Organism

Our living world is fascinatingly diverse and amazingly complex. We can try to understand its complexity by investigating processes at various levels of biological organisation – macromolecules, cells, tissues, organs, individual organisms, population, communities and ecosystems and biomes. At any level of biological organisation we can ask two types of questions – for example, when we hear the bulbul singing early morning in the garden, we may ask – ‘How does the bird sing ?’ Or, ‘Why does the bird sing?’ The ‘how-type’ questions seek the mechanism behind the process while the ‘why- type’ questions seek the significance of the process.

For the first question in our example, the answer might be in terms of the operation of the voice box and the vibrating bone in the bird, whereas for the second question the answer may lie in the bird’s need to communicate with its mate during breeding season. 

Ecology is basically concerned with four levels of biological organisation – organisms, populations, communities and biomes. 

Levels of Ecological Organization

• Organisms: Organisms are the basic unit of study in ecology. Organisms belonging to same species interbreed and produce fertile offspring under natural condition. Species is a genetically and reproductively closed group, consisting of same type of organisms. 
• Population: Group of individuals of plants or animals of a species inhabiting a particular area is called population. 
• Community: All the populations of different species in a particular area is called community. Various types of interaction occur in between the organisms of the same species as well as between the members of different species, in a community. 
• Biome: It is a large regional unit delimited by a specific climatic zone having a particular major vegetation zone and its associated fauna e.g., tropical rain forest, desert biome, temperate coniferous forest, etc.

Organism and Its Environment

• Ecology at the organismic level is essentially physiological ecology which tries to understand how different organisms are adapted to their environments in terms of not only survival but also reproduction. 
• The rotation of our planet around the Sun and the tilt of its axis cause annual variations in the intensity and duration of temperature, resulting in distinct seasons. These variations together with annual variation in precipitation (remember precipitation includes both rain and snow) account for the formation of major biomes such as desert, rain forest and tundra (Figure).

• Regional and local variations within each biome lead to the formation of a wide variety of habitats. Major biomes of India are shown in Figure. 
• On planet Earth, life exists not just in a few favourable habitats but even in extreme and harsh habitats – scorching Rajasthan desert, perpetually rain-soaked Meghalaya forests, deep ocean trenches, torrential streams, permafrost polar regions, high mountain tops, boiling thermal springs, and stinking compost 

Environment Habitat and Niche

Environment: Sum total of all living or biotic and non-living or abiotic factors that influence an organism is called environment. Some environmental components function as resources while others work as regulatory factors.

Climate: Short-term properties of the atmosphere such as precipitation, temperature, humidity, sunshine, pressure, wind and cloud cover at a particular area and time are called weather. Average weather conditions of that area over a long period of time, say for a season or an year is called its climate. Temperature and rainfall are the two most important factors which determine the climate of an area. On the basis of variations in mean temperature along latitude the main climatic regions are tropical (0°–20° latitude) subtropical (20°–40° latitude), temperate (40°–60° latitude), Arctic and Antarctic (60°–80° latitude). As we move from equator to poles or from lower latitude to higher latitude temperature decreases. Similarly temperature also decreases as we move from lower altitude to higher altitude.

Microclimate: Often environment in which organisms live is quite different from overall climate of the area e.g., under a tree temperature and humidity are quite different in comparison to average climate of the area. The climatic conditions that prevail at a local scale or in area of limited size is called microclimate.

Habitat and Niche

• The place where an organism lives is called its habitat. Functional role of an organism in the ecosystem along with physical space, which it occupies, is called its ecologicalniche. Term niche was given by Grinnel.
• Each species has a distinct niche in the ecosystem. No two species can occupy same niche in the same ecosystem for long and one species replaces another. This is called competitive exclusion principle or Gause hypothesis. Organisms, which occupy the same niche in different habitats are called ecological equivalents.
• The key elements that lead to so much variation in the physical and chemical conditions of different habitats are temperature, water, light and soil. 
• The physico-chemical (abiotic) components alone do not characterise the habitat of an organism completely; the habitat includes biotic components also – pathogens, parasites, predators and competitors – of the organism with which it interacts constantly. It is assumed that over a period of time, the organism has through natural selection, evolved adaptations to optimise its survival and reproduction in its habitat.

Environment Factors

Environmental factors affect the life of living organisms. These factors are broadly categorized into two groups, abiotic and biotic factors.

Physical or abiotic factor: 

• Climatic factors such as air, light, atmospheric humidity, rain, temperature, etc. 
• Edaphic factors which are related with soil.
• Topographic factors, which are related with, surface behavior of earth like steepness, altitude, direction of mountain chains and valleys.
• Fire as an ecological factor also play important role in distribution of plants and animals.

Biotic factors: These include producers or plants, consumers or animals and decomposers or microorganisms.

Major Abiotic Factors

Temperature:

• Temperature is the most ecologically relevant environmental factor the average temperature on land varies seasonally, decreases progressively from the equator towards the poles and from plains to the mountain tops. The vertical temperature gradient over earth surface is called lapse rate. Its value is 6.5°C per 1000 meters elevation.
• It ranges from subzero levels in polar areas and high altitudes to >500C in tropical deserts in summer. There are, however, unique habitats such as thermal springs and deep-sea hydrothermal vents where average temperatures exceed 1000 C. It is general knowledge that mango trees do not and cannot grow in temperate countries like Canada and Germany, snow leopards are not found in Kerala forests and tuna fish are rarely caught beyond tropical latitudes in the ocean. 
• The significance of temperature to living organisms is that it affects the kinetics of enzymes and through it the basal metabolism, activity and other physiological functions of the organism. 
• A few organisms can tolerate and thrive in a wide range of temperatures (they are called eurythermal), but, a vast majority of them are restricted to a narrow range of temperatures (such organisms are called stenothermal). The levels of thermal tolerance of different species determine to a large extent their geographical distribution. 
• Inwater bodies temperature varies with depth which is called thermal stratification. During summer, temperature of upper layer of water is higher than the lower layer. The upper layer is called epilimnion while lower layer is called hypolimnion. These two layers are separated by a narrow transition zone called thermocline. During winter, in a temperate lake, water is at freezing temperature on the surface, whereas in the lower layer temperature is about 4°C. The surface water is cooled during autumn, and warmed in spring. This results in a free mixing of water in the whole water body and known as autumn and spring turnover respectively. It results in vertical mixing of oxygen and nutrients, which results in rich growth of algae or bloom formation.
• Altitude (height above the sea level) and latitude (distance from equator) have effect on temperature, which in turn affect type of vegetation. Effect of altitude and latitude are almost same on temperature and vegetation. The types of vegetation from sea level to increasing altitudes are tropical or evergreen forest, xerophytic plants or deserts, deciduous vegetation, conifers (e.g., Pinus, Taxus, Cedrus, Picea, etc.) alpine vegetation (Rhododendron and Juniperus) which are found at 3600 m or 12,000 ft above sea level and tundra region where small grasses like Saxifraga, Primula and lichens, etc., occur. Almost similar change in vegetation occurs with latitude, i.e., from equator to poles.

Effect on reproduction (flowering): Temperature is also an important factor affecting flowering in plants. Low temperature treatment (0–6°C), i.e., vernalization or springification is responsible for enhancing flowering in plants.

Temperature and Morphology: Several structural modifications are induced in animals by temperature.

• According to Jordan, fishes living in water characterized by low temperature are said to have more vertebrae than those of the warm water forms (Jordan’s rule).
• Animals living in cold regions tend to be large, for example polar bears and whales, whilst animals living in hot climates are generally smaller. This phenomenon is known as Bergman’s rule. 
• Protruding parts such as tails, ears or legs tend to be shorter in colder climates than in warmer climates (Allen’s rule). 
• Colours tend to be darker in warm moist climates and lighter in cold dry climate (Gloger’s rule)
• Birds of colder areas have narrow and acuminate wings while those of warmer areas have broader wings (Rensch’s rule).

Fig.: variation in ear length shown by three species of fox, each occupies a different geographical region (Allen’s rule)

Temperature and Cyclomorphosis: Cyclomorphosis is a phenomenon exhibited by certain planktonic organisms like Daphnia, which change their body form with the seasonal changes of temperature. In the winter, these crustaceans have a round head. During spring a helmet like projection develops, which gets reduced in autumn and disappears in winter.

Fig.: Seasonal variation in body form of Daphnia

Water:

• Next to temperature, water is the most important factor influencing the life of organisms. In fact, life on earth originated in water and is unsustainable without water. Its availability is so limited in deserts that only special adaptations make it possible to live there. The productivity and distribution of plants is also heavily dependent on water. 
• Those organisms living in oceans, lakes and rivers should not face any water-related problems, it is not true. For aquatic organisms the quality (chemical composition, pH) of water becomes important. The salt concentration (measured as salinity in parts per thousand), is less than 5 per cent in inland waters, 30-35 per cent the sea and > 100 per cent in some hypersaline lagoons. 
• Some organisms are tolerant of a wide range of salinities (euryhaline) but others are restricted to a narrow range (stenohaline). Many freshwater animals cannot live for long in sea water and vice versa because of the osmotic problems, they would face.
• Rainfall is the chief source of precipitation. Out of the total rainfall, 45% is run away or runoff water, which flows to rivers and oceans, 35% is evaporated and rest 20% is absorbed by the ground. Amount and timings of rainfall determine the type of vegetation of an area, e.g.,
• If no rainfall or scanty rainfall throughout the year then xerophytic vegetation or deserts occur. 
• If more rainfall in summer and less in winter, then grasslands will develop.
• If more rainfall in winter and less in summer, then thick, dwarf shrubs with leathery leaves, i.e., sclerophyllous vegetation occurs. 
• If rainfall occurs throughout the year then evergreen forests, e.g., tropical rain forests occur.

Light:

• Since plants produce food through photosynthesis, a process which is only possible when sunlight is available as a source of energy, we can quickly understand the importance of light for living organisms, particularly autotrophs. 
• Visible part of light falls between 400–700 nm, which is mainly utilized by plants. It is called photosynthetically active radiation (PAR). Radiation beyond 700 nm is called infrared while between 100-400 nm wavelength is called ultra violet (UV). UV radiation can be categorized into three types UV-C (100 nm–280 nm), UV-B (280nm–320nm) and UV-A (320 nm–400 nm). Out of these, UV-C radiation is lethal for organisms while UV-B is also harmful.
• Many species of small plants (herbs and shrubs) growing in forests are adapted to photosynthesise optimally under very low light conditions because they are constantly overshadowed by tall, canopied trees. Many plants are also dependent on sunlight to meet their photoperiodic requirement for flowering. For many animals too, light is important in that they use the diurnal and seasonal variations in light intensity and duration (photoperiod) as cues for timing their foraging, reproductive and migratory activities. The availability of light on land is closely linked with that of temperature since the sun is the source for both. But, deep (>500m) in the oceans, the environment is perpetually dark and its inhabitants are not aware of the existence of a celestial source of energy called Sun. 
• The spectral quality of solar radiation is also important for life. The UV component of the visible spectrum are available for marine plants living at different depths of the ocean. 
• Light is an important limiting factor for plants in deep water bodies such as lakes and oceans. Different zones in water bodies on the basis of penetration of light are–

• Littoral zone: It is the shallow zone around the edges where depth of water is not much, so the rooted plant grows.
• Limentic zone: It is open water zone beyond the littoral zone where depth of the water is so much that only phytoplankton or free-floating plants can grow. Its depth may be between 20–40 metres depending upon the turbidity of water.
• Profundal zone: It is the dark zone present below limnetic zone where light cannot penetrate. Blue light has maximum penetration power while red light has minimum for the water. The floor of the water bodies is called benthic region and organisms living there are called benthic organism, e.g., snails, slugs and micro-organisms.

Effect of light on growth and development: Different plants have different light requirements and hence plants are divided into two main types on the basis of light requirement for growth and development.

• Heliophytes or Photophilous or Sun loving plants: These grow in high light condition, e.g., Sunflower. 
• Sciophytes or Heliophobous or Photophobous or Shade loving plants: These grow in poor light intensity, e.g., Abies, Picea, Taxus, etc.

Soil:

• The factors, which affect the plants through soil, are called edaphic factors. Soil is very important ecological factor as it provides water, minerals or nutrients and support. Warming (1909) was the first ecologist who recognized the importance of soil as ecological factor and he divided plants into five different types on the basis of the soil conditions, i.e., halophytes (plants growing in salty or saline soil), oxylophytes (plants growing in acidic soil), psammophytes (plants growing in sandy soil), chasmophytes (plants growing in rock crevices) and lithophytes (plants growing on rocks).
• The nature and properties of soil in different places vary; it is dependent on the climate, the weathering process, whether soil is transported or sedimentary and how soil development occurred. 
• Various characteristics of the soil such as soil composition, grain size and aggregation determine the percolation and water holding capacity of the soils. These characteristics along with parameters such as pH, mineral composition and topography determine to a large extent the vegetation in any area. This is in turn dictates the type of animals that can be supported. Similarly, in the aquatic environment, the sediment-characteristics often determine the type of benthic animals that can thrive there.

Components of soil complex and their characteristic

Mineral particles: These are the chief components of soil complex and are formed by weathering of rocks. Depending upon size of mineral particles, soils are of following types:

Clay: If particle size is less than 0.002 mm

Silt: If particle size is 0.002–0.02 mm

Fine sand: If the particles size is 0.02–0.2 mm

Coarse sand: 0.2 –2 mm

Gravel or Stone: More than 2 mm

• Thus the correct sequence according to increasing particle size is 
• Clay > Slit > Fine sand > Coarse sand < Gravel.

Soil texture: the proportion of particles of different sizes determine the soil texture and on this basis soils are of following types: 

• Clayey: 85% clay + 15% sand or silt or both.
• Sandy: 80% or more of sandy the remaining being silt and clay.
• Loamy: 50% clay + 50% sand or silt or both. 
• Loamy soil is best for plant growth.
• Soil texture determines, soil air, soil water and root penetration properties.

Response to Abiotic Factors

The abiotic conditions of many habitats may vary drastically in time, how do the organisms living in such habitats cope or manage with stressful conditions ? Why a highly variable external environment should bother organisms after all. One would expect that during the course of millions of years of their existence, many species would have evolved a relatively constant internal (within the body) environment that permits all biochemical reactions and physiological functions to proceed with maximal efficiency and thus, enhance the overall ‘fitness’ of the species.

This constancy, for example, could be in terms of optimal temperature and osmotic concentration of body fluids. Ideally then, the organism should try to maintain the constancy of its internal environment (a process called homeostasis) despite varying external environmental conditions that tend to upset its homeostasis. 

Regulate: Some organisms are able to maintain homeostasis by physiological (sometimes behavioural also) means which ensures constant body temperature, constant osmotic concentration, etc. All birds and mammals, and a very few lower vertebrate and invertebrate species are indeed capable of such regulation (thermoregulation and osmoregulation). Evolutionary biologists believe that the ‘success’ of mammals is largely due to their ability to maintain a constant body temperature and thrive whether they live in Antarctica or in the Sahara desert.

The mechanisms used by most mammals to regulate their body temperature are similar to the ones that we humans use. We maintain a constant body temperature of 370C. In summer, when outside temperature is more than our body temperature, we sweat profusely. The resulting evaporative cooling, similar to what happens with a desert cooler in operation, brings down the body temperature. In winter when the temperature is much lower than 370C, we start to shiver, a kind of exercise which produces heat and raises the body temperature. Plants, on the other hand, do not have such mechanisms to maintain internal temperatures.

Conform: An overwhelming majority (99 per cent) of animals and nearly all plants cannot maintain a constant internal environment. Their body temperature changes with the ambient temperature. 

In aquatic animals, the osmotic concentration of the body fluids change with that of the ambient water osmotic concentration. These animals and plants are simply conformers. Considering the benefits of a constant internal environment to the organism, we must ask why these conformers had not evolved to become regulators. Recall the human analogy we used above; much as they like, how many people can really afford an air conditioner? Many simply ‘sweat it out’ and resign themselves to suboptimal performance in hot summer months. Thermoregulation is energetically expensive for many organisms. This is particularly true for small animals like shrews and humming birds. Heat loss or heat gain is a function of surface area. Since small animals have a larger surface area relative to their volume, they tend to lose body heat very fast when it is cold outside; then they have to expend much energy to generate body heat through metabolism. This is the main reason why very small animals are rarely found in polar regions. 

During the course of evolution, the costs and benefits of maintaining a constant internal environment are taken into consideration. Some species have evolved the ability to regulate, but only over a limited range of environmental conditions, beyond which they simply conform.

If the stressful external conditions are localized or remain only for a short duration, the organism has two other alternatives. 

Migrate: The organism can move away temporarily from the stressful habitat to a more hospitable area and return when stressful period is over. In human analogy, this strategy is like a person moving from Delhi to Shimla for the duration of summer. Many animals, particularly birds, during winter undertake long-distance migrations to more hospitable areas. Every winter the famous Keolado National Park (Bhartpur) in Rajasthan host thousands of migratory birds coming from Siberia and other extremely cold northern regions.

Suspend: In bacteria, fungi and lower plants, various kinds of thick-walled spores are formed which help them to survive unfavourable conditions – these germinate on availability of suitable environment. In higher plants, seeds and some other vegetative reproductive structures serve as means to tide over periods of stress besides helping in dispersal – they germinate to form new plants under favourable moisture and temperature conditions. They do so by reducing their metabolic activity and going into a date of ‘dormancy’.

In animals, the organism, if unable to migrate, might avoid the stress by escaping in time. The familiar case of bears going into hibernationduring winter is an example of escape in time. Some snails and fish go into aestivation to avoid summer–related problems-heat and desiccation. Under unfavourable conditions many zooplankton species in lakes and ponds are known to enter diapause, a stage of suspended development.

Adaptations

• While considering the various alternatives available to organisms for coping with extremes in their environment, we have seen that some are able to respond through certain physiological adjustments while others do so behaviourally (migrating temporarily to a less stressful habitat). These responses are also actually, their adaptations. So, we can say that adaptation is any attribute of the organism (morphological, physiological, behavioural) that enables the organism to survive and reproduce in its habitat. 
• Many adaptations have evolved over a long evolutionary time and are genetically fixed. 

Example: 

• In the absence of an external source of water, the kangaroo rat in North American deserts is capable of meeting all its water requirements through its internal fat oxidation (in which water is a by product). It also has the ability to concentrate its urine so that minimal volume of water is used to remove excretory products.
• Many desert plants have a thick cuticle on their leaf surfaces and have their stomata arranged in deep pits to minimise water loss through transpiration. They also have a special photosynthetic pathway (CAM) that enables their stomata to remain closed during day time. 
• Some desert plants like Opuntia, have no leaves – they are reduced to spines– and the photosynthetic function is taken over by the flattened stems.
• Mammals from colder climates generally have shorter ears and limbs to minimise heat loss. (This is called the Allen’s Rule.) 
• In the polar seas aquatic mammals like seals have a thick layer of fat (blubber) below their skin that acts as an insulator and reduces loss of body heat.
• Some organisms possess adaptations that are physiological which allow them to respond quickly to a stressful situation at high altitude place (>3,500m Rohtang Pass near Manali and Mansarovar, in China occupied Tibet) we experienced what is called altitude sickness. Its symptoms include nausea, fatigue and heart palpitations. This is because in the low atmospheric pressure of high altitudes, the body does not get enough oxygen. But, gradually you get acclimatised and stop experiencing altitude sickness. The body compensates low oxygen availability by increasing red blood cell production, decreasing the binding capacity of hemoglobin and by increasing breathing rate. 
• In most animals, the metabolic reactions and hence all the physiological functions proceed optimally in a narrow temperature range (in humans, it is 370C). But there are microbes (archaebacteria) that flourish in hot springs and deep sea hydrothermal vents where temperatures far exceed 1000C. How is this possible?
• Many fish thrive in Antarctic waters where the temperature is always below zero. 
• A large variety of marine invertebrates and fish live at great depths in the ocean where the pressure could be >100 times the normal atmospheric pressure that we experience. Organisms living in such extreme environments show a fascinating array of biochemical adaptations.
• Some organisms show behavioural responses to cope with variations in their environment. Desert lizards lack the physiological ability that mammals have to deal with the high temperatures of their habitat, but manage to keep their body temperature fairly constant by behavioural means. They bask in the sun and absorb heat when their body temperature drops below the comfort zone, but move into shade when the ambient temperature starts increasing. Some species are capable of burrowing into the soil to hide and escape from the above-ground heat.

Strategies of Adaptions of Plants

Plants possess special traits that help them to tolerate wide range of light regimes, dry condition, high temperature, water saturated conditions as well as saline environments. Flowers have evolved special structure to ensure pollination by insects or other animals. Plants have developed various mechanisms to deal with stress conditions of the environment.

Plant Adaptations to Light Regime

• Not only individual plant but also plant communities show adaptations to different intensities of light. Plants that have adapted to bright sunlight are called sun plants or heliophytes while those growing in partial shade or low intensity light are called shade plants or sciophytes.
• In a forest, plants get arranged in various strata according to their shade tolerance. The phenomenon is called stratification or layering.
• Heliophytes are adapted to high intensity of light, and have higher temperature optima for photosynthesis, and have high rates of respiration. On the contrary, sciophytes possess low photosynthetic, respiratory and metabolic activities. Ferns and herbaceous plants growing on the ground under the dense canopy of three, is shade tolerate plants.

Adaptations in Plants to Water Scarcity and Heat

Plants living under water scarcity are called xerophytes. They are of four types: 

Ephemerals or Drought Escapers: These plants grow during brief rainy season. During that period they flower, set seed and during rest of the unfavourable xeric condition they survive as seed and remain dormant, e.g., Boerhaavia, Tribulus terrestris, Euphorbia prostrata.

Annuals or Drought Evaders: These plants grow during rainy season but live for a few months even after stoppage of rain. They have morphological and anatomical adaptations to reduce transpiration, e.g., Echinops echinata, Solanum surattense.

Succulents or Drought Resistant: Such plants have fleshy organs, which store water. Succulents face scarcity of water in external environment only though internally plenty of water is available. Fleshy organ responsible for storage of water may be fleshy stem (chylocauly, e.g., Opuntia, Euphorbia, Asparagus) or fleshy leaves (chylophylly, e.g., Aloe, Agave) or fleshy roots (chylorhizy, e.g., Asparagus). Leaves are reduced to spines, whereas stems are modified into phylloclade or cladode (e.g., cacti). Stems and leaves possess very thick cuticle and sunken stomata, which open during night only (scotoactive). Succulents perform CAM pathway of photosynthesis.

Non succulent Perennial or Drought Endurer Xerophytes: These are considered as true xerophytes, which face scarcity of water in both external and internal environment, e.g., Nerium, Casuarina, Acacia, etc. These have extensive root system that spreads along the soil surface to absorb maximum amount of water. They possess waxy coating on leaves, sunken stomata, reduced leaf blades, etc., to reduce transpiration. 

Many tropical plants, particularly grasses which grow in heat and arid climates, possess C4 pathway of photosynthesis and perform better in low soil water environments. Such plants, therefore, use less water to achieve higher rates of photosynthesis, particularly at higher temperature.

Many xerophytes may accumulate proline (an amino acid) in the cells to maintain osmotic and water potential in their leaves. The heat shock proteins (Chaperonins) provide physiological adaptations to plants to high temperature. These proteins help

Plant adaptations in Aquatic environment

• Plants living is aquatic environment are called hydrophytes. Hydrophytes are of five types– free floating (e.g., Lemna, Wolffia, Pistia, Trapa, etc.,) floating leaved anchored (e.g., Marsilea), Submerged floating (e.g., Ceratophyllum, Utricularia, etc,), Submerged rooted(e.g., Hydrilla, Vallisnaria, Potamogeton, Chara, etc.), and rooted emergent (e.g., Nymphaea, Typha, Ranunculus, Sagittaria, etc.).
• Hydrophytes are characterized by the presence of mucilage, absence of poorly developed roots, absence or poorly developed roots, absence of root hair and mechanical tissue. They possess aerenchyma in the leaves and petioles. Aerenchyma helps to transport oxygen produced during photosynthesis and permits its free diffusion to other parts, including roots located in anaerobic soil. These tissues also impart buoyancy to the plants.
• Inflated roots in Ludwigia and petioles and Eichhornia keep the plants floating on the surface of water. Heterophylly is common in partially submerged hydrophytes (e.g., Ranunculus).

Plant adaptation in Saline environment 

• Plant growing is saline soil are called halophytes. Such plants are adapted to grow in high concentration of salts (e.g., NaCl, MgCl2, MgSO4, etc) in soil or water. They occur in tidal marshes and coastal dunes, mangroves and saline soils. They have the ability not only to tolerate high concentrations of salts but also to obtain water from the same.
• Halophytes inhabiting hot and dry conditions may become succulent and store water and mucilage in their large cells to dilute the ion concentration of salts.
• Mangrove plants are found in marshy conditions of tropical deltas and along ocean edges. Some species of mangroves (e.g., Avicennia, Atriplex, Spartina, Aegiatilis, etc.) excrete salts through chalk or salt glands on the leaves. Others excrete salt from their roots. Many mangrove plants have high salt concentration and osmotic potential. Dunaliella species (green and holophytic algae found in hyper saline lakes) can tolerate saline conditions by accumulating glycerol in the cells, which helps in osmoregulation.
• In mangrove forests, Avicennia and Rhizophora are the dominant. Halophytes have developed special adaptations like pneumatophores, prop and stilt roots and vivipary to cope with saline and anaerobic conditions in wetlands. Pneumatophores help to take up O2 from the atmosphere and transport it to the main roots. Prop and stilt roots give support to the plants. Vivipary permits plants to escape the effect of salinity on seed germination.

Population Attributes

• In nature, we rarely find isolated, single individuals of any species; majority of them live in groups in a well defined geographical area, share or compete for similar resources, potentially interbreed and thus constitute a population. 
• Although the term interbreeding implies sexual reproduction, a group of individuals resulting from even asexual reproduction is also generally considered a population for the purpose of ecological studies. 
• All the cormorants in a wetland, rats in an abandoned dwelling, teakwood trees in a forest tract, bacteria in a culture plate and lotus plants in a pond, are some examples of a population. 
• Although an individual organism is the one that has to cope with a changed environment, it is at the population level that natural selection operates to evolve the desired traits. Population ecology is, therefore, an important area of ecology because it links ecology to population genetics and evolution.
• A population has certain attributes that an individual organism does not. An individual may have births and deaths, but a population has birth ratesand death rates. In a population these rates refer to per capita births and deaths, respectively. 
• The rates, hence, are expressed is change in numbers (increase or decrease) with respect to members of the population. Here is an example. If in a pond there are 20 lotus plants last year and through reproduction 8 new plants are added, taking the current population to 28, we calculate the birth rate as 8/20 = 0.4 offspring per lotus per year. If 4 individuals in a laboratory population of 40 fruitflies died during a specified time interval, say a week, the death rate in the population during that period is 4/40 = 0.1 individuals per fruitfly per week.
• Another attribute characteristic of a population is sex ratio. An individual is either a male or a female but a population has a sex ratio (e.g., 60 per cent of the population are females and 40 per cent males).
• A population at any given time is composed of individuals of different ages. If the age distribution (per cent individuals of a given age or age group) is plotted for the population, the resulting structure is called an age pyramid. For human population, the age pyramids generally show age distribution of males and females in a combined diagram.  The shape of the pyramids reflects the growth status of the population - (a) whether it is growing, (b) stable or (c) declining.

Age Pyramids

• The graphic representation of the proportions of different age groups in the population of any organisms is called age pyramid. The pre-reproductive group is placed at the base, reproductive in the middle and post reproductive group at the top. The age pyramids can be of three types–

Triangular age pyramid: It indicates a high percentage of young individuals. In rapidly growing young populations birth rate is high and population growth may be exponential as in yeast, house fly, Paramecium etc. Under such conditions each successive generation will be more numerous than the preceding ones and thus a triangular pyramid with broad base will result.

Bell shaped age pyramid: In this case the pre-reproductive and reproductive age groups becomes more or less equal in size while post-reproductive group; remains the smallest. This results in bell shaped, structure of the pyramid. This type of age distribution results in stable population.

Urn shaped age pyramid: It indicates a low percentage of young individuals. If the birth rate is drastically reduced the pre-reproductive group dwindles in proportion to the other two groups and it results in Urn shaped pyramid. Such a population shows negative growth and decreases in size.

• The size of the population tells us a lot about its status in the habitat. Whatever ecological processes we wish to investigate in a population, be predator or the effect of a pesticide application, we always evaluate them in terms of any change in the population size. 
• The size, in nature, could be as low as <10 (Siberian cranes at Bharatpur wetlands in any year) or go into millions (Chlamydomonas in a pond). Population size, more technically called population density (designated as N), need not necessarily be measured in numbers only. Although total number is generally the most appropriate measure of population density, it is in some cases either meaningless or difficult to determine. In an area, if there are 200 Parthenium plants but only a single huge banyan tree with a large canopy, stating that the population density of banyan is low relative to that of Parthenium amounts to underestimating the enormous role of the Banyan in that community. In such cases, the per cent cover or biomass is a more meaningful measure of the population size. Total number is again not an easily adoptable measure if the population is huge and counting is impossible or very time-consuming.
• Sometimes, for certain ecological investigations, there is no need to know the absolute population densities; relative densities serve the purpose equally well. For instance, the number of fish caught per trap is good enough measure of its total population density in the lake. We are mostly obliged to estimate population sizes indirectly, without actually counting them or seeing them. The tiger census in our national parks and tiger reserves is often based on pug marks and fecal pellets.

Population Growth

The size of a population for any species is not a static parameter. It keeps changing in time, depending on various factors including food availability, predation pressure and reduce weather. In fact, it is these changes in population density that give us some idea of what is happening to the population – whether it is flourishing or declining.

Whatever might be the ultimate reasons, the density of a population in a given habitat during a given period, fluctuates due to changes in four basic processes, two of which (natality and immigration) contribute an increase in population density and two (mortality and emigration) to a decrease.

• Natality: refers to the number of births during a given period in the population that are added to the initial density.
• Mortality: is the number of deaths in the population during a given period.
• Immigration: is the number of individuals of the same species that have come into the habitat from elsewhere during the time period under consideration.
• Emigration: is the number of individuals of the population who left the habitat and gone elsewhere during the time period under consideration.

So, if N is the population density at time t, then its density at time t +1 is

Nt+1 = Nt + [(B + I) – (D + E)]

From the above equation that population density will increase if the number of births plus the number of immigrants (B + I) is more than the number of deaths plus the number of emigrants (D + E), otherwise it will decrease. 

Under normal conditions, births and deaths are the most important factors influencing population density, the other two factors assuming importance only under special conditions. For instance, if a new habitat is just being colonised, immigration may contribute more significantly to population growth than birth rates. 

Growth Models

Exponential growth : Resource (food and space) availability is obviously essential for the unimpeded growth of a population. Ideally, when resources in the habitat are unlimited, each species has the ability to realise fully its innate potential to grow in number, as Darwin observed while developing his theory of natural selection. Then the population grows in an exponential or geometric fashion. It in a population of size N, the birth rates (not total number but per capita births) are represented as b and death rates (again, per capita death rates) as d, then the increase or decrease in N during a unit time period t (dN/dt) will be

dN/dt = (b-d) × N, 

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 parameter chosen for assessing impacts of any biotic or abiotic factor on population growth.

To give you some idea about the magnitude of r values, for the Norway rat the r is 0.015, and for the flour beetle it is 0.12. In 1981, the r value for human population in India was 0.0205. 

The above equation describes the exponential or geometric growth pattern of a population and results in a J-shaped curve when we plot N in relation to time. If you are familiar with basic calculus, you can derive the integral form of the exponential growth equation as

Nt = N0 ert, where

N = Population density after time t 

N0 = Population density at time zero 

r = intrinsic rate of natural increase

e = the base of natural logarithms (2.71828)

• Any species growing exponentially under unlimited resource conditions can reach enormous population densities in a short time. Darwin showed how even. Population growth curve a when responses are not limiting the growth, plot is exponential, b when responses are limiting the growth, plot is logistic, K is carrying capacity a slow growing animal like elephant could reach enormous numbers in the absence of checks. 

Logistic growth:No population of any species in nature has its disposal unlimited resources to permit exponential growth. This leads to competition between individuals for limited resources. Eventually, the ‘fittest’ individual will survive and reproduce. The governments of many countries have also realised this fact and introduced various restraints with a view to limit human population growth. 

• In nature, a given habitat has enough resources to support a maximum possible number, beyond which no further growth is possible. Let us call this limit as nature’s carrying capacity(K) for that species in that habitat.
• A population growing in a habitat with limited resources show initially a lag phase, followed by phases of acceleration and deceleration and finally an asymptote, when the population density reaches the carrying capacity. A plot of N in relation to time (t) results in a sigmoid curve. This type of population growth is called Verhulst-Pearl Logistic Growthand is described by the following equation:

Where N = Population density at time t

r = Intrinsic rate of natural increase, K = Carrying capacity

Since resources for growth for most animal populations are finite and become limiting sooner or later, the logistic growth model is considered a more realistic one.

Life History Variation

• Populations evolve to maximise their reproductive fitness, also called Darwinian fitness (high r value), in the habitat in which they live. Under a particular set of selection pressures, organisms evolve towards the most efficient reproductive strategy. Some organisms breed only once in their lifetime (Pacific salmon fish, bamboo) while others breed many times during their lifetime (most birds and mammals). 
• Some produce a large number of small-sized offspring (Oysters, pelagic fishes) while others produce a small number of large-sized offspring (birds, mammals). 
• Ecologists suggest that life history traits of organisms have evolved in relation to the constraints imposed by the abiotic and biotic components of the habitat in which they live. Evolution of life history traits in different species is currently an important area of research being conducted by ecologists.