Ecology - Organisms and Populations

Bee Pollination
Bee Pollination
Ecology is the branch of biology which studies the relationship of an organism with different biotic (other living organisms) and abiotic (water, mineral, light and temperature) components of the environment. This interaction or relationship between organisms and their environment is always a two-way process in which they complement each other.

Ecology is closely related to Evolution (Evolutionary Biology) and they are often called sister disciplines of biology. However, It is vastly different from Environmentalism. The area of interest for an ecologist include biodiversity, biomass, organism population, and competition between species. An ecologist tries to answer

1. Life processes, interactions and adaptations
2. Successional development of ecosystems
3. Movement of food, material and energy
4. The distribution of organism and biodiversity around the globe

It has practical application in energy conservation, wetland management, agriculture, forestry, fisheries, urban planning, community health and human interactions.

For example, the interaction between a bee and a flower. i.e. when a bee visits a flower, it brings the pollination.

Ecology at the Organismic Level
At the organismic level, ecology tries to understand how different living organisms adapt to their surrounding environment in terms of survival and reproduction. The major factors which affect the formation of an environment are the rotation of the earth, variation in rain and variation in temperature. The local factors such as soil and topography add another layer of complexity.

It is the most important ecological environmental factor. The temperature varies from places to place, it decreases steadily from the equator to poles and from plains to the mountains. The thermal tolerance capacity of the species determines their survival. On the basis of thermal tolerance capacity, organisms are divided into eurythermal and stenothermal.

Eurythermal organisms are that organism which can survive in a wider range of the temperature while as Stenothermal organisms only survive in a narrow range of temperature.

After temperature, the water is the second most important environmental component in the list. The importance of water can be gauged from the fact, life first begins under the water. The qualitative attributes of water such as chemical composition and pH values play a major role in the survival of the organism.  The salinity or salt concentration in parts per thousand is less than 5 in inland waters, 30-35 in the sea and >100 in some hypersaline lagoons. Some aquatic organisms are tolerant to a wide range of salt concentration but others are limited to a small range. Moreover, many freshwater animals cannot survive for long in sea water and vice versa.

The organisms which show tolerance to a wider range of salinity are known as Euryhaline and opposite which show the tolerance to a smaller range of salinity are known as Stenohaline.

It may not seem an important factor for humans and other land animals but it is a major environmental factor for plants, birds, fishes and other organisms. The plants produce food through photosynthesis, which is only possible thanks to the sunlight. On this basis, organisms can be defined as autotrophs who can produce their own food and heterotrophs who cannot produce their own food. Next, there are organisms who take cues from sunlight for foraging, reproduction and migration. Again under the deep sea (deeper than 500 m), there are numerous creatures which have not seen a flicker of light.

The nature and properties of soil vary from one place to another. The qualitative properties of soil such as pH, mineral content, water retention property along with topography of the place determine the growth of certain crops, plants and trees. The soil is the major factor behind some plants or trees are indigenous and cannot be found elsewhere on the earth.

Response (Homeostasis)
Different organisms response to different external situations in a different manner. However, each organism maintains its internal environmental through a process known as homeostasis. Homeostasis is the condition of optimal functioning of the organism. It includes variables such as internal body temperature, fluid balance and mineral concentration. It can be also achieved through artificial means. For example, a man or woman sitting in his or her car to escape sudden extreme weather.

Different Ways to Maintain Homeostasis
Many organisms including humans maintain homeostasis by regulating their body temperature. In summer, many organisms sweat whereas, in winters, they shiver to maintain their internal temperature. Plants generally do not have this capability with exception to plants like Lotus (Nelumbo Nucifera) and Skunk Cabbage (Symplocarpus foetidus). The creatures are known as regulators.

Majority of organisms (99 per cent of all organism) cannot maintain their body temperature. They simply adjust with changes in temperature. Similarly, aquatic animals change their osmotic concentration with a change in water osmotic concentration. These organisms are known as conformers.

Thermoregulation is an energy consuming activity and it depends upon the surface area of the animal. The small animals which have a larger surface area in comparison to their volume can not afford thermoregulation. Hence, the majority of them are conformers. If stressful conditions continue for more than their persistent timing, they opt for other alternatives such as migration and suspension.

The organisms move away from stressful conditions to more suitable conditions and return to their original habitats when condition become favourable again.

Many organisms if not able to migrate, avoid stressful condition by suspending into time. The bacteria, fungi and many lower plants form thick-walled spores to avoid unfavourable conditions and germinates from them again when favourable conditions are achieved. Higher plants survive these conditions through seeds and other vegetative reproduction mechanisms. Many animals such bear goes into hibernation, animals such tortoise, crocodile and salamander opt for aestivation and many zooplankton goes under diapause in unfavourable conditions.

Adaptation refers to any morphological, physiological or behavioural attribute of the organism that enables its survival and reproduction in its habitat. For example, some desert plants like Opuntia has no leaves, they are reduced to spines and photosynthesis abilities are taken over by the stem. Mammals in colder climatic regions have shorter ears and limbs to minimise heat loss.

The major evolutionary changes through natural selection happen at the population level. Henceforth, population ecology is one of the most important branches of ecology. A population can be defined as a group of the organism of same species sharing or competing for similar resources in a defined an area.

Populations exhibit attributes which cannot be noted through a single organism such as birth rates, death rates, sex ratio and age distribution. The graphical representation of the proportion of different age groups of males and females in a population is known as the age-sex pyramid or simply population pyramid. This graph allows one to understand how a population is changing over time, i.e. stationary, growing or declining.

The ecological effects of any biotic or abiotic factors on a population are generally examined through population density, which can be again expressed in a number of ways such as numbers, biomass and per cent cover depending on the species. In the case of humans, we use numbers, but in the case of plants and small organisms such as fungi, bacteria, we calculate the data on the basis of biomass and per cent cover.

Population Growth
The population is not a static parameter. It depends upon food availability, predation pressure and climatic conditions. The changes in the population give us an idea of whether a species is growing or declining. There are four major processes which cause population change. These processes are

Natality: Number of births during a given period in the population that are added to the initial population
Mortality: Number of deaths during a given period in the population that are removed from the initial population
Immigration: Number of individuals of same species that have entered the habitat from another habitat during a specific period of time
Emigration: Number of individuals of same species that have gone somewhere else during a specific period of time

Immigration and Emigration might not seem a bigger factor on a global scale, but they make a huge impact on the regional level.

Population growth can be expressed as if N(t) is the initial population at time t, then population N(t+1) at t+1 is

N(t+1) = N(t) +  [(B + I) – (D + E)]

where B is the Natality or New Births, I is the Immigration or Number of Immigrants, D is the Mortality or the Deaths and E is the Emigration or the Number of emigrants.

Population Pyramids
Population Growth Model
The population growth largely depends upon the availability of the resources. When resources are abundant in nature, the population grows exponentially and when the resources are scarce, the growth becomes limited.

Exponential Growth: As noted by Charles Darwin, when resources are abundant in nature, the population grows in an exponential method. If N is the size of the population, b is the birth rate per capita and d is the death rate per capita, then increase or decrease in population during a time period t will be

dN/dt = (b – d) × N
Let (b–d) = r, then
dN/dt = rN

The 'r' in the above equation is known as the ‘intrinsic rate of natural increase’ or Malthusian parameter. In 1981, the r-value for the human population in India was 0.0205 and as of 2016, it has dropped down to 0.01019 (~0.0102).

Logistic Growth: However, the resources are finite in nature. Beyond a number, nature cannot support a population. This number is known as the carrying capacity. A population in such a habitat first show a lag phase, followed by successive phases of acceleration and deceleration and then finally an asymptote when the population reaches the carrying capacity. This model of population growth is known as Verhulst-Pearl Logistic Growth model, mathematically described as

dN/dt = rN [(K – N)/K]

where N is the population, r is the intrinsic rate of natural increase and K is the carrying capacity.

Life History Variation
Different organisms have evolved different strategies to maximise their reproductivity and sustain the population. Some organisms breed only once in a lifetime (Pacific salmon) and some breeds multiple time (mammals). Some produce a large number of offsprings (fish) and some are content with a smaller number of offspring (birds and mammals).

Population Interaction
In reality, no population of different species lives in isolation, they live along with each other. They interact with each other and affect their population consequently. These interactions could be positive, negative or neutral. The two interacting species gains in mutualism and loses in the case of competition. One species loses and other gains in case of parasitism and predation. Finally, one species gains and other remains neutral in case of commensalism and one species is harmed and other remains neutral in case of amensalism.

Source: NCERT Class 12 Biology

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