B2: Organisms in their environment

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Organisms in their Environment

  • Many factors affect living organisms.
  • Temperature- In cold climates temperature is a limiting factor which means that plants are generally smaller there and less herbivores are found in that area.
  • Availability of Nutrients - Most plants struggle to grow when mineral ions are in short supply and again few animals survive in that area. However, carnivorous plants like the Venus flytraps can survive on low levels of nitrate ions because they can trap, digest and get mineral ions from animal prey.
  • Amount of light - Some plants are adapted to living in low light levels-they might have more chlorophyll or bigger leaves, but most plants need higher light levels which is shown as very few plants grow on the forest floor.
  • Availability of water - As a rule, there are less plants and animals in the desert due to the low availability of water.
     However, when it rains, plants will grow very quickly and then many animals will come to eat them
  • Availability of Oxygen - Water animals can be affected by lack of oxygen. Some invertebrates can live at very low oxygen levels, but mosr fish need high levels of oxygen dissolved in water.
  • Carbon Dioxide - Lack of carbon dioxide will affect plant growth and consequentley the food available for animals.
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Measuring the Distribution of Organisms

  • Quadrats
    • This is the simplest way to sample and area (count the number of organisms). It is usually a wooden or metal square frame that you lay on the ground to outline your sample area.
    • A quadrat with 0.5m sides gives a 0.25m2 sample area.
    • You use the same size one each time and sample as many areas as possible to make the data reliable.
    • Sample size is very important, and sample areas must be selected randomly in order to make sure that results reflect the true distributions. To make it random, a person could spin around, walk a certain amount of paces and drop the quadrat. A more scientific way would be to use a random number generator on a scientific calculator.
    • After a number of random samplings, a mean number of organisms per m2 has to be taken. This is quantatative sampling. It can be used to compare distribution of the same organism in different habitats.
    • You could also find the median, mode and range. Median- the middle value of readings when they are in order. Mode-the most common value. Range-the highest value subtracting the smallest value.
  • Transect
    • You could sample along a transect, although it is not random. The most common is a line transect.
    • You could stretch a tape between two points, and then sample along the tape at regular intervals using a quadrat, showing the distribution along it. You can also measure physical factors like light levels and pH that would affect growth of plants along the transect.
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How Valid is the Data?

  • Investigating distribution of organisms can be difficult, so have to be done over a long time and not all variables can be controlled.
  • Reproducible and valid methods must be used to collect data.
  • Reproducible-other people can do the same investigation and get similar or the same results as you.
  • Valid-the investigation must answer the question or prove/disprove your hypothesis.
  • Sample size is very important-the bigger, the more reproducible and valid data you get.
  • Method of sampling should be appropiate-random quadrats for the distribution of plants in an area. To measure change in distribution over a range of habitats, a transect is better.
  • If you are measuring over time, you must be able to replicate your method each time you repeat readings.
  • Changes in distribution is often used to prove environmental change.
  • When carrying out an investigation, as many variables as possible have to be controlled, so others could do the same investigation too, making your results more likely to be reproducible.
  • Outside, it is harder to control variables, but as many as possible still should be controlled.
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