Problems with population size

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  • Created by: ava.scott
  • Created on: 19-10-14 21:21


Population- a group of organisms of a single species interbreeding and occupying a particular area.

Birth rate- the reproductive capacity of the population.

Immigration- the movement of individuals into a population.

The growth of a population is determined by the birth rate, death rate, immigration and emmigration.

Ecosystems are dynamic and subject to change.

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Bacteria pattern of growth

Bacteria pattern of growth (over 4 days):

  • The lag phase: Lasts a few minutes to a few days, period of adaptation and preparation for growth, with intense metabolic activity  amd notable enzyme synthesis. Slow growth, as individuals are few at the start.
  • The exponential phase: A phase with no limiting factor, and population increases hugely. Bacteria, splitting by binary fission, cause the population to double per unit of time, causing the cell population to increase geometrically. This cannot be maintained. Cell production is greater than death rate. NOT JUST GROWTH.
  • The stationary phase: Cell production rate is equal to that of death rate, and factors limit population growth. Population has reached its maximum size known as the 'carrying capacity' for the particular environment- the limit to the number of indiviuals the area can support.
  • The death phase: The death rate is greater than the cell production rate; key reasons: nutrients used up; toxins waste build up; space becomes limiting; disease may occur as result of overcrowding. Toxins = ethanol in yeast cell

This S-shape is typical of any organism in a new environment. A slow adaption phase,a period of rapid growth, carrying capacity and then death phase.

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Limiting factors of growth

This can also be called environmental resistance. This includes available food, disease, overcrowding and toxins. In unartificial settings, biotic factors can include predation, parasitism, competition and disease. Abiotic factors are temperature, pH  and light.

Biotic= factor which makes up the living part of the environment of an organism.

Abiotic- non living part of the environment of an organism.


There are factors thats slow down growth, and those which cause a population crash.

  • Some factors are density dependent- their impact increases as population increases. Examples are toxins, disease, parasitism and food depletion. Resources provided by the environment are density dependent.
  • Others are density independent; all individuals are affected no matter how many there are. This is usally a violent change is a abiotic factor e.g. fire, flood.


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Generally, size is determined by birth and death rates. Fluctuation occurs, though not usually dramatically.

  • Most popualtions reach a equilibrium (set point), and if it rises above this a density dependent factor will kill individuals off to reach the set point again.
  • If it goes below this set point, the environmental resitance is relieved, and the population can grow again.

This fluctuation can be on a regular basis, and the result of weathering patterns. They are determined by a process called negative feedback.

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Plants compete for water, light and nutrients; animals compete for food, shelter, space and reproductive partners.

  • Intraspecific competition is between individuals of the same species, and is density dependent. Individuals tend to produce more offspring than the environment can support, so the best adapted survive.
  • Interspecific competion is between different species. Each species has a specific niche in a place and also within the ecosystem. No two species can occupy the same niche in a specific habitat.
  • Niches: the ecological role and space that an organism fills in an ecosystem.

Carrying capacity: the maximum population a habitat can sustain.

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Pests: any organism that competes with or adversely affects a population of organisms that are of economic importance.

Pests cause reduction in yield and massive economic loss for farmers by:

  • feeding on crops and animals.
  • competing with crop organisms for resources.
  • directly causing disease.
  • making infection by pathogens more likely.
  • spoiling food during transportation or storage.

Money is invested is reducing pests and their impact, either through biological or chemical means.

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Chemical control

Pesticides: poisonous chemicals used to control organisms considered harmful to agriculture or organisms involved in disease transmisssion.

Insecticides should be:

  • specific
  • non-persistent
  • not accumlate
  • NOT passed along food chains

The overuse of pesticides such as DDT have lead to environmental problems and growing resistance within pest populations.

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Pros and Cons of Chemical Control


  • Very effective
  • quick and inexpensive
  • can be used on a small scale
  • no skill required


  • Chemicals are non-specific and can eradiate useful insects e.g. bees are a biological control agent, and their removal can lead to an increase in pests.
  • Resistance can build
  • pesticides can kill fish, birds and mammals by food contamination
  • long term exposure to pesticides can cause health problmes in humans- sheepdipping chemicals are carcinogenic. 


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The population of a pest and their natural enemy fluctuates naturally until the addition of a pesticide. The pest and their natural enemy both decrease in population, until pest resurgance (because of relief from resistance.) The natural enemy remains decreased allowing the pest to suddenly grow and thrive.

Its in this way that pesticides actually increases pest population.

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Biological Control

Biological control- controlling a pest using organisms that are predators or parasites of the pest organism.

  • A specific type of interspecific competiton involving predator-prey relationships. This method causes huge fluctuations in population of both species.
  • Predator need their prey, and without them they die. Therefore levels of both species is controlled by negative feedback (when there is a change in a monitored variable a response is triggered to counteract the fluctuation.)

Early Success:Introduction of prickly pear cactus into N.America and Australia, was completely unthreatened and spread hugely, becoming a pest. A burrowing moth was introduced 25 years later, and the caterpillars stropped the spread of the cactus, reclaiming farmland.

This use of natural enemies is used commonly now. The agent is deployed against the target, to keep levels of the pest artifically low by increasing numbers of the agent. Insect parasites and microorganisms are also used in this.

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Biological Control 2

Eradicating the pest completely would be counterprodcutive, as the predator would die out, leading to a potential resurgences of the pest.

Farming and agriculture can upset the natural system e.g.the destruction of hedgerows in making farmland removes predators habitats.

The introduction of foreign agents can results in eradication of the pest, but the agent can change its food source and become a pest itself.

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Pros and Cons of Biological Control


  • highly specific
  • long term control if equilibrium is established.
  • Initial research is expensive and skilled, but long term, it is inexpensive method.
  • No environmental contamination.
  • no resistance
  • can be used in a glasshouse situation.


  •  Slow- biological agents need to become established to the environment, so cannot deal with a sudden increase of pest.
  • Few successful examples.
  • Detailed knowledge needed of the ecosystem and life cycles- research =expensive. This is to ensure a adequate numbers are introduced at the optimum time.
  • exotic organisms released can have unforseen consequences
  • frequent input needed to maintain balance.
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Integrated pest control

Despite effective pesticides such as organophosphates and synthetic pyrethroids, it is now consdiered best to combine bio and chem methods.

Biological control agents are used to create pest restistant crops, and then supplemented by highly selective pesticides.

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Recycling nutrients: Carbon Cycle

Plants remove carbon during the day by photosynthesis. All organisms release CO2 through respiration. CO2 levels have increased because of: The combustion of fossil fuels and deforestation of large quantities of photosynthesising biomass.

The basis of the carbon cycle:

  • CO2 is added to the air by respiration, the combustion of fossil fuels, and the decay of organisms.
  • Photosynthesis takes place on a great scale so that it re-uses nearly all the CO2 released into the atmosphere.
  • The production of carbohydrates, proteins and fats contributes to plant growth and subsequently to animal growth. The dead remains on organisms are acted upon by saprobionts in the soil, which ultimately releases CO2 into the atmosphere

CAN YOU DRAW THIS: photosynthesis, respiration, decomposition, fossilisation and combustion

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Nitrogen Cycle

This is the flow of organic and inorganic nitrogen within an ecosystem where there is an inter change between nitrogenous compunds and atmospheric nitrogen.

Living organisms need amino acids, nucleic acids and proteins. Plants take in nitrates and make these products, which are then eaten by animals. Minerals are returned to the soil when animals and their excrement decompose.

Main processes:

  • Ammonification
  • Nitrification
  • Nitrogen fixation
  • Denitrification 
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Ammonification and Nitrification

Ammonifiaction/ decomposition

This is when bacteria and fungi decompose dead plants, animals, faeces and urine into ammonium ions. Also called putrefaction.


The ammonia formed in putrefation is converted via nitrites to nitrates. Many aerobic bacteria are used in the process.

Ammonia >> Nitrite is done by Nitrosomas.

Nitrites >>> nitrates is done by Nitrobacter.

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Nitrogen Fixation

Atmospheric Nitrogen is converted by reduction directly to ammonia by nitrogen-fixing bacteria. This includes free living Azotobacter. There is also symbiotic nitrogen fixing bacteria, Rhizobium, found in the root nodules of legumes.

They use the enzyme nitrogenase, which is inhibited by oxygen. Therefore, the root nodules of legumes etc are often pink/red because they contain haemoglobin. This captures oxygen and stops inhibition of nitrogenase.

Nodules and bacteria allow legumes to survive in nitrate scarce conditions, by providing ammonia to make DNA, proteins for growth.

Soil fertility is improved upon death of the plant because the nodules release ammonia straight into the soil.

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Denitirification and human efforts

This is how nitrogen is lost from ecosystems. This is a particular problem in waterlogged soils where anaerobic bacteria reduce nitrates and ammoinum ions into nitrogen.

How humans improve nitrogen circulation:

  • Artificially fixing nitrogen by chemical processes to use as fertilisers.
  • Large amounts of animal waste used as manure.
  • Sewage disposal boosts organic nitrogen supplies.
  • Micro-organisms can be used to make compost and silage..
  • Farming practices such as: planting fields of clover to encourage nitrogen fixation, draining land and reducing anaerobic conditions, ploughing fields to improve aeration
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