation in their
environment depends on the severity of the change and how often it occurs. In general, the
more extreme the event, the less frequently it occurs.
Spatial environmental variation
Environmental variation also occurs from place to place due to large-scale variation in
climate, topography and soil type.
Phenotypic trade offs
Individuals with mutations that allow them to produce multiple phenotypes that are
uniquely suited to each environment would experience relatively high fitness in both
environments and therefore be favoured by natural selection. Different traits can change at
different rates and these environmentally induced traits can be either reversible or
irreversible. By changing its traits, an individual often maintains a high level of performance
when the environmental changes. This means that phenotypically plastic traits often are a
mechanism of achieving homeostasis. The fitness advantage of phenotypic plasticity occurs
whenever environmental variation in space or time occurs frequently. If environmental
conditions frequently change, then the phenotype favoured by natural selection also
changes frequently and this gives the plastic genotype a higher average fitness than the
non-plastic genotype. If spatial or temporal variation is not common, a single phenotype will
be favoured; the phenotype with the highest fitness in the stable environment will be
favoured.
Environmental cues
For an organism to alter its phenotype in an adaptive way, it must first be able to sense its
environmental conditions. These can take many forms, including smells, sights, sounds and
changes in abiotic conditions.
Response speed and reversibility
Phenotypically plastic traits respond to changes in the environment at different rates. Some
of the trait changes are irreversible. The most rapid responses are typically behavioural
traits, which can be altered in seconds. Physiological plasticity can also be relatively rapid.
Changes in morphology can take a longer time. Differences in response speed have
implications for the reversibility of the induced traits. Behavioural traits that are induced by
a change in the environment typically can be rapidly reversed if the environment reverts to
its original condition. Induced changes in morphology and life history are more difficult to
reverse. The differences in the speed of phenotypic changes, and the ability to reverse
phenotypic changes, influence which traits are favoured by natural selection. When
environments fluctuate more slowly, selection can favour many more types of traits,
including morphological and lift history traits that are slow to respond and are often much
less reversible.
Phenotypic plasticity
Many types of environmental variation can induce phenotypic plasticity. Among biotic
environments, three of the best-studied types of environmental variation involve the
occurrence of enemies, competitor and mates
, Enemies
Because enemies (including predators, herbivores, parasites and pathogens) pose a major
risk to organisms that are consumes, we would expect that many organisms have evolved
defences against their enemies. Predators respond to these with plastic abilities as well.as
well as morphological changes prey also use behavioural defences against predators.
However, these behavioural changes come at a cost as when prey become less active or
congregate in refuges, they spend less time feeding. Also, the supply of food in and around
crowded refuges can be depleted quickly. As a result, behavioural defences commonly come
at a cost of slower growth, development or reproduction. In the absence of predators, prey
become more active and leave the refuges to find more food; this changes in behaviour
allows more rapid growth.
Competition for scarce resources
Most organisms face the challenge of scarce resources, which leads to competition.
However, the intensity of competition varies across and within habitats. As a result,
organisms have evolved a variety of phenotypically plastic strategies for high and low
competition. As we would expect, responses to high competition often exhibit phenotypic
trade-offs that favour the evolution of phenotypically plastic responses. The range of
densities has an effect on the intensity of competition among the plants for sunlight. A
plastic phenotype is an effective way to gain high fitness when the intensity of competition
varies over time and space.
Abiotic variation
Abiotic conditions, including temperature, water availability, salinity, and oxygen vary faced
with this abiotic variation, many species have evolved phenotypically plastic traits that allow
them to improve their fitness.
Temperature
Organisms have evolved a number of plastic responses to temperature variation. Isozymes
are actually a form of phenotypic plasticity with a rapid response time. Many animals
respond to changing temperatures by moving to habitats with more favourable abiotic
conditions.
Water availability
When faced with changes in water availability, most animals can move among different
microhabitats. Because plants are rooted in one place, they have a number of
phenotypically plastic adaptations for coping with water variability these adaptations
include closing the stomata, devoting more energy to grow shoots or roots and shape of the
plants.
Salinity
Freshwater and saltwater organisms have evolved numerous adaptations to handle their
aquatic environments. However, some organisms live in aquatic environments characterized
by solute concentrations that fluctuate widely over short periods of time. To survive, these
organisms must have the ability to make rapid physiological adjustments. When salt
concentrations are high, individuals synthesise large quantities of certain amino acids such
as alanine and proline. These small molecules increase the osmotic potential of the body
fluids to match that of the environmental without the deleterious physiological
consequences that come with high levels of salts or urea.
Oxygen
Some animals are able to adjust their physiology to this variation in oxygen concentration.
Initial changes in higher altitude include more rapid breathing and an increased heart rate.
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