Populations (general)

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Population growth

Lag phase- slow population growth as limited number of individuals of reproductive age in the population

Log - plentiful supply of nutrients, little intraspecific competition for nutrients and resources, waste products at low concentrations, birth rate at maximum

As the population nears the carrying capacity, the growth rate slows by environmental resistance. These are either density dependent (effect increases as population increases e.g. food availability, competition, predation and disease) or density independent (effect is the same regardless of population size e.g. sudden temperature changes)


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Paramecium investigation

P. bursaria, caudatum and aurelia grown in tubes of liquid medium containing yeast cells. P bursaria feeds on yeast cells on bottom of tubes, P aurelia and caudatum feed on suspended yeast cells.

When the three species are grown seperately, each reaches a maximum population size.

P. Caudatum and P. aurelia - P aurelia outcompetes P caudatum and reaches its carrying capacity, P caudatum becomes locally extinct.

P. Caudatum and P. Bursaria - coexist but with smaller populations than when seperate - can coexist as feed in different areas, but still are competing with each other for the same source.

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Pest control introduction

Pests are unwanted organisms that interfere directly or indirectly with human activity, usually associated with agriculture and food production.

Aim of pest control is to reduce the pest population to a level that doesnt result in an economic loss to the farmer (costs of pest control cannot exceed additional economic value of the crop). 

Chemical control used pesticides (a chemical poison used to kill pests)

Biological control exploits natural enemies of the pest species using them to regulate the pest population. Control agent could be natural predators, parasites or disease causing organisms. Does NOT aim to kill the pest population. Only aims to reduce it below the economic damage threshold - killing the pest species would be counter productive as it would kill the control agent too.

Integrated pest managment involves the use of biological control agents together with minimal, well targeted application of highly selective pesticides.

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Advantages and disadvantages of chemical control

Advantages - fast acting before pest has a chance to increase

relatively cost effective (does not exceed additional economic value)

chemicals can be applied on a small scale, does not require high skill level

Disadvantages - non selective - can kill beneficial organisms like pollinating insects and natural                                                              pest predators

Persistent - often arent biodegradable so remain active in the soil for many years. Tend to be fat soluble, so accumulate in the tissues and arent excreted. Bioaccumulation: as organism grows, level of toxins in its body grows. Biomagification: as the pesticides passes up the trophic levels it becomes more concentrated, until the top lvels which it may be fatal

Some pest individuals may contain genetic resistance alleles to the pesticde. They are the only ones that will survive treatment, so will result in the evolution of resistant populations

Long term exposure to pesticides (e.g. sheep dips) can harm humans

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Advantages and disadvantages of biological control

Advantages - only target pest species (highly specific)

Environment is not contaminated/non toxic to other species

Pest species cannot become resistant to the control agents

Once the population is established, control of pest is self perpetuating (predator-prey cycle established)

Inexpensive in the long term

Very effective in greenhouses

Disadvantages - slows as takes time for control agent population to reach an effective level,                                repeated applications of agent maybe to achieve long term population balance

Initially expensive for high level of skill for research and implementation

Exotic control agents may become pests e.g. cane toad

Only works on large scale

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The carbon cycle

Carbon found in carbohydrates, lipids, proteins and nucleic acids

Carbon in the atmosphere and oceans made available to organisms by photosynthesis/carbon fixationn by green plants and algae, which convert it to complex organic molecules like glucose.

Carbon dioxide is produced by all organisms by respiration, enters the atmosphere.

Carbon is stored in wood and fossil fuels (fossilised animal and plant remains). Combustion of these releases the stored carbon into the atmosphere as carbon dioxide,

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The nitrogen cycle

Putrefaction Saprophytic decomposers metabolise nitrogen-containg organic matter sucg as urea in urine, faeces, amino acids and nucleic acids in dead organisms, releasing ammonium ions.

Nitrification Under aerobic conditions nitrifying bacteria convert:

- ammonia to nitrites (Nitrosomonas)

- nitrites to nitrates (Nitrobacter)

Nitrates then taken up from soil by producers.

Denitrification denitrifying bacteria such as Psuedomonas convert nitrates and ammonium ions into atmospheric nitrogen, removing nitrogen from ecosystems, in anaerobic conditions.

Nitrogen fixation nitrogen fixing bacteria convert atmospheric nitrogen into ammonium ions

Rhizobium is found in the root nodules of legumes where it has a symbiotic relationship: plant rapidly converts the ammonium ions into nitrogen-containing organic compounds.Bacteria gains energy and nutrients e.g. carbohydrates from the plants.

Azobacter found free living in the soil.

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