Ecology at the basic level – An Organism.

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. Here thus comes to the fore, the very basic factor of ecological significance of flora and fauna from the fact that by way of such ecological adaptations, the organisms not only make almost whole of the biosphere habitable and hence, cause the distribution of biodiversity to different regions (biomes) of the globe, but also drives the ecosystems of such particular biomes.

As we already know that how the rotation of our planet earth around the Sun and the tilt of its axis becomes responsible for annual variations in the intensity and duration of temperature across the globe and hence, in the resultant appearance of distinct seasons. These variations together with annual variation in precipitation (remember precipitation includes both rain and snow) eventually, account for the formation of major biomes, such as desert, rain forest and tundra as shown in the (Fig.) below:

Figure showing:  Biome distribution with respect to annual temperature and precipitation.

It must be noted that regional and local variations within each biome lead to the formation of a wide variety of habitats. While the fact remains that on the planet Earth, life exists not just in a few favorable 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 pits, to name only a few. Surprisingly enough, even our own intestine is itself a unique habitat for hundreds of species of microbes, notably, E.coli.

Now the pertinent question is: What are the key elements that lead to so much variations in the physical and chemical conditions of different habitats?

 The most important ones obviously are: temperature, water, light and soil. We must remember that the physico-chemical (abiotic) components alone do not characterize the habitat of an organism completely; the habitat includes biotic components as well that include – pathogens, parasites, predators and competitors – of the organism with which they interact constantly. These factors thus become responsible for the formation of distinct ecological niches within the habitat found in a particular biome as such.

Another relevant question that also comes to the fore in the context that how has an organism become so fit to survive in a particular habitat it lives in?

 We can argue that over a period of time, the organism had through natural selection, evolved such adaptations in the form of physiological or morphological modifications so as to optimize its survival and reproduction in its very habitat. In order to understand such of the modifications and adaptations, it is important to go into little details of such environmental or abiotic factors to which an organism has learnt to acclimatize itself with which even holds equally true to human beings as well…

Major Abiotic Factors

Determinants of distinct ecologically important habitats & Niches:

Temperature: Temperature is the most ecologically relevant environmental factor. Every one of us may be aware that the average temperature on land varies seasonally, decreases progressively from the equator towards the poles and from plains to the mountain tops. 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 100⁰C. 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. We can thus, readily appreciate the significance of temperature to living organisms when we realize that it affects the kinetics of enzymes and through it the basal metabolism, so called as BMR 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, say upto 99% of them are restricted to a narrow range of temperatures (such organism are called stenothermal). The levels of thermal tolerance of different species determine to a large extent their geographical distribution. In this regard, Birds & Mammals including insects and human beings with all flowering plants (Angiosperms), can be cited as the examples of so called eurythermal animals and plants whereas, rest of all animals with xerophytes or cryophytes, can be quoted as stenothermal animals and plants.

In recent years, there has been a growing concern about the gradually increasing average global temperatures. If this trend continues, we can certainly reason out that the distributional range of some species would be severely affected. This is why the contemporary phenomenon of global warming today, has become a war cry of global warning in the context of global biodiversity.

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. Every layman can think that organisms living in oceans, lakes and rivers should not face any difficulty surviving there, while the fact remains that the quality (chemical composition, pH etc.) of water becomes equally important a factor for their survival. The salt concentration (measures 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 hyper saline lagoons. Akin to the adaptability of organisms (flora & fauna) to temperature ranges, similar is the case of some organisms which are tolerant of a wide range of salinities and thus are referred to as (euryhaline), while others are restricted to a narrow range of such salinities in terms of their survival and hence, are called as (stenohaline). Many freshwater animals cannot live for long in sea water and vice versa because of the osmotic problems, they would face owing to the lack of such adaptive physiological mechanisms (absence of enzymes etc.) which otherwise are present in such organisms which are adapted to living under such hostile conditions.

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. Many species of small plants (herbs and shrubs) growing in forests are adapted to photosynthesize 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, a phenomenon referred to as photoperiodism in plants. For many animals too, light is important in that they use the diurnal (wake and sleep) 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. 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. What then is their source of energy? Obviously, such organisms meet their requirement of energy by feeding on the organic matter synthesized by other organisms in the ocean. At the same time, the spectral quality of solar radiation is also important for life. The UV components of the spectrum is harmful to many organisms while not all the colour components of the visible spectrum are available for marine plants living at different depths of the ocean. Among the red, green and brown algae that inhabit the sea, we can reasonably argue that it is the red algae that thrive in the deepest waters of the ocean as they have learnt to photosynthesize optimally well by absorbing the low wavelength solar radiations of the visible spectrum.

Soil: 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-characteristic often determine the type of benthic animals that can thrive there.

Responses to Abiotic Factors

How do organisms (Flora & Fauna) adapt to different environmental conditions?   

Having realized that the abiotic conditions of many habitats may vary drastically in time, we now ask-how do the organisms living in such habitats cope or manage with stressful conditions? But before attempting to answer this question, we should perhaps ask first why a highly variable external environment should brother organisms after all. One would except 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 organisms 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. Let us take an analogy to clarify this important concept. Suppose a person is able to perform his/her best when the temperature is 25⁰C and wishes to maintain it so, even when it is scorchingly hot or freezingly cold outside. It could be achieved at home, in the car while traveling and at workplace by using an air conditioner in summer and heater in winter. Then his/her performance would be always maximal regardless of the weather around him/her. Here the person’s homeostasis is accomplished, not through physiological, but artificial means. How do other living organisms cope with the situation? Let us look at various possibilities (Fig.)

(i)   Regulate: Some organisms are able to maintain homeostasis by physiological (sometimes behavioral 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 temperatures. Plants, on the other hand, do not have such mechanisms to maintain internal temperatures.

(ii) 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 changes 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 organisms, 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 simple ‘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 reasons 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 organisms have two other alternatives.

(iii) Migrate: The organisms 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 hospital areas. Every inter the famous Keolado National Park (Bhartpur) in Rajasthan host thousands of migratory birds coming from Siberia and other extremely cold northern regions.

(iv) 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 organisms, if unable to migrate, might avoid the stress by escaping in time. The familiar case of bears going into hibernation during winter is an example of escape in time. Some snails and fish go into aestivation to avoid summer-related problems-heat and desiccation. Under unfavorable conditions many zooplankton species in lakes and ponds are known to enter diapauses, a stage of suspended development.   

Morphological and Physiological adaptations in organisms: Ecological significance

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 behaviorally (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. 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 byproduct). It also has the ability to concentrate its urine so that minimal urine so that minimal volume of water is used to remove excretory products.

Many deserts plants have a thick cuticle on their leaf surfaces and have their stomata arranged in deep pits to minimize 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 functions is taken over by the flattened stems. (Question asked in CSP-2013).

Mammals from colder climates generally have shorter ears and limbs to minimize 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.

Physiological adaptations: Some organisms possess adaptations that are physiological which allow them to respond quickly to a stressful situation. Say for example, we as humans, whenever we go to higher altitudes say, to mountains, every one of us would experience what is called altitude sickness. Its symptoms include nausea, fatigue and heart palpitations due to the reason that our body does not get enough oxygen as higher the altitude, the ambient air gets thinner and hence, lesser in oxygen content. But, gradually then, we get to acclimatize to the surrounding environment and stop experiencing what is called as altitude sickness.

How does our body solve this problem?

The answer lies in the fact that our body compensates for low oxygen availability in the altitudes by increasing our red blood cell production; by decreasing the binding capacity of hemoglobin and by increasing breathing rate. Many tribes living in the high altitude of Himalayas necessarily have high RBC count in their blood for obvious reasons. This speaks for the reason that the people living in the hilly areas generally have more red blood cell count in their body than those in the plains and depict rosy cheeks.

In most animals, the metabolic reactions and hence all the physiological functions proceed optimally in a narrow temperatures 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? The reason is that they have special body adaptations to survive under such boiling temperatures say, in the form of a special material; their cell walls are made of.

Many fish thrive in Antarctic waters where the temperature is always below zero for the similar reasons of having special physiological and morphological adaptations say, fatty blubber in seals that acts as an insulator to prevent the loss of heat from their body.  

Similarly, 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 on the surface of the earth. How do they live under such crushing pressures and do they have any special enzymes? Obviously, the organisms living in such extreme environments show a fascinating array of biochemical adaptations in their body.

Some organisms on the other hand, show some behavioral 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 behavioral means. They bask in the sun and absorb heat when their body temperature start decreasing and move back to shade as soon as the ambient temperature starts increasing. Some species are even capable of burrowing into the soil to hide and escape from the above-ground heat.