By Nasif Nahle, biologist

Definition of Ecology

Ecology is a branch of biological sciences which studies the interactions among organisms and their environment (chemical and physical factors).

Ecology is a multidisciplinary science related with Biology, Climatology, Chemistry, Biochemistry, Physics, Biophysics, Geology and Geography.


Ecology needs Physics because all biotic processes are related with energy transfer, from producers, which take advantage of light energy for producing complex organic compounds, to bacteria, which obtain chemical energy by the disintegration of the molecular structures of other organisms.

Ecology resorts to Chemistry because all the metabolic and physiological processes of biosystems depend on chemical reactions.

Ecology relates to Geology because biomes depend on the geological structure of the environment.

For Ecology, Geography is a very important discipline because of the specific distribution of living beings on Earth.

Mathematics is crucial for Ecology, for example for the calculus, statistics, projections and extrapolations when Ecologists deal with specific information about the number and distribution of species, the evaluation of biomass, population growth, extension of communities and biodiversity, and to quantify the environmental pressures at a given biome.

Climatology and Meteorology are significant disciplines which help Ecologists to understand how the variation on climatic conditions at a given region affects the regional or global biodiversity. The knowledge of Climatology and Meteorology help Ecologists to know how the regional or global climate increases or reduces the probabilities of survival of individuals, populations and communities in any region under study and how the distribution of species is related to the regional climate.

There are many disciplines working into Ecology. We have only mentioned the more important disciplines, those which are intimately related to this science.



Ecosystem is the aggregate of all organisms (biotic factors) living in a community and all the nonliving (abiotic factors) which interact with them.

There is a sensible 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 the same 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 established in a specific area.

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



The abiotic factors are the inert factors of the ecosystem; for example, UV radiation, light, heat, chemical substances, water, air, etc.


From the total of solar radiation incoming to the Earth (1.94 calories per square centimeter per minute), the atmospheric dust and the clouds reflect nearly 0.582 calories toward the space. The layers of the atmosphere absorb near 0.4 calories, and 0.97 calories of the solar radiation strike on the surface of the Earth.

Light is an essential abiotic factor of all ecosystems because all the living beings on Earth take advantage of it. The photosynthetic organisms, i.e. cyanobacteria, algae and plants, can transform visible light into chemical energy. The chemical energy produced during photosynthesis is stored in complex organic compounds (food). Without light, there would not be living beings on Earth.

Besides this significant function, the light regulates biological rhythms of most species.

Visible light is not the only form of energy emitted by the Sun. The Sun radiates several forms of energy, from long wave radiation to high energy gamma radiation. Ultraviolet light (UV) and infrared radiation (heat) are found among these forms of solar radiation. Both ultraviolet sunlight and infrared radiation are very significant abiotic ecological factors.

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


Heat is fundamental for the survival of exothermic organisms, explicitly, for those organisms which are not adapted for regulating their inner body’s temperature (by example plants, fish, amphibians and reptiles). Plants use a little amount of heat during the photosynthesis. Plants are adapted to survive between minimum and maximum extremes of temperature. This is valid for all living organisms, from Archaea to Mammals. Although some microorganisms can tolerate extreme temperatures (thermophiles), they would perish if they would be placed in other environment different to the rigorous environments with high temperature where they live.

When the shortwave infrared radiation (SWIR) incoming from the Sun breaks through the Earth’s atmosphere, the water vapor in the atmosphere absorbs a percentage deferring the radiation of heat towards the outer space; consequently, the atmosphere is kept warm thanks to the capability of water vapor for storing radiant, convective and conductive energy (greenhouse effect).

The oceans play an important role on the stability of the Earth’s climate. The difference of temperature in different volumes of oceanic water, in combination with airstreams and Earth’s rotation, generates the oceanic currents. The transfer of the heat emitted or absorbed by the oceanic water permits the warming of colder volumes of air and the cooling of warmer volumes of air.


The emergence of life on this planet would be hardly possible without the atmosphere. Many planets of our solar system have atmospheres, but the structure of the terrestrial atmosphere is ideal for the origin and perpetuation of living beings as we know them. The atmosphere’s composition makes the terrestrial atmosphere to be very special and ideal for life.

The Earth’s atmosphere has four main layers that extend from the surface of the planet upwards to about 80 kilometers.  We can distinguish the layers of the Earth’s atmosphere by the difference of temperature between a given layer and another upper or lower layer.

The layer which extends from the Earth’s surface up to 10 kilometers in altitude is called troposphere. In this layer the temperature decreases in inverse proportion with height, i.e., at a larger height, a lower temperature. The minimum temperature at the top of the troposphere is -50°C.

The troposphere contains more than three quarters of all the molecules of the atmosphere (almost 75% of the atmosphere's mass is concentrated in the troposphere). This layer is moving continuously, and virtually all the meteorological phenomena take place in this layer.

Each boundary layer amidst two different atmospheric layers is suffixed with the term “pause”. The corresponding prefix to the lower layer is placed before the term "pause". For example, the border between the troposphere and the upper layer, i.e. the stratosphere, is called “tropopause”.

The next layer above the tropopause is the Stratosphere, which extends from 10 km up to 50 km in height. In this layer, the temperature increases with height; as the altitude increases, the temperature also increases. Near the tropopause, that is, in the lower layer of the stratosphere, the temperature is about -60 °C, while at the top of the stratosphere is almost -3 °C. The cause of the increase of the stratosphere’s temperature is the photochemical process which is induced in the stratosphere by the incoming ultraviolet radiation captured by the oxygen molecules in the ozonosphere, which is a layer into the stratosphere:

The ultraviolet sunlight strikes on molecules of Oxygen (O2) in the stratosphere and splits them, generating free Oxygen atoms (O=), which combine with complete molecules of oxygen (O2) for building Ozone molecules (O3). In this type of chemical reactions, the transformation of sunlight energy into chemical energy generates heat which causes a greater molecular motion (kinetic energy), the measurement of which is known as temperature.

The ozonosphere is an essential layer for living beings because it stops from 95% to 99% of the ultraviolet sunlight which could be lethal for every kind of living organisms.

The ozonosphere is not stable; it changes with seasons and it is affected by the atmospheric electrical activity. If we imagine the ozone layer as if it was the surface of a soccer ball, we would see the Ozone Layer Depletion as if it was a deep depression on any of the panels of the ball’s cover, as if it had been severely deflated on any of its poles.

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


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

Water works as a thermoregulator for climate and into the bodies of the living systems: Thanks to water the climate on Earth is maintained in a quasi-stable state. The water functions also as a thermoregulator into the living systems, especially in homoiothermic animals.

Thermoregulation is possible due to the Specific Heat of water (specific heat is the energy needed for increasing the temperature of one gram of a substance by one degree Celsius), which is one calorie for water. In biological terms it means that before an elevation of the temperature in the surrounding environment, the temperature of a mass of water will rise slower than the temperature of other materials. In the same way, if the surrounding temperature diminishes, the temperature of a mass of water will diminish slower than the temperature of other materials. Thus, this thermal quality of water permits that the aquatic organisms live relatively comfortably in an environment with almost 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 well known territoriality behavior is shown by dogs. Dogs delimit a territory around the place where they are dwelling. The dogs resort to the emission of urine with an odor that is easily 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 due to this territoriality stuff. We 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.



The massive icebergs that crumble from the glaciers of Greenland and Antarctica are the good news for all us because they are an indicator about the growing in thickness of the ice crust in Greenland and Antarctic. The glaciers are huge masses of ice in continuous movement (moraines), pushed by their own weight - thickening towards the free edge of the glacier, or by the force exerted by the increase in the mass-volume from the center of the main glacier toward its periphery. When a glacier obtains too much weight and/or it has reached the outer border of the glacier with oceans, it is cracked out from the main ice core and moves with the flow of the oceanic currents (glacier’s drift). The heat melts the ice; atmospheric heat does not cut as if it was a saw nor cracks the ice like a hammer.

The last Ice Age began 1.5 million years ago and it has not yet finished. When the Ice Age finishes -within ca. 100 thousand years- the variability in the tropospheric temperature would be again 3 C, at the outset. Within ca. 500 thousand years it would be again 10 C. At present, we are in a period of transition between the last Ice Age and a Warming Period in the Solar System; for that reason the variation in the temperatures of our planet increases every decade by almost 0.0185 C. This periodic natural phenomenon has been called erroneously “global Warming” and it is being exploited by some ambitious groups to get social and economic supremacy for the rest of the times of humanity.

The glaciers are important for the life on the planet for several reasons:

1. 75% of the fresh water is stored in glaciers, which at this moment constitute 10% of the terrestrial surface.

2. When the continental glaciers melt, water flows into currents that can be taken advantageously for the irrigation of crops, for the humidification of rustic soils, for utilization by the wild animals and human beings.

3. When the oceanic glaciers melt, the local salinity of oceanic waters diminishes and their temperature increases, producing then a beneficial flow of sea currents that regulate the climate of the planet. This is beneficial for marine and fresh water species and for terrestrial organisms because the temperature of oceanic and continental waters and atmosphere is regulated into the ideal for life limits, as much for the exothermic organisms as for the endothermic organisms.

4. Rainy periods on continental areas are longer and more abundant when glaciers diminish than when glaciers expand. Human communities must adapt to environmental circumstances, not the environmental circumstances to human communities.

So you want to know a bit more about glaciers? Read:

Paterson, W. S. B. The Physics of Glaciers. Reed Educational and Professional Publishing Ltd. Woburn, MA. 1994.

You can get it from Amazon.com


ENERGY PYRAMID (See diagram here)
By Nasif Nahle

An energy pyramid is the graphical representation of the trophic levels (nutritional) by which the incoming solar energy is transferred in an ecosystem. Approximately we can say that the absolute source of energy for living beings on Earth is the Sun. The energy that the Sun emits at the present time is of 1366.75 W/m^2 (400 years ago, it was of 1363.48 W/m^2). When the studies of the arrest of energy by the producers (photosynthetic organisms) were made, the Solar Irradiance (SI) was of 1365.45 W/m^2. At present, the energy by usable the photosynthetic organisms is of 697.04 W/m^2; nevertheless, the photosynthetic organisms only take 0.65 W/m^2 and the rest dissipates to the abiotic surroundings (the oceans, ground, atmosphere, etc.) and from there, to sidereal space and Gravity field. The atmosphere absorbs 191.345 W/m^2, maintaining the world’s tropospheric temperature in the hospitable 95.72 °F (35.4 °C).

In the diagram, the amounts in the boxes to the left of the pyramid represent the energy that takes advantage of each individual. For example, the amount of energy profited by the herbivores is equivalent to the ingestion of a gram of organic material of the photosynthetic organisms. Each subsequent amount of energy (green rectangle) in the pyramid (towards the peak) is the energy that would be obtained by each gram of organic material of the underlying level. Detritivores or detritivorous are organisms that fed on remnants of organic matter, like corpses, excrements, etc. Detritivores take advantage of ca. 57% from the energy stored by the producers. (See diagram here)

Author of this page: Biol. Nasif Nahle









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Published on August 27, 1999Updated on November 15, 2009
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