Chapter 24


Population growth

Population griwth cannot be sustained indefinitely as limiting factors will prevent the population from rising above a certain level. A limiting factor is am environmental resource or constraint that limits population growth. If the growth of a new population over time is plotted on a graph, regardless of the organism, most natural populations will show the same characteristics. This is known as a population growth curve. The graph can be divided into 3 main phases:

  • Phase 1 - a period of slow growth. The small number of individuals that are initially present reproduce increasing the total population. As the birth rate is higher than the death rate te population increases in size.
  • Phase 2 - a period of rapid growth. As the number of breeding individuals increases, the total population multiplies exponentially. No constraints act to limit this exponetial growth.
  • Phase 3 - a stable state. Further population growth is prevented by external constraints. During this time the population size fluctuates, but overall its size remains relatively stable. Birth rates and death rates are approximately equal. Slight increases and decreases can be accounted for by fluctuations in limting factors, such as the presence of predators.
1 of 16

Limiting factors`

In theory if all resources were plentiful a population would continue to grow exponentially. However this is rarely seen in nature and instead a short period of growth occurs when conditions are ideal and the maximum growth rate is achieved. Limiting factors prevent further growth of a population and in some cases cause it to decline. Examples of limiting factors include competition between organisms for resources, the build up of the toxic by products of metabolism, or diease. Limiting factors can be divided into abiotic and biotic factors: abiotic factors are non living and include temperature, water availability, light and pH. Biotic factors include predators, disease and competition. The maximum population sixe that an environment can support is known as its carrying capacity, although individual years can show fluctuations in population size, it reamins stable as the number of births and deaths is approximately equal.

Another important variable which affects the population size is migration. Immigration is where the movement of individual organisms into a particular area increases population size. Emmigration is the movement of individuals of a species away from a particular area which decreases population size. Density independant factors are factors which have an effect on the whole population regardless of its size which can dramatically change a population or remove one, eg a volcanic eruption.

2 of 16

Types of competition

Competition is an example of a biotic limiting factor as it is the result of interactions between living organisms. There are two main types of competition: Interspecific competition is competition between different species and intraspecific competition which is competiton between members of the same species.

Interspecific competition occurs when two or more different species of organism compete for the same resource. This interaction results in a reduction of the resource available to both populations. For example if both species compete for the same food source there will usually be less available for organisms of both species. As a result of less food, organisms will have less energy for growth and reproduction resulting in smaller species compared to if only one of the species was present. If two species of organism, however, are both competing for the same resource but one is better adapted, the less well adapted species is likley to be outcompeted. If conditions remain the same, the less well adapted species will decline in number until it can no longer exist in the habitat alongside the better adapted species. This is known known as the competitive exlusion principle, where two species are competing for limted resources, the one that uses the resources more effectively will ultimately outcompete the other, for example the red and greq squirrels in the UK.

3 of 16

Types of competition II

Intraspecific competition occurs when memberts of the same species compete for the same resource.. The availability of the resource determines the population size, the greater the availability the larger the population that can be supported. This results in fluctuations in the number of organisms present in a particular population over time, and can be divided in 3 stages:

  • Stage 1 - When a resource is plentiful in a habitat, all organisms have enough of the resource to survive and reproduce, which results in an increased population size.
  • Stage 2 - As a result of the increased population, there are many more individuals that share the resources available, and therefore they will become limtied and will not be enough for all organisms to survive, resulting in the population decreasing in size.
  • Stage 3 - Less competition exists as the smaller population means less organisms are competing for the same resources. This means more organisms survive and reproduce, resulting in population growth. 

The cycle of event will the repeat.

4 of 16

Predator-prey relationships

Another major biotic factor that has an influence on population size is the role of predation where an organism killls and eats another organism. The size of the predator and prey popluations are interlinked. As the population of one changes, it causes a change in the size of the other. In general all predator-prey relationships follow the same pattern. The peaks/troughs of the prey population are mirrored by those in the predator population after a time delay:

  • Stage 1 - An increase in the prey population provides more food for the predators, allowing more to survive and reproduce, increasing in the predator population.
  • Stage 2 - The increasd predator population eats more prey organisms, causing a decline in the prey population, and the death rate of the prey population is greater than its birth rate.
  • Stage 3  - The reduced prey population can no longer support the large predator population, Intraspecific competition for food increases, decreasing the size of the predator population.
  • Stage 4 - Reduced predator numbers result in less of the prey population being killed. More  prey organisms survive and reproduce, increasing the the prey population - the cycle restarts. 

Rarely in the wild is the link between predator and prey this simple, as other factors will also influence the population size like biotic factors or season changes in abiotic factors.

5 of 16

Conservation and preservation

Conservation means the maintenance of biodiversity through human action or management. This includes maintaing biodiversity between species, maintaing genetic diversity within a species, and the maintenance of habitats. Conservation involves the managment of ecosystems so that the natural resources in them can be used without running out. This is known as sustainable development. Conservation also includes reclamation, which is the process of restoring ecosystems that have been damaged or destoryed, for example due to a flood or a new building projet. Reclamation also involves techniques like controlled burning of forests to halt succession and increase biodiversity. Conservation is dynamic and needs to adapt to constant change.

Preservation is the protection of an area by restricting or banning human interference, so that the ecosystem is kept in its original state. It is most commonly used when preserving sensitive resources which can easily be destroyed or damaged by distrubance. For example newly discovered caves contain very sensitive geological fomrations and unique ecosystems have their entrances barred to preserve them.

6 of 16

The importance of conservation

Conservation is important for many reasons, which can broadly divided into 3 groups:

  • Economic - to provide resources that humans need to survive and provide income. For example rainforest species provide medicine, clothes and food, and materials like timber.
  • Social - many people enjoy the natural beauty of the world and they are useful for exercise and relaxtation which helps lower stress in humans.
  • Ethical - all organisms have a right to exist, and most play an important role within their ecosystem. Many people believe that we should not have the right to decide which organisms should survive amd which we could live without. We also have a moral responsibility for future generations to conserve the wide variety of existing natural ecosystems.

The gray bat is an example of a species which has been successfully supported through conservation measures. The species became endagered following human exploitation of its habitat and through pesticide bioaccumulation. Measures were taken including gating the caves the bats where present in to prevent human access, strictly controlling the exploitation of land around the caves as it provides the bats with their food source, along with raising awareness about the endagrerd species. 

7 of 16


An increasing global population results in greater demand for resources.The world's natural resources have conflicting demans on them. To conserve natural resources for future generations, sustainable managment of the natural environment is necessary. A sustainable resource is a renewable resources that is being economicall exploited in such a way that it will not diminish or run out. The aims of sustainability are to preserve the environment, ensure resources are available for future generations, allow humans in all societies to live comfortably, enable LEDCs to develop through exploiting their natural resources, and to create more of an even balance in the consumption of these resources between MEDCs and LEDCs.

Alongside the sustainable managment of resources, existing resources should be used more efficiently, helping to prevent finitie resources being used up so quickly. For example many products can be reused or recycled. As technology develops alternatives may be developed that could ease the strain on current finite resources, but these may take years to develop, be more costly, and have negative environmental effects of their own.

8 of 16

Sustainable timber production

The sustainable managment of forests allows for the maintenance of a forest's biodiversity while sustaining both our supply of wood to meet demands and the econmic viability of timber production. The technqiues used depend on the scale of timber production. 

A technique known as coppicing is used to produce sustainable timber on a small scale where a tree is trunk is cut close to the ground. New shoots form from the cut surface and mature. Eventually these shoots are cot and in their place more a produced. these shoots have many uses including fencing. In most managed woodlans, rotational coppicing takes place, where the woodland is divided into sections and trees are only cut in a particular section until they have all be coppiced, and then coppicing begins in another area, allowing time for the newly coppiced trees to grow, and by the time the first coppiced trees are reached they have matured, and the cycle begins again. Rotational coppicing maintains biodiversity as the trees never grow enough to block out light, so succession does not occur so more species can survive. An alternative technique to coppicing is pollarding where the trees are cut higher up so animals cannot eat the new shoots as they appear.

9 of 16

Sustainable timber production II

Sustainable timber production on a large scale is based around the technique of felling large areas of forest. The felled trees are destroyed and will not regrow. To ensure that production is sustainable, timber companies:

  • Practise selective cutting, which involves removing only rhe largest trees.
  • Replace trees through replanting rather than waiting for natural regeneration. This also helps to ensure that the biodiversity and mineral and water cycles are maintained.
  • Plant trees an optimal distance apart to reduce competition. This results in higher yields as more wood is produced per tree as they can grwo larger.
  • Manage pests and pathogens to maximise yields.
  • Ensure that areas of forest remain for indigineous people.

The makro disadvantage of this technique is that the habitats are destroyed, soil minerals are reduced, and the bare soil which is left suseptible to soil erosion. Trees are important for binding soil together, removing water from soil, and maintaing nutrient levels through their role in the carbon and nitrogen cycles.

10 of 16

Sustainable fishing

As well as the increased for fuel and materials due to population growth, the demand for food is ever-increasing. Overfishing has led to the populations of some species decreasing significantly. These fish populations are then unable to regenerate, meaning they will no longer be able to provide humans with a food source in the future. Tp try and overcome this problem there are internation agreements so that fishing quotas are put in place to limit the number of certain species of fish that are allowed to be caught in a particular area. The aim is to maintain a natural population of these species that allows the fish to reproduce sufficiently before being hunted to maintain their population. Other techniques that have been used include: 

  • The use of nets with different mesh sizes. For example mesh sizes can be made large enough so thag immature fish can escape and allowinfg mature fish are caught, allowing breeding to continue.
  • Allowing commerical fishing only at certain times of the year protects the breeding season of some fish species and allows the fish levels to increase back to a sustainable level.
  • The introduction of fish farming. This prevents the loss of species whilst maintaing the supply of food. For example salmon can easily be farmed due their diet, tolerance of high densities and rapid growth.
11 of 16

Masai Mara

The MMNR is an example of an ecosystem that is actively managed to balance the needs of humans and the need for conservation. It is a large area of savannah home to zebras, lions, cheetahs and the Masai people. Traditionally this are has been used by the Masai people for farming and rearing livestock. In the past the Masai practised semi nomadic farming where tribes would move frequently depending on climate variation and the presence of tseste flies, and this allowed time for vegetaion to recover from animal graxing. However grazing is now limtied to the edges of the reserve and populations have grown in this area. The Masai Mara rekues on tourism for most of its economic tourism/ Ecotourism is tourism directed towards natural environments and aims to ensure tourism doesn't exploiu the natural environment or local people, to consult and engage with local communities on planned projects and to ensure that infrastructrue improvements benefit local people as well as tourists, and is a type of sustaibable developemnt that aims to reduce the impact that tourism has on the environment. The nature reserve also plays an important role in the conservation of endagered species such as the Black Rhino.

A constnt balance has to be maintained between the human and animal populations, elephans threaten cultivation as they trample crops, so areas have to be fenced, legal hunting is used to cull excess animals which brings money into conservation work, but levels must be constantly monitored. Livestock can also pass or recieve diseases from wild populations.

12 of 16

Terai region of Nepal

The Terai arc is an area of forest and grasslands which is rich in plant nutrients and has extreme biodiversity of sub tropical plants and animals, and it is also an important source of income, but large areas have been removed to sell timber and create space for housing. The removal of large parts of the forest has exacerbated the effects of monsoon flooding. The aim of sustainable forest managment is tp provide a livelihood for local people and ensure the conservation of forests, so several community forestry groups have been set up to improve soil and water managment across the region, increase the retail price of forestry products, employment and income generation through forest production, securing biodiversity of forested areas and increasing forest area and density. The Terai region also has strategies in place for sustainable land use through imrpoving irrigation facilities to improve crop yields, promoting new crops in areas away from the Terai region so prevent further itensification, the growth of nitrogen fixing crops to improve soil fertility, and improviing fertilisation techniques so that less land is needed to produce higher yields of crops.

13 of 16

Peat Bogs

Peat forms when plant material is inhibited from fully decaying by acidic and anaeropbic conditions, which normally occurs in wet and boggy areas, and the plants that grow in these areas have adapted to thrive in wet conditions with few nutrients, and bogs also support a wide range f insects along with the lack of predators. Lowland raised bogs are one example of a peatland ecosystem which are rate and threatened due to afforestation, peat extraction and instensive agriculture. It is essential that the remaining areas are conserved to main biodiversity and also their maintenance will contribute to flood managment, erosion control and carbon storage. Conservation of these bogs focuses on restoring or maintaining the appropriate water levels, eg:

  • Ensuring that the peat and vegetation of the bog surface is as undisturbed as possible, including ditch blocking to raise the water table.
  • Removal of seedling trees as trees have a high water requirment due to transpirtation so removing them maintains the water level in that area.
  • Using controlled grazing helps to maintain the biodiversity of the peatland as it ensures a diverse wetland surface by hlating succession and providing a habitat for many rare species.
14 of 16

Environmentally sensitive ecosystems

Some regions are less resistant to changes than others. These regions are known as environmentally sensitive ecosystems. 

The Galapagos Islands have many animals including some unique to the species like the giant tortoise, the flightless cormorant and the marine inguana, and has a coastal, arid and humid zone all with their own plant ecosyste,s. Human activities are strictly controlled on the islands are prior to this non native animals were introduced that decreased numbers of native species and non native plants competet with the native plants causing a decrease in their populations. Since then measures have been taken to protect the living and non living parts of the unique ecosystem including limitig human access, controlling migration to and from the islands, and strictyl controlling the movement of livestock.

Antarctica is one of the harshest climates on Earth and all the endothermic animals living on and arounf Antarctica rely on thick layers of blubber for insulation, for example the emperor penguin. Visitors to this continent have caused pollution by dumping sewage, oil spills and hunting and whaling have reduced population numbers, along with plant wide impacts such as global warming. To control this all waste must be taken away by shops, there are tourist restrictions and designated protected areas and the banning of hunting and whaling.

15 of 16

Environmentally sensitive ecosystems II

Snowdonia is a National Parl that has a rich diversity of habitats leading to a range of birds such like oystercatchers to sparrowhawks, along with a diverse range of artic alpine plants along with woodland plants. The key purposes of the Park authority are to conserve and enhance the natural beauty, wildlife and hertiage of the area and to enhance the econmic social wellbeing of communities within the park. 

The Lake District is Englands largest national parl that has a wide range of animals include bats, deer, water voles and natterjack toads and a number of native species are under threat of extinction. There are also rare arctic-alpine plant communities including specialised trees that have evolved in these harsh conditions along with the UK's few rare carniverous plants. The park controls human activities in the same way as in the Snowdonia through carrying out regular reoaur abnd maintenance of paths, regrowing damaged vegetation and educating hikers about the importance of sticking to paths.

16 of 16


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all Population size and sustainability resources »