AQA A2 Unit 4 Populations

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populations and Ecosystems

- A communitity is a group of organisms of different species, all living in the same place at the same time. A population is all the members of one species, living together at the same time. the community is non-living environment make up an ecosystem, in which the living and non-living components interact.

-Each species has its own ecological niche- its role in the community. The niche is determined by the adaptions that the species has evolved, so that it is able to be successful in a particular range of abiotic and biotic conditions

-Abiotic factors are those resulting from non-living parts of an ecosystem. Biotic factors result from living organisims

-No two species can occupy exactly the same niche in a community, although niches do often overlap. However, in different ecosystems there may be different species that have evolved similar adaptations that make the successful in similar niches to one another

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Sampling Populations

-An area can be sampled to find out which species live there, and to estimate the size of thier populations. Quadrats are used to delineate an area within which data wil be colleted

-Quadrats may be placed randomly, using random numbers as coordinates. The percentage of quadrats in which a species occurs gives the species frequency. The mean percentage area that a species covers in all the quadrats gives the mean percentage cover. an abundance scale can be useed where it is difficult or too time-consuming to determine percentage cover.

-Quadrats may be of different sizes, including point quadrats, which have a tiny area.

-A transect is a line along which sampling takes place. Quadrats may be placed all along the line, or at intervals. This is useful to get an indication conditions, for example, at different heights on a rocky shore.

-The mark-release-recapture technique can be used to estimate the size of a population of mobile animals. Animals can be caught using suitable technique, then marked and released. The proportion of marked animals in a second sample can be used to calcuate the population, using the lincoln index

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Population Size

-Factors that influence the distrubution and abundance (population size) of organisms can be categorised as abiotic and biotic factors.

-Abiotic factors result from the non-living part of the environment, such as temperature or water supply. Biotic factors result from the living parts of the ecosystem, such as competition and predation

-Abiotic factors can vary greatly i habitat, forming different microclimates that are important in determining the distribution of a species within a habitat

-Biotic factors include food supply, predation, parasitism, disease, intraspecific and interspecific competition. 

-In some cases, one particular factor has a very strong influence on population size. For example, if a predator depends on one species of prey for food, and if that predator kills mostly that prey, then the populations of one may be largerly determined by the other. This can result in repeated oscillations of the population sizes of both species

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Human Populations

-The human popultion is growing, and may eventually reach a point at which abiotic or biotic fators cause it to decrease. Growth rate is generally higher in deveoping countries than in developed countries.

-Population pyramids show the numbers (or percentages) of people in each age group at any one point in time. A pyramid with a wide base shows an increasing population.

-A survival curve shows the percentage of people who survive to each age within the total age range. A curve that falls near the beginning indicates a high infant mortality. In  a developed country, the curve is generally high and flat in the early years, with the sharpest downward gradient from aged 60 or beyond

-As a country develops, it gradually changes from high birth rates and death rates to low birth rates and death rates. This i called the demographic transition.

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