Ecosystems - biotic and abiotic factors - interaction between organisms - interdependence - parasitic and mutual relationships

Doc Brown's Biology Revision Notes

Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent

 This page will help you answer questions such as ...

 What is an ecosystem?

 What are biotic factors?

 What are abiotic factors?

 Why do populations of species rise and fall?

 How do environmental changes affect communities?


Introduction to ecosystems

An ecosystem is all the living organisms (plant or animal) in a specified area and such a description includes all the non-living conditions such as temperature, soil quality, water sources (or lack of them).

Within an ecosystem there is continuous competition for food and other resources and coping with several conditions such as availability of water or the temperature of the environment.

You can consider that ecosystems have different levels of organisation.

Levels of organisation - in increasing size

1. Individual - a single organism of any species in the ecosystem.

2. Population - the total number of one particular species in a specified habitat.

3. Community - all the organisms of all the different species living in a specified habitat.

4. Ecosystem - the community of organisms and the conditions under which they live.

Here habitat means a specified area of the ecosystem involved e.g. a wood, a pond, a sea shore etc.

Here conditions means the 'non-living' abiotic factors like temperature, pH, moisture level or light intensity.

Species is defined as a group of similar organisms that can reproduce to produce fertile offspring.



Competition for resources

All organisms need a variety things from their environment and other organisms to survive and reproduce.

Plants, the primary food producers at the base of many food chains, need light, mineral ion nutrients, space to grow, water and oxygen and carbon dioxide from the atmosphere.

Animals need food, mates for reproduction, water and space to live in (the 'territory' of their habitat)

Organisms may compete with other species or with members of their own species for the same resources in the same habitat.

Different predators might compete for the same prey.

e.g. in the oceans sharks and dolphins compete for the same shoals of smaller fish.

Animals might compete for the same plant food.

e.g. red and grey squirrels compete for the same food like nuts in the same woodland habitat. Grey squirrels compete more strongly-efficiently for food resources, depleting them to the point where red squirrel populations are in decline.



The effect of environmental changes on communities.

Environmental conditions are changing all the time and are caused by many factors.

There are non-living (abiotic) factors and living (biotic) factors (described below).

These affect communities in several different ways e.g.

populations of species may rise or fall due to availability of food,

the distribution might change - may involve movement from one location to another,


Examples of abiotic factors (non-living factors)

Light intensity - all 'green' plants need good access to light for photosynthesis, but in growth they produce areas of shade from sunlight (and sometimes more moist conditions too). This means grass can be replaced to some extent by fungi and mosses which can cope much better with lower intensity sunlight.

Moisture level - most plants need access to a continuous source of water. Some plants are adapted to survive in relatively dry conditions, but most cannot. However, with the opposite situation, most plants cannot survive in very boggy waterlogged ground, but a few species are adapted to such conditions.

pH of the soil - some plants are adapted to live in quite acid conditions e.g. moorland peat conditions. Other plants are adapted to live in the mildly alkaline soils in limestone country. If you swap the locations for these groups of plants they will not flourish to the same extent as when grown in their native habitat.

Temperature - many populations are only stable if the ambient environmental temperature lies within a certain range. Populations of fish and other aquatic life are quite sensitive to the temperature of the water. There is evidence of populations of certain marine species moving to more northerly waters as sea temperatures rise around the UK coast. Similarly there is evidence of both bird and butterfly species extending their territory further north on mainland Britain. Species of birds associated with Mediterranean countries are now appearing in northern European countries.


Examples of biotic factors (living factors)

Food - the availability of food is one of the most important biotic factors that supports a stable population of a species. The more abundant the food supply, the more organisms can survive and reproduce to maintain or increase their population. The food ranges from plants for animals, prey for predators etc.

Predators - the relative populations of predators and their prey will affect each other. If a prey is abundant at one point in time, then the population of a predator can increase because there is more food available to eat. However, in doing so, the population of the prey will decline soon after. This leads to complex cycles in the rise and fall of the populations of the species involved.

A classic example is the relationship between rabbit (the prey) and fox (predator) populations in a particular community.

The population of a species (e.g. foxes) is often limited by the amount of food available (e.g. rabbits).

If the population of prey increases e.g. lots of rich green grass for the rabbits, their population can increase - the crests in the upper graph line.

This means more food for the foxes!, so their population increases too - upper crests on the lower graph line. There is a time lag to allow for reproduction!

BUT, the extra foxes eat more rabbits and so the number of rabbits decreases - troughs in the upper graph line.

The decline in rabbits once again limits the food for the foxes, so their population begins to decrease - troughs on the lower graph line.

This rise and fall in prey-predator populations is very typical and produces these 'wave-like' graphs and is a classic case of interdependence.

The two interdependent populations will always be 'out of phase' with each other because there will be a time lag (dotted line) as one change in population gradually affects the other population.



More examples of interactions between organisms

Types of interdependence - parasitic and mutual relationships winners and loses!

An organism may depend entirely on another species to survive.

Therefore, where an organism lives and the size of its population can be influenced by the distribution and abundance of this other species.

Parasites live off a host (the other organism). From the host they extract what they need to survive without giving anything in return. This process can be harmful to the host, one organism gains and the other loses e.g.

Fleas are a common parasite on humans, cats and dogs amongst other animals. The bite through the skin to feed on blood for nutrients and lay eggs after! There can be allergic reactions to the presence of fleas.

Tapeworms happily live in the intestines of many animals. In their parasitic action they consume large quantities of nutrients, depriving the host animals of some of its digested food causing malnutrition.

Mutualism is a situation where both organisms benefit from the relationship. Two winners! e.g.

Many plants are pollinated by insects, because of the pollen grains carried on their legs. The pollination allows the plants to sexually reproduce. In the process the insects have access to food in the form of a sugary solution - nectar. Both organisms benefit from the pollination.

Microorganisms in a cow's stomach survive and flourish by helping to break down the hard to digest grass. Both the host cow and microorganisms benefit from this interaction, hence one is mutually dependent on the other.



Extra thoughts on ecosystems

  • Know that living organisms form communities, and we need to understand the relationships within and between these communities.

  • Know that these relationships are affected by external influences.

  • You should be able to use your skills, knowledge and understanding to:

    • suggest reasons for the distribution of living organisms in a particular habitat,

    • evaluate methods used to collect environmental data, and consider the validity of the method and the reproducibility of the data as evidence for environmental change,

      • at the end of an investigation and analysis, can you distinguish whether differences in distributions of an organism are due to one or more environmental factors?

      • is it possible to control, or allow for, different environmental factors?

    • and you should understand:

      • the terms mean (average of all of a data set), median (middle value in a data set) and mode (the most common value in a data set - could be more than one value)

      • that sample size is related to both validity and reproducibility,

        • the larger the sample, random sampling from many locations and the more times the experiment is repeated, the more reliable will be the final analysis and conclusions,

        • reproducibility is the key to a successful valid investigation,

  • Know and understand the physical factors that may affect an organism in its habitat (area where the organism lives):

    • To study the distribution of a species of animal or plant you must measure the population of the organism in different sample areas.

    • A habitat is where an organism lives (plant or animal) and its distribution is the areas where the organisms live an this may depend on environmental factors such as ...

    • ambient temperature,

    • availability of nutrients in the soil or water,

    • amount of light falling on the habitat,

    • availability of water in soil for plants,

    • availability of oxygen and carbon dioxide in the air or water.

      • Organisms will be adapted by evolution to fill a niche in a particular area of the environment,

      • but if there is a change in any of the factors above, then organism populations will be affected, some will increase and some will decrease,

      • in extreme cases, one species might die out in an area and another species may take advantage and move in.

      • An organism will be more common in an area, where environmental factors make conditions more suitable for the organism to survive and reproduce e.g.

        • shade for plants that need little sunlight, or out in the open for plants that need bright sunlight,

        • some creatures may prefer damp conditions, others adapted to dry conditions,

        • greater density/availability of the organism's specific food requirements

  • Know and understand that quantitative data on the distribution of organisms can be obtained by:

    • (i) random sampling with quadrats to cover a large area without sampling all of it

      • A sampling quadrat is usually a 1m x 1m (1 m2) square frame of wood or plastic,

      • Therefore if you measure the number of organisms in a quadrat you get the density in organisms per square metre.

      • You can average the random individual quadrat results to get the mean value for a particular organism/m2.

      • To work out the total population of an organism in the area you have been randomly sampling, you multiply the mean by the total area.

      • The more samples you take, the more reliable is your data, and therefore any deductions made will also be more reliable, but the data is only statistical, never completely precise, but

      • In presenting data make sure you know how to use the terms mean (average of all of a data set), median (middle value in a data set) and mode (the most common value in a data set - could be more than one value).

    • (ii) sampling with quadrats along a linear transect to look for changes across an area of land e.g. to see how a population changes across a wider area.

      • You can mark out the transect with two sticks and a long piece of string.

    • Note (iii) Whatever field work you do, the only really reliable data, are data that are consistent, i.e. always show the same pattern of organism distribution (plant or animal), and in that way the data is repeatable and reproducible.

      • This involves random sampling with samples using many quadrats and transects.

      • Does the data support the question posed about organism distribution?

      • Are differences in population due to environmental factors?

      • What are the variables?

      • Have the variables been controlled properly in your survey design?

  • Your practical work to develop your skills and understanding may have included the following:

    • investigative fieldwork involving sampling techniques and the use of quadrats and transects; which might include, on a local scale, the:

      • patterns of grass growth under trees,

      • distribution of daisy and dandelion plants in a field,

      • distribution of lichens or moss on trees, walls and other surfaces,

      • distribution of the alga Pleurococcus on trees, walls and other surfaces,

      • leaf size in plants growing on or climbing against walls, including height and effect of aspect.

    • analysing the measurement of specific abiotic factors in relation to the distribution of organisms

    • the study of hay infusions

    • the use of sensors to measure environmental conditions in a fieldwork context.



See also

Carbon cycle, nitrogen cycle, water cycle and decomposition  gcse biology revision notes

Food chains, food webs and biomass   gcse biology revision notes


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