unit 2.

CONCEPTS OF AN ECOSYSTEMIntroduction - What is an Ecosystem?

The term ecosystem was coined in 1935 by the Oxford ecologist Arthur Tansley to encompass the interactions among biotic and abiotic components of the environment at a given site. The living and non-living components of an ecosystem are known as biotic and abiotic components, respectively.

Ecosystem was defined in its presently accepted form by Eugene Odum as, “an unit that includes all the organisms, i.e., the community in a given area interacting with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity and material cycles, i.e., exchange of materials between living and non-living, within the system”.

Smith (1966) has summarized common characteristics of most of the ecosystems as follows:

1. The ecosystem is a major structural and functional unit of ecology.

2. The structure of an ecosystem is related to its species diversity in the sense that complex ecosystem have high species diversity.

3. The function of ecosystem is related to energy flow and material cycles within and outside the system.

4. The relative amount of energy needed to maintain an ecosystem depends on its structure. Complex ecosystems needed less energy to maintain themselves.

5. Young ecosystems develop and change from less complex to more complex ecosystems, through the process called succession.

6. Each ecosystem has its own energy budget, which cannot be exceeded.

7. Adaptation to local environmental conditions is the important feature of the biotic components of an ecosystem, failing which they might perish.

8. The function of every ecosystem involves a series of cycles, e.g., water cycle, nitrogen cycle, oxygen cycle, etc. these cycles are driven by energy. A continuation or existence of ecosystem demands exchange of materials/nutrients to and from the different components.

 Components of an Ecosystem

The structure of an ecosystem is basically a description of the species of organisms that are present, including information on their life histories, populations and distribution in space. It is a guide to who’s who in the ecosystem. It also includes descriptive information on the non-living (physical) features of environment, including the amount and distribution of nutrients

The structure of ecosystem provides information about the range of climatic conditions that prevail in the area

1. Abiotic Substances

These include basic inorganic and organic compounds of the environment or habitat of the organism. The inorganic components of an ecosystem are carbon dioxide, water, nitrogen, calcium, phosphate, all of which are involved in matter cycles (biogeochemical cycles).

The organic components of an ecosystem are proteins, carbohydrates, lipids and amino acids, all of which are synthesized by the biota (flora and fauna) of an ecosystem and are reached to ecosystem as their wastes, dead remains, etc,

The climate, temperature, light, soil, etc., are other abiotic components of the ecosystem

Air

Air from Earth's atmosphere contains 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% carbon dioxide, and traces of hydrogen, helium, and other "noble" gases (by volume), but generally a variable amount of water vapour is also present, on average about 1% at sea level.

Earth's atmosphere consists of a number of layers, that differ in properties such as Air composition, temperature and pressure.

The first layer is called the troposphere. The depth of this layer varies from about 8 to 16 kilometers. Greatest depths occur at the tropics where warm temperatures causes vertical expansion of the lower atmosphere. From the tropics to the Earth's polar regions the troposphere becomes gradually thinner. The depth of this layer at the poles is roughly half as thick when compared to the tropics. Average depth of the troposphere is approximately 11 kilometers.

About 80 % of the total mass of the atmosphere is contained in troposphere. It is also the layer where the majority of our weather occurs.

Maximum air temperature also occurs near the Earth's surface in this layer. With increasing height, air temperature drops uniformly with altitude at a rate of approximately 6.5° Celsius per 1000 meters. This phenomenon is commonly called the Environmental Lapse Rate. At an average temperature of -56.5° Celsius, the top of the troposphere is reached. At the upper edge of the troposphere is a narrow transition zone known as the tropopause.

Above the tropopause is the stratosphere. This layer extends from an average altitude of 11 to 50 kilometers above the Earth's surface. This stratosphere contains about 19.9 % of the total mass found in the atmosphere. Very little weather occurs in the stratosphere.

Occasionally, the top portions of thunderstorms breach this layer. In the first 9 kilometers of the stratosphere, temperature remains constant with height. A zone with constant temperature in the atmosphere is called an isothermal layer.

From an altitude of 20 to 50 kilometers, temperature increases with an increase in altitude. The higher temperatures found in this region of the stratosphere occurs because of a localized concentration of ozone gas molecules. These molecules absorb ultraviolet sunlight creating heat energy that warms the stratosphere. ‘

Ozone is primarily found in the atmosphere at varying concentrations between the altitudes of 10 to 50 kilometers. This layer of ozone is also called the ozone layer. The ozone layer is important to organisms at the Earth's surface as it protects them from the harmful effects of the sun's ultraviolet radiation. Without the ozone layer life could not exist on the Earth's surface.

Separating the mesosphere from the stratosphere is a transition zone called the stratopause.

In the mesosphere, the atmosphere reaches its coldest temperatures (about -90° Celsius) at a height of approximately 80 kilometers. At the top of the mesosphere is another transition zone known as the mesopause.

The atmospheric layer has an altitude greater than 80 kilometers and is called the thermosphere. Temperatures in this layer can be as high as 1200°C.

These high temperatures are generated from the absorption of intense solar radiation by oxygen molecules (O2). The air in the thermosphere is extremely thin with individual gas molecules being separated from each other by large distances.

The temperature is quite hot and may be as high as thousands of degrees.

Exosphere: The exosphere is the region beyond the thermosphere.

The most common molecules within Earth's exosphere are those of the lightest atmospheric gasses. Hydrogen is present throughout the exosphere, with some helium, carbon dioxide, and atomic oxygen near its base. Because it can be difficult to define the boundary between the exosphere and outer space, the exosphere may be considered a part of interplanetary or outer space.

Ionosphere: The ionosphere overlaps the other atmospheric layers, from above the Earth. The air is ionized by the Sun’s ultraviolet light. These ionized layers affect the transmittance and reflectance of radio waves. Different ioniosphere layers are the D, E (Heaviside-Kennelly), and F (Appleton) regions.

Water

Water covers 71% of the Earth's surface. It is vital for all known forms of life.

On Earth, 96.5% of the planet's crust water is found in seas and oceans, 1.7% in groundwater, 1.7% in glaciers and the ice caps of Antarctica and Greenland, a small fraction in other large water bodies, and 0.001% in the air as vapor, clouds (formed of ice and liquid water suspended in air), and precipitation.

Only 2.5% of this water is freshwater, and 98.8% of that water is in ice (excepting ice in clouds) and groundwater. Less than 0.3% of all freshwater is in rivers, lakes, and the atmosphere, and an even smaller amount of the Earth's freshwater (0.003%) is contained within biological bodies.

A greater quantity of water is found in the earth's interior.

Water has many distinct properties that are critical for the proliferation of life. The human body contains from 55% to 78% water, depending on body size. It carries out this role by allowing organic compounds to react in ways that ultimately allow replication. All known forms of life depend on water. Water is vital both as a solvent in which many of the body's solutes dissolve and as an essential part of many metabolic processes within the body.

Water is fundamental to photosynthesis and respiration. Photosynthetic cells use the sun's energy to split off water's hydrogen from oxygen. All living cells use such fuels and oxidize the hydrogen and carbon to capture the sun's energy and reform water and CO2 in the process (cellular respiration).

Water is also central to acid-base neutrality and enzyme function.

The earliest life forms appeared in water; nearly all fish live exclusively in water, and there are many types of marine mammals, such as dolphins and whales. Some kinds of animals, such as amphibians, spend portions of their lives in water and portions on land. Plants such as kelp and algae grow in the water and are the basis for some underwater ecosystems. Plankton is generally the foundation of the ocean food chain.

Soil

Soil is the mixture of minerals, organic matter, gases, liquids, and the countless organisms that together support life on Earth. Soil is a natural body known as the pedosphere and which performs four important functions: it is a medium for plant growth; it is a means of water storage, supply and purification; it is a modifier of Earth's atmosphere; it is a habitat for organisms; all of which, in turn, modify the soil.

Soil is the end product of the influence of the climate, relief (elevation, orientation, and slope of terrain), organisms, and its parent materials (original minerals) interacting over time. Soil continually undergoes development by way of numerous physical, chemical and biological processes, which include weathering with associated erosion.

Soil acts as an engineering medium, a habitat for soil organisms, a recycling system for nutrients and organic wastes, a regulator of water quality, a modifier of atmospheric composition, and a medium for plant growth. Since soil has a tremendous range of available niches and habitats, it contains most of the Earth's genetic diversity. A gram of soil can contain billions of organisms,belonging to thousands of species.

Most soils have a density between 1 and 2 g/cm3.

Soil contamination or soil pollution is caused by the presence of xenobiotic (human-made) chemicals or other alteration in the natural soil environment. It is typically caused by industrial activity, agricultural chemicals, or improper disposal of waste. The most common chemicals involved are petroleum hydrocarbons, polynuclear aromatic hydrocarbons (such as naphthalene and benzo(a)pyrene), solvents, pesticides, lead, and other heavy metals. Contamination is correlated with the degree of industrialization and intensity of chemical usage.

Contaminated or polluted soil directly affects human health through direct contact with soil or via inhalation of soil contaminants which have vaporized; potentially greater threats are posed by the infiltration of soil contamination into groundwater aquifers used for human consumption.

Health consequences from exposure to soil contamination vary greatly depending on pollutant type, pathway of attack and vulnerability of the exposed population. Chronic exposure to chromium, lead and other metals, petroleum, solvents, and many pesticide and herbicide formulations can be carcinogenic, can cause congenital disorders, or can cause other chronic

health conditions. Industrial or man-made concentrations of naturally occurring substances, such as nitrate and ammonia associated with livestock manure from agricultural operations.

Chronic exposure to benzene at sufficient concentrations is known to be associated with higher incidence of leukemia. Mercury and cyclodienes are known to induce higher incidences of kidney damage and some irreversible diseases. PCBs and cyclodienes are linked to liver toxicity. Organophosphates and carbomates can induce a chain of responses leading to neuromuscular blockage. Many chlorinated solvents induce liver changes, kidney changes and depression of the central nervous system. There is an entire spectrum of further health effects such as headache, nausea, fatigue, eye irritation and skin rash for the above cited and other chemicals.

There are radical soil chemistry changes which can arise from the presence of many hazardous chemicals even at low concentration of the contaminant species. These changes can manifest in the alteration of metabolism of endemic microorganisms and arthropods resident in a given soil environment. The result can be virtual eradication of some of the primary food chain, which in turn could have major consequences for predator or consumer species. Even if the chemical effect on lower life forms is small, the lower pyramid levels of the food chain may ingest alien chemicals, which normally become more concentrated for each consuming rung of the food chain. Many of these effects are now well known, such as the concentration of persistent DDT materials for avian consumers, leading to weakening of egg shells, increased chick mortality and potential extinction of species.

Effects occur on agricultural lands which have certain types of soil contamination. Contaminants typically alter plant metabolism, often causing a reduction in crop yields as well as soil erosion.

Sunlight

Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light.

On Earth, sunlight is filtered through Earth's atmosphere, and is obvious as daylight when the Sun is above the horizon.

Sunlight takes about 8.3 minutes to reach Earth from the surface of the Sun.

The total amount of energy received at ground level from the Sun at the zenith depends on the distance to the Sun and thus on the time of year. It is about 3.3% higher than average in January and 3.3% lower in July.

The direct sunlight at Earth's surface when the Sun is at the zenith is about 1050 W/m2, but the total amount (direct and indirect from the atmosphere) hitting the ground is around 1120 W/m2.

In terms of energy, sunlight at Earth's surface is around 52 to 55 percent infrared (above 700 nm), 42 to 43 percent visible (400 to 700 nm), and 3 to 5 percent ultraviolet (below 400 nm).

At the top of the atmosphere, sunlight is about 30% more intense, having about 8% ultraviolet (UV),with most of the extra UV consisting of biologically damaging short-wave ultraviolet.

The existence of nearly all life on Earth is fueled by light from the Sun. Most autotrophs, such as plants, use the energy of sunlight, combined with carbon dioxide and water, to produce simple sugars—a process known as photosynthesis.

The ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a principal source of vitamin D3 and a mutagen.A dietary supplement can supply vitamin D without this mutagenic effect,but bypasses natural mechanisms that would prevent overdoses of vitamin D generated internally from sunlight. Vitamin D has a wide range of positive health effects, which include strengthening bones and possibly inhibiting the growth of some cancers. Sun exposure has also been associated with the timing of melatonin synthesis, maintenance of normal circadian rhythms, and reduced risk of seasonal affective disorder.

Long-term sunlight exposure is known to be associated with the development of skin cancer, skin aging, immune suppression, and eye diseases such as cataracts and macular degeneration.Short-term overexposure is the cause of sunburn, snow blindness, and solar retinopathy.

Temperature

A temperature is an objective comparative measure of hot or cold.

A common range of 18°C (64°F) to 23°C (73°F) is thought desirable, though differences in climate may acclimate people to higher or lower temperatures.

Scales measuring temperature, the most common being Celsius (denoted °C; formerly called centigrade), Fahrenheit (denoted °F), and, especially in science, Kelvin (denoted K).

The coldest theoretical temperature is absolute zero, at which the thermal motion of atoms and molecules reaches its minimum.

The differential heating of the atmosphere resulting from temperature variation over the earth’s surface produces a number of ecological effects, including local and trade winds and hurricanes and other storms and Green house effect, but more importantly it determines the distribution of precipitation.

It has significant role on the cells, morphology, Physiology, behaviour, growth, ontogenetic development and distribution of plants and animals.

Since 1970, the earth’s average temperature has risen by 0.8 degrees Celsius.

2. Biotic Substances

Producers:

Producers are autotrophic organisms like chemosynthetic and photosynthetic bacteria, blue green algae, algae and all other green plants. They are called ecosystem producers because they capture energy from non-organic sources, especially light, and store some of the energy the form of chemical bonds, for the later use.

Algae of various types are the most important producers of aquatic ecosystems, although in estuaries and marshes, grasses may be important as producers. Terrestrial ecosystems have trees, shrubs, herbs, grasses, and mosses that contribute with varying importance to the production of the ecosystem.

Since heterotrophic organisms depend on plants and other autotrophic Organisms like bacteria and algae for their nutrition, the amount of energy that the producers capture, sets the limit on the availability of energy for the ecosystem. Thus, when a green plant captures a certain amount of energy from sunlight, it is said to “produce” the energy for the ecosystem.

Consumers:

They are heterotrophic organisms in the ecosystem which eat other living creatures. There are herbivores, which eat plants, and carnivores, which eat other animals. They are also called phagotrophs or macroconsumers. Sometimes herbivores are called primary macroconsumers and carnivores are called secondary Macroconsumers.

Reducers or Decomposers:

Reducers, decomposers, saprotrophs or Macroconsumers are heterotrophic organisms that breakdown dead and waste matter. Fungi and certain bacteria are the prime representatives of this category. Enzymes are secreted by their cells into or onto dead plant and animal debris. These chemicals digest the dead organism into smaller bits or molecules, which can be absorbed by the fungi or bacteria (saprotrophs).

The decomposers take the energy and matter that they harvest during this feeding process for their own metabolism. Heat is liberated in each chemical conversion along the metabolic pathway.

No ecosystem could function long without decomposers. Dead organisms would pile up without rotting, as would waste products. It would not be long before an essential element, phosphorus, for example, would be first in short supply and then gone altogether, because the dead corpses littering the landscape would be hoarding the entire supply.

The Geography of Ecosystems

There are many different ecosystems: rain forests and tundra, coral reefs and ponds, grasslands and deserts. Climate differences from place to place largely determine the types of ecosystems we see. How terrestrial ecosystems appear to us is influenced mainly by the dominant vegetation.

The word "biome" is used to describe a major vegetation type such as tropical rain forest, grassland, tundra, etc., extending over a large geographic area (Figure 3). It is never used for aquatic systems, such as ponds or coral reefs. It always refers to a vegetation category that is dominant over a very large geographic scale, and so is somewhat broader than an ecosystem.

We can draw upon previous lectures to remember that temperature and rainfall patterns for a region are distinctive. Every place on earth gets the same total number of hours of sunlight each year, but not the same amount of heat. The sun's rays strike low latitudes directly but high latitudes obliquely. This uneven distribution of heat sets up not just temperature differences, but global wind and ocean currents that in turn have a great deal to do with where rainfall occurs. Add in the cooling effects of elevation and the effects of land masses on temperature and rainfall, and we get a complicated global pattern of climate.

A schematic view of the earth shows that, complicated though climate may be, many aspects are predictable. High solar energy striking near the equator ensures nearly constant high temperatures and high rates of evaporation and plant transpiration. Warm air rises, cools, and sheds its moisture, creating just the conditions for a tropical rain forest.

TYPES OF ECOSYSTEM

There are essentially two kinds of ecosystems; Aquatic and Terrestrial. Any other sub-ecosystem falls under one of these two headings.

The Forest Ecosystems

They are the ecosystems in which an abundance of flora, or plants, is seen so they have a big number of organisms which live in relatively small space. Therefore, in forest ecosystems the density of living organisms is quite high.

They are further divided into:

Tropical evergreen forest: These are tropical forests that receive a mean rainfall of 200 cms or more. The forests are characterised by dense vegetation which comprises tall trees at different heights. Each level is shelter to different types of animals.

Tropical rainforests of India are found in the Andaman and Nicobar Islands,the Western Ghats,the coastline of peninsular India, and the greater Assam region in the north-east.

Small remnants of rainforest are found in Odisha state. Semi-evergreen rainforest is more extensive than the evergreen formation partly because evergreen forests tend to degrade to semi-evergreen with human interference.

Some of the trees found in Indian Tropical Forests are Rosewood, Mahagony and Ebony.

Tropical deciduous forest: Shrubs and dense bushes rule along with a broad selection of trees.

They thrive where rainfall is about 70cm- 200cm.

The trees in this forest shed their leaves for about six to eight weeks in summer.

Two types are there-Dry Deciduous(70-100cms) and Moist deciduous(100-200 cms).

Found in rainier parts of peninsular India. Bihar, U.P, Chattisgarh etc

Temperate evergreen forest: Those have quite a few number of trees as mosses and ferns make up for them. Trees have developed spiked leaves in order to minimize transpiration.

Temperate deciduous forest: The forest is located in the moist temperate places that have sufficient rainfall. Summers and winters are clearly defined and the trees shed the leaves during the winter months.

Annual precipitation over 140 cm (55 in)

Mean annual temperature is between 4 and 12 °C

Taiga: Situated just before the arctic regions, the taiga is defined by evergreen conifers. As the temperature is below zero for almost half a year, the remainder of the months, it buzzes with migratory birds and insects.

Tundra

The vegetation is composed of dwarf shrubs, sedges and grasses, mosses, and lichens. Scattered trees grow in some tundra regions.

The ecotone (or ecological boundary region) between the tundra and the forest is known as the tree line or timberline.

There are three types of tundra: arctic tundra, alpine tundra and Antarctic tundra.

The Desert Ecosystem

Desert ecosystems are located in regions that receive an annual rainfall less than 25. They occupy about 17 percent of all the land on our planet. Due to the extremely high temperature, low water availability and intense sunlight, fauna and flora are scarce and poorly developed. The vegetation is mainly shrubs, bushes, few grasses and rare trees.

The stems and leaves of the plants are modified in order to conserve water as much as possible. The best known desert ones are the succulents such as the spiny leaved cacti. The organisms include insects, birds, reptiles all of which are adapted to the desert (xeric) conditions.

The Grassland Ecosystem

Grasslands are located in both the tropical and temperate regions of the world though the ecosystems vary slightly. The area mainly comprises grasses with a little number of trees and shrubs. The main vegetation includes grasses, plants and legumes that belong to the composite family. A lot of grazing animals, insectivores and herbivores inhabit the grasslands. The two main kinds of grasslands ecosystems are:

Savanna: The tropical grasslands are dry seasonally and have few individual trees. They support a large number of predators and grazers.

Prairies: It is temperate grassland, completely devoid of large shrubs and trees. Prairies could be categorized as mixed grass, tall grass and short grass prairies.

The Mountain Ecosystem

Mountain land provides a scattered and diverse array of habitats where a large number of animals and plants can be found. At the higher altitudes, the harsh environmental conditions normally prevail, and only the treeless alpine vegetation can survive.

The animals that live there have thick fur coats for prevention from cold and hibernation in the winter months. Lower slopes are commonly covered with coniferous forests.

Aquatic Ecosystem

Aquatic ecosystem is an ecosystem in a body of water. Communities of organisms that are

dependent on each other and on their environment live in aquatic ecosystems. The two main

types of aquatic ecosystems are freshwater and  marine ecosystems.

1. Freshwater

Freshwater community consists of an array of organisms depending on the physico-chemical and

biological characteristics of the freshwater environment.

Freshwater habitats are divided into two major categories, lotic (lotus = washed, or running

water), and lentic habitats.

Lotic habitats are those existing in relatively fast running streams, springs, rivers and brooks.

Lentic habitats are represented by the lakes, ponds, and swamps.

The above classification of the freshwater environments is based on two conditions: currents and

the ratio of the depth to surface area. Since lakes and ponds often contain currents or at least

wave action and since streams often harbour quiet pools or calm backwaters, the difference

between lotic and lentic waters is not very precise. However, temperature, light, currents, amount

of respiratory gases, and concentration of biogenic salts are important limiting factors

influencing the organisms of all freshwater habitats.

Lentic Community:

Lentic waters are generally divided into three zones or sub-habitats: littoral, limnetic, and pro-

fundal. A small pond may consist entirely of littoral zone. However, a deep lake with an abruptly

sloping basin may possess an extremely reduced littoral zone.

Lake Zonation:

The three major zones of a lake described as follows

(a) Littoral zone:

The littoral zone adjoins the shore (and is thus the home of rooted plants) and extends down to a

point called the light compensation level, or the depth at which the rate of photosynthesis equals

the rate of respiration. Within the littoral zone producers are of two main types: rooted or benthic

plants, and phytoplankton (plant plankton) or floating green plants, which are mostly algae.

The littoral zone is the home of greater variety of consumers than are the other zones. The

zooplankton (animal plankton) of the littoral zone is rather characteristic and differs from that of

the limnetic zone in preponderance of heavier, less buoyant crustacea which often cling to plants

or rest on the bottom when not actively moving their appendages. Important groups of littoral

zooplankton are large, weak-swimming species of Daphia and Simocephalus, some species of

copepods, many families of ostracods and some rotifers.

The nekton of littoral zone is often rich in species and numbers. Adult and larval diving beetles

and various adult Hemipetra are conspicuous. Various Diptera larvae and pupae remain

suspended in the water, often near the surface. Pond fish, frogs, turtles, and water snakes are

almost exclusively the members of the littoral zone community. Tadpoles of the frogs are

important primary consumers, feeding on algae and other plant material.

Periphyton of the littoral zone exhibits a zonation paralleling that of the rooted plants, but many

species occur almost throughout the littoral zone. Among the periphyton forms, for example,

pond snails, damselfly nymphs and climbing dragonfly nymphs, rotifers, flatworms, bryozoa,

hydra, and midge larvae rest on, or are attached to stems and leaves of the plants.

Another group containing both primary and secondary consumers may be found resting or

moving on the bottom or beneath silt or plant debris— for example, sprawling odonata nymphs

(which have flattened rather than cylindrical bodies), crayfish, isopods, and certain mayfly

nymphs. Descending more deeply into the bottom mud are burrowing odonata and

ephemeroptera, clams, true worms, snails, chironomids (midges), and other diptera larvae.

(b) Limnetic Zone:

The limnetic zone includes all the waters beyond the littoral zone and down to the light

compensation level. The limnetic zone derives its oxygen content from the photosynthetic

activity of phytoplankton and from the atmosphere immediately over the lake’s surface. The

atmospheric source of oxygen becomes significant primarily when there is some surface

disturbance of water caused by wind action or human activity. The community of the limnetic

zone is composed only of plankton, nekton, and sometimes neuston (organisms resting or

swimming on the surface).

Phytoplankton producers consist of diatoms, green algae, blue- green algae, and algae- like green

flagellates, chiefly the dinoflagellates. The limnetic zooplankton consists of few species but the

number of individuals may be large. Copepods, cladocerans, and rotifers are generally of first

importance; but their species are largely different from those found in the littoral zone. The

limnetic nekton consists almost entirely of fish. In ponds, the fish of the limnetic zone are the

same as those of the littoral zone, but in large bodies of water a few species may be restricted to

the limnetic zone.

(C) Profundal Zone:

The bottom and deep water area of a lake, which is beyond the depth of effective light

penetration is called the pro-fundal zone. In north-temperate latitudes, where winters are long

and severe, this zone has the warmest water (4°C) in the lake in winter and coldest water in

summer.

The major community consists of bacteria and fungi and three groups of animal consumers:

(a) Blood worms, or haemoglobin containing chironomid larvae and annelids,

(b) Small clams, and

(c) Phantom larvae, or Chaoborus (corethra).

2. Marine

Marine ecosystems cover approximately 71% of the Earth's surface and contain approximately

97% of the planet's water. They generate 32% of the world's net primary production. They are

distinguished from freshwater ecosystems by the presence of dissolved compounds,

especially salts, in the water. Approximately 85% of the dissolved materials

in seawater are sodium and chlorine. Seawater has an average salinity of 35 parts per

thousand (ppt) of water. Actual salinity varies among different marine ecosystems.

A classification of marine habitats.

Marine ecosystems can be divided into many zones depending upon water depth and shoreline

features. The oceanic zone is the vast open part of the ocean where animals such as whales,

sharks, and tuna live. The benthic zone consists of substrates below water where many

invertebrates live. The intertidal zone is the area between high and low tides; in this figure it is

termed the littoral zone. Other near-shore (neritic) zones can include estuaries, salt

marshes, coral reefs, lagoons and mangrove swamps. In the deep water, hydrothermal vents may

occur where chemosynthetic sulfur bacteria form the base of the food web.

Classes of organisms found in marine ecosystems include brown

algae, dinoflagellates, corals, cephalopods, echinoderms, and sharks. Fishes caught in marine

ecosystems are the biggest source of commercial foods obtained from wild populations.

Environmental problems concerning marine ecosystems include unsustainable exploitation of

marine resources (for example overfishing of certain species), marine pollution, climate change,

and building on coastal areas.

1. Intertidal zone: The littoral zone covers the region between low and high tide and represents

the transitional area between marine and terrestrial conditions. It is also known as

the intertidal zone because it is the area where tide level affects the conditions of the region. This

is the place is the top layer where the fish live by the tides and the hard waves. Half the day the

zone is underwater and the other half is where the organisms are exposed to air. To live here you

have to be able to avoid injury. Some organisms are crabs, green algae, mussels, sea stars, snails,

and some marine vegetation. The intertidal zone, also known as the foreshore and seashore and

sometimes referred to as the littoral zone, is the area that is above water at low tide and under

water at high tide (in other words, the area between tide marks). This area can include many

different types of habitats, with many types of animals, such as starfish, sea urchins, and

numerous species of coral. The well-known area also includes steep rocky cliffs, sandy beaches,

or wetlands (e.g., vast mudflats). The area can be a narrow strip, as in Pacific islands that have

only a narrow tidal range, or can include many meters of shoreline where shallow beach slopes

interact with high tidal excursion. Organisms in the intertidal zone are adapted to an environment

of harsh extremes. The intertidal zone is also home to many several species which cut across

different taxa including Porifera, Annelids, Coelenterates, Mollusks, Crustaceans, Arthropods,

and etc. Water is available regularly with the tides but varies from fresh with rain to

highly saline and dry salt with drying between tidal inundations. The action of waves can

dislodge residents from the littoral zone. With the intertidal zone's high exposure to the sun,

the temperature range can be anything from very hot with full sun to near freezing in colder

climates. Some microclimates in the littoral zone are ameliorated by local features and larger

plants such asmangroves. Adaptation in the littoral zone allows the use of nutrients supplied in

high volume on a regular basis from the sea which is actively moved to the zone by tides. Edges

of habitats, in this case land and sea, are themselves often significant ecologies, and the littoral

zone is a prime example.

2. Pelagic zone: Any water in a sea or lake that is neither close to the bottom nor near the shore

can be said to be in the pelagic zone. The word "pelagic" is derived

from Greekπέλαγος (pélagos), meaning "open sea". The pelagic zone can be thought of in terms

of an imaginary cylinder or water column that goes from the surface of the sea almost to the

bottom. Conditions differ deeper in the water column such that as pressure increases with depth,

the temperature drops and less light penetrates. Depending on the depth, the water column, rather

like the Earth's atmosphere, may be divided into different layers. The pelagic zone occupies

1,330 million km3 (320 million mi3) with a mean depth of 3.68 km (2.29 mi) and maximum

depth of 11 km (6.8 mi). Fish that live in the pelagic zone are called pelagic fish. Pelagic life

decreases with increasing depth. It is affected by light intensity, pressure, temperature, salinity,

the supply of dissolved oxygen and nutrients, and the submarine topography, which is

called bathymetry. In deep water, the pelagic zone is sometimes called the open-ocean zone and

can be contrasted with water that is near the coast or on the continental shelf. In other contexts,

coastal water not near the bottom is still said to be in the pelagic zone.

3. Abyssal zone: The abyssal zone is the abyssopelagic layer or pelagic zone. "Abyss" derives

from the Greek word ἄάβυσσος, meaning bottomless. At depths of 4,000 to 6,000 metres (13,123

to 19,685 feet), this zone remains in perpetual darkness and never receives daylight. These

regions are also characterised by continuous cold and lack of nutrients. The abyssal zone has

temperatures around 2 °C to 3 °C (35 °F to 37 °F) through the large majority of its mass. It is the

deeper part of the midnight zone which starts in the bathypelagic waters above. Its permanent

inhabitants (for example, Riftia pachyptila, (the Giant tube worm) found near hydrothermal

vents in the pacific ocean and the giant squid are able to withstand the immense pressures of the

ocean depths, up to 76 megapascals (11,000 psi).

4. Benthic zone: The benthic zone is the ecological region at the lowest level of a body of

water such as an ocean or a lake, including the sediment surface and some sub-surface layers.

Organisms living in this zone are called benthos, e.g. the benthic invertebrate community,

including crustaceans. The organisms generally live in close relationship with the substrate

bottom and many are permanently attached to the bottom. The superficial layer of the soil lining

the given body of water, the benthic boundary layer, is an integral part of the benthic zone, as it

greatly influences the biological activity which takes place there. Examples of contact soil layers

include sand bottoms, rocky outcrops, coral, and bay mud. Generally, these include life forms

that tolerate cool temperatures and low oxygen levels, but this depends on the depth of the water.

Coral reefs: Coral reefs are the epicenter for immense amounts of biodiversity, and are a key

player in the survival of an entire ecosystem. They provide various marine animals with food,

protection, and shelter which keep generations of species alive. Furthermore, coral reefs are an

integral part of sustaining human life through serving as a food source (i.e. fish, mollusks, etc.)

as well as a marine space for eco-tourism which provides economic benefits. Unfortunately,

because of human impact of coral reefs, these ecosystems are becoming increasingly degraded

and in need of conservation. The biggest threats include "overfishing, destructive fishing

practices, and sedimentation and pollution from land-based sources."  This in conjunction with

increased carbon in oceans, coral bleaching, and diseases, there are no pristine reefs anywhere in

the world.

Abyssal zone

In fact, up to 88% of coral reefs in Southeast Asia are now threatened, with 50% of those reefs

at either "high" or "very high" risk of disappearing which directly effects biodiversity and

survival of species dependent on coral.

Estuary: An estuary is a partly enclosed coastal body of brackish water with one or more rivers

or streams flowing into it, and with a free connection to the open sea.

Estuaries form a transition zone between river environments and maritime environments. They

are subject both to marine influences—such as tides, waves, and the influx of saline water—and

to riverine influences—such as flows of fresh water and sediment. The inflows of both sea water

and fresh water provide high levels of nutrients both in the water column and in sediment,

making estuaries among the most productive natural habitats in the world.

FOOD CHAIN

The food chain describes who eats whom in the wild. Every living thing—from one-celled algae to giant blue whales—needs food to survive. Each food chain is a possible pathway that energy and nutrients can follow through the ecosystem.

For example, grass produces its own food from sunlight. A rabbit eats the grass. A fox eats the rabbit. When the fox dies, bacteria break down its body, returning it to the soil where it provides nutrients for plants like grass.

Of course, many different animals eat grass, and rabbits can eat other plants besides grass. Foxes, in turn, can eat many types of animals and plants. Each of these living things can be a part of multiple food chains. All of the interconnected and overlapping food chains in an ecosystem make up a food web.

Examples:

Different habitats and ecosystems provide many possible food chains that make up a food web.

In one marine food chain, single-celled organisms called phytoplankton provide food for tiny shrimp called krill. Krill provide the main food source for the blue whale, an animal on the third trophic level.

In a grassland ecosystem, a grasshopper might eat grass, a producer. The grasshopper might get eaten by a rat, which in turn is consumed by a snake. Finally, a hawk—an apex predator—swoops down and snatches up the snake.

In a pond, the autotroph might be algae. A mosquito larva eats the algae, and then perhaps a dragonfly larva eats the young mosquito. The dragonfly larva becomes food for a fish, which provides a tasty meal for a raccoon.

FOOD WEB

Food webs consist of a number of food chains meshed together. Each food chain is a descriptive diagram including a series of arrows, each pointing from one species to another, representing the flow of food energy from one feeding group of organisms to another.

Food web is an important ecological concept. Basically, food web represents feeding relationships within a community. It also implies the transfer of food energy from its source in plants through herbivores to carnivores.

Food web offers an important tool for investigating the ecological interactions that define energy flows and predator-prey relationship.

Fig below shows a simplified food web in an ecosystem. In this food web, locusts, rats and rabbits feed on plant derived materials and in turn are predated upon by snakes and other secondary consumers which are utilised by owls and other higher tertiary consumers.

While the food web showed here is a simple one, most feed webs are complex and involve many species with both strong and weak interactions among them.

For example, the predators of a scorpion in a desert ecosystem might be a golden eagle, an owl, a roadrunner, or a fox.

Food webs are constructed to describe species interactions (direct relationships).They can be used to illustrate indirect interactions among species.

A simple food web

ECOLOGICAL PYRAMID

An ecological pyramid is an illustration of the reduction in energy as you move through each feeding (trophic) level in an ecosystem.

The base of the pyramid is large since the ecosystem's energy factories (the producers) are converting solar energy into chemical energy via photosynthesis.

There are three ways an ecological pyramid can be represented.

1. A Pyramid of Numbers can be generated by counting all the organisms at the different feeding levels. As you might guess, this can be a very difficult task since we are not just

identifying each species in the ecosystem. We are also counting how many of each species is present. On occasion, this approach will not work

2. A second type of pyramid is called a Pyramid of Biomass where organisms are collected from each feeding level, dried and then weighed. This dry weight (biomass) represents the amount of organic matter (available energy) of the organisms. [Note that there are alternate, nonlethal ways to determine biomass.] While this approach will generally create a pyramid that illustrates energy flow, its use can also produce an inverted pyramid. For example, in aquatic ecosystems, phytoplankton could reproduce and then be eaten rapidly by zooplankton. Therefore, it would be possible to have few herbivores and a lot of carnivores when a collection is taken.

3. A third type of pyramid called a Pyramid of Energy Flow tends to resolve these problems. This approach necessitates measuring the caloric value of the different organisms that make up the community. It nicely shows how energy is continually decreasing along the food chain from producers to top level carnivores.

BIOGEOCHEMISTRY

The term Biogeochemistry is defined as the study of how living systems influence, and are controlled by, the geology and chemistry of the earth. Thus biogeochemistry encompasses many aspects of the abiotic and biotic world that we live in.

There are several main principles and tools that biogeochemists use to study earth systems. Most of the major environmental problems that we face in our world toady can be analyzed using biogeochemical principles and tools.

These problems include global warming, acid rain, environmental pollution, and increasing greenhouse gases. The principles and tools that we use can be broken down into 3 major components: element ratios, mass balance, and element cycling.

Biogeochemical Cycles

The term biogeochemical is a contraction that refers to the consideration of the biological, geological, and chemical aspects of each cycle.

Biogeochemical cycles describe the circulation of matter, particularly plant and animal nutrients,through ecosystems. These cycles are ultimately powered by solar energy, fine-tuned and directed by energy expended by organisms.

In a sense, the solar-energy-powered hydrologic cycle acts as an endless conveyer belt to move materials essential for life through ecosystems.

Most biogeochemical cycles can be described as elemental cycles involving nutrient elements such as carbon, oxygen, nitrogen, sulfur and phosphorus. Many are gaseous cycles in which the element in question spends part of the cycle in the atmosphere – O2 for oxygen, N2 for nitrogen, CO2 for carbon.

Others, notably the phosphorus cycle, do not have a gaseous component and are called sedimentary cycles.

All sedimentary cycles involve salt solutions or soil solutions that contain dissolved substances leached from weathered minerals that may be deposited as mineral formations or they may be taken up by organisms as nutrients.

The sulfur cycle, which may have H2S or SO2 in the gaseous phase or minerals (CaSO4 2H2O) in the solid phase, is a combination of gaseous and sedimentary cycles.

WATER CYCLE

The water cycle, also known as the hydrological cycle or the H2O cycle, describes the continuous movement of water on, above and below the surface of the Earth.

The mass of water on Earth remains fairly constant over time but the partitioning of the water into the major reservoirs of ice, fresh water, saline water and atmospheric water is variable depending on a wide range of climatic variables.

The water moves from one reservoir to another, such as from river to ocean, or from the ocean to the atmosphere, by the physical processes of evaporation, condensation, precipitation, infiltration, runoff, and subsurface flow. In doing so, the water goes through different phases: liquid, solid (ice), and gas (vapor).

The water cycle involves the exchange of energy, which leads to temperature changes. For instance, when water evaporates, it takes up energy from its surroundings and cools the

environment. When it condenses, it releases energy and warms the environment. These heat exchanges influence climate.

The evaporative phase of the cycle purifies water which then replenishes the land with freshwater. The flow of liquid water and ice transports minerals across the globe. It is also involved in reshaping the geological features of the Earth, through processes including erosion and sedimentation. The water cycle is also essential for the maintenance of most life and ecosystems on the planet.

PROCESSES

Many different processes lead to movements and phase changes in water.

Precipitation

Condensed water vapor that falls to the Earth's surface . Most precipitation occurs as rain, but also includes snow, hail, fog drip, graupel, and sleet.

Approximately 505,000 km3 (121,000 cu mi) of water falls as precipitation each year, 398,000 km3 (95,000 cu mi) of it over the oceans.

The rain on land contains 107,000 km3 (26,000 cu mi) of water per year and a snowing only 1,000 km3 (240 cu mi). 78% of global precipitation occurs over the ocean.

Canopy interception

The precipitation that is intercepted by plant foliage, eventually evaporates back to the atmosphere rather than falling to the ground.

Snowmelt

The runoff produced by melting snow.

Runoff

The variety of ways by which water moves across the land. This includes both surface runoff and channel runoff. As it flows, the water may seep into the ground, evaporate into the air, become stored in lakes or reservoirs, or be extracted for agricultural or other human uses.

Infiltration

The flow of water from the ground surface into the ground. Once infiltrated, the water becomes soil moisture or groundwater. A recent global study using water stable isotopes, however, shows that not all soil moisture is equally available for groundwater recharge or for plant transpiration.

Subsurface flow

The flow of water underground, in the vadose zone and aquifers. Subsurface water may return to the surface (e.g. as a spring or by being pumped) or eventually seep into the oceans. Water returns to the land surface at lower elevation than where it infiltrated, under the force of gravity or gravity induced pressures. Groundwater tends to move slowly, and is replenished slowly, so it can remain in aquifers for thousands of years.

Evaporation

The transformation of water from liquid to gas phases as it moves from the ground or bodies of water into the overlying atmosphere.

The source of energy for evaporation is primarily solar radiation. Evaporation often implicitly includes transpiration from plants, though together they are specifically referred to as evapotranspiration. Total annual evapotranspiration amounts to approximately 505,000 km3 (121,000 cu mi) of water, 434,000 km3 (104,000 cu mi) of which evaporates from the oceans.

i.e. 86% of global evaporation occurs over the ocean.

Sublimation

The state change directly from solid water (snow or ice) to water vapor.

Deposition

This refers to changing of water vapor directly to ice.

Advection

The movement of water — in solid, liquid, or vapor states — through the atmosphere. Without advection, water that evaporated over the oceans could not precipitate over land.

Condensation

The transformation of water vapor to liquid water droplets in the air, creating clouds and fog.

Transpiration

The release of water vapor from plants and soil into the air.

Percolation

Water flows vertically through the soil and rocks under the influence of gravity

Plate tectonics

Water enters the mantle via subduction of oceanic crust. Water returns to the surface via volcanism.

CARBON CYCLE

The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth, the carbon cycle comprises a sequence of events that are key to making the Earth capable of sustaining life; it describes the movement of carbon as it is recycled and reused throughout the biosphere, including carbon sinks.

Main components

The global carbon cycle is now usually divided into the following major reservoirs of carbon interconnected by pathways of exchange:

The atmosphere;The terrestrial biosphere;The oceans, including dissolved inorganic carbon and living and non-living marine biota

The sediments, including fossil fuels, fresh water systems and non-living organic material.

The Earth's interior, carbon from the Earth's mantle and crust. These carbon stores interact with the other components through geological processes

The carbon exchanges between reservoirs occur as the result of various chemical, physical, geological, and biological processes. The ocean contains the largest active pool of carbon near the surface of the Earth.

The natural flows of carbon between the atmosphere, ocean, terrestrial ecosystems, and sediments is fairly balanced, so that carbon levels would be roughly stable without human influence

OXYGEN CYCLE

The oxygen cycle is the biogeochemical cycle that describes the movement of oxygen within its three main reservoirs: the atmosphere (air), the total content of biological matter within the biosphere (the global sum of all ecosystems), and the lithosphere (Earth's crust).

Failures in the oxygen cycle within the hydrosphere (the combined mass of water found on, under, and over the surface of planet Earth) can result in the development of hypoxic zones. The main driving factor of the oxygen cycle is photosynthesis, which is responsible for the modern Earth's atmosphere and life on earth

NITROGEN CYCLE

The nitrogen cycle is the process by which nitrogen is converted between its various chemical forms. This transformation can be carried out through both biological and physical processes. Important processes in the nitrogen cycle include fixation, ammonification, nitrification, and denitrification.

The majority of Earth's atmosphere (78%) is nitrogen,making it the largest pool of nitrogen. However, atmospheric nitrogen has limited availability for biological use, leading to a scarcity of usable nitrogen in many types of ecosystems.

Human activities such as fossil fuel combustion, use of artificial nitrogen fertilizers, and release of nitrogen in wastewater have dramatically altered the global nitrogen cycle.

Nitrogen fixation

Atmospheric nitrogen must be processed, or "fixed", in a usable form to be taken up by plants.

A lot is fixed by lightning strikes, but most fixation is done by free-living or symbiotic bacteria known as diazotrophs. These bacteria have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia, which is converted by the bacteria into other organic compounds.

Most biological nitrogen fixation occurs by the activity of Mo-nitrogenase, found in a wide variety of bacteria and some Archaea. Mo-nitrogenase is a complex two component enzyme that has multiple metal-containing prosthetic groups.

An example of the free-living bacteria is Azotobacter. Symbiotic nitrogen-fixing bacteria such as Rhizobium usually live in the root nodules of legumes (such as peas, alfalfa, and locust trees).

Assimilation

Plants take nitrogen from the soil by absorption through their roots as amino acids, nitrate ions, nitrite ions, or ammonium ions. Most nitrogen obtained by terrestrial animals can be traced back to the eating of plants at some stage of the food chain.

Plants can absorb nitrate or ammonium from the soil via their root hairs. If nitrate is absorbed, it is first reduced to nitrite ions and then ammonium ions for incorporation into amino acids, nucleic acids, and chlorophyll.

In plants that have a symbiotic relationship with rhizobia, some nitrogen is assimilated in the form of ammonium ions directly from the nodules. It is now known that there is a more complex cycling of amino acids between Rhizobia bacteroids and plants.

Utilization of various N sources is carefully regulated in all organisms.

Ammonification

When a plant or animal dies or an animal expels waste, the initial form of nitrogen is organic. Bacteria or fungi convert the organic nitrogen within the remains back into ammonium (NH4+), a process called ammonification or mineralization. Enzymes involved are:

GS: Gln Synthetase (Cytosolic & Plastic)

GOGAT: Glu 2-oxoglutarate aminotransferase (Ferredoxin & NADH dependent)

GDH: Glu Dehydrogenase:

Nitrification

The conversion of ammonia to nitrate is performed primarily by soil-living bacteria and other nitrifying bacteria. In the primary stage of nitrification, the oxidation of ammonium (NH4+) is performed by bacteria such as the Nitrosomonas species, which converts ammonia to nitrites (NO2−). Other bacterial species such as Nitrobacter, are responsible for the oxidation of the nitrites into nitrates (NO3−).

It is important for the ammonia to be converted to nitrates or nitrites because ammonia gas is toxic to plants.

Due to their very high solubility and because soils are largely unable to retain anions, nitrates can enter groundwater.

Elevated nitrate in groundwater is a concern for drinking water use because nitrate can interfere with blood-oxygen levels in infants and cause methemoglobinemia or blue-baby syndrome.

Where groundwater recharges stream flow, nitrate-enriched groundwater can contribute to eutrophication, a process that leads to high algal population and growth, especially blue-green algal populations. While not directly toxic to fish life, like ammonia, nitrate can have indirect effects on fish if it contributes to this eutrophication.

Denitrification

Denitrification is the reduction of nitrates back into the largely inert nitrogen gas (N2), completing the nitrogen cycle. This process is performed by bacterial species such as Pseudomonas and Clostridium in anaerobic conditions.

They use the nitrate as an electron acceptor in the place of oxygen during respiration. These facultatively anaerobic bacteria can also live in aerobic conditions.

Denitrification happens in anaerobic conditions e.g. waterlogged soils. The denitrifying bacteria use nitrates in the soil to carry out respiration and consequently produce nitrogen gas, which is inert and unavailable to plants.

Anaerobic ammonia oxidation

In this biological process, nitrite and ammonia are converted directly into molecular nitrogen (N2) gas. This process makes up a major proportion of nitrogen conversion in the oceans.

SPECIES AND GENUS CONCEPT

Species is one of the basic units of biological classification and a taxonomic rank.

Kingdom Phylum Class Order Family Genus Species

A species is often defined as the largest group of organisms where two individuals are capable of reproducing fertile offspring, typically using sexual reproduction.

Genus is a principal taxonomic category that ranks above species and below family, and is denoted by a capitalized Latin name.

Species hypothesized to have the same ancestors are placed in one genus, based on similarities. The similarity of species is judged based on comparison of physical attributes, and where available, their DNA sequences. All species are given a two-part name, a "binomial name", or just "binomial".

The first part of a binomial is the generic name, the genus to which the species belongs. The second part is either called the specific name (a term used only in zoology) or the specific epithet (the term used in botany, which can also be used in zoology).

For example, Boa constrictor is one of four species of the Boa genus. While the genus gets capitalized, the specific epithet does not.

The binomial is written in italics when printed and underlined when handwritten.

Presence of specific locally adapted traits may further subdivide species into "infraspecific taxa" such as subspecies (and in botany other taxa are used, such as varieties, subvarieties, and formae).

CONCEPT OF HABITAT

It is the place where an organism lives within a community. You might think of an animal's habitat as its "home" or its "address" in the community. Habitats can be very different in size, from a small pond (for a tadpole) to an entire forest (for a tiger).

Habitat must have four essential things: 1. Food 2. Water 3. Cover 4. Space

REFERENCES AND RECOMMENDED READING

Borman, F.H. and G.E. Likens. 1970. "The nutrient cycles of an ecosystem." Scientific American, October 1970, pp 92-101.

Cain, M. L., Bowman, W. D. & Hacker, S. D. Ecology. Sunderland MA: Sinauer Associate Inc. 2008.

Cebrian, J. Patterns in the fate of production in plant communities. American Naturalist 154, 449-468 (1999)

Golley, F. B. 1993. A History of the Ecosystem Concept in Ecology: More Than the Sum of the Parts. Yale University Press, New Haven. ISBN: 0300066422.

Elton, C. S. Animal Ecology. Chicago, MI: University of Chicago Press, 1927, Republished 2001.

Knight, T. M., et al. Trophic cascades across ecosystems. Nature 437, 880-883 (2005)

Krebs, C. J. Ecology 6th ed. San Francisco CA: Pearson Benjamin Cummings, 2009.

Marquis, R. J. & Whelan, C. Insectivorous birds increase growth of white oak through consumption of leaf-chewing insects. Ecology 75, 2007-2017 (1994)

Molles, M. C. Jr. Ecology: Concepts and Applications 5th ed. New York, NY: McGraw-Hill Higher Education, 2010.

Odum, E. P. 1971. Fundamentals of Ecology. Third edition. W. B. Saunders, Philadelphia.

Paine, R. T. The Pisaster-Tegula interaction: Prey parches, predator food preferences and intertide community structure. Ecology 60, 950-961 (1969)

Pimm, S. L., Lawton, J. H. & Cohen, J. E. Food web patterns and their consequences. Nature 350, 669-674 (1991)

Power, M. E. Top-down and bottom-up forces in food webs: do plants have primacy? Ecology 73, 733-746 (1992)

Schoender, T. W. Food webs from the small to the large. Ecology 70, 1559-1589 (1989)

Smith, T. M. & Smith, R. L. Elements of Ecology 7th ed. San Francisco CA: Pearson Benjamin Cummings, 2009.

Tansley, A. G. 1935. The use and abuse of vegetational concepts and terms. Ecology 16:284-307.

Wessells, N.K. and J.L. Hopson. 1988. Biology. New York: Random House. Ch. 44.