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Structure and Function

The Ecosystem – Structure & Function Side 2

Environmental Systems and Societies June 2009

THE ECOSYSTEM – Structure and Function

By the end of this topic you should be able to:- N.B. Many practical aspects of this topic will be reinforced during field work. SYLLABUS

STATEMENT ASSESSMENT STATEMENT CHECK NOTES

2.1

STRUCTURE

2.1.1 Distinguish between biotic and abiotic (physical) components of an ecosystem.

2.1.2 Define the term trophic level.

2.1.3 Identify and explain trophic levels in food chains and food webs selected from a local environment i.e. _____________________.

2.1.4 Explain the principles of pyramids of numbers, pyramids of biomass and pyramids of productivity, and construct such pyramids from given data.

2.1.5 Discuss how the pyramid structure affects the functioning of an ecosystem.

2.1.6 Define the terms species, population, habitat, niche, community and ecosystem with reference to local examples.

2.1.7 Describe and explain population interactions using examples of named species.



SYLLABUS STATEMENT ASSESSMENT STATEMENT CHECK NOTES

2.5

FUNCTION

2.5.1 Explain the role of producers, consumers and decomposers in the ecosystem.

2.5.2 Describe photosynthesis and respiration in terms of inputs, outputs and energy transformations.

2.5.3 Describe and explain the transfer and transformation of energy and material as they flow through an ecosystem.

2.5.4 Describe and explain the transfer and transformation of materials as they cycle within an ecosystem.

2.5.5 Define the terms gross productivity, net productivity, primary productivity, secondary productivity, gross primary productivity and net primary productivity.

2.5.6 Define the terms and calculate the values of gross primary productivity (GPP) and net primary productivity (NPP) from given data.

2.5.7 Define the terms and calculate the values of both gross secondary productivity (GSP) and net secondary productivity (NSP) from given data,



THE ECOSYSTEM

Humans share the Earth with millions of other kinds of organisms. Unlike any other planet in our solar system, Earth has a protective, oxygen-rich atmosphere and an abundance of water, and it is this which has allowed life to develop. The part of the Earth including air (atmosphere), water (hydrosphere) and minerals (lithosphere) where life can exist is called the BIOSPHERE. The biosphere is less than 20km thick, extending approximately 9km above sea level and approximately 11km below it. Within the biosphere, organisms interact. By providing oxygen, green plants enable animals to aerobically respire, a process that in turn releases the carbon dioxide which the plants use in photosynthesis. Plants provide food and habitats but rely upon animals and microorganisms to release the nutrients they need to grow. Animals such as insects are also frequently involved in the reproductive processes of plants. The biosphere existed long before humans came along. In fact, in terms of geological time, we are the latest of latecomers, and just as dependent as any other organism on the complex inter-relationships which support life on the planet. If we are to play our part and maintain the biosphere, we need to understand it.

Fig 2.1 Structure of the biosphere



THE ECOSYSTEM

– a community of interdependent organisms and the physical environment they inhabit.

ECOLOGY – the study of the inter-relationships of organisms with each other and the environment.

Basic terms

Using local examples where possible, explain the meaning of the following terms:-

• Species

• Population

• Habitat

• Niche

• Community

• Environment



Factors affecting the distribution of species

The environment is a collective term to include all the conditions in which an organism lives and it can be divided into two main parts:- ABIOTIC FACTORS – non-living, physical factors that may influence an organism or ecosystem. These can be further divided into edaphic (soil), climatic and topographic factors. BIOTIC FACTORS – living, biological factors that may influence an organism or ecosystem. Complete the following table using the factors list below:- Temperature, light, soil pH, territory, salinity, mineral availability, interspecific competition (between different species), disease, predation, slope, soil structure, parasitism, wind speed, aspect (which direction a slope is facing), grazing, intraspecific competition (between organisms of the same species), fire, oxygen availability, pollution, agriculture. ABIOTIC FACTORS (PHYSICAL) BIOTIC FACTORS (LIVING)



Although abiotic factors are often presented as if they were discrete entities, this is not the case. For example, two edaphic factors, soil pH and inorganic ion concentration interact with each other. Furthermore, the concentration of inorganic ions in the soil also depends on the activity of decomposers, mainly bacteria, a biotic factor. The activity of these bacteria also depends on soil pH. The interrelationships between these factors are shown in the diagram below. * You should always look for such interactions when you think about the effects of abiotic and biotic factors on organisms.

Fig 2.2 One way in which abiotic and biotic factors interact in the soil. Construct a diagram based on the one above to summarise the interrelationships between some of the environmental factors which may affect the structure of a woodland (use the diagram on the next page to help you).



Abiotic factors in an oak woodland.

Fig 2.3 Abiotic factors in an oak woodland



Ecosystems

An ecosystem is the interrelationship between plants, animals, and their living and non-living environments. Biogeography is the geographic distribution of soils, vegetation, and ecosystems – where they are and why they are there.

THE POND AS AN ECOSYSTEM

Ecosystems can be divided into two main components: • Abiotic elements (non-living), e.g. air,

water, heat, nutrients, rock, and sediments.

• Biotic elements (living), e.g. plants and animals. These can be divided into: Autotrophs (or producers) – organisms cable of converting sunlight energy into food energy by photosynthesis. Heterotrophs (or consumers) – organisms that must feed on other organisms, e.g. herbivores – plant eaters carnivores – meat eaters ominivores – plant and meat eaters detritivores - decomposers

The trophic classification or system is based on feeding patterns. Typically there is a trophic pyramid, showing a larger plant biomass and a smaller consumer biomass. This occurs because: (i) no energy transfer is 100%

efficient – the transfer of light energy to food energy is only 1% efficient;

(ii) there are large losses of energy at each trophic level due to respiration, growth, reproduction, mobility, and so on.



Food chains and webs

The terms producer, primary consumer, secondary consumer and tertiary consumer refer to the organisms feeding level in a food chain or web – this is called its TROPHIC LEVEL. TROPHIC LEVEL – the position that an organism occupies in a food chain, or a group of organisms in a community that occupy the same position in food chains. Q1. Describe what is meant by a food chain.

e.g. phytoplankton → zooplankton → fish →otter Q2. Give three realistic examples of food chains.

a) b) c)

Q3. Define the following terms (giving examples where possible):-

a) Producer (autotroph) b) Consumer (heterotroph) c) Detritivore



d) Saprotroph (decomposer) e) Herbivore f) Carnivore g) Top carnivore

Q4. Describe what is meant by a food web.

e.g.

Fig 2.4 Woodland food chains and a food web: the white arrows indicate links between

the food chains that make a food web



Q5. Define the following terms using the diagram below for reference.

Fig 2.5 Biotic of an oak woodland ecosystem

a) Primary (1º) consumer b) Secondary (2º) consumer c) Tertiary (3º) consumer d) Which of the above is a herbivore? e) Which of the above are carnivores? f) What is an omnivore?



Examples Trophic levels of a food chain in each of 5 different habitats

Trophic level Habitat

Grassland Woodland Freshwater pond Rocky marine shore Ocean

Quaternary consumers (3º carnivores)

Mammal e.g. stoat

Bird e.g. thrush

Large fish e.g. pike

Bird e.g. gull

Marine mammal e.g. seal

Tertiary consumers (2º carnivores)

Reptile e.g. grass snake

Arachnid e.g. spider

Small fish e.g. stickleback

Crustacean e.g. crab

Large fish e.g. herring

Secondary consumer (1º) carnivores)

Amphibian e.g. toad

Carnivorous insect e.g. ladybird

Annelid e.g. leech

Carnivorous mollusc e.g. whelk

Small fish e.g. sand eel larvae

Primary consumers (herbivores)

Insect larva e.g. caterpillar

Herbivorous insect e.g. aphid

Mollusc e.g. freshwater snail

Herbivorous mollusc e.g. limpet

Zooplankton e.g. copepods

Primary producers (e.g. photosynthetic organisms)

Grass e.g. Festuca

Tree e.g. oak leaves

Aquatic plant e.g. Elodea

Seaweed e.g. sea lettuce

Phytoplankton e.g. diatom

Q6. Look at the food chain/web in Q4.

a) How many trophic levels are there in food chain A? b) Name the producer. c) Label each trophic level in food chain B. d) At which level is the sparrow hawk? e) The caterpillar in food chain A is also eaten by a beetle (the devil’s coachhorse

beetle) which is itself also eaten by the great tit. Add the beetle to the web. f) How has this addition changed our understanding of the great tit’s trophic level?

N.B. It is often difficult to classify organisms into definite trophic levels. They may occupy more than one level if they have a varied food source or change feeding patterns during different stages of their life cycle.



Q7. Data analysis practice question:-

Fig 2.6 A food web for an oak woodland.

a) Draw a food chain of 5 organisms from the web. Label it fully. b) Name one animal which is exclusively a secondary consumer. c) Name one animal which is at both the primary and secondary consumer trophic level i.e.

an omnivore. d) Name one animal which is both a secondary and a tertiary consumer. e) The woodland is a bird sanctuary managed by a warden. In the summer of 1995 the

oak trees were being eaten away by a large population of caterpillars. The warden sprayed the sanctuary (by light aircraft) with an insecticide which only killed insects. The caterpillar population was mostly destroyed. In late 1996 he noticed a reduction in the number of great tits in the wood but an increase in the number of young blackbirds. Explain this.



Q8. Below is an excerpt from a Year 12 student’s lab notebook during a visit to a tropical rainforest.

Date 14/05/2007 Location

Bukit Timah Nature Reserve

Feeding relationships observations

TRF TREES (many species) all eaten by a caterpillar in a silken tube called a BAGWORM. TERMITES eating tree branches. TREE SHREW (insectivorous mouse) eating termites and bagworms. PYTHON found dead. Gut contained bones from tree shrew and a young MONKEY skull. Monkeys seen eating fruits from trees. BULBUL (a type of tropical bird) seen eating bagworms, termites and PRAYING MANTIS (an insect predator). Praying mantis found with a bagworm in its jaws. WOLF SPIDER found in crevice on tree trunk. It had termites and bagworms stored in silk bundles.



a) Using only the information from the notebook, construct a well organized food web. Your diagram should be organized into trophic levels as far as possible. Label the web.

b) Suggest a likely feeding relationship in this web which the student did not see.



PYRAMIDS OF NUMBERS, BIOMASS AND PRODUCTIVITY.

Ecological pyramids represent numerical relationships between successive trophic levels in a community. Which pyramid is chosen for examination depends on the type and quantity of data collected. One way to simplify food webs is to assign the organisms to trophic levels and then count the number of organisms in each trophic level. e.g.

A pyramid of numbers

e.g.

represents the numbers of individual plants and animals present in the food web. It does not take into account the relative size of any organisms. Consequently, it is possible to have an inverted pyramid of numbers.

As with all pyramids of numbers, the numbers of individuals have been counted. The primary producer is one oak tree which can support and entire food chain containing many organisms. A pyramid of biomass takes body size into account. Such a pyramid for the oak tree is shown on the next page. This now seems to make sense but it requires much more data to construct i.e. you have to find the dry mass of the organisms in your sample and their energy content (this must be measured in a fixed area). However, it does seem to be logical i.e. big organisms support lots of little ones.



Above is the pyramid of biomass for the woodland food chain. Biomass is a measurement of body size (dry mass/g m-2). One oak tree is very large. Why does biomass decrease at each trophic level? Biomass decreases at each trophic level because the transfer of energy from the primary producer to the primary consumer and between the consumers is inefficient. Energy is lost at each stage, that is, at each of the transfers. e.g. green plant → zebra → lion • Zebras only eat a proportion of the plant i.e. roots are left • Zebras lose energy because they can not digest and break down all the material they

consume and so energy is lost in the faeces. • Most of the energy that is obtained from the grass is used for respiration to keep

the zebra alive or is lost as heat when energy is transformed, during respiration from one form to another. This energy cannot be used to create biomass.

• The lion will not always consume the entire zebra i.e. bones etc. Consequently, it may take 100kg of grass to make 10kg of zebra which supports 1kg of lion (Ref:- 2nd law of thermodynamics) The apparent rule of nature, that a large biomass supports a small biomass, is broken by the pyramid shown overleaf. Here a huge biomass of zooplankton seems to be supported by a much smaller biomass of phytoplankton. To produce a pyramid like this we would need to collect a sample of sea water and establish the standing crop of phytoplankton and zooplankton in it. But we would just be taking a snapshot, not taking into account how fast the standing crop of phytoplankton is producing more phytoplankton.



In reality, the phytoplankton is growing very fast and its productivity, or amount of energy it can pass on to the next trophic level, is very high, (analogy – mowing the lawn i.e. if we collected all the lawn clippings for a year, we would have a much clearer idea as to how productive the grass has been). Consequently, the zooplankton is not being supported by the standing crop of phytoplankton we measure at the moment when we take our single sample, but by their productivity over time. So what we need is a pyramid of productivity

.

Pyramids of productivity require much more data to construct since many samples, separated by intervals of time, are needed to estimate productivity, but they provide a coherent view of how an ecosystem works. Production by primary producers must be greater than production by secondary consumers and so on. We do not need to consider numbers or differences in size because all organisms can be converted to their energy equivalent.

A pyramid of productivity for a pond ecosystem is shown above. The energy of each trophic level has two parts i.e. NP (Net Production) and R (Respiration) and is measured in KJ m-2 yr-1.



How can pyramid structure affect the functioning of an ecosystem?

The pyramid pattern of energy flow in ecosystems has an important ramification – biological magnification, the tendency for toxic substances to increase in concentration in progressively higher levels of the food chain. Many chemical insecticides such as DDT resist degradation in the environment and tend to be stored in body fats. If farmers spray DDT on their cabbage plants to control cabbage looper caterpillars, some of the chemical inevitably runs off into streams and lakes. There, instead of breaking down, some of it may enter water plants later eaten by herbivorous fish. Fish and other animals cannot break down or excrete the toxin, and it is instead stored in their body fats. The magnification continues when carnivorous fish such as pike eat herbivorous fish, and when top carnivores such as ospreys devour pike. The concentration of DDT in the birds’ bodies can reach levels 10,000 times higher than in the water plants that originally took it up. This amount of DDT interferes with calcium metabolism and results in thin-shelled, easily broken eggs. During the two decades that DDT was commonly used in the USA, the numbers of ospreys, falcons, hawks, eagles, and condors dropped significantly as a result of a lethal chain of events. Although the use of DDT was banned in 1968, the chemicals are still widely used in overpopulated and developing nations to control mosquitoes that spread malaria, the greatest killer of humans, and they continue to magnify in the food chain as a consequence of ecological pyramids.

Fig 2.7 The bioaccumulation of DDT in the fatty tissue of organisms in an estuary. The harmful pesticide has become concentrated as it passed through the food chain. The tertiary consumers, the fish-eating birds, are most affected.



Case Study: DDT and the Bald Eagle

The Bald Eagle (Haliaeetus leucocephalus) is an American National Symbol, but although it is one of the most legally protected organisms in the country, it is on the danger list. A population of Bald Eagles in northwestern Ontario was studied from 1966 to 1981. The birds nest in a part of Canada which has suffered little exposure to DDT. However, they spend the winter in the United States, where they consume prey contaminated with DDT (DDE). During the sixteen year study, the average number of young produced was 0.82 per breeding area per year. The mean concentration of DDE in the birds eggs was 57 parts per million.

Fig 2.8 Summary of average annual bald eagle reproduction and DDE residues in addled eggs in north-western Ontario, 1966 to 1981. The dashed lines indicate weighted mean concentrations of DDE residues in clutches before (94 p.p.m.) and after (29 p.p.m.) the ban of DDT

Later studies showed that DDT (DDE) caused the eggs to have thinner shells. Summarise what the graph shows.



Q1. Construct a hypothesis to explain the decline and recovery of the Bald Eagle population in the area shown.

Q2. DDE in food webs does not break down. Suggest two reasons why the average DDE

concentration in Bald Eagle Eggs declined from 1974 to 1981. N.B. To illustrate how insecticides pass along and accumulate in the whole world ecosystem, quite high levels of DDT have been found in the fat of penguins at the South Pole!



INTERACTIONS BETWEEN SPECIES

POPULATION – a group of organisms of the same species living in the same area at the same time

, and which are capable of interbreeding.

Although food webs and ecological pyramids link all the individuals of an ecosystem, there are some important relationships that do not depend on feeding alone. You will study:- competition, parasitism, mutualism and feeding interactions (herbivory and predation).

Competition

COMPETITION – the interaction between two individuals who share the same limited resources in the same place at the same time

.

Q1. List some resources which may be competed for. Q2. Give two local examples of competition. Competition may be:- a) Intraspecific – between two members of the same species b) Interspecific – between members of two different species



Species in competition have overlapping niches. The competition will be harmful to their population. There is an advantage to avoiding competition. Often species evolve to partition resources rather than compete by sharing. This is called NICHE SEPARATION. e.g.

Fig 2.9 The niches of two morphologically similar birds, the shag and cormorant. Notice

how differences in diet and nesting habits means that the niches do not overlap. If two species are in competition they might:- a) Coexist – if their niches overlap less i.e. they do not share all the same

- if the environment keeps changing and favouring one species more than the other at different points in time.

Q3. Give one local example of two species in competition but coexisting. What allows

them to coexist? b) Exclude – if their niches overlap a lot i.e. they share almost or all the same resources. THE COMPETITIVE EXCLUSION PRINCIPLE – if two populations share the same limited resources at the same time in the same place they will not coexist. One will survive and one will die out. One will out-compete

the other.



e.g.

Fig 2.10 The effect of competition. Paramecium aurelia and P. caudatum eat the same food but P.aurelia can capture and ingest it faster than P.caudatum

Parasitism

PARASITISM – a relationship between two species in which one species (the parasite) lives in or on another (the host), gaining

all or much of its food from it.

The association between a parasite and its host can be of two types:-

a) Ectoparasites – a parasite which lives and feeds on the surface of its host e.g. a tick feeding on a deer.

b) Endoparasites – a parasite which lives inside its host e.g. a tapeworm living inside the human gut.

Q4. Give a local example of:-

a) an ectoparasite b) an endoparasite



Mutualism

MUTUALISM – a relationship between individuals of two or more species in which

all benefit and none suffer

Lichens are composed of two organisms i.e.

• a photosynthetic BLUE GREEN ALGAE • a FUNGUS

They are MUTUALISTS.

Q5. List the advantages to the fungus and the algae.



Q6. Describe another mutualistic relationship e.g. a coral polyp, and list the advantages to each organism.

Predation

PREDATION – The interaction between two consumers, the predator (higher trophic level) and the prey (lower trophic level). Predators differ from parasites in that they kill their prey before eating it. Q6. Give an example (local) of the predation of a primary consumer by a secondary

consumer. Q7. Give an example (local) of the predation of a secondary consumer by a tertiary

consumer. The predator population regulates the carrying capacity for the prey and in turn its own carrying capacity is regulated by the prey population. This means the two populations regulate each other. N.B. CARRYING CAPACITY

– the maximum number of a species or “load” that can be sustainably supported by a given environment.



e.g. Predator prey cycles.

Fig 2.11 The connection between the number of snowshoe hares and the number of lynx from 1845 to 1925 as shown by the number of pelts taken by the Hudson Bay Company

Q8. Explain the cycles that are shown above.

Hint:-

Q9. Predict the effect of removing the predator.



In reality, the cycles in the snowshoe hare continue (but are less obvious) even when the lynx are absent. Clearly there is some other factor contributing to these cycles. Q10. What other factor could create these cycles?

Herbivory

HERBIVORY – the interaction between a primary consumer (herbivore) and a producer. Sometimes called grazer interactions. Q11. Give two examples of herbivory. N.B. The Net Primary Productivity (NPP) of the producer (i.e. the amount of energy available to the next trophic level) regulates the carrying capacity for the primary consumer. The grazing of the primary consumer regulates the carrying capacity for the producer. This means the two populations regulate each other’s size. In a grassy wildlife park, three areas are isolated by circular fences. Inside one isolated area all deer are removed. Inside the other, deer are introduced to twice the population density of that in the rest of the park. The third area is left un-touched so deer are at the normal density.



Q12. Discuss the outcomes in each of the three fenced areas

a) short term b) long term

Q13. In the overstocked area, the deer may over-graze the grass and cause permanent

damage to the soil by soil erosion. What effect will this have on carrying capacity for:-

a) the grass b) the deer

N.B. Interactions within populations should be understood in terms of the influences each species has on the population dynamics of others, and upon the carrying capacity of the other’s environment.



THE ROLE OF PRODUCERS, CONSUMERS AND DECOMPOSERS IN THE ECOSYSTEM

PRODUCER

– an organism which synthesises organic molecules e.g. glucose from simple inorganic molecules e.g. carbon dioxide and water, using an external energy source (e.g. the Sun – photoautotrophs; inorganic compounds – chemoautotrophs). Examples – algae, moss, grass, shrubs, trees, some bacteria etc.

Producers are mainly green plants which use the sun’s energy during PHOTOSYNTHESIS to convert water and gaseous carbon dioxide into carbohydrates, releasing oxygen as a waste product. The producers then use these carbohydrates, plus minerals such as nitrates absorbed from the soil, to make all the other complex organic compounds they require. Plants are a source of food for other organisms in the ecosystem. CONSUMER

Examples - 1º = elephant, deer etc. 2º /3º = tiger, hawk etc.

– an organism which obtains its energy in the form of complex organic molecules by feeding on producers i.e. primary consumers or feeding on other consumers i.e. secondary and tertiary consumers.

Those primary consumers that only feed on plants are called HERBIVORES e.g. buffalo, and secondary, tertiary or higher order consumers that kill other organisms are called CARNIVORES e.g. wolf. Consumers that eat both plants and animals are called OMNIVORES e.g. bear, chimpanzee. When an animal such as a lion attacks and kills an antelope, the lion is said to be acting as a PREDATOR and the antelope that is killed is called the PREY. PARASITES are plants and animals that live on or in another organism, called the host, causing it harm but usually without killing it.



DECOMPOSER

Examples – bacteria and fungi.

– an organism which breaks down organic material into smaller molecules.

DETRITUS includes partly broken-down dead plant and animal material, such as fallen leaves and berries, animal faeces and dead animals. The organisms that feed on the dead material are called detritus feeders or DETRITIVORES. An earthworm is a detritivore. DECOMPOSERS such as bacteria or fungi normally digest every type of dead organism and their waste products. Bacteria and fungi that are involved in this breakdown are known as SAPROTROPHS. Decomposers play a vital down role in an ecosystem. By breaking down (rotting) the dead remains of other organisms, they release NUTRIENTS back into the soil. These nutrients can then be absorbed by plants, and so the nutrient cycle continues. The diagram below shows the flow of energy through a community. Only some of the energy stored in a trophic level goes to the trophic level above. The rest is respired and lost as heat or passes to the decomposers. This is why pyramids of productivity (energy) can never be inverted.



ENERGY TRANSFORMATIONS

PHOTOSYNTHESIS

Photosynthesis is a vital process to all life on Earth for the following reasons:-

• The Sun provides the ultimate source of energy for life on Earth, and by trapping it and using it to produce food, green plants form the start of most food chains.

• Photosynthesis also helps to regulate atmospheric carbon dioxide levels and therefore tropospheric (lowest level of atmosphere) temperatures.

• It provides the oxygen which is essential for aerobic organisms and for the formation of the protective ozone layer in the stratosphere (layer above troposphere).

Q1. Define photosynthesis. The basic process of photosynthesis can be summarised by the following equation:-

sunlight energy absorbed

carbon dioxide + water glucose + oxygen by chlorophyll



Carbon dioxide

The atmosphere contains approximately 0.035% of gaseous carbon dioxide. This is absorbed by plants and provides the carbon needed to make carbohydrates, polysaccharides, fats and proteins, which make up the body of the plant. The carbon dioxide diffuses into the leaves of the plant through microscopic pores called stomata.

Water

Water is essential for every chemical reaction in the plant, which all occur in solution. Water is absorbed into the very fine root hairs by osmosis. It is then transported through the plant in tiny tubes called xylem vessels. The xylem vessels extend all the way through the plant – from the roots, up the stem, and into the veins of the leaves where most photosynthesis takes place. Water is directly involved in the chemical reactions of photosynthesis but also allows plant cells to maintain their shape (turgidity).

Light and chlorophyll

The green pigment, chlorophyll, absorbs light from the visible part of the electromagnetic spectrum. The absorption spectrum describes the amount of absorption at different wavelengths. Chlorophyll is really a mixture of several different pigments. Each of these pigments absorbs slightly different wavelengths. The absorption spectrum shows that chlorophyll absorbs a lot of red and blue light but only a little green light. Most of the green light is reflected, which is why chlorophyll and the leaves containing chlorophyll appear green. Chlorophyll is contained in chloroplasts that are found within the cells of green plants and are in particular abundance in the upper tissue layers of the leaves. Q2. Visible light i.e. sunlight is a mixture of colours. We classify them into the seven

colours of the visible spectrum which are:-



(a) Absorption spectrum: measured using a spectrometer

(b) Action spectrum: a record of the amount of photosynthesis occurring at each wavelength

Fig 2.12 The absorption spectrum and the action spectrum of chlorophyll Q3. Green light is sometimes shone onto plants in displays to make them appear very

green. If this is continued for too long, the plant loses mass and eventually dies. Explain why.

Q4. What colour would a leaf appear in red light?



Q5.

Fig 2.13 The production of organic molecules from water and carbon dioxide using the energy of red and blue light

As you know, the carbohydrate produced is made of carbon, hydrogen and oxygen atoms. What is the origin of each atom? Q6. At the end of this process, where has the light energy been stored? Q7. What is the energy transformation that has occurred?



Q8. What effect will this process have on the biomass and energy content of the producer?

Q9. Using the diagram of leaf structure, outline the ways in which the structure and

arrangement of the leaf help to promote photosynthesis.



Monitoring photosynthesis

CO2 taken in removes CO2 from the solution and makes it less acidic and more alkaline. Hydrogencarbonate indicator solution around the plant turns from yellow to red to purple as the pH rises (more alkaline). This colour change indicates PHOTOSYNTHESIS has occurred

.

Oxygen bubbles given off from a submerged pond weed can be counted in a fixed period of time and a rate of oxygen production calculated as bubbles per hour. This is a measure of the RATE OF PHOTOSYNTHESIS.

Leaf discs can be floated on water and illuminated. If they are weighed before and after the experiment, the mass increase can be found. The rate of mass increase in a fixed period of time is a measure of the

RATE OF PHOTOSYNTHESIS.

This can also be done with whole leaves hung up or even whole potted plants.

The plants (e.g. grass) can be harvested and the biomass per m2

measured at time intervals. This can be done by cutting the same square metre or by cutting different square metres (samples).

The increase in biomass in a fixed period of time indicates the

RATE OF PHOTOSNTHESIS.

None of these methods are perfect. Some are indirect methods, most make assumptions and neglect other variables effecting the results obtained.



Q10. Discuss

these methods critically. Select the one which you feel is the best monitor of photosynthesis. Justify your selection.

Limiting factors

The rate of photosynthesis is affected by:- • Temperature • Light intensity and wavelength • Carbon dioxide concentration • Water and mineral availability • Pollution

Q11. Describe

the relationship between the rate of photosynthesis and:-

a) Light intensity b) Carbon dioxide concentration c) Temperature



Q12. Below is a table to show the effect of light intensity on the release of oxygen by elodea pondweed.

LIGHT INTENSITY (in kilolux) 5 15 25 40 55 60 65 70 75 85 Rate of photosynthesis (ml. O2 released per hour)

0.6 2.1 3.5 5.5 6.8 7.0 7.1 7.0 7.05 7.0

a) Draw a graph to display these results

Title - _________________________________

b) Interpret the graph. Describe

the relationship between light intensity and the rate of photosynthesis:-

• from 0 – 45 kilolux.





c) Explain

the relationship from 0 – 45 kilolux.

d) Explain

the relationship from 60 – 85 kilolux.

e) Predict

the rate of photosynthesis at 35 kilolux.

f) Predict

the effect on the rate of photosynthesis of:-

• an increase in light intensity from 10 – 29 kilolux.

• an increase in light intensity from 60 – 70 kilolux.



SUMMARY

The rate of photosynthesis is proportional to light intensity, carbon dioxide concentration and temperature when these are at lower levels. An increase in the amount will cause a rise in the rate of photosynthesis. At higher levels of each, the rate of photosynthesis remains constant if the amount of light, carbon dioxide or temperature is increased. However, if temperatures exceed the optimum for the photosynthetic enzymes to function, photosynthetic rate will start to decrease and eventually stop altogether. TASK – Extend the temperature graph to show this occurring.

THE LAW OF LIMITING FACTORS – the limiting factor of a chemical process is that factor which is nearest its minimum value.

Limiting factors may affect the rate individually or interact together. If we are to maximise photosynthesis and hence global food production, it is essential that we have some understanding of how and why these factors affect the rate of photosynthesis.



e.g.

LIMITING FACTOR FUNCTION / EFFECT RESPONSE

Water availability

Solvent for all chemical reactions; transport of minerals and sugars around the plant; support (turgidity); source of hydrogen atoms to ‘fix’ CO2 into carbohydrates.

Irrigate in arid areas; timed sprinklers in greenhouses; add organic matter to soil to increase its water-holding capacity; shelterbelts reduce wind speed so less water loss

Carbon dioxide concentration

Source of carbon for synthesis of carbohydrates, fats, protein etc.

Release more carbon dioxide in greenhouses to encourage growth e.g. by burning fuel.

Light intensity and wavelength

Provides energy for the chemical reactions of photosynthesis; splits water to provide hydrogen atoms to manufacture carbohydrates.

Artificial light may be provided if competition for light is severe or to control light/dark regimes; some trees removed to leave more light for the rest.

Temperature

Affects the rate of enzyme activity; controls the rate of photosynthesis and the speed of seed germination.

Greenhouse temperature may be thermostatically controlled; soil temperature may be increased by adding mulches.

Mineral availability For healthy growth Artificial fertilizers; liming to increase pH; plants legumes (see Nitrogen Cycle)

Pollution Many effects e.g. acid rain damaging leaves etc.

Pollution control strategies e.g. flue gas desulphurisation in power stations (see Pollution Management unit).

Pests Attack the plant causing damage/disease/reduced biomass.

Pest control measures e.g. pesticides/biological control/genetic modification etc.



RESPIRATION

RESPIRATION – the controlled breakdown of organic compounds e.g. carbohydrates, which releases energy and produces carbon dioxide and water OR

the controlled release of energy from food.

Respiration proceeds most efficiently when oxygen is available. This is known as AEROBIC RESPIRATION. Q1. State the general equation for aerobic respiration. ANAEROBIC RESPIRATION occurs when oxygen is not available. It releases a much smaller amount of energy. The remaining energy that is not released is retained in the molecules of lactic acid (lactate) or ethanol. i.e. glucose → lactate + energy (in animals) glucose → ethanol + carbon dioxide + energy (in waterlogged plants, yeast, fungi etc.) N.B.

The process of respiration occurs in ALL living cells of ALL organisms ALL the time!!!! Q2. What are the main differences between aerobic and anaerobic respiration?



Q3. Describe the energy transformation taking place during respiration. Q4. The energy released during the breakdown of complex organic molecules (food) is

used to do useful work or is “lost” as heat. What is meant by useful work?

Q5. Which process stored this energy in the organic molecules originally? Q6. The carbon dioxide released has come from the broken down carbohydrate

molecule. By doing this, respiration is performing the opposite operation to photosynthesis. Explain this idea using the diagram below.



THE ENERGY FLOWS (ENTERS AND LEAVES)

THE MATERIALS CYCLE (ARE REUSED)

SUMMARY

• Photosynthesis STORES energy and carbon dioxide within organic molecules (food). • Respiration RELEASES the energy and the carbon dioxide that was stored. • Eventually, once used, the energy is lost from the ecosystem as HEAT. • Energy FLOWS whereas materials CYCLE.



ENERGY FLOW AND THE ECOSYSTEM

It has been said that two great principles of ecology are the circulation of materials within an ecosystem and the flow of energy through it. These determine the number of organisms and the rate at which they live. However, the main difference between the two is that materials circulate, but energy does not; it FLOWS THROUGH a system. Once energy has been used by an organism it is lost from the system so a continuous renewal for energy supply is necessary. This is brought about by continuous inflow of sunlight. All life depends on this, either directly or indirectly because of the process of PHOTOSYNTHESIS. The effective running of an ecological system depends on the transfer of energy from organism to organism, via the FOOD CHAIN

. An overall impression of the energetics of an ecosystem can be obtained by studying the efficiency with which all the autotrophs convert solar energy into the chemical energy of the plant biomolecules and the efficiency with which it is utilised by heterotrophs. One species, if it is efficient, can actually facilitate energy flow through an ecosystem, so this too can be studied.

The sun powers most of the energy cycles on Earth.

INSOLATION is a term used to describe the amount and duration of incoming solar radiation. This radiation provides the energy for life on Earth and also powers the climatic systems. The sun emits solar radiation, much of which is short wave, and the cooler Earth emits mainly long wave radiation.



The fate of incoming solar radiation

The effects of the atmosphere on incoming and outgoing radiation are as follows:-

• Some of the incoming radiation is reflected back into space from clouds and the atmosphere.

• Some radiation is absorbed by the gases in the atmosphere. This warms the atmosphere.

• Some radiation is absorbed by the Earth’s surface. • As the Earth’s surface and atmosphere are warmed by the absorption of incoming

radiation, they begin to re-radiate heat, some of which escapes to space. • Some of the outgoing radiation emitted from the Earth’s surface is absorbed,

scattered or reflected by the clouds and gases in the atmosphere, delaying the release of energy. This trapping in of the heat in the atmosphere is called the greenhouse effect, and the gases which are responsible are called greenhouse gases.



Of the 45% of incoming solar radiation which reaches the Earth’s surface, less than 5% is available for photosynthesis and converted into the chemical energy of plant tissues. The diagram below shows the fate of solar energy once it has initially reached a plant, assuming 1000 units of energy.

Q1. Summarise what the diagram shows in terms of the eventual amount of energy

which is converted into plant biomass.



Q2. Calculate the net conversion efficiency i.e. the proportion of energy represented by growth in the plant.

Q3. What factors will tend to prevent a plant from achieving this theoretical maximum?

(Hint – think of the basic photosynthesis equation)



Energy flow diagrams

ENERGY FLOW – the passage of energy through the trophic levels in an ecosystem. The energy in sunlight is absorbed by the leaves of green plants. It is then transferred from one trophic level to the next. At each stage energy is lost as heat by respiration and egestion. Ultimately the rest of the energy is lost by decomposer respiration. i.e.

However, a more accurate method of drawing an energy flow diagram is shown below:-

* The diagram above shows the flow of energy through an ecosystem. The amount

of energy stored in each trophic level is approximately proportional to the area of the rectangles; the rate of flow from one store to another is approximately proportional to the width of the arrow linking them.



Below is an example of an energy flow diagram for a community in Florida.

Fig 2.18 The energy flow in the Silver Springs community, Florida. The units are kJ m-2 yr-1. Notice

how little of the sunlight that strikes the plants (insolation) ends up trapped in glucose Q1. Which process is represented by the arrows labelled, 4599, 193 and 45 kJ m-2 yr-1

on the diagram? Q2. What percentage of the available food is utilised by the 3rd consumer? Why is the

percentage so small?



Energy flow diagram for a grazing food chain.

Q3. Label the four trophic levels on the diagram above. Q4. The pasture (field) studies were 10,000m2. Calculate the amount of sunlight

energy which shone on the whole pasture for one year. Q3. Calculate the percentage of incident (hitting the surface) solar energy which is

captured into gross primary production (GPP).

Fig 2.19 Energy flow through a grazing food chain, such as a grazed pasture. Figures represent kJ m-2 yr-1



This photosynthetic efficiency would vary for different ecosystems. Q4. State the differences you would expect in photosynthesis efficiency for tropical

rainforest, temperate forest, desert and polar ecosystems. Explain these differences.

Q5. Calculate the net primary production and add it to the diagram. Q6. Calculate the energy lost in excretion and egestion by the primary consumers. Q7. How else is energy “lost” from the food chain?



Q8. Using the information in the diagram, draw a scale diagram of the productivity pyramid for this grazing food chain on a separate sheet of graph paper.

THE CYCLING OF MATERIALS

ENERGY FLOWS BUT MATERIALS (NUTRIENTS) MUST CYCLE



Biochemical cycles

All chemical elements (e.g. carbon) in living things are part of cycles which involve the land, the water and the atmosphere. These cycles involve the movement of elements from INORGANIC “POOLS” (e.g. atmosphere, rocks etc.) into COMPLEX ORGANIC MOLECULES by assimilation. These move through the living world via FOOD CHAINS and are eventually BROKEN DOWN back into simpler inorganic forms again by SAPROTROPHS (decomposers). The element has undergone a cycle. e.g.

Fig 2.20 A generalised nutrient cycle. Transfers with the atmosphere have not been included. There are many important cycles to consider but you only need to understand the transfers and transformations of carbon, nitrogen and water

as they cycle within an ecosystem.



The Carbon Cycle

Left to itself, the cycle is self regulating – the oceans act as massive reservoirs of carbon dioxide, absorbing excess when it is produced and releasing it when it is in short supply. However, the enormous increase in the production of carbon dioxide by people is now threatening the balance of the carbon cycle, and the results could be disastrous for many forms of life. Q1. Label fossilisation and combustion on the cycle diagram. Q2. Indicate the position of saprotrophs on the cycle diagram. Q3. Identify:-

• The inorganic pool

• The process assimilating the inorganic carbon into organic compounds

• Three processes recycling the carbon back into the inorganic pool



Atmospheric carbon

– the troposphere contains approximately 0.035% gaseous carbon dioxide. Carbon dioxide is removed from the atmosphere by photosynthesis and is added when plants, animals and microorganisms respire or when carbon-containing material such as coal or wood is burned.

Organic compounds in plants

– plants use carbon in carbon dioxide to produce organic substances such as carbohydrates, fats, proteins, cellulose and chlorophyll.

Organic compounds in consumers

– through eating plants, primary consumers obtain their organic compounds, the energy from the release of which can be passed on through the trophic levels.

Carbon in freshwater and marine ecosystems

– carbon dioxide dissolves in water and large quantities of carbon are also trapped in the form of carbonates in sea-bed sediments, which over millions of years will form sedimentary rock. In some coastal waters in the tropics, coral reefs lock up carbon as calcium carbonate. Areas where carbon is locked up e.g. in the oceans, tree stems etc., are known as CARBON SINKS.

Human effect on the carbon cycle

– the combustion of fossil fuels and the destruction of important carbon sinks e.g. rainforests, have led to an increase in atmospheric carbon dioxide.



The Water Cycle

Q1. What is precipitation? Q2. Identify three processes which involve evaporation. Q3. Label condensation on the diagram. Q4. What would be the effect of deforestation on this cycle?



The Nitrogen Cycle

Nitrogen gas from the atmosphere cannot be used directly by green plants. It must be in the form of NITRATES (NO3

-) or AMMONIUM (NH4+).

The process of converting nitrogen gas into forms useful to the living world is called NITROGEN FIXATION.

Nitrogen fixation

Nitrogen gas can be fixed into ammonium ions by:- • Lightning • Industrial processes to make fertilisers e.g. The Haber Process. • Microbes

There are two main types of microbial nitrogen-fixers:-

1. Blue green algae (Cyanobacteria) 2. Nitrogen-fixing bacteria – these may be either free living in the soil e.g.

Azotobacter on live in root swellings (nodules) e.g. Rhizobium. They use energy and an enzyme called nitrogenase to convert nitrogen to ammonium ions.



Q1. Describe the relationship between Rhizobium and the leguminous plants. Q2. Identify the nitrogen-fixation processes on the diagram by shading the areas a

certain colour (create a key at the side of the diagram as you go through the tasks).

Nitrification

The process of oxidizing ammonium ions into nitrate ions is called NITRIFICATION. It adds nitrates to the soils. The process is carried out by NITRIFYING BACTERIA. There are two types as shown below:- Nitrosomonas Nitrobacter NH4

+ NO2- NO3

- (Ammonium) Oxygen (Nitrite) Oxygen (Nitrate) Q3. Identify nitrification on the cycle and shade the arrows a certain colour. Label

Nitrosomonas and Nitrobacter on the diagram. Q4. What conditions might inhibit nitrification?

Absorption, Assimilation and Recycling

Nitrates are very soluble. Plant roots absorb them, convert them into amino groups (NH2) and add them to carbohydrates to make amino acids. These are made into plant proteins. They may be eaten by herbivores and so pass along the food chain. Some of the nitrogen will be excreted as nitrogenous waste such as urea. Some will return to the soil as dead organic matter or detritus. Both these will be recycled in the soil. Q5. Shade the arrows representing absorption, food chain and return to the soil in

different colours.



Ammonification

Saprotrophic bacteria and fungi decompose the detritus and nitrogenous waste using protease enzymes. They remove the amino group (NH2) from their excess amino acids (deamination) and so release ammonia (NH3) and ammonium (NH4+) back to the soil. Q6. Identify, shade and label the arrow for ammonification on the nitrogen cycle

diagram. Q7. What now happens to the ammonium in well aerated soils?

Denitrification

This process removes nitrate from the soil. In poorly aerated soils (with a lack of oxygen), denitrifying bacteria such as Pseudomonas denitrificans obtain oxygen by breaking down nitrates. This releases nitrogen gas. Q8. Identify, shade and label denitrification and Pseudomonas denitrificans on the

nitrogen cycle diagram. Q9. Farmers and gardeners increase the nitrogen fertility of the soil by:-

• Adding fertiliser • Ploughing / digging • Using legumes in crop rotation

Discuss the purpose of each strategy.



Nitrifying bacteria as chemoautotrophs

A chemoautotroph is an organism which uses the oxidation of inorganic substances as a direct energy source to synthesise ATP. They do not use light. Nitrosomonas and Nitrobacter are always the bacteria which act as chemoautotrophs. Q10. Which inorganic substance does each of these oxidise?

SUMMARY OF ENERGY FLOW AND NUTRIENT CYCLING

Summary of energy flow (white arrows) and nutrient cycling (black arrows) in an ecosystem (R = Respiration). Energy from the sun is progressively lost as heat. Nutrients are eventually recycled thanks to the decomposers.



PRODUCTIVITY

Productivity is basically production per unit time

but this can be subdivided into many different types for which you must learn and understand the definitions and be able to use the equations to make simple calculations.

GROSS PRIMARY PRODUCTIVITY (GPP) – the quantity of organic matter produced, or solar energy fixed, by photosynthesis in green plants per unit area per unit time. However, not all of this energy is available for the animals that eat the plants, the herbivores, because some of the energy is used by the plant itself in staying alive. The energy lost in this way is called the RESPIRATORY LOSS. The amount of energy left after respiration is called the NET PRIMARY PRODUCTIVITY (NPP). A simple equation can be used to show this relationship:- GROSS PRIMARY PRODUCTIVITY (GPP) – RESPIRATION (R) = NET PRIMARY PRODUCTION (NPP) (solar energy assimilated) (chemical energy of plant growth)

Herbivores only assimilate about 10% of their food intake, so up to 90% is lost as faeces.

Fig 2.14 The fate of energy in the grass consumed by a cow. Some of the energy consumed is lost in urine,

faeces and respiratory heat. The remaining energy is termed the production of the trophic level.



Carnivores assimilate 30 – 75% of the food eaten.

N.B. Food eaten = energy source We can write an equation which accounts for the fate of all the energy which is consumed by heterotrophs:-

Energy =

Energy used +

Secondary +

Energy lost +

Energy lost Consumed in respiration Production in urine in faeces

C = R + P + U + F

Rather than just looking at the fate of the energy entering one consumer, we can look at the fate of all energy entering a trophic level. The equation above would still apply. All the energy entering a trophic level (C) must be accounted for in terms of R, P, F and U.

Fig 2.15 The fate of the food eaten by a consumer - a lion is shown but the situation is similar in any herbivore, omnivore or carnivore. Much of the food consumed is assimilated by the lion, but only a small proportion is used to produce new lion tissue. Some of the food is never assimilated at all and is simply lost as faeces – this proportion will be significantly larger in a plant-eater than in a carnivore.



i.e.

C = energy entering the trophic level (consumed) R = respiratory heat loss F = energy lost in faeces U = energy lost in urine (usually very small) P = production of the trophic level Fig 2.16 The pattern of energy flow through a trophic level.

N.B. At each stage of the food chain, energy is lost. The nearer the organism is to the beginning of a food chain, the greater the available food energy. From producer to primary consumer, the energy is reduced about 100 times. From each stage thereafter, it is reduced by approximately 10 times. Each energy transfer in a system conforms to the two Laws of Thermodynamics

:-

• Energy may be transformed from one type to another, but it cannot be created or destroyed. In a food chain, energy is converted from light energy to chemical energy.

• No process involving an energy transfer will occur unless there is a degradation of

energy from a non-random form e.g. light to a random form e.g. heat. The food chain energy transformation always involves dispersal of energy as unavailable heat.



Define the following terms

:-

• Gross productivity

• Net productivity

• Primary productivity

• Secondary productivity

• Gross primary productivity (GPP)

• Net primary productivity (NPP)



Q4. Modern beef production – the values concern bullocks (young castrated males) grazing on specially maintained rye grass / clover pasture.

A typical square metre of pasture in Britain receives about 1046700 kJ of visible light energy from the sun each year. This energy can be accounted for as follows:- Energy reflected by leaves 165,002kJ Energy leaving the plant as a result of the evaporation of water 523,350kJ Energy transmitted to the ground 334,944kJ Energy locked up in new growth of plants 21,436kJ Energy transferred during respiration of plants 1,968kJ

• How much energy is trapped by the plants in photosynthesis? This is Gross Primary Production

• What is the value of net primary production over the year, expressed as a % of total visible light energy?

Now look carefully at the diagram below.



• What proportion

of the net primary production was eaten by the bullocks? Why is it so low?

• How much energy was actually assimilated by the bullocks? (called Gross Secondary Production - GSP).

• What percentage

of the energy assimilated by the bullocks was converted into new growth? (Net Secondary Production - NSP).

• Calculate the conversion efficiency of the bullocks. The conversion energy of a population will depend on:-

• The proportion of available energy taken in. • The proportion of that taken in which is assimilated. • The proportion of that assimilated which is used for growth.



The conversion efficiency of the bullocks will depend upon:- • The proportion of plant mass ingested (other herbivores? roots?) • The proportion absorbed by the gut wall (egestion? undigestible parts?) • How much is used in respiration rather than stored as new growth (used to keep warm,

move etc.) Q5. The energy within each trophic level reduces along the food chain. The energy lost

from the chain is released as heat eventually. Explain the causes of this reduction in energy within trophic levels. Why can’t there be as much secondary consumer as primary consumer? Discuss this.



Example questions



2.



KEY WORDS

TERM DEFINITION

ABIOTIC FACTOR A non-living, physical factor that may influence an organism or ecosystem, e.g. temperature, sunlight, pH, salinity, precipitation etc.

AUTOTROPH An organism that can synthesise organic molecules (food) from simple inorganic molecules using an external energy source e.g. the sun. All producers are autotrophs.

BIOMAGNIFICATION The increasing accumulation of non-biodegradable substances up through the food chain.

BIODIVERSITY The amount of biological or living diversity per unit area. It includes the concepts of species diversity, habitat diversity and genetic diversity.

BIOMASS

The mass of organic material in organisms or ecosystems, usually per unit area. Sometimes the term “dry weight biomass” is used where mass is measured after the removal of water. Water is not organic material and inorganic material is usually relatively insignificant in terms of mass.

BIOSPHERE

That part of the Earth inhabited by organisms, i.e. the narrow zone (a few kilometres in thickness) in which plants and animals exist. It extends from the upper part of the atmosphere (where birds, insects and wind-blown pollen may be found) down to the deepest part of the Earth’s crust to which living organisms venture.

BIOTIC FACTOR A living, biological factor that may influence an organism or ecosystem, e.g. predation, parasitism, disease, competition.

CARBON SINK Part of the carbon cycle where large quantities of carbon accumulate e.g. sedimentary rocks, forests etc.



CARRYING CAPACITY The maximum number of a species or “load” that can be sustainably supported by a given environment.

COMMUNITY A group of populations living and interacting with each other in a common habitat.

COMPETITION A common demand by two or more organisms upon a limited supply of a resource (e.g. food, water, light, space, mates, nesting sites). It may be intraspecific or interspecific.

DECOMPOSITION The degradation of organic material into smaller, soluble molecules by fungi and bacteria.

DETRITIVORE

The organisms in a food chain which consume dung, debris and other organic litter e.g. worms, consequently increasing the surface area and hence allowing for faster decomposition.

ECOSYSTEM A community of interdependent organisms and the physical environment they inhabit.

ENVIRONMENT An organism’s physical and biological surroundings.

FEEDBACK, NEGATIVE Feedback that tends to damp down, neutralise or counteract any deviation from an equilibrium, and promotes stability.

FEEDBACK, POSITIVE Feedback that amplifies or increases changes; it leads to exponential deviation away from an equilibrium.

FOOD CHAIN The transfer of energy and matter in a sequence of trophic levels in which organisms of a lower trophic level become the food of organisms in a higher level.

FOOD WEB The interconnection of organisms within several food chains.

GROSS PRIMARY PRODUCTIVITY (GPP)

The quantity f organic matter produced, or solar energy fixed, by photosynthesis in green plants per unit area per unit time.



HABITAT The environment in which a species normally lives.

HERBIVORY The interaction between a primary consumer (herbivore) and a producer. Sometimes called grazer interactions.

HETEROTROPH

All organisms which cannot synthesise their own food and consequently rely on food provided from a lower trophic level to survive. All animals are heterotrophs (often called consumers.)

INSOLATION The amount and duration of incoming solar radiation.

LEGUMINOUS PLANTS (LEGUMES)

Plants which have root nodules which contain nitrogen-fixing bacteria as part of a mutualistic relationship. The bacteria gain a habitat and food and he plants gain an important source of nitrogen.

LIMITING FACTORS Various factors which limit the distribution or numbers of an organism.

MUTUALISM A relationship between individuals of two or more species in which all benefit and none suffer.

NET PRIMARY PRODUCTIVITY (NPP)

The gain by producers in energy or biomass per unit area per unit time remaining after allowing for respiratory loss (R). This is potentially available to the primary consumers in an ecosystem.

NICHE A species’ share of a habitat and the resources in it. An organism’s ecological niche depends not only on where it lives but on what it does.

NITRIFICATION The conversion ammonium ions (NH4

+) to nitrite (NO2

-) then nitrate (NO3-) by aerobic bacteria

as part of the nitrogen cycle.



NITROGEN-FIXATION The conversion of atmospheric nitrogen into ammonium ions by free-living bacteria, or bacteria within the roots of leguminous plants.

PARASITISM A relationship between two species in which one species (the parasite) lives in or on another (the host), gaining all or much (in the case of a partial parasite) of its food from it.

PHOTOSYNTHESIS The process by which autotrophs (plants) make their own food by converting light energy into chemical energy.

POPULATION A group of organisms of the same species living, in the same area at the same time, and which are capable of interbreeding.

RESOURCE PARTITIONING The division of environmental resources between similar species which appear to share overlapping niches.

RESPIRATION The breakdown of food to release energy.

SAPROPHYTE/SAPROTROPH An organism e.g. fungi which uses a method of nutrition in which it feeds by digesting dead organic matter before absorbing it.

SECONDARY PRODUCTIVITY

The biomass gained by heterotrophic organisms, through feeding and absorption, measured in units of mass or energy per unit area per unit time.

SOIL The loose aggregate of mineral and other particles that covers the land, and in which terrestrial plants generally grow.

SPECIES A group of organisms that interbreed and produce fertile offspring.



SYSTEM An assemblage of parts and the relationships between them, which together constitute an entity or whole.

TROPHIC LEVEL The position that an organism occupies in a food chain, or a group of organisms in a community that occupy the same position in food chains.

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