By Nasif Nahle, biologist

What is Ecology?

Ecology is the branch of biological sciences dealing with the interactions between living beings and their environment (chemical and physical factors).

Ecology is a multidisciplinary science that is closely related to Biology,  Climatology, Chemistry, Biophysics, Engineering, Mechanics, Architecture, Geology, Physics and Mathematics.


Ecology takes advantage of Physics because every biotic process implies the transfer and storing of energy. Producer organisms, like plants, take advantage of light energy to produce organic compounds. Other organisms, like bacteria and fungus, obtain energy by the disintegration of the molecular structures of other organisms.

Ecology applies Chemistry because metabolic processes and physiology of biosystems involve chemical reactions.

Ecology is connected with Geology because the distribution of the biomes depends on the geological structure and composition of the Earth's surface.

Geography is important to Ecology because of living beings are distributed on Earth following specific patrons. The ecologist, i.e. professionals dedicated to the study of Ecology, must know where to find a particular ecosystem or biome which is being investigated.

Mathematics is a central science to Ecology to make calculations, statistics, projections and extrapolations of the number and distribution of species, amount of biomass, population growth, extension of communities, biodiversity, incoming energy, outgoing energy, etc. Additionally, ecologists use mathematics to quantify the environmental conditions that have a direct or indirect influence on the living beings inhabiting a given biome.

Climatology and Meteorology are disciplines which help Ecologists to understand how the variations of climatic conditions affect the regional biodiversity. Climatology and Meteorology help Ecologists to identify the effects of regional and global climate changes on the probabilities of survival of individuals, populations and communities inhabiting a certain geographic locality, and how the regional climate relates with the distribution of living beings on Earth.

Ecology resorts to many other disciplines like Astronomy, etcetera; however, I have only mentioned the most important sciences that are essential for the study and development of the Ecology.



Ecosystem is the sum of living beings (biotic factors) interacting with inert entities (abiotic factors) in a given location.

There is a fine equilibrium between biotic and abiotic factors in the ecosystems.

Levels of Organization in Ecology

The main levels of organization in Ecology are five:

1. Individual: Individual is any living being. Individuals act reciprocally with the environmental abiotic factors, which limit their distribution.

2. Population: A group of individuals of a given species that live in a specific geographic area.

3. Community: All the living beings distributed into a specific geographical area. A community includes organisms of different species.

4. Ecosystem: The term refers to all the abiotic factors (physical and chemical constituents) and all the communities that established in a specific area.

5. Biosphere: It is the whole portion of Earth colonized by living beings. Biosphere is the sum of all the ecosystems established on Earth.



The abiotic factors are those inert factors of the ecosystem; for example, light, temperature, chemical products, water and atmosphere.


Dust and clouds in the terrestrial atmosphere reflect nearly 0.582 calories toward deep space from the total of solar energy that reaches Earth (1.94 calories per square centimeter per minute). About 0.4 calories are absorbed by the atmosphere's layers, and 0.97 calories arrive at the terrestrial ground.

Light is an essential abiotic factor in the ecosystems because it constitutes the main supply of energy for organisms. The plants with chlorophyll can change the light energy into chemical energy thanks to the process known as photosynthesis. This chemical energy is stored in complex organic substances (food). It is useless to say that without light there would not be living beings on Earth.

Besides this significant function, the light regulates many biological rhythms of a large amount of species.

Visible light is not the only form of energy emitted by our Sun. The Sun propels several forms of energy, from radio waves to gamma energy. Ultraviolet light (UV) and infrared radiation (heat) are found among these forms of solar radiation. Both UV light and infrared radiation are abiotic factors of high significance for Ecosystems.

Insects use ultraviolet light to differentiate one flower from another. Humans cannot perceive UV radiation. UV light works also limiting some biochemical reactions that could be harmful for the living beings; besides, UN light annihilates pathogens and can cause favorable mutations in all life forms. Recently, GB scientists discovered that the birds orient themselves by means of the perception of small differences in the reflection of light UV by the objects on land, for example edifice, trees, water surfaces, rocks, etc.


Heat is useful for ectothermic animals, to be precise those that are not adapted to regulate their corporal temperature (by example fishes, amphibians and reptiles). Plants make use of a little amount of heat to perform the photosynthetic process. Plants are adapted to survive between a minimum and a maximum limits of temperature. This is valid for all the organisms, from archaea to Mammalia. Although some microorganisms unusually tolerate extreme temperatures, they would perish if they would be retired from those rigorous environments.

When the infrared radiation incoming from the Sun penetrates into Earth’s atmosphere, the atmospheric water vapor absorbs it and delays the release of heat towards the space; consequently, the atmosphere is kept warm (greenhouse effect).

The oceans play an important role in the stability of the terrestrial climate. The difference of temperatures in different masses of oceanic water in combination with winds and Earth’s rotation creates the oceanic currents. The transfer of heat emitted or absorbed by oceanic water permits that some cold volumes of the atmosphere warm up and that warmer volumes of the atmosphere are cooled.


The happening of life upon our planet would be barely possible without our existing atmosphere. Many planets in our solar system have an atmosphere, but the structure of the terrestrial atmosphere is the ideal for the origin and the perpetuation of the as we know life. Its constitution does that terrestrial atmosphere be so special.

The terrestrial atmosphere is formed by four concentric superimposed layers, which are extended up to 80 kilometers.  Divergence in their temperatures differentiates these layers.

The layer that is extended over the terrestrial surface close to 10 kilometers is called troposphere. In this layer the temperature diminishes in inverse proportion with the height, that is to say, to greater height a smaller temperature. The minimum temperature at the top of the troposphere is -50°C.

Troposphere contains three quarters of all the molecules of the atmosphere. This layer is in continuous movement, and nearly all the meteorological phenomena take place in this layer.

Each border among two atmospheric layers is called pause, and the prefix pertaining to the lower layer is placed before the word "pause". By this method, the border among the troposphere and the immediate higher layer (stratosphere) is called tropopause.

The next layer is the Stratosphere, which extends starting at 10 km up to 50 km in altitude. The temperature increases proportionally to the height here; to greater height, greater the temperature. At the top of the stratosphere, the temperature almost reaches 25 °C. the cause of this ascent in the temperature is the ozone layer  (ozonosphere).

Ozone absorbs the Ultraviolet radiation that breaks the molecules of Oxygen (O2), generating freed atoms of Oxygen (O), which are connected again to build Ozone molecules (O3). In this type of chemical reactions, the transformation of luminous energy into chemical energy generates heat that provokes a greater molecular movement. This is the reason for the ascent of the temperature in the stratosphere.

The ozonosphere has an unmatched consequence for the life, given that it stops the solar emissions that are lethal for the totality of the organisms.

If we imagine the layer of ozone like a football, we would see the Ozone Layer Depletion similar to a deep depression upon the skin of the ball, as if it was barely deflated.

Above the Stratosphere is Mesosphere. Mesosphere covers from the limit of the stratosphere (stratopause) up to 80 km.


Water (H2O) is an indispensable factor for the life. Living beings were originated in water, and all living beings need water to subsist. Water is involved in diverse organic chemical processes, for example water molecules are used during the photosynthesis, freeing oxygen atoms of water.

Water works as a thermoregulator for climate and living systems: Thanks to water the climate on Earth is maintained stable. The water functions also as thermoregulator in the living systems, especially in endotherm animals.

Thermoregulation is possible thanks to the Specific Heat of water (specific heat is the heat in calories necessary to elevate the temperature of one gram of a substance one degree Celsius), which is of one calorie for the water. In biological terms it signifies that before an elevation of the temperature in the surrounding environment, the temperature of a mass of water will rise with greater slowness than that of other materials. Equally, if the surrounding temperature diminishes, the temperature of that mass of water will diminish with more slowness than that of other materials. Thus, this quality of water permits that aquatic organisms live relatively placidly in an environment with stable temperature.

Evaporation is the change of the physical state of a substance from a liquid physical state to a gaseous physical state. We need 540 calories to evaporate a gram of water. At this point water boils (point of boiling). This means that we have to rise the temperature at 100°C to do water boils. When evaporates from the surface of the skin, or from the surface of leaves of a plant, water molecules drag large quantities of heat with it. This works in the organisms like a cooling system.

Another advantage of the water is its melting point. With the intention that a liquid substance change from a physical state liquid to a physical state solid, it should be extracted heat from that substance. The temperature at which a substance change from a physical liquid state to the physical solid state is called melting point. To change the water physical state from liquid to solid we have to diminish the surrounding temperature at 0°C.To revert melting, this is to say to change one gram of ice to liquid water, it requires a supply of 79.7 calories. When water melts, the same amount of heat is liberated to the surrounding environment. This allows that in winter the environmental temperature does not decrease to the point of annihilate all the life on the planet.



Living beings are constituted by matter. From the 92 known natural elements, only 25 elements are found in the living matter. From these 25 elements, four elements, Carbon, Oxygen, Hydrogen and Nitrogen, are present in the 97% of the molecules of life. The remaining elements compose only the 3% of the living matter, being the most important Phosphorus, Potassium, Calcium and Sulfur.

Molecules containing Carbon in their structures are called Organic compounds; for example, the Carbon Dioxide, which is formed by an atom of Carbon and two atoms of Oxygen (CO2). Compounds that have not Carbon in their structure are known as Inorganic Compounds; for example, a molecule of water, which is formed by an atom of Oxygen and two of Hydrogen (H2O).



Biotic factors are all the organisms that share an environment.

Biotic Components are all the living beings in an environment, from protists to mammalians. Individuals should have specific behavioral and physiological characteristics that permit their survival and reproduction in a defined environment. The condition of sharing an environment generates a competence among the species, competence that is given for food, space, etc.

We can say that the survival of an organism in a given environment is limited so by the abiotic factors as by the biotic factors of that environment.

The biotic components of an ecosystem are found in the ecological organizational categories, and they constitute the food chains in the ecosystems.


Energy and nutrients pass through various alimentary levels. Each one of those levels is called "trophic levels" in Ecology. The sum of all the trophic levels of an ecosystem is called "food chain". The alimentary relationships in an ecosystem are called "food web".

In an uncomplicated ecosystem, the trophic levels would be Primary Producers (plants or phytoplankton) Primary Consumers (herbivores or zooplankton) Secondary Consumers (carnivores) Tertiary Consumers (carnivores) Quaternary Consumers (carnivores).  Let me show it by example:


Primary Producers: grass, shrubs and trees.

Primary Consumers: grasshoppers (plant-eaters).

Secondary Consumers: birds (insectivores).

Tertiary Consumers: snakes (bird-eaters).

Quaternary Consumers: Owls (snake-eaters).

Finally, the biotic factors and their products are recycled (decomposed) by detritivores (Bacteria, fungi, and some animals).


February 20, 2006

The interspecific interactions are those which happen among individuals of different species.

The interspecific interactions can be positive, neutral or negative:

The positive interspecific interactions are those through which at least one of the species obtains a benefit from another species without damaging to the second individual or altering the course of its life.

The neutral interspecific interactions are those in which there is not a direct damage or benefit from any of both species. The damage or benefit is taken only indirectly.

The negative interspecific interactions are those interactions by which one of the species obtains a benefit in detriment of other species.

The positive interspecific interactions are:

Commensalism: It occurs when an individual obtains a benefit from a different species without damaging it.

For example, Percebes (barnacle) is a crustacean that adheres to the body of whales, turtles, etc. Adult percebes are sessile, that is, they remain fixed to a substrate being not able to displace from one place to another to look for their own food. In this case, percebes obtains the benefit of free transportation toward diverse zones rich in food (plankton) offered by whales and other marine species.

Mutualism: It occurs when an individual obtains a benefit from another species and, at the same time, the second species obtains a benefit from the first one. The mutualism is not obligated, which makes it different from symbiosis. The concept mutualism derives from the mutual aid between two individuals that belong to different species.

A classical example of mutualism happens between surgeon fishes and sharks. The surgeon fish feeds from the parasites tied to the skin of sharks and other fishes. In this case, the surgeon fish obtains food from the sharks and the sharks are cleaned from displeasing parasites.

Symbiosis: we say that two organisms are symbiotic when both of them belong to different species and are benefited mutually in an obliged relation.  If one of the symbiotic individuals perishes, the other also will perish by losing the source from which it was obtaining a profit.

A well known case of symbiosis corresponds to lichens. Lichens organize from an obliged interaction between a fungus and a green alga. The interaction is extreme because the individuals not only do not belong to the same species, but they do neither belong to the same kingdom. The fungus provides an adequate humid bed to the algae and this provides with free food to the fungus. The relation has occurred so narrowly in the course of the evolution that an isolated species cannot subsist without the other.

We know only one neutral interspecific interaction:

Competence: Occurs when two different species into a community have the same needs for one or more factors from the environment. The individuals of the species that possesses advantages to obtain those factors from the environment will be the ones that will succeed. The struggle is not a physical clash, but a selective non-violent struggle. There can be unintentional encounters between the two individuals, but it is not the general rule.

The best example on interspecific competence is the case of two carnivorous species seeking for food in the same area and both feed from the same species; for example, lions and cheetahs which feed of antelopes. Lions take advantage over the other carnivorous species by the tendency of cooperation between them and by their social behavior. 

The negative interspecific interactions are:

Predation: It occurs when an individual from one species kills at once to another individual of another species for feeding from it.

The killer is named a predator. The killed individual is called a prey

Examples of predators and preys are: a lion (predator) and a zebra (prey), a chicken (predator) and an earthworm (prey), a mantis (predator) and a butterfly (prey), a spider (predator) and a homefly (prey), etc.

Parasitism: It takes place when a species obtains a benefit from another species provoking a gradual damage that does not cause the immediate death of its victim.

The species that obtains the benefit causing a gradual damage to the host is called a guest or a parasite; while the species that receives the damage is called a host. When the parasite requires of an intermediate species between it and its final host, the intermediate species is called a reservoir.

Examples of parasites are: Amoebas, pig's round worm, liver flat worms, lice, flies, ticks, mites, wasp larvae, mistletoes, etc. The list of parasites is extraordinarily large.


1. Considering the interspecific interactions, how would you classify to humans?
2. What kind of interspecific interaction is given between humans and cows?
3. What kind of interaction exists between humans and dogs?
4. Is cats-humans interaction a mutualism?
5. When eating veggies, how would you classify to human beings?
6. In view of interspecific interactions, how would you classify a carnivorous plant?
7. Is it a cricket that eats another cricket from the same species a predator?
8. Is cannibalism a kind of interspecific interaction or an intraspecific one?



The Intraspecific Interactions are those that occur among organisms of the same species.

Social Dominance: Is the stratification of groups into a society given by the influence that one individual or one group of individuals has on the other individuals or groups into the same society.

For example, in an ant’s population, we can distinguish several castes by their rank of influence; for example, fertile queens, infertile soldiers, infertile workers and fertile male, where fertile queens occupy the main rank in the colony and the fertile males are in the lowest rank.

Social Hierarchy: Is the stratification of the individuals that consists in the domination that an individual has on the other individuals of the same population.

For example, in a poultry pen, the stronger adult rooster has the absolute control of the other members of that population (into that poultry coop). The dominant rooster is called alpha male. Below the alpha male, all the hens and the weakest roosters occupy a range of hierarchies. The alpha male has preferences to a particular hen, which becomes into an alpha female that dominates to the other hens and roosters. The alpha female has the "right" to peck to every one of the remainder hens and to the weakest roosters. The second in hierarchy, or beta hen, is able to peck to the other individuals of the poultry coop, except to alpha rooster and alpha hen. Thus, the hen’s rights keep going down in order to pecked individuals to end on the pariah of the society: the chicken that eats when all its brothers and sisters have satisfied their hunger, the chicken that only eats the food scraps, the chicken that is always relegated to a corner of the corral and that we observe severely injured and plucked due to a lot of pecks that it suffers from the other members of its own family.

Territoriality: It is the demarcation and defense of a physical area that is defined by an individual or by a group of individuals.

A common case of territoriality is well displayed by dogs. Dogs delimit a surrounding territory, in connection with the place where they inhabit, using emissions of urine that has an odor detectable by other dogs. Have you seen a Great Dane frightened by a Pekinese puppy? Well, it happens frequently among the dogs, and it precisely occurs by this stuff of territoriality. Humans are highly territorial: we define the boundaries of our rooms, our homes, our towns, our counties, our states, our countries, our continents, and probably in the future we will define the limits of our planet, our Solar System, our Galaxy, our Supergalaxy, etc.

Intraspecific Competition: It happens when two or more individuals of a population try to obtain a factor needed by all individuals from the environment where they inhabit. The competition can occur effectively if the competition is brought to a harmful struggle between two of the stressed individuals of a population, or unintentionally if the competition does not imply a deadly or harmful ritual, but a natural application of abilities to achieve a required factor. If the contest implies a risk-free ritual of threatening and submissive behavior, it is called Agonistic Behavior. If the competition does not imply a ritual, it is called a non ritualistic competition.

Some examples on intraspecific harmful competition are: buffaloes fighting for a female, angelfish struggling for the best territory into a fishbowl, chimpanzees that go into ferocious battles to achieve the supremacy in the tribe, etc. Can you demonstrate the occurrence of harmful competition rituals in humans?

A good example of intraspecific non harmful ritualistic competition is when a female is conquered through odors, colors, sounds and/or a good exhibition of power and gallantry displayed by males (in some species, for example in humans, the exhibition could be performed so by females as by males). This kind of exhibitions is frequently reported in birds, like turkeys, swallows, parrots, etc., but it occurs in many animal species (frogs, lions, gorillas, etc).

May be, the cruelest form of non ritualistic competition in animals is when a weak member of a group falls under a predator’s claws and teeth. In the last example, the contestants are all the members of the prey group, surviving only the better skilled to run away from the enemy. As you can see, many times competition is extremely subtle. Describe some examples of unintentional competition, some examples of harmful competition and some examples of Agonistic Behavior in humans.

Author of this page: Biol. Nasif Nahle








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Published on August 27, 1999Restructured and Corrected on February 14, 2007
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