OCR A2 UNIT F215 POPULATIONS AND SUSTAINABILITY

Specification:

1) Explain the significance of limiting factors in determining the final size of a population;

2) Explain the meaning of the term carrying capacity;

3) Describe predator-prey relationships and their possible effects on the population sizes of both the predator and the prey;

4) Explain, with examples, the terms interspecific and intraspecific competition;

5) Distinguish between the terms conservation and preservation;

6) Explain how the management of an ecosystem can provide resources in a sustainable way, with reference to timber production in a temperate country;

7) Explain that conservation is a dynamic process involving management and reclamation;

8) Discuss the economic, social and ethical reasons for conservation of biological resources;

9) Outline, with examples, the effects of human activities on the animal and plant populations in the Galapagos Islands

Population Dynamics

The size of a population of organisms (one species in an area) changes over time.

The population size is determined by birth rate (reproduction rate), death rate (mortality), immigration and emigration as illustrated below

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Carrying Capacity and Limiting Factors

Definitions

Limiting FactorA limiting factor is a biotic or abiotic factor that prevents a population from increasing in size at its maximum rate

Limiting factors include:

Availability of resources such as food, water, oxygen, nesting sites (animals) and water, carbon dioxide, mineral ions and light (plants)

Effects of other species such as predators and parasites (of animals) and grazers or parasites (of plants)

Competition between members of the same species (intraspecific) and between members of different species (interspecific)

Limiting factors are also described as environmental resistance

Carrying Capacity

The carrying capacity is the maximum population size that can be maintained in a habitat over a period of time. The carrying capacity is determined by the limiting factors in the environment

Sigmoid Population Growth Curve

The concept of population growth curves was covered in the biotechnology module. The principles of population growth apply for many species (the biotechnology module was concerned with the growth of populations of micro-organisms)

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Summary of Phases in the Sigmoid Growth Curve

a) The Lag Phase

The population size is very low as the individuals acclimatise to their habitat

The rate of reproduction is zero or very low (you must check the graph you are required to describe in an examination question)

Many species of plants and animals have a breeding season and members of the species entering a new habitat must wait until the breeding time before the population size can begin to increase. This may explain the lag phase of their growth curve

b) The Log Phase/Exponential Phase

The rate of reproduction is fast and is greater than the death rate

The population size increases rapidly

There are no limiting factors – resources are plentiful and the abiotic factors are favourable to population growth

c) The Stationary Phase

The population size has levelled out at its carrying capacity of the habitat

The reproduction rate and death rate are equal

The population size remains stable, or has small fluctuations up and down around a mean as the environmental factors change. These environmental changes may be seasonal

Population Curve for k-strategists

The sigmoid curve on page 2 is typical of species called k-strategists

The limiting factors exert a greater and greater effect as the population size gets closer and closer to the carrying capacity

The population growth curve includes a deceleration phase just before the stationary phase is reached

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Population Growth Curve for r-strategists

Some species have a different type of population growth

The log phase shows a very rapid increase in population size such that the carrying capacity is exceeded before the limiting factors have an effect

This type of population growth is called ‘boom and bust’ and is shown by species called r-strategists

When the carrying capacity is exceeded, there are not enough resources for reproduction and increased competition leads to more individuals dying

R-strategists include species with a short generation time such as bacteria and pioneer species

Pioneer species colonise a new habitat before k-strategists

Predator-Prey Relationships

Definitions:

Predator – a predator is an animal that hunts and kills other animals (its prey) for food

Prey – a prey is an animal that is hunted and killed by another animal (a predator) for food

Predator-Prey Relationship between a Predatory Mite and a Prey Mite – a laboratory experiment

This laboratory experiment with populations of a predatory mite and a prey mite was carried out in the 1950’s

Observations and Explanations of the Graphs

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The population sizes of both mites oscillate. The increases and decreases in population size of the predator lag behind those of the prey

The population of the prey species rises first, followed by an increase in population size of the predator. This is explained as the predatory mites feed on the prey mites. As the prey population size increases, there is more food for the predators, so their population size increases also, following that of the prey

The rise in population size of the prey is followed by a decrease. This occurs because with the increased predator population size, more and more prey are eaten

The decrease in population size of the prey means there is less food for the predators so their numbers decrease also, following the prey population

As the predator population falls, fewer prey are eaten and their numbers increase as the reproduction rate in the prey can increase

Predation by the predatory mite is density dependent (dependent upon the density of the prey mite)

Predator-Prey Relationships in Natural Ecosystems

In the laboratory example, there is one predator species and one prey

In complex food webs a single predator/single prey situation is rare. A predator usually eats many different types of prey and the prey are food for more than one type of predator

In addition, it is unlikely that the availability of its prey is the only limiting factor for the predator population. Likewise, the predator numbers is not the only limiting factor for the prey population

A predator-prey relationship has been studied over many years in Northern Canada and is shown graphically below. The predator is the lynx and snowshoe hares are the prey

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This graph confirms a relationship between the population sizes of hares and lynxes

However, other limiting factors could also be involved. The maximum population size of the hares in each cycle is variable and the lower peaks could be due to lack of food for the hares, in the cold ecosystem in which they live

The maximum lynx population sizes are also variable. Other limiting factors affecting the predator population size could be disease and hunting by humans

Competition

Organisms compete when a factor is in short supply. As the competition increases, the reproduction rate decreases and the death rate increases. There are two types of competition:

1) Intraspecific Competition – this is competition between members of the same species

This is important in the process of natural selection when the individuals in a population that are best adapted to obtaining food or mates will survive to reproduce

Intraspecific competition is also very important in limiting the size of a population when a population enters the stationary phase. Although the population size fluctuates in this phase, the population size is fairly stable since if the population increases, there is more competition and the population size then falls. Also, if the population size decreases, there is less competition and the population size increases

Remember that ‘intra’ means ‘within’ – the ‘intranet’ is set up within an organisation or company

An example is competition between robins in the gardens of a village for food and nesting sites

2) Interspecific Competition – this is competition between members of different species

Interspecific competition only occurs if the niches of the two different species overlap

Remember that the ‘internet’ or international are worldwide

Some Examples of Interspecific Competition

Interspecific competition has been studied in laboratory experiments

Competition between Two Species of Flour Beetle in a Container of Wholemeal Flour

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If small numbers of each species were placed into some wholemeal flour to which a little yeast was added, the population of Tribolium castaneum always increased and then fluctuated. The population size of Tribolium confusum gradually fell until it died out completely

Similar results have been found in many other cases where two species with similar requirements/niches are living and reproducing in the same restricted habitat. One species often out-competes the other. This idea is known as the competitive exclusion principle

However, in the Tribolium experiments, a very small change in the temperature could alter the species that survived. If the temperature was > 29oC, T.castaneum survived. Below 29oC, T.confusum survived

Competition between Two Species of Paramecium

Paramecium species belong to the kingdom Protoctista. What features do you think they have?

When the two Paramecium species, Paramecium caudatum and Paramecium aurelia were grown together, the population of P.aurelia increased and that of P. caudatum decreased and died

Competition between Plants

Some plants release chemicals into their habitat that interfere with the metabolism of other competing plants

This concept is called allelopathy – it is a form of competition because it prevents other plants using the resources

The chemicals may inhibit growth, seed germination or mineral ion uptake

The chemicals may be secreted from the roots or be leached out of leaves/fruit when they are shed from the plant (abscission)

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Sustainable Management of Ecosystems

The world human population is increasing at an exponential rate

Increasing numbers of humans puts increasing demands on the Earth’s resources

Without careful planning, the intensive methods used by humans can easily disrupt/destroy ecosystems, reduce biodiversity and use up the resource completely

For wood and timber production, it is possible to exploit forests and woodlands without destroying them. The methods employed require sustainable management of these ecosystems

Summary of Sustainable Management of Forest/Woodland

Involves harvesting timber without destroying the forest/woodland so that the forest ecosystem is maintained, maintaining biodiversity

Timber can be removed from the forest year after year for a long period of time

Trees are cut down but new trees are planted to replace them – reforestation

Techniques such as coppicing, pollarding, selective felling and rotational felling are used

Woodland Management for Small Scale Timber Production

Pollarding

The trunk of a deciduous tree is cut about 2 metres from the ground. New shoots will grow from the cut region and will mature into stems that can be harvested as a wood source. Cutting the trees at this higher level prevents deer feeding on the new shoots

Coppicing

The trunk of a fast growing deciduous tree(one that loses its leaves in the winter) is cut close to ground level, leaving a stool

New shoots will grow from the stool and mature into stems of narrow diameter – called poles

The poles can be cut and used for firewood, fencing, garden furniture and making charcoal

After cutting the poles, new shoots grow and the cycle starts again

Sweet chestnut, hazel and ash are trees suitable for coppicing

Usually, the woodland is divided into areas, with one area only coppiced each year. When all areas have been coppiced once, the first area is coppiced again. This is called rotational coppicing

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Often, a ‘coppice with standards’ system is used. This means that some of the trees (particularly the slower growing ones like oak) are not coppiced but allowed to grow into full sized trees called standards. The oak trees may be cut when they are big enough to provide large timber

Coppicing is only suitable for small scale timber production since it is labour intensive

Benefits of rotational coppicing

Rotational coppicing increases the biodiversity of the woodland. When unmanaged, a woodland goes through succession to a climax community. The canopy of the tallest trees blocks out light to the woodland floor, reducing the number of species that can grow there. Coppicing opens up parts of the woodland, increasing light intensity on the woodland floor. This allows ground level herbaceous plants to grow. Since different parts of the woodland are at different stages of the coppice cycle, biodiversity in maximised

Habitats are preserved and different nesting sites are available for different bird species in different areas of coppiced woodland

Preservation of tree species prevents the soil erosion that occurs when deforestation is carried out

There is less disturbance by machinery

Managing Large Scale Timber Production

It is more difficult to carry out large scale timber production in a sustainable way

Rather than clear felling (removal of all the trees in an area), selective felling is carried out. Selective felling involves cutting down some of the largest commercially valuable trees only and leaving the others to grow

By selective felling, much of the habitat and biodiversity is maintained. However, large machines are used to fell and remove the cut tree so there is disturbance to the habitat

Sound forestry practices optimise the growth of healthy trees such as controlling pests and pathogens, selecting suitable trees to plant according to soil type and climate and planting trees at the correct distance apart – if too close, they will grow very tall and thin

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Comparison of Large and Small Scale Timber Production

Differences Similarities Different sized timbers produced. Coppicing produces narrow diameter poles. Felling of large trees produces large timbers

Selective cutting in large scale and cutting of standards in small scale are similar strategies to obtain high quality large timbers

More habitat destruction in large scale production. Coppicing increases biodiversityLarge scale production reduces soil quality – it leads to soil erosion Large scale production requires the planting of new trees. Coppicing does not (unless coppicing with standards)

Conservation

Definitions

Biodiversity – the variety of different species in the area, the variety of habitats and the genetic variation within each species

Conservation – the active management of habitats in order to maintain or increase their biodiversity

Preservation – keeping ecosystems and biodiversity as they are now

Reclamation – this involves reversing the effects of human activity by using management techniques to restore habitats that have been lost. An example is reclaiming wet meadows that have been drained to use as farmland

Ecologists conserve ecosystems and do not aim to preserve them. This means that ecologists have to manage ecosystems since they are dynamic and will change over time, by succession processes

You should review you conservation notes covered in Unit F212 Maintaining Biodiversity when preparing for F215, since this is relevant to synoptic questions

Conservation involves establishing National Parks, green belt land, SSSIs (sites of special scientific interest) and nature reserves

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Reasons for ConservationReasons for Conservation ExamplesEconomic So that we can harvest resources from our environment for

years to come such as timber from woodland and fish from the oceans

Natural predators of pests can be used in biological control to reduce expenditure on synthetic pesticides

Maintaining habitats for pollinating insects is important for pollination of crops. Without them, harvests would fail and farmers go out of business

Microbes are important in sewage treatment and water purification, preventing disease

Genetic resources - wild relatives of crop plants and domesticated animals are important sources of genes and alleles for crop and animal improvement. Maintaining wild species maintains the genetic diversity for future use in breeding disease resistance and drought tolerance into domesticated species (genetic engineering)

So that we can use more plants and animals from tropical rainforests as a source of medicinal drugs

Ecotourism – raise money from visitors such as in the Galapagos Islands

Ecological Important to preserve biodiversity. Organisms are part of food chains. Loss of species disrupts food chains and the interactions between species eg hunting top predators such as lions leads to an increase in herbivores that leads to overgrazing and this leads to soil erosion

Important to preserve the whole habitat since its disruption will affect many species

Over hunting animals and overfishing can endanger species

To support indigenous people

Bacteria and fungi are important in the recycling of minerals in all habitats. This removes dead carcasses from habitats

Maintaining forests is important for the regulation of the O2 and CO2 concentrations of the atmosphere, local rainfall and soil condition

Aesthetic and Recreational Value Most natural habitats and man-made ecosystems are beautiful tranquil places that people enjoy and can relax in. They should be conserved for future generations also

Many people derive pleasure from observing wildlife in its natural habitats

Maintaining the fishing and timber industries ensures that workers in these industries do not lose their jobs

Ethical Humans have a duty to conserve habitats/ biodiversity for future generations

Organisms have a right to exist

Humans and the Galapagos Islands

Background

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The Galapagos Islands are a small group of isolated islands lying on the equator to the west of Ecuador, which owns them

These islands are 600 miles away from South America. They consist of 15 main islands, 3 smaller islands and 107 rocks and islets. All of the islands are the tips of volcanoes that erupted under the sea millions of years ago

They are famous for the many species of plants and animals that live only there – these are endemic species including Darwin’s finches, giant tortoises and land iguanas. The islands received World Heritage Site status in 1978

They are also famous because they were visited by Charles Darwin on HMS Beagle in 1835. The observations Darwin made there were important in developing his ideas of evolution

The plants and animals of Galapagos have been living in isolation from each other and the mainland of South America for hundreds/thousands of years. They have been geographically isolated

They provide excellent examples of speciation, for example, the many species of finches on different islands

Different islands presented different selection pressures to the organisms. Different adaptations were selected for. Because the organisms were reproductively isolated, speciation occurred

The population sizes were small resulting in greater genetic drift

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Threats to Species on the Galapagos

50% of vertebrate species and 25% of plant species are endangered

Activities of humans on and around the Islands is a serious threat to their biodiversity

The resident human population size is increasing, as more people leave mainland Ecuador to find work in the Islands tourist industry

Resident human population sizes: 1990 - <10,000 2008 - >28,000

2014 - estimated 40,000

The tourist trade has also increased as more visitors arrive to experience the beauty of the ecosystems. In 1980, the number of visitors was 16000. This figure had increased to 125000 by 2005. Calculate the percentage increase in visitors between these two dates.

Ways in which human activity has put the endemic species at risk of extinction

Habitat Destruction

Habitats and ecosystems are disturbed or destroyed

Trees are cut down – deforestation. Many forests of Scalesia trees and shrubs (an endemic genus) have been destroyed on Santa Cruz and San Cristobal

Deforestation provides land for building houses, schools, factories and roads (urbanisation)

Land is also used for farming

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There is an increased demand for oil and an oil tanker spill in 2001 damaged marine and coastal ecosystem

Tourism increases the risks to the ecosystem. The National Park Service regulates where boats can land and where visitors can walk

Increased Pollution

The increased rubbish that more humans create is disposed of in dumps or burned, releasing more polluting gases

Increased sewage is dumped in the sea causing eutrophication

Increased hunting, poaching and over-fishing

Humans’ over-fishing is a problem. Many new immigrants could not find work on the islands and resorted to fishing. Far too many sea cucumbers have been removed for the Asian market, such that this species is under threat. Quotas have been brought in but the over-fishing continues

In the 19th century, whaling boats and fur traders took up residence, harvesting whales and seals to sell internationally. Giant tortoises were taken on the boats as a source of food for the whalers. 200,000 tortoises were taken in less than half a century

Introduced Species

The deliberate introduction of non-native species such as feral cats, feral dogs and feral goats to the islands is a real problem

Feral cats hunt a number of endemic species, including the lava lizard and young iguanas

Feral dogs eat tortoise eggs

Goats eat most vegetation including the endemic rock purslane

On several islands, the grazing of the goats is threatening the tortoise populations, because the goats out-compete the giant tortoises for food and water. The goats also change the habitat and this reduces the number of tortoise nesting sites

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On Northern Isabella Island, the goats have transformed forest into grassland, leading to soil erosion

Fruit and vegetable plants have also been deliberately brought onto the islands

The red quinine tree is an invasive introduced species. Its seeds are wind dispersed. The highland ecosystem has changed from low scrub and grassland to a dense forest canopy. These trees have out-competed the native Cacaotillo shrub from Santa Cruz and the Galapagos petrel (bird) has lost its nesting sites. The red quinine also successfully out-competes native Scalesia trees

Some insect species have been carried to the islands accidentally. Alien insect species eat native species, out-compete other native species and bring diseases such as avian (bird) malaria onto the islands

Conservation Projects

In 2007, the United Nations put the Galapagos Islands on its Red List of endangered sites. The Galapagos government’s response included making new laws and placing restrictions on human activity, issuing eviction orders and culling introduced species of animals

In 1999, the Charles Darwin Research Station began strategies to prevent the introduction of alien species and to treat the problems caused by such species

Boats and tourists arriving on the islands are searched for alien species

Natural predators are being used to eradicate pest species – the release of ladybirds wiped out a scale insect, a plant pest

Feral goats have been culled on Isabella Island

Problems arising from this 2007 UN decision

Economic : fewer jobs, smaller profits, reduced tourism, business closure, less income

Ethical : animal welfare issues ensuring that the culling of animals is humane and the ethical issues of people suffering through losing their jobs and homes

The Charles Darwin Research Station has also started a captive breeding programme to increase tortoise numbers

A marine reserve has been set up around the Islands. At least 36% of the coastal zones have been designated ‘No-Take’ areas, where no extraction of resources is allowed

It is important that the residents on the Islands are educated about the value of the islands ecosystems and the importance of conservation

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