Option G.1 – Community Ecology

Option G.1 – Community Ecology

G.1.1 РOutline the factors that affect the distribution of plant species, including temperature, water, light, soil pH, salinity and mineral nutrients

All species are restricted to certain geographical areas and particular habitats. Abiotic factors are conditions caused by non-living elements. The main factors that determine the distribution of green plants are:


This determines the rate of biochemical reactions in organisms, as enzymes have optimum temperatures for function. Temperature also determines the rate of evaporation from plants, called transpiration. Plants have adapted to resist variation in temperature.


This is vital to life, as it is necessary in reactions of photosynthesis and respiration, along with other uses. Rainfall will restrict plant distribution and density. Plants have evolved to retain water according to their environment, include waxy cuticles and extensive root systems, including the xerophytes.


This is necessary for photosynthesis by autotrophic organisms, which provide energy for the whole ecosystem. Light intensity and wavelength vary, in turn affecting temperature and humidity in the area. Also, day length is used to control some daily or seasonal rhythms, including flowering. In this way, light can affect the structure of plant communities, controlling which types of plants can grow in certain places.

Soil pH

This affects the availability of essential ions. Also, highly acidic or alkaline soils may cause the denaturation of proteins, disrupting important physiological processes. It also affects population and soil bacteria activity that decompose organic matter. While diversity is generally greatest at pH7, many plants have adapted or acid or alkaline conditions.


This affects the distribution of water plants. Salts and ions can accumulate to high levels in water. When salt concentration in the soil is high, the plant may lose too much water through osmosis. Salt marshes have a variable salinity based on the tide, and often contain halophytes, which survive high salt conditions by retaining enhanced levels of ions and resisting loss of water to their environment.

Mineral nutrients

These are found dissolved in water of the soil solution, usually at low concentrations, and are essential for the growth of the plant. The essential elements are released by decay of dead plants and animals and their waste matter, including K and Ca, NO3 and PO3. They are absorbed then reused by the plant as ions, and endlessly cycled. Stocks can be added to by the weathering of rocks, but nitrates are generated from atmospheric nitrogen gas by the actions of nitrogen-fixing microorganisms, and as an outcome of lightning. The availability of minerals is affected by pH and bacteria.

G.1.2 – Explain the factors that affect the distribution of animal species, including
temperature, water, breeding sites, food supplies and territory

The dominant plants in an ecosystem will often determine animal distribution as the animals are highly dependent on them for their sources of energy and carbon. It also determines the type of habitat that exists, along with the critical abiotic conditions. This should be kept in mind when investigating animal distribution.


This can influence the metabolism and growth of animals, as well as their behaviour. There is strong correlation between amount of light and temperature. Ectotherms are able to survive very low or high temperatures using behavioural means, but cannot survive if it gets too extreme. Homeotherms are animals that regulate their own temperature.

Poikilotherms maintain less control, and are more dependent on the external temperature.



The bodies of animals are mostly made up of water. It is lost by evaporation during the processes of gaseous exchange, regulation of body temperature and disposal of waste matter. Some animals, such as insects, reptiles, birds and mammals, have adapted to retain more water by having an impervious body covering, and can occupy a wider range of habitats. Some may use it as their permanent habitat, such as fish, or just for breeding, such as amphibians. However, all animals need a reliable water source in their habitat.

Breeding sites

These are sites in which young are fed and reared, typically placed at the heart of the territory. These sites often have special requirements, including temperature and light. Food resources and protection have a large effect on the chances of survival for the young. Without these sites, the species is likely to become extinct.

Food supplies

This affects the existence and size of local populations. They may be dependent on plant matter, other animals or both for meeting their special dietary requirements. When supplies are short, individuals must compete for a limited resource. This can occur between the same species, intraspecific competition, or different species, interspecific competition. This competition is a major factor determining population size. If food supplies run out due to over-predation, the effects are felt all the way up the food chain.


This is a defended area of a habitat, established by individuals, breeding pairs or family groups. Organisms will have specific requirements for their territory. Herds will mark their territory using scent marking and defend their territory through active behaviour, although fighting will be kept to a minimum; instead, ritualised aggression through body language is usually used. They may occupy habitat on either a temporary or permanent basis.

G.1.3 – Describe one method of random sampling, based on quadrat methods, that is used to compare the population size of two plant or two animal species

Random sampling means that every individual of the population has an equal chance of being selected, so a representative sample is assured. A quadrat is a square frame used to outline an area for sampling. The size of the quadrat used will depend on the size of the individuals in the population being analysed. Many quadrats will need to be placed over the area for an accurate population size estimate.

When the running mean stabilises, then enough quadrats have been used.

The optimum size of a quadrat will depend on the size of the area being analysed and the size of the individuals.

G.1.4 – Outline the use of a transect to correlate the distribution of plant or animal species with an abiotic variable.

When there is a gradient in an abiotic factor, such as altitude, salinity or light, then communities will often show a trend in variation. Transects are used to measure this trend. The transect is placed at a right angle to the impact of these factors.

A single transect may not give an adequate sample, so multiple samples should be done. When there are changes such as height of the land, these can be measured and recorded as a profile transect.

A belt transect is when quadrats are placed in a line, or at intervals. The number of individuals within each transect is used.

A line transect is when the number of individuals found along a line is measured at certain intervals. The individuals that are touching the string are counted, along with the distance along the line.

G.1.5 – Explain what is meant by the niche concept, including an organism’s spatial habitat, its feeding activities and its interactions with other species

A niche is an ecological term to define how an organism feeds, where it lives, and how it behaves in relation to other organisms in its habitat. This concept is useful because identifies the precise conditions which a species needs. It is these combinations of environmental conditions are necessary for the species to tolerate their physical environment, obtain energy and nutrients, and to avoid predators. Competition is the interaction between two organisms striving for the same resource in

Competition is the interaction between two organisms striving for the same resource in the same place due to overlap in their niches. Potential competitors in fact have evolved from different niches.

G.1.6 – Outline the following interactions between species, giving two examples of each: competition, herbivory, predation, parasitism and mutualism

As resources are in limited supply, organisms must then compete for them. This includes space, light and mineral ions.

Intraspecific competition – when individuals of the same species compete for resources or a
mate. Interspecific competition – when individuals of a different species compete for resources.

  • Different species of Paramecium compete for plankton
  • Beech and oak trees in England compete for minerals and water


When an animal feeds on plant matter.

  • Leaf-cutter ants in Costa Rica, feeding of leaves
  • Wildebeest in Africa eat grass


When an animal preys on another for food. This will often require patient stalking and a determined final attack because most prey species will help to defend their whole herd.

  • Lions in Africa prey on wildebeest, usually going after young or injured ones as they are easier to catch.
  • Lady beetles feed on aphids inside plant step tissue.


When two organisms lives together, where the relationship is harmful to the host, or at least unhelpful.

  • Plasmodium is a protozoan that lives in the blood and liver of humans, causing malaria. It is transferred via mosquitoes. As it resides inside the host, it is an endoparasite.
  • Ticks are also parasites, which attach themselves to warm-blooded, furry mammals and suck the blood. They are ectoparasites. When they are satiated with blood, they may drop off, though usually somewhere in the vicinity of the host so that they can reattach later.


This is a form of symbiosis. Two organisms of a different specieslive in intimate association to a mutual advantage. This is a form of favourable interaction between organisms.

  • Microorganisms, usually bacteria or fungi, will live in the rumen of ruminant mammals. The rumen is a large, stomach-like fermentation vessel. The microorganisms break down cellulose and other polymers in plant matter, producing organic acids (along with methane). The ruminant is dependent on this enzymic digestion in order to gain nutritional benefit from its diet. The rumen forms the habitat for the microorganisms (until they are digested by ruminant enzymes further into the gut of the host.
  • Lichens are made of a fungal and algal part. The algal part performs photosynthesis, and the fungal part absorbs mineral and other nutrients.

G.1.7 – Explain the principle of competitive exclusion

When two species occupy the same niche or have overlapping niches, only one will be able to exclusively use the resources, and the other will either move away or die off. The overlap leads to competition between the species. If the overlap is small, then they may be able to co-exist.

The competitive exclusion principle is the idea that ecological separation of closely related or otherwise similar species is the inevitable outcome. If two species share the same resource at the same place and same time, the dominant, or stronger species will outcompete the other, which will die out or move away. This may be the reason that closely related species living in close proximity have evolved clearly defined but separate niches, with no competition between them. This occurs over an extended period of time. This is also referred to as Gause’s Law, saying that two species competing for the same resources cannot coexist, as one will always have an advantage.

G.1.8 – Distinguish between fundamental and realised niches

Most species can survive in a range of conditions, so their niches are fairly unrestricted ones. For example, animals can mostly eat a wide range and variety of food sources.

The fundamental niche of a species is the potential mode of existence, given the adaptations of the species. It is the total range of conditions the organism is suited to with influence of intraspecific competition or predation from other species.

It is only when organisms experience resources and conditions totally outside their range are they fatally threatened by their environment or competition, so they will die out.. The portion of their fundamental niche that is available to them becomes the realised niche.

The realized niche of a species is the actual mode of existence, which results from its adaptations and competition with other species. It is the segment of the fundamental niche that the organism actually occupies.

The niche of an organism refers to how an organism or population responds to the distribution of resources and competitors, and how it alters those factors, acting as a food source and a consumer.

G.1.9 – Define biomass

Biomass is the total dry weight (or volume, or energy equivalent) of living organisms in a given area (e.g. a quadrat)

G.1.10 – Describe one method for the measurement of biomass of different trophic levels in an ecosystem

The trophic levels in a community are:

To measure the biomass at different trophic levels, we must first estimate the number of organisms of each type at each trophic level of the community. If the mass is to be expressed as dry mass (as it commonly is) then the dry mass of a representative sample must be found for each type of organism The dry mass can be found by heating a weighed sample to about 80oC, removing water without burning any organic

The dry mass can be found by heating a weighed sample to about 80oC, removing water without burning any organic matter. The cooled samples are weighed and heated, showing that all the water has been evaporated. The heating is repeated until there is no change in the mass of the organism. Unfortunately, to obtain the dry mass of samples they

Unfortunately, to obtain the dry mass of samples they must first be killed. As a result, communities of organisms are destroyed by this technique, reducing their habitat to a ‘desert,’ at least until it is repopulated by surrounding communities.

There are many ethical issues arising from this technique, especially when biodiversity is under threat. To take a less destructive approach, the fresh mass of a small representative sample could be taken, which can then be returned to the environment. However, this approach is less accurate.