Organism and its Environment
Biomes
Rotation of our planet around the Sun and tilt of its axis causes annual variations in the intensity and duration of temperature, resulting in seasons. These variations together with precipitation account for formation of major biomes. These include:
Desert
Tropical rainforest
Tundra
Niche
The ecological niche of an organism represents the physical space occupied by it, the resources it utilizes and its functional role in the ecological system.
Major Abiotic Factors Affecting the Organisms: Temperature
Temperature has a significant effect on the metabolism of living beings The level of thermal tolerance of species determines the geographical distribution of various species. It ranges from sub zero to hot summers in tropicals.
Eurythermal:
Stenothermal:
Organisms that can survive in a narrow range of temperatures are called stenothermal.
Water
Life cannot be sustained without water. Productivity and distribution of plants depends on water.
For aquatic organisms, quality of water is very important. Chemical composition and pH are the criteria for determining the quality of water.
Euryhaline:
Organisms which can thrive in a wide range of salinities are called euryhaline organisms.
Stenohaline:
Organisms which can sustain only in a narrow range of salinities are stenohaline organisms
Light
Plants are dependent on sunlight to meet their photoperiodic requirements for flowering and for photosynthesis.
Many animals use the diurnal and seasonal variations in light intensity and duration as signs for timing their foraging and migratory activities.
The spectral quality of solar radiation is also important for living beings. UV radiations are harmful for almost all living beings.
Soil
Important characteristics of soil are - soil composition, grain size and aggregation. These attributes determine the percolation and water holding capacity of soil.
Other parameters like pH, mineral composition and topography determine the vegetation in any area.
The vegetation determines the type of animals which can be sustained in that area.
Response of Organisms to the Abiotic Factors
Regulation
Some organisms are able to maintain homeostasis and maintain a constant body temperature, osmotic concentration, etc.
All birds and mammals, and a very few lower vertebrate and invertebrate species are capable of such regulation.
Due to this, mammals and birds are found in almost all parts of the Earth as they are capable of regulation.
Conformation
These organisms cannot maintain a constant internal environment. Their body temperature changes with the ambient temperature.
Osmotic concentration of the body fluids of aquatic animals changes with that of the ambient water osmotic concentration.
These organisms are called conformers because they conform to the surrounding environmental conditions.
Thermoregulation
Small animals have a larger surface area in comparison to their volume. Hence, they start losing body heat very fast when it is cold outside. In such a condition, they need to spend too much energy to generate body heat through metabolism. This is the reason why very small animals are rarely found in polar regions.
3. Migration
Movement of organisms temporarily from the stressful habitat to a more hospitable area and returning when the stressful period is over is referred to as migration.
Many species of birds migrate from Siberia to India during the winter in the northern hemisphere. They migrate to escape the harsh climate during these months and return to their original habitat once the winter season is over.
4. Suspension
Many organisms suspend metabolic activities to pass over unfavorable conditions.
Bacteria, fungi and lower plants form thick-walled spores to tide over unfavorable conditions. The spores germinate when the conditions become favorable.
In higher plants, seed dormancy is a means to do so. Many vegetative structures help the same during harsh conditions.
Bears go into hibernation during winter months.
Frogs go into summer aestivation to avoid harsh summers.
Many zooplankton species in lakes and ponds enter diapause, i.e. a stage of suspended development.
Adaptations in Organisms
Any attribute of the organism that enables the organism to survive and reproduce in its habitat is called adaptation.
Adaptations can be morphological, physiological and behavioural.
For example, the kangaroo rat in North American deserts is capable of satisfying all its water requirements by fat oxidation in which water is a byproduct.
Allen's Rule
This rule states that animals adapted to cold climates have shorter limbs and body appendages than animals, adapted to warm climates.
This rule states that the body volume-to-surface area ratio for homeothermic animals varies inversely with the average temperature of the habitat. In cold climates, the ratio is high as compared to hot climates.
For example, mammals from colder climates normally have shorter ears and limbs to reduce heat loss.
Physiological Adaptation
It allows an organism to respond quickly to a stressful situation.
Altitude sickness is often seen in a person who lives in the plains and goes to a hilly area. Within a few days, the person gets accustomed and symptoms of altitude sickness disappear. This occurs because of increased production of red blood cells. This is an example of physiological adaptation.
Behavioural Adaptation
Desert lizards, when their body temperature drops below a certain level, bask in the sun to absorb heat. When the surrounding temperature increases, these lizards go into shade.
Population
Population Attributes
Birth Rate:
Number of live births per thousand members is generally taken as birth rate.
Death Rate:
Number of deaths per thousand members is generally taken as death rate.
Sex Ratio:
The ratio of females to males in a population is called sex ratio.
Age Pyramid:
When the age distribution of population is graphically represented, the resulting structure on the graph is called age pyramid.
Growing/Expanding:
When the population of pre-reproductive age group is highest, followed by reproductive and post-reproductive age group in that order, the age pyramid represents a growing population.
Stable:
When the population of the pre-reproductive age group is almost the same as that of the reproductive age group, followed by the population of the post-reproductive age group, the age pyramid represents a stable population.
Declining:
When the population of the pre-reproductive age group is lower than that of the reproductive age group, the age pyramid represents a declining population.
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Measurement of Population Size
Population size is generally measured in terms of population density, i.e. number per unit geographical area.
But it is difficult to determine the absolute numbers in some cases. For example - There can be a single banyan tree in a huge lawn full of grass. Percent cover or biomass is a more correct measure of the population size in this case.
In some cases, indirect measurement is a more practical way of assessing the population size, e.g. population of bacteria in a laboratory setup, or population of tigers.
Population Growth
Natality:
The number of births during a given period in the population.
Mortality:
The number of deaths in the population during a given period.
Immigration:
The number of individuals of the same species that have come into the habitat from elsewhere during the given time period .
Emigration:
The number of individuals of the population who left the habitat and went elsewhere during the given time period. N is the population density at time t, then its density at time t +1 is Nt+1 = Nt + [(B + I) - (D + E)] This equation shows that population density will increase if the number of births and the number of immigrants (B + I) is greater than the number of deaths and the number of emigrants (D + E), otherwise it will reduce.
Growth Models
Exponential Growth
Ideally, in the presence of unlimited resources, each species has the ability to grow in number.
In this case, the population grows in an exponential or geometric fashion.
If in a population of size N, the birth rates are represented as b and death rates as d, then the change in N during a unit time period t (dN/dt) will be dN/dt = (b - d)xN Let (b-d) = r, then dN/dt = rN The r in this equation is called the 'intrinsic rate of natural increase'and is a very important factor considered for estimating impacts of any biotic or abiotic factor on population growth.
Logistic Growth
Resources are limited and there is competition for resources. So, exponential growth is not possible in actual situations.
A given habitat has enough resources to support a maximum possible number. No further growth is possible beyond that number. This number is known as the carrying capacity (K) of nature, for a particular species.
In a habitat with limited resources, the population growth initially shows a lag phase. This is followed by phases of acceleration and deceleration, and lastly an asymptote.
A plot of N in relation to time (t) results in a sigmoid curve. This type of population growth is called Verhulst-Pearl Logistic Growth and is understood by the following equation: dN/dt=rN((K-N)/K) Where N = Population density at time t r = Intrinsic rate of natural increase K = Carrying capacity The logistic growth model is a more realistic growth model.
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Life History Variations
Populations evolve to maximize their reproductive fitness in the habitat in which they live which is called Darwinian fitness.
Considering a particular set of selection pressures, organisms evolve towards the most efficient reproductive strategy.
Some organisms breed only once in their lifetime, e.g. Pacific salmon fish, bamboo, etc.
Some organisms breed many times during their lifetime.
Some produce a large number of small-sized offspring.
Some others produce a small number of large-sized offspring. The life history traits of organisms have grown in relation to the limitations imposed by abiotic and biotic components of the habitat.
Population Interactions
Predation
Predation is nature's way of transferring the energy (fixed by plants) to higher trophic levels.
Act as mediators for energy transfer across trophic levels.
They keep the prey population under control.
This helps prevent ecosystem instability.
Help maintain species diversity in a community. Prey species have evolved various defense mechanisms to reduce the impact of predation. Camouflage, thorns, poisoned armory, etc. are examples of some defenses.
Competition
Competition for resources exists between closely related species, or between entirely unrelated species.
It is logically defined as a process in which the fitness of one species (measured in terms of r) is markedly lower in the presence of another species.
Species who face competition may evolve mechanisms that promote coexistence rather than exclusion.
One such alternative is resource partitioning. Species may choose different times for feeding or maybe different patterns.
Commensalism
This is the interaction in which one species benefits and the other is neither harmed nor benefited.
Orchids that grow as an epiphyte on a mango branch and barnacles growing on the back of a whale benefit while neither the mango tree nor the whale get any benefit.
The cattle egret and grazing cattle in close association, is an example of commensalism. When the cattle graze, they stir up and flush out the insects from vegetation. Such insects would be difficult for the egrets to find and catch.
Mutualism
This interaction benefits both the interacting species.
Lichens and mycorrhizae are examples of mutualism.
Relationships between plants and animals show fascinating examples of mutualism.
Plants need the help of animals for cross pollination. In exchange for that, an animal gets nectar or fruit as food.
