Option G.2 – Ecosystems and Biomes

Option G.2 – Ecosystems and Biomes

G.2.1 – Define gross production, net production and biomass

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

Gross Production is the total amount of organic matter produced by plants in an ecosystem measured in kJ per square metre per year

Net Production is organic matter of organisms less the amount needed to fuel respiration

G.2.2 – Calculate values for gross production and net production using the equation: gross production – respiration = net production

The units for this are kJ m-2 yr-1

Values for this may be given in a data table. If you are missing certain variables, rearrange the equation to find it.

G.2.3 – Discuss the difficulties of classifying organisms into trophic levels

Trophic levels are defined by the method the organism uses to obtain food. Within one trophic level, all the organisms are the same number of energy transfers from the original energy source – the Sun and photosynthesis.

The main difficulty is when organisms obtain their energy from food sources that are a variety of transfers from the first energy source. Thus, some of the top carnivores are at trophic levels 2 to 5 at any one time.

This is especially the case with omnivores, which feed on producers and consumers. Thus, they are primary consumers and secondary consumers, making them difficult to place on the food pyramid

Another difficulty arises due to the fact that many animals change their diets over the course of their growth and development.

Also, the detritivores and decomposers are not always featured on food webs, yet up to 80% of plant matter produced may be consumed this way.

It is for this reason that the food web was developed, showing the organisms in relation to each other in a complex network instead of in a hierarchal structure. Also, individual animals themselves are shown, instead of simple hubs of trophic levels.

Another solution is to classify them according to their main food source.

G.2.4 – Explain the small biomass and low numbers of organisms in higher trophic levels

At higher trophic levels, the small biomass and lower number of organisms is due to the energy loss at each level. As we know, this loss comes as a result of respiration, using their energy for different activities. When glucose or another respiratory substrate is oxidised, the energy is released for use then lost as heat.

Also, a portion of the biomass at each level is not eaten by the next. Biomass losses occur in the form of excretion of faeces and urine, which will pass directly to decomposers.

As a result, an average of 10% of the matter and energy at any one level will be passed on to the next. The amount of energy per gram does not decrease in the higher trophic levels, in fact it may increase. However, there the total amount of biomass available to the trophic levels is less. This is why there are fewer organisms at the higher trophic levels, resulting in a low biomass per unit area.

As well as this, top carnivores may hunt for their food, which must then be killed. During the chase, more energy is transferred, prey may be injured and die elsewhere, and possibly become food for other organisms at lower trophic levels.

G.2.5 – Construct a pyramid of energy, given appropriate information

Pyramids of numbers of organisms present, or of biomass, or of energy, are a proportional representation of the organisms present at successive trophic levels. They should be represented on graph paper by areas proportional to the data being used.

The units are kJ m-2 yr-1, so the pyramid should be drawn on a grid.

The bottom bar of the pyramid is always the energy flowing through the producers, or gross production. The trophic level must always be labelled on the pyramid.

G.2.6 – Distinguish between primary and secondary succession, using an example of each

Ecological succession is the process of a sequence of communities developing with time. In this, plant and animal communities increase in complexity. The climax community is when the succession has ended and it has all of its characteristics. It can also be seen as a series of changes to an ecosystem, caused by the interaction between the living organisms and the abiotic factors.

Primary Succession

This is when the succession starts on entirely new land without established soil. This may happen at river deltas, at sand dunes, and from cooled volcanic lava. These also develop in aquatic habitats, such as a pond fed from a spring. The first significant development in this case is the formation of soil. Another example may be a new island that was created through volcanic activity. This occurs when a major catastrophic disturbance takes place. Examples include

This occurs when a major catastrophic disturbance takes place. Examples include glacial retreat and volcanic eruptions. All the plants in the area are destroyed, and new species may colonise.

Secondary Succession

This is a succession that starts from existing soil. Communities may be disrupted and destroyed, such as during bushfires when a lot of vegetation is destroyed. The soil is already formed and present; however, the existing biota has been removed.

There must have been a change in conditions, such as a farm being abandoned, and becoming a forest. Unlike in primary succession, not all the plants are wiped out completely, leaving behind a seedbank or vegetable propagules. Dominance is usually achieved by the fast growing plants.

G.2.7 – Outline the changes in species diversity and production during primary succession

At the initial site of a primary succession, all that may be present is parent rock, from which the bulk of the soils is formed by erosion. This is the breaking of solid rock into smaller particles by the effects of extremes of temperature, the action of wind and water, and chemical reactions, such as when acid rain falls. Mineral particles may also be blown or washed in from elsewhere. The resulting mineral skeleton is of particles of a wide range of sizes from small stones and coarse sand to the finest clay particles. Succession can be seen as a directional change in a community with time. Initially,

Succession can be seen as a directional change in a community with time. Initially, abiotic factors have the greater influence on the survival and growth of organisms. Later, as the numbers of living organisms build up, biotic factors increasingly affect survival too. A xerosere is

A xerosere is succession under dry, exposed conditions where water supply is an abiotic factor limiting growth of plants.

The eventual outcome of the succession is the climax community. An important feature of succession is the progressive increase in the number of species present. As more and more species occur in the habitat, the food webs are likely to be more diverse and complex. As a result, if one population crashed, such as due to a disease, then alternative food chains may be sufficient to supply the higher trophic levels.

If primary succession takes place in aquatic habitats, then the sequence of pioneer plants differs from a dry land primary succession, although it may still result in a woodland community. The succession is called a hydrosere. Plants adapted to aquatic or permanent swamp conditions are known as hydrophytes.

Changes to the abiotic conditions during succession include:

  • Increase in the amount organic matter in the soil
  • Soil becomes deeper
  • Soil structure improves, resulting in better water retention
  • Soil erosion is reduced
  • The amount of mineral recycling increases.

G.2.8 – Explain the effects of living organisms on the abiotic environment, with reference to the changes occurring during primary succession

Soil, when fully formed, has organic matter called humus wrapped around the particles of the mineral skeleton. Humus is a substance derived from dead plant and animal remains, together with animal faeces, that have been decomposed by the actions of microorganisms. Humus is a dark-coloured, sticky substances, that continues to be decayed, releasing mineral nutrients. Humus also helps the soil to hold water. Between mineral particles and

Between mineral particles and humus linings are innumerable pockets of air. Also present in a developed soil is a huge community of microorganisms and small animals, including earthworms, all adapted to life in this habitat.

Humus is first added by plant invaders of the primary succession, known as pioneer plants. These are typically lichen and cushions of moss, and after them, tiny herbaceous plants, many with features that help them survive where water is scarce.

Until the soil is fully formed it retains little water, even when water is freely available. Plants able to survive drought are called xerophytes, and their special features are called xeromorphic features. When a sere starts from dry conditions, then the sere is called a xerosere.

The growth and death of the early plant communities continue to add humus, and so more soil water is retained. Nutrients are added to the soil when organisms die, and the range of nutrients available to plants increases steadily. Nutrients are the ions essential for plant growth, such as nitrates, phosphates, and a range of macronutrients. Different plants now grow – various herbaceous weeds may start to shade out the pioneers. The conditions in the soil are increasingly favourable to microorganisms and soil animals, which continue to invade the habitat from the surroundings. Herbaceous plants are followed by shrubs and small trees, all growing from seeds that are carried in the wind, water or activities of animals. The first-formed soil accumulates relatively

The first-formed soil accumulates relatively slowly, because it is vulnerable to erosion and dissipation by wind and heavy rains. Increasingly, as plant life becomes established, the growth of plant roots and the cover provided by plant vegetation prevent or reduce loss by soil erosion.

G.2.9 – Distinguish between biome and biosphere

A major life zone characterised by the dominant plant life present.

Also defined as a climatic and geographically defined area of ecologically similar communities of plants, animals and soil organisms, often referred to as ecosystems. Biomes are defined based on factors such as plant structures (trees, shrubs and grasses), leaf types (broadleaf and needleleaf), plant spacing (forest, woodland, savannah) and other factors like climate. Unlike ecozones, biomes are not defined by genetic, taxonomic or historical similarities. Biomes are often identified with particular patterns of ecological succession and climax vegetation.


The part of the Earth, including air, land, surface rocks and water within which life occurs, and which biotic processes in turn alter or transform. Also defined as the restricted zone which living things can inhabit.

From the broadest biophysiological point of view, the biosphere is the global ecological system integrating all living things and their relationships, interaction with the elements of the lithosphere, hydrosphere and atmosphere. This process is said to have evolved, beginning at a process of biogenesis or biopoesis around 3.5 billion years ago or more.

G.2.10 – Explain how rainfall and temperature affect the distribution of biomes

The main factors that affect distribution of the biomes are rainfall and temperature. The interaction of these factors can vary with latitude, longitude, position within land masses and proximity to the sea.

Temperature is influential because it affects the metabolism of organisms. The phases in the life cycles of many plants and animals require certain temperatures for them to succeed, such as seed germination.

In the same way, the availability of water in the soil is critical for the growth and nutrition of plants. The adaptations of plants to varying water levels influence their ability to grow successfully.

G.2.11 – Outline the characteristics of six major biomes

Although they are found in completely different areas, many places have similar climatic conditions. They will also have comparable ecosystems due to natural selection. These major ecosystems are called biomes. Biomes are identified by the dominant plant found in their communities, as well as the diversity of animal life and subdominant plant forms, which are controlled by the abiotic environment.



Deserts typically consist of shrub covered land where the plants are well dispersed. They are influence by the descending air currents, which limit the formation of precipitation.

The dominant plants include drought resistant shrubs and water-storing succulents like cacti. Other species are short-lived annuals that complete their life cycles during the infrequent, short rainy periods. Deserts are often void of vegetation due to the low rainfall.

Desert mammals are usually nocturnal to avoid the high temperatures, often lizards and snakes as the conditions promote the success of cold-blooded species. Evaporation tends to concentrate salts on the surface of the soil, and the litter layer is limited. The organic content of surface soil layer is low.

Deserts receive very limited and unpredictable rainfall. The plants in this habitat either remain dormant until rain comes, then completing their cycle in days or weeks, or are succulents that survive above ground all year round, as they can store water.

The limited animal life responds to the danger of extreme dehydration in many ways. Camels, for example, have adapted to allow their body temperature to vary by up to 7oC, allowing for conservation of water in daylight hours.


In areas where precipitation is less, buffalo grass and other grasses that are only a few inches tall are common. There are also flowering herbs, which are still very common. Very few trees can be found, usually close to streams and in low-lying areas.

In these areas, the soil is rich and black chernozemic. Chernozems are rich in nutrients and is the most fertile in the world. In drier parts, the soil can be influenced by salinisation. Humans often use these areas to grow grain and other dryland crops because of their fertility.

Smaller, burrowing herbivores such as prairie dogs, jack rabbits, ground squirrels and gophers dominate the grassland mammals, as well as larger running herbivores such as bison, pronghorn, antelope and elk. Carnivores in these areas include badger, coyote, ferret and cougar. Habitat destruction has had a dramatic impact on population size in these areas, many on the edge of extinction.

This can also occur as steppe and pampas, in areas of moderately dry climate, with hot summers and cold winters. The most common plants are perennial grasses, along with broad-leaved flowering plants. Grassland can provide natural pasture for grazing animals, and where rainfall is sufficient, have been converted into grain-crop grassland for human use.

Savannah is tropical grassland with scattered, individual trees, usually with three distinct seasons: hot and dry, cool and dry, and warm and wet. There is typically an abundance of wildlife, with large herds of herbivores and their predators, often from the cat family. A limiting factor for the growth and spread of trees is destruction by herbivores like elephants and fires caused by electrical storms.

Shrubland (chaparral)

This biome has a very specific spatial distribution. These areas have a dry climate because of their subtropical high temperatures during autumn, summer and spring. Precipitation falls during the winter because of seasonal movement of the polar front. As a result, the vegetation has adapted to withstand drought and fire. The trees and shrubs will tend to have hard, evergreen leaves. They do not lose their leaves in autumn because it is expensive to replace them. The dry climate slows the rate of decomposition, so the plant do not have nutrients available to make new leaves, instead producing ones that withstand arid conditions.

The main species in this area include cork oak, olive, eucalyptus, arbutus, acacia, maritime pine, shrub oak and live oak. Many of the plants have thorns to protect them from herbivore damage. These areas have abundant winter rainfall and dry summers.

Temperate Deciduous Forest

This biome is characterised by a moderate climate and deciduous trees. This biome has been affected by human activity, much of it being converted into agricultural fields or urban development. Dominant plant species include maple, beech, oak, hickory, basswood, cottonwood, elm and willow. There are also many well-developed and richly diversified shrubs and herbs. There are many different types of herbivores and carnivores, as well as reptiles and amphibians. Temperate deciduous forests also have brown forest soils. Their surface has a litter layer that is very thin, as decomposition takes place rapidly.

These forests typically have several layers, with enough light reaching the bottom layer of herbaceous plants. The leaves fall at the end of summer, leaving the trees dormant during winter. The leaf loss is only possible if the soil contains enough nutrients for the leaves to regrow in spring.

Tropical Rainforest

Tropical rainforests usually occur near the equator. Rainfall in these areas is usually distributed evenly throughout the year. Temperature and humidity is usually high. Plant species are diverse. The trees in this biome are closer together, forming a canopy 25-35 metres tall. Characteristic species include vines and orchids, as well as ferns and palms. Most plants are evergreen with large, dark green, leathery leaves.

Tropical rainforests also contain a great variety of animals, believed to be 30-50% of all the Earth’s species

The rate of decomposition is high because of high temperatures and abundance of moisture. Frequent rains cause the soils to be subject to extreme chemical weathering and leaching. These soils are acidic and nutrient poor

Because little light can penetrate the canopy to reach the forest floor, much of the animal life is confined to the canopy. The soil below along with the canopy above support a huge fauna of non-vertebrates


Tundra means marshy plain. It is characterised by the absence of trees, and the presence of dwarf plants, as well a ground surface that it wet, spongy and hummocky. Soil in this biome are usually permanently frozen, called permafrost, up to a metre or more deep (though usually only a few centimetres). This line acts as a physical barrier to plant root growth. The amount of air in the soil is also limited because of the permafrost.

Conditions sometimes improve enough in summer to support plant life, along with the brief appearance of insects.

Temperature, precipitation and evaporation all tend to be at a minimum. In summer months of most tundra biomes, the temperature will average at below 10oC. The precipitation would be no higher than 25mm. The ground of tundra tends to be waterlogged because of the low rates of evapotranspiration.

There is a relatively small species diversity of tundra vegetation. The plant communities are typically composed of a few species of dwarf shrubs, some grass species, sedges and mosses. The most characteristic plants are lichens (such as reindeer moss). The principal herbivores would include caribou, musk, ox, arctic hare, voles and lemmings. Most bird species of tundra are able to migrate to warmer places during the cold winter months. The herbivores support a few carnivore species such as wolves, polar bears, snow owls and arctic fox. There are very few reptiles and amphibians because of the low temperatures.

Alpine tundra is very similar to arctic tundra, although without the presence of permafrost and the presence of a better drainage system. It occurs on the highest mountains, well above the tree-line. The night-time temperatures are below freezing, though the day-time temperature will usually rise above freezing, allowing for slow growth of plants.