Ecology, Section 8

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Ecosystems, Definitions

Habitat - the places where an organism lives Eg, a rocky shore or a field.

Population - All the organisms of one species in a habitat.

Community - All the different species in a habitat.

Ecosystem - All the organisms living in a particular area and all the non-living (abiotic) conditions, Eg, temperature, climate, soil-type.

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Estimating population size

How to estimate population size...

Use a quadrat, frame enclosing a known area, eg 1m2.

How to use a quadrat...

1) Place a 1m2 quadrat on the ground at a random point within the area you're investigating. The areas you choose to sample must be selected randomly to avoid bias.

2) Count all the organisms with in the quadrat.

3) Multiply the number of organisms by the total areas (in M2) of the habitat.

4) Then do this again in another are and compare the population sizes.

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Important things...

1)  The sample may not be representative of the populationI I.e, what you find in your particular sample might be different from what you'd have found if you'd looked at other areas of your chosen habitats.

2) The sample size effects the accuracy of the estimate - the bigger your sample, the more accurate your estimate of the total population is likely to be. So its better to use the quadrat at several points, get an average value for the number of organisms in a 1M2 quadrat, then multiply that by the total area.

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Distribution of organisms

You can use quadrats to help find out how organisms (like plants) are distributed across their habitat. - Eg how species change from a hedge towards the center of a field.  The quadrats are laid out along the line, called a transect. Transects can be used in any ecosystem.

What to do:

1) Mark out a line in the area you want to study, eg from the hedge to the middle of the field.

2) The collect the data along the like using quadrats placed next to eachother.

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Food chains

1) Food  chains always start with the producers, eg. a plant. Producers make their own food using energy from the sun.

2) Producers are eaten by primary consumers. Primary consumers are eaten by secondary consumers who are then eaten by tertiary consumers.

3) Eventually when all these animals die, their remainings are eaten by decomposers, eg. bacteria. Decomposers break down dead materials and waste.

4) Each stage (eg produvers, primary consumers) is called a trophic level.

Eg 5000 Dandlions....feed...100 rabbits....which feed...1 fox.

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Pyraimids of numbers

1) Each bar on a pyramid of numbers show the number of organisms at that stage of the food chain.

2) So the dandelions bar on a pyramid (where the anmount of dandelions is 5000 and the amount of rabbits is 100) would be longer than the rabbits bar.

3) the bottom line of the pyramid must be the the organism at the bottom of the food chain. In this case this is the dandelions)

4) In a typical pyramid of numbers, everytime you go up a trophic level, the number of organisms goes down. This is becuase it takes a lot of food from the level below to keep one animal alive.

5) There are cases where a number pyramid is not a pyramid at all. For example 1 fox may feed 500 fleas.

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Pyramid of biomass

1) Each bar on a pyramid of biomass shows the mass of living material at that stage oof the food chain - basically how much all the organisms at each level 'weigh' if you put them all together.

2) So the one fox would have a big biomass and the hundreds of fleas would have a very small biomass. Biomass pyramids are practically always the right shape.

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Pyramids of energy

1) Pyramids of energy show the energy transferred to each trophic level in a food chain. E.g. when a rabbit eats a dandelion it gets energy, which the danelions got from the sun.

2) Pyramids of energy transfer are always the right shape - a nice, regular pyramid.

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Energy transfer and food webs

1) Energy from the sun is the source of energy for nearly all life on earth.

2) Plants use light energy from the sun to make food during photosynthesis. This energy then works its wa through the food chain as animals eat the plants and eachother.

3) Not all the energy that's available to the organisms in a trophic level is passed on to the next trophic level  - around 90% of the energy is lost in various ways.

4) Some parts of food, eg roots or bones, aren't eaten by organisms so the energy isn't taken in. Some parts of food are indigestible (fibre) so pass through organisms and come out as waste eg. faeces.

5) A lot of the energy that does get taken in is used for staying alive, ie respiration, which powers all life processes.

6) Most of this energy is eventually lost to the surroundings as heat.

7) This is the energy that's transferred from one trophic level to the next.

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Food chains

1)  There are many different species within an environment - which means lots of different possible food chains.

2) All the species in a food web are interdependant, which means if one species changes, it affects all the others. For example if the grass hoppers were to suddenly die, then there is less food for the snakes, therefore their numbers might decrease.


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The water cycle means water is endlessly recycled.

1) Heat from the sun makes water evaporate form the land and sea turning it in to water vapour. Water also evaporates from plants, this is known as transportation.

2) The warm water vapou is carried upwards (as trhe warm air rises) When it gets highe up it cools and condenses to form couds.

3) Water falls from the clouds as precipitation (usually rain but sometimes snow or hail.) and is returned to the land and sea.

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water cycle picture...


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The carbon cycle shows how carbon is recycled...

1) The whole thing is powered by photosynthesis. Green plants use the carbon from CO2 in the air to make carbohydrates, fats and protein.

2) Eating passes the carbon compounds in the plant along to animals in a food chain or web.

3) Both plant and animal respiration while the organisms are alive it releases CO2 back in to the air.

4) Plants and animals eventually die and decompose or are killed and turned in to useful products.

5) When plants and animals decompose they're broken down by bacteria and fungi. These decomposers release CO2 back in to the air by respiration, as they break down the material.

6) Some useful plant and animal products, EG wood and fossil fuels, are burned (conbustion). This also releases CO2 back in to the air.

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Nitrogen is also recycled...

1) The atmosphere contains aboutt 78% nitrogen gas, N2. This is very unreactive and so it can't be used directly by plants or animals.

2) Nitrogen is needed for making proteins for growth, so living organisms have to get it somehow.

3) Plants get their nitrogen from the soil, so nitrogen in the air has to be turned in to nitrogen compounds (such as nitrates) before plants can use it. Animald canonly get proteins by eating plants (or each other.)

4) Nitrogen fixation isn't an obsession with nitrogen - it's process of turning N2 from the air into nirtogen compounds in the soil which plants can use. There are  two main ways that this happens:

A) Lighting - there's so much energy in a bolt of lightening that it's enough to make nitrogen react with oxygen in the airto give nitrates.

B) Nitrogen fixing bacteria in roots and soil.  CONTINUES....

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There are 4 different types of bacteria involved in the nitrogen cycle:

A) Decomposers - break down proteins (in rotting plants and animals) and urea (in animal waste) and turn them in to ammonia (a nitrogen compound.

B) Nitrifying bactria - turn ammonia in decaying matter in decaying matter in to nitrates.

C) Nitrogen fixing bacteria - turn atmospheric N2, in to nitrogen compounds that plants can use.

D) Denitrifying bacteria - turn nitrates back in to N2 gas. This is of no benefit to living organisms.

Some of these bacteria live in the soil and some of them live in nodules on plant roots.

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Air pollution

Carbon monoxide is poisonous...

1) When fossil fuels are burnt with out enough air supply they produce the gas carbon monoxide (CO)

2) It's a poisonous gas. If it combines with red blood cells, it prevents them from carrying oxygen.

3) Carbon monoxide's mostly released in car emmisions. Most modern cars are fitted with catalytic converters that turn the carbon monoxide in to carbon dioxide, decreasing the amount of CO that's released into the atmosphere.

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Acid rain is caused by sulfur dioxide...

1) Burning fossil fuels releases harmful hases like CO2 (a greenhouse gas) and sulfur dioxide (SO2)

2) The sulfur dioxide comes from sulfur impurities in th fossil fuels.

3) When this has mixes with rain clouds it forms diltue sulfuric acid.

4) This then falls as acid rain.

5) Iternal combustion engines in cars and power stations are the main causes of acid rain.

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Acid rain kills fish and trees...

1) Acid rain can cause a lake to become more acidic. This has a severe effect on the lake's ecosystem. Many organisms are sensitive to changes in pH and can't survive in more acidic conditions. Many plants and animals die.

2) Acid rain can kill trees. The acid damages leaves and releases toxic substances from the soil, making it hard for the trees to take up nutrients.

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Greenhouse gases trap heat from the sun...

1) The temperature of the earth is a balance between the heat it gets from the sun and the heat it radiates back out into space.

2)Gases in the atmosphere absorb most of the heat would normally be radiated out in to space, and re-radiate it in all directions, (including back towards the earth)

3) If this didn't happen then at night there'd ne nothing to keep any heat in and we'd quickly get very cold.

4) There are several different gases in the atmosphere that help kee heat in. They're called 'Green house gases' they include water vapour, carbon dioxide and methane.

5) Human being are increasing the amount of carbon dioxide in the atmosphere. We are also increasing levels of other gases that can act as green house gases, eg. CFCS and nitrous oxide.This has enhanced the green house gas effect.

6) As a result of all of this, the earth is heating up - this is global warming. Global Warming is a type of climate and causes other types of climate change, Eg changing rainfall patterns. Climate change could lead to things like changing crop growth patterns or flooding due to the polar caps melting.

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Human activity produces lots of greenhouse gases..

1)Humans are constantly releasing greenhouse gases everyday, in car exhausts, industrial processes, as we burn fossil fuels.

2) People around the world are also cutting down trees (deforestation) for timber and to clear land for farming - this activity effects the level of carbon dioxide in the atmosphere.

1) Methane gas is also produced naturally from various sources eg rotting plants in marshalnd.

2) However two man made sources of methane are ont he increase: rice growing and cattle rearing - its the cows "pumping" that is the problem.

1) nitrous oxide is released naturally by bacteria in soils and the ocean,.

2) a lot more is released from soils after fertiliser is used. Its also released from vehicle engines &industry. 

1) CFCS are man made chemicals that were once used in deodorant and also fridges. They are very powerful greenhouse gases. Most countries have agreed not to use them due to the damage they do to the ozone layer.which prevents UV radiation from reaching the earth.

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Water pollution

1) Nitrates and phosphates are put on to fields as mineral fertilisers.

2) If too much fertiliser is applied and it rains afterwards, nitrates are easily leached (washed through soil) in to rivers and lakes. The result is euthrophication which causes problems with lakes and rivers:

A) first fertilisers eneter the water adding extra nutrients (nitrates and phosphates)

B) the extra nutgrients cause algae to grow fast and block out the light.

C) Plants can't photosynthesise due to the lack of light and they begin to die.

D) With more food available mocroorganisms that feed on dead plants increase in number and deplete (use up) all the oxygen in the water.

E) Organisms that need that oxygen (e.g. Fish) die.

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another cause of eutrophication...

Another cause of eutrophication is pollution by sewage. Sewage contains lots of phosphates from detergents, Eg. washing powder. It also contains nitrates from urine and faeces.

these extra nutrients cause euthrophication in the same way that fertilisers do.

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Deforestation affects the soil, water&carbon cycle

Leaching - trees take up the nutrients from the soil before they can be washed away (leached) by rain, but return them to the soil before leaves die. When trees are removed nutrients gets leached away, but dont get replaced, leaving infertile soil.

Soil erosion - Tree roots hold the soil together, when trees are removed, soil can be washed away by the rain  (eroded) leaving intertile ground.

Disturbing the water cycle - trees stop rainwater reaching rivers too quickly, when they're cut down rainwater will run straight in to rivers - this can lead to flooding.Transpiration from trees releases some of the rainwater back in to the atmosphere. When they're cut down this can make the local climate drier.

Disturbing the balance of carbon dioxide and oxygen - forests take up CO2 by photosynthesis, store it in wood and slowly release it when they decompose (microorganisms feeding on bits of dead wood release CO2 as a waste product of respiration.) When trees are cut down and burnt the stored carbon dioxide is released at onoce as CO2. This contributes to global warming. Fewer trees in the forest also means that less photosynthesis takes place, releasing less oxygen. This causes the oxygen level in the atmosphere to drop.

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