Environmental Systems and Societies for the IB Diploma Paul Guinness and Brenda Walpole Cambridge University Press’s mission is to advance learning, knowledge and research worldwide. Our IB Diploma resources aim to: • encourage learners to explore concepts, ideas and topics that have local and global significance • help students develop a positive attitude to learning in preparation for higher education • assist students in approaching complex questions, applying critical-thinking skills and forming reasoned answers.
CAMBRIDGE UNIVERSITY PRESS
Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo, Delhi, Mexico City Cambridge University Press The Edinburgh Building, Cambridge CB2 8RU, UK www.cambridge.org Information on this title: www.cambridge.org/9781107609204 Š Cambridge University Press 2012 This publication is in copyright. Subject to statutory exception and to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published 2012 Printed in the United Kingdom by Latimer Trend A catalogue record for this publication is available from the British Library ISBN 978-1-107-60920-4 Paperback Cambridge University Press has no responsibility for the persistence or accuracy of URLs for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate. This material has been developed independently by the publisher and the content is in no way connected with nor endorsed by the International Baccalaureate Organization.
Contents Introduction
v
Topic 1: Systems and models
1
Chapter 1.1
1
Systems and models
Topic 2: The ecosystem
13
Chapter 2.1
Structure
13
Chapter 2.2
Measuring abiotic components of the system
23
Chapter 2.3
Measuring the biotic components of the system
29
Chapter 2.4
Biomes
37
Chapter 2.5
Function
42
Chapter 2.6
Changes
56
Chapter 2.7
Measuring changes
70
Topic 3: Human population, carrying capacity and resource use
78
Chapter 3.1
Population dynamics
78
Chapter 3.2
Resources – natural capital
93
Chapter 3.3
Energy resources
109
Chapter 3.4
The soil system
121
Chapter 3.5
Food resources
137
Chapter 3.6
Water resources
145
Chapter 3.7
Limits to growth
152
Chapter 3.8
Environmental demands of human populations
158
Topic 4: Conservation and biodiversity
175
Chapter 4.1
Biodiversity in ecosystems
175
Chapter 4.2
Evaluating biodiversity and vulnerability
185
Chapter 4.3
Conservation of biodiversity
200
Topic 5: Pollution management
212
Chapter 5.1
Nature of pollution
212
Chapter 5.2
Detection and monitoring of pollution
217
Chapter 5.3
Approaches to pollution management
225
Chapter 5.4
Eutrophication
235
Chapter 5.5
Solid domestic waste
243
Chapter 5.6
Depletion of stratospheric ozone
249
Contents
iii
Chapter 5.7
Urban air pollution
262
Chapter 5.8
Acid deposition
269
Topic 6: The issue of global warming Chapter 6.1
The issue of global warming
Topic 7: Environmental value systems Chapter 7.1
Environmental value systems and philosophies
276 276
296 296
Answers
309
Glossary
315
Index
322
Acknowledgements
329
iv
Contents
Introduction This book covers the syllabus for the IB Diploma Programme Environmental Systems and Societies, which is offered at Standard Level only. Our understanding of the environment and its importance to our lives has grown rapidly over recent decades and the Environmental Systems and Societies course is a transdisciplinary subject combining the knowledge and techniques of science with social and cultural aspects of a more anthropocentric approach. The intention of the Environmental Systems and Societies course is to give you the opportunity to study the close relationship between environmental systems and human societies so that you can understand the range of environmental issues that we all face. Environmental issues may either be very local or global. They may affect us directly or indirectly but it is important that we all consider how we use the Earth’s resources. Every individual has a part to play in conserving living things and their environments either on a small scale by recycling or conserving water, or by considering the global response to these issues, for example through the work of organisations such as Greenpeace and UNEP (United Nations Environment Programme). The book follows the sequence of the syllabus in terms of the seven topics and the sub-sections within these topics. The overall objective is to provide comprehensive coverage of all the topics in an up-to-date and interesting format. Each topic is covered in a separate chapter and the key concepts are listed at the start of each section. Case studies have been chosen to represent a wide range of geographical locations and biological examples so that, as you read, you can reflect on the international element of this course. Examples from across the environmental and economic spectrums highlight how our impact on the environment is not just an issue for one country or section of society but is something important to us all. There is a diversity of opinions on the environment which are influenced by culture and location. Some issues are controversial because they occur across cultural and geographic boundaries. The book considers a range of environmental issues from small-scale local events to largescale global issues. The use of ICT and technology in general has made all of us more aware than ever before of what is happening elsewhere in the world and the implications that changes in other parts of the world can have on us. By the end of the course you should be able to reflect on your own personal environmental value system and how it affects the way you behave and react to the environment.
How to use this book Self-assessment question boxes appear at regular intervals in the text so that you can test your own knowledge and understanding of key concepts and terms as you go along. Most questions require relatively brief answers so that you can move on swiftly to the next section of the book. In addition, Discussion points and Research ideas offer the opportunity for more extensive investigation and group work. Exploring the ways in which people’s perceptions differ is an important aspect of this course and you should try to develop your own thinking skills and reflect on your own views about the world in which you live. You are asked to investigate patterns and processes in your own country and compare them to trends elsewhere in the world. Interesting note boxes appear throughout the book to add points of particular interest, which are not necessarily part of the syllabus. You will also find TOK (Theory of Knowledge) boxes, which invite you to reflect on the central role of Theory of Knowledge in the IB Diploma Programme and on our knowledge and understanding of environmental philosophy. In this course you can approach Theory of Knowledge in a very practical context, for example by comparing the way in which scientific quantitative facts are used alongside qualitative value judgements made in politics and ethics. It is also interesting to contrast the ‘systems’ approach of the course with the other methodologies used in science. Topic 1 Systems and models explains the essential systems approach to the study of this subject, identifying some of the underlying principles that can be applied to living systems. Your syllabus advocates a holistic approach to the analysis of environmental systems and societies so that you can arrive at informed personal viewpoints while being aware of the continuum of environmental philosophies. Topic 2 The ecosystem presents much of the basic scientific knowledge and understanding for the topics that follow. Understanding how to measure components of systems and how they can change are central to this topic. Topic 3 Human population, carrying capacity and resource use begins with an analysis of population dynamics before moving on to examine natural capital and energy resources, soil and food, and water resources. This topic consider the limits to growth and the environmental
Introduction
v
demands of human populations. The key concept of sustainability is central to this topic and to the study of environmental systems and societies in general. Topic 4 Conservation and biodiversity raises issues of debate concerning the moral justification for both exploiting species and conserving them. Topic 5 Pollution management aims to provide a basic overview of pollution and its management with examples from aquatic, terrestrial and atmospheric systems. This topic provides the foundation for Topic 6 The issue of global warming where this controversial global issue is examined in considerable depth. Topic 7 Environmental value systems relates back to Topic 1. Thus, you and your teachers may prefer to consider the content of this topic alongside Topic 1 at the beginning of the course. However, it also provides a very useful conclusion to the course.
vi
Introduction
The International Baccalaureate programme aims to help students create a better world through understanding and respecting the world and its different cultures. The phrase ‘Think globally, act locally’ was first used by Scottish town planner and social activist Patrick Geddes, who wrote Cities in Evolution in 1915. It has since become a concept widely used by environmentalists and taken into consideration by governments, educators and communities. The fact that we all share guardianship of our Earth, no matter how small our contribution may be is an idea on which we can all reflect in our daily lives and at the end of this book. Full details of the Assessment Objectives for the Diploma Programme Environmental Systems and Societies course can be found in the relevant IBO guide or website.
Paul Guinness Brenda Walpole
Systems and models
1.1 Systems and models KEY QUESTIONS • What are the characteristics • • •
• • • • •
•
of systems? How can the concept of the system be applied on a range of scales? How are the terms open system, closed system and isolated system defined? How are the first and second laws of thermodynamics relevant to environmental systems? What types of equilibrium are there and how stable is each one? What is meant by the terms positive feedback and negative feedback? How are transfer and transformation processes described? What are the differences between flows and storages in relation to systems? How are quantitative models which involve flows and storages constructed and analysed? What are the strengths and limitations of models?
FPO
The characteristics of systems The environmental systems which we will be studying in this book are one example of the type of complex systems which can be studied using a systems approach. Others include biological systems, transport systems and communication systems. The systems approach looks at the environment as a set of components that work together as integrated units. We may study plants, animals or the atmosphere separately, but using the systems approach we consider them together as components of the complex environments in which they are found. Using this approach, we can obtain an integrated picture of the environment and the relationships and interactions within it. Integrated study is what makes the systems approach very different from the separate study of botany, zoology or geography. A system is defined as an assemblage of parts and the relationships between them which enable them to work together to form a functioning whole. Systems may be living or non-living, large or small. A single cell, a whole body, an ecosystem, or non-living examples such as a banking system or a social system are all examples of systems. In this course, you will be studying natural systems which include individual organisms, ecosystems (communities of organisms and their environment) and biomes (groups of ecosystems with a similar climate). You will have the opportunity to examine the interactions within systems that are often represented as diagrams with inputs, flows and outputs. All systems have components which are represented in diagrams showing their interconnections (Figure 1.1). Components and their commonly used representations are shown in Table 1.1. Component of system
Shown as
storages (stores of matter or energy)
boxes
flows (into, within and out of the system)
arrows
inputs
arrows into the system
outputs
arrows out of the system
processes (which transfer or transform energy or matter from one storage to another)
labels such as respiration, consumption or photosynthesis
Table 1.1 System components and their representation.
photo 1.1
Earthrise. This photograph of the Earth from space became an early symbol of the global ecology movement. It was taken in 1968 on the Apollo 8 mission. 1
Theory of Knowledge
carbon dioxide water
water
oxygen
waste
biomass to next trophic level
nutrients from food biomass to decomposers
Figure 1.1 The systems approach can be used to consider an individual fish in a pond ecosystem. A systems diagram can be drawn to show storages, inputs and outputs.
The scale of a system The scale of a system can range from a small part of a larger ecosystem, such as a tree in a forest, or on a larger scale, the whole ecosystem and, on a significantly larger scale, the Earth itself could be regarded as a system. Whatever their size, each of these systems has inputs, stores and outputs. One example of a small-scale natural system is a pond within a woodland. The pond has inputs which include the light energy, water and nutrients needed to sustain the system and also outputs such as the oxygen, decomposing material and nutrients which are generated in the pond. The pond in turn forms part of the whole woodland ecosystem which also has inputs, stores and outputs. On a much bigger scale, the woodland can also be thought of as part of an even larger system which includes all the woodland biomes in different parts of the world. These woodlands share the same climatic conditions and we can study them together as one large-scale system.
Reductio Reductionist versus systems approach A reductionist view of a natural system looks at a single object that can be clearly recognised and identified by its properties. The organisms found within a pond are individually described in reference books which describe them in terms of the characteristics that they have, such as whether they are a plant or an animal. At the next level, animals may be described as invertebrates or vertebrates and so on. This reductionist approach does not try to consider how the pond works as a dynamic system. A systems approach gives a holistic view of the pond. The reductionist view does not allow interconnections and interrelationships that go on in the pond to be taken into account. The systems approach considers any system as a set of interrelated objects. In the pond, the most obvious interrelationship between the plants and animals is that some plants and animals are food for other animals; this relationship is called a food chain, and without it, animals would die of starvation. Imagining the pond system as flows of energy and matter (food) between objects (plants and animals) means that a picture of the pond system’s structure (the objects and their relationships) and function (the purpose of the various interactions) can be built up. 1 What are the advantages and disadvantages of the systems approach compared with a reductionist approach to the study of an ecosystem? 2 In science, the reductionist and the systems approach may use similar methods of study. What is the most important difference between the philosophies of the two approaches? 3 Discuss the following statement. Do you think it has any validity? ‘The reductionist view that currently dominates society is rooted in unlimited economic growth, unperceptive to its social and environmental impact. It cannot resolve the converging environmental, social and economic crises we now face.’
The Gaia hypothesis
one system which is self-regulating and maintains the conditions for life on Earth.
In the 1960s, James Lovelock first suggested the Gaia hypothesis. This proposed that the Earth can be regarded as a single functioning system. The hypothesis was developed further by Lynn Margulis in the 1970s. These two scientists suggested that all living organisms and their non-living environments are closely integrated to form
Environmental scientists who work on the Gaia hypothesis gather observations on how the biosphere (the living component of the Earth) and the evolution of life contribute to the stability of the Earth’s temperature, ocean salinity and oxygen content of the air to establish a balance that provides the optimum conditions for life.
2
Systems and models
When it was first proposed, the hypothesis received a hostile reception from the scientific community, but now it is widely accepted and considered in the study of Earth science and geophysiology as well as biogeochemistry and systems ecology.
Interesting note
(a) Open system energy in
energy out
matter (mass) in
matter (mass) out
(b) Closed system
For many centuries, people have discussed ideas of a holistic view of the Earth as an integrated, living whole. In ancient Greek mythology, Gaia was the goddess who personified the Earth. James Lovelock gave her name to his hypothesis following a suggestion from the novelist William Golding. Gea is an alternative spelling for the goddess Gaia that is used as prefix in geology, geophysics and geochemistry. The Gaia hypothesis became well known during the 1960s at the time of the ‘space race’ between the Soviet Union and the USA when people first saw images of the whole Earth taken from space (see page 1).
Open, closed and isolated systems Systems are divided into three types: open, closed and isolated (Figure 1.2).
Key terms An open system exchanges both matter and energy within its surroundings across the boundaries of the system. A closed system exchanges energy but not matter across the boundaries of the system. An isolated system exchanges neither energy nor matter with its environment.
An open system exchanges both matter and energy with its surroundings across the boundaries of the system. Most living systems and all ecosystems are open systems which exchange energy, new matter and wastes with their environment. These open systems and the exchanges which take place can be seen in any living environment, even in remote locations such as Antarctica or tiny isolated islands.
energy
energy
(c) Isolated system
Figure 1.2 Open systems exchange energy and matter, closed systems exchange only energy, and isolated systems do not exchange energy or matter.
In a woodland ecosystem, the main inputs include light and carbon dioxide which plants use for photosynthesis. Further inputs come from woodland herbivores that return mineral nutrients to the soil in faeces and bacteria in the soil which fix nitrogen from the air. Outputs may include water which is lost during respiration and transpiration, nutrients which flow away in waterways and heat which is exchanged with the environment around the woodland. In a closed system, energy but not matter is exchanged across the boundaries of the system. These systems are very rare in nature. Most examples are used for experiments and are artificial. A bottle garden or an aquarium can be set up so that light and heat are exchanged across their boundaries but matter cannot be exchanged or leave the system. In most cases, these systems do not survive because they become unbalanced. Organisms may die as oxygen is depleted or as food runs out or waste matter builds up to toxic levels. An isolated system exchanges neither energy nor matter with its environment. No such systems exist although some people regard the entire universe as an isolated system.
1.1 Systems and models
3
Self-assessment questions 1
2 3
Outline the difference between the systems approach and the conventional approach to the study of an ecosystem. Construct a table to compare the exchange of matter and energy in open, closed and isolated systems. Discussion points: a What are the benefits and drawbacks of using the systems approach in other subjects such as engineering or economics? b Do you think that it is useful to have the concept of an isolated system which does not exchange energy or matter with its surroundings?
Laws of thermodynamics and their relevance to environmental systems The first law of thermodynamics states that energy cannot be created or destroyed but can be converted from one form to another. Energy exists in the form of light, heat, chemical energy, electrical energy, sound and kinetic energy. Different forms of energy are interconvertible but, in a living system, heat energy cannot be converted to other forms. The most obvious non-living conversion of heat energy is seen in a steam engine when it is converted to kinetic energy. In an ecosystem, useful energy enters the system as light energy which is converted to chemical energy during photosynthesis and used to build the bonds found in plant biomass (biological organic matter). This energy in plant biomass is passed along a food chain in a series of transfers (page 9) as organisms feed on plants and are themselves eaten. At each stage in the transfer processes, some energy is transformed (page 9) to other forms, including heat, as organisms respire and use the energy from their food to fuel their life processes. Energy leaves the system as heat because heat energy cannot be transformed in a living process. In living systems, no new energy has been created but the input energy has been converted from one form to another. Although the total amount of energy in the system does not change, the amount that is available to living things gradually reduces as energy is used for growth, movement, reproduction and other processes. Energy transfer and transformation are not very efficient in living systems and at each transfer there is less energy available after the transfer 4
Systems and models
than at the start; less than 10% of useable energy is passed from one organism to the next in a food chain (Figure 1.3).
energy lost and unavailable
fox
rabbit
grass
energy lost and unavailable
Figure 1.3 Only 10% of the energy is transferred at each link in a food chain.
The second law of thermodynamics states that in isolated systems entropy tends to increase. Entropy is a measure of the evenness of energy distribution in a system. Energy is used to create order and hold molecules together. This means that if less energy is available, entropy (disorder) increases. As entropy increases, energy and matter change from a concentrated to a more dispersed form. The availability of energy to carry out processes becomes reduced and the system becomes less orderly. The most concentrated form of energy is the Sun and the most dispersed form is heat. If the universe is considered as an isolated system, the level of entropy is gradually increasing as energy is distributed within it. Eventually, in billions of years’ time, energy may run out. Natural systems are never isolated (see above) and living systems require a constant input of energy from the Sun to maintain their order and replace energy that is lost.
Equilibrium Equilibrium is a state of balance which exists between the different parts of any system. Most systems tend to return to their steady, balanced state after any disturbance. As we have seen, natural systems are open and most are in a state of equilibrium. There are fluctuations in the system but these are within narrow limits and the system usually returns to its original state after being disturbed. Steady-state equilibrium (also known as dynamic equilibrium) is a stable form of equilibrium which allows a system to return to its steady state after a disturbance. Regulation of body temperature in mammals is an example of a steady-state equilibrium. If the temperature of a mammal rises above 37 °C, processes occur in the body to
37 °C
A pendulum swinging from a suspended string is said to be in stable equilibrium because if it is pushed to the side it will return to its original position. But a ruler balanced vertically on a finger is in unstable equilibrium – if it is disturbed, it will fall and continue to fall until it hits the ground creating a new and different equilibrium.
System state
System state (body temperature)
return the temperature to normal. If the temperature falls, the processes are reversed to enable the body to warm up (Figure 1.4) Another example of a steady-state equilibrium can be seen in a population of animals which remains approximately the same size. Some animals may be born and others may die, but if birth and death rates are equal, there is no net change in the population size.
system state unchanged
Time
Figure 1.5 Nothing changes in a static equilibrium.
Figure 1.4 The body temperature of a human varies between 36 °C and 39 °C but remains at an average of 37 °C. Small rises and falls in temperature are corrected by processes such as sweating or shivering so that a steady state is maintained.
(a) Stable equilibrium
ball
At the level of the ecosystem, a steady state can be achieved after a disturbance either in the short term, for example as a woodland recovers after heavy rainfall, or in the long term, as new growth occurs to replace damaged plants in an area of the wood. If a tree dies or is felled, a new area is opened up in the woodland and a phase of new growth can take place. New plants will receive extra light and young trees in the clearing can grow rapidly. After a long time, a new tree will become established to replace the one that was lost. Eventually, the system will return to its previous equilibrium.
Stable and unstable equilibria are two examples of situations in a system where change occurs but in each case the final result is different. In a stable equilibrium, the system tends to return to the same equilibrium after a disturbance, while in an unstable equilibrium a new equilibrium is formed after the disturbance (Figure 1.6).
bowl
bowl ball
After a small nudge the ball returns to the centre, the equilibrium point.
System state
A static equilibrium is a type of equilibrium in which there are no changes over time because there are no inputs and outputs to the system. These systems are not living and they remain unchanged for long periods of time. We can observe static equilibrium in a rock formation where the rocks do not move their position or change their state over time. On a domestic level, objects such as a sofa or armchair or a bottle placed on a table can be said to be in static equilibrium. A graph showing static equilibrium is shown in Figure 1.5.
(b) Unstable equilibrium
A small nudge sends the ball away from the equilibrium point.
System state
Time
Time
Time
Figure 1.6 (a) The ball inside the bowl will return to its original position after a disturbance and is said to be in stable equilibrium. (b) A ball balanced on top of the ball is in unstable equilibrium because a new equilibrium is formed if it is disturbed.
1.1 Systems and models
5
Positive and negative feedback Natural systems are able to regulate themselves through feedback systems. Information, which may come from inside or outside the system, starts a reaction which affects the processes of the system. Changes in these processes lead to changes in the level of output and this in turn affects (feeds back) to the level of input. This whole cycle is known as a feedback mechanism or feedback loop. Feedback loops can be either positive or negative (Figure 1.7). Feedback can change a system to a new state or maintain a system at a steady state.
input
two years of unchecked growth the number of plants can reach 109 (1 billion). At this level, serious problems occur for other species and for navigation across the lake. The plant prevents sunlight reaching the water which is starved of oxygen, fish die and the boats cannot move across the water. The exponential growth of the water hyacinth population is a positive feedback relationship between the population size and the number of new organisms added to the population. The greater the population, the greater the number of new additional organisms, so the faster the population grows (Topic 2, page 56). births
+
output
+
population (n)
account balance grows
feedback
Figure 1.7 A feedback loop.
Key terms
more interest earned
Figure 1.8 Exponential population growth is an example of positive feedback. A bank account earning interest is another.
Positive feedback
Other examples involving positive feedback include the increase in the Earth’s temperature through global warming. Higher atmospheric temperatures increase the evaporation of water from the Earth’s surface, this increases water vapour in the atmosphere and, because water vapour helps to trap heat in the atmosphere, the outcome will be more heat trapped and a further increase in atmospheric temperature. In addition, if warmer air temperatures cause polar ice to melt, the reflection of heat from the white surface of the ice will decrease. More of the Sun’s energy will be absorbed and the temperature will increase still further.
A positive feedback loop allows a system to change rapidly (Figure 1.8). One example is the population growth of a plant, the water hyacinth, which has spread into new environments from South America. If one water hyacinth is introduced into a large, uncolonised lake, the plant will reproduce exponentially: one plant divides to become two; two become four and so on. At first, the growth of the plants does not seem to be significant but after about
Positive feedback must eventually come to an end as the resources which allow the rapid change will also come to an end. But there is no guarantee that the situation will revert back to its original state. Lakes in many parts of the African continent remain covered with the invasive water hyacinth. A system affected by positive feedback may reach a tipping point when it is unstable and a new equilibrium must form.
Positive feedback results in a change in the system which leads to more and greater change. Information enhances the change and destabilises the system. Negative feedback stabilises a system so that a change in the system allows it to eliminate any deviation from the preferred conditions.
6
Systems and models
Negative feedback Negative feedback works to counteract any deviation from the stable state or equilibrium. It stabilises a system and allows it to regulate itself and eliminates any deviation from the preferred conditions. Negative feedback leads to stability (Figure 1.9). In engineering, one of the first examples of negative feedback was a device known as a governor, used by the Scottish engineer James Watt (1736–1819) who incorporated it into his steam engines in order to maintain their speed. If the speed of the engine increased, the governor cut off the supply of steam to slow it down to the correct speed. If the engine speed decreased, the governor allowed in more steam, so that the engine ran faster. In this way a constant speed was achieved. Another similar example of negative feedback is the thermostat on a heating system which can be set to maintain a constant temperature. In the human body, negative feedback helps to maintain a constant body temperature (Figure 1.9). (a)
body temperature rises
body temperature drops
body sweats more (b)
conditions in the body change from set point
corrective mechanisms switched off
change detected
conditions returned to set point
corrective mechanisms activated
Maintenance of a steady-state equilibrium involving negative feedback is vital to keep the internal conditions of animals’ bodies relatively constant. The temperature of a mammal’s body must be maintained at about 37 °C so that life processes can take place in their optimum conditions. An increase in temperature leads to increased sweating and widening of the capillaries in the skin so that heat is lost. As the body is cooled by these processes, it returns to its normal temperature. Many other body functions including the regulation of sugar in the bloodstream by the hormone insulin and the maintenance of the correct level of water in the body are controlled by physiological processes that involve negative feedback. In an ecosystem, negative feedback leads to the control of the relative numbers of species in food webs (a food web is a complex interacting set of food chains). If one species becomes too successful and its numbers rise excessively, it will use up its resources or be overcome by its waste. Negative feedback ensures that its numbers fall back to sustainable levels. On a global scale, the increase in carbon dioxide released into the air from burning fossil fuels provides more carbon dioxide for plants which can increase their rate of photosynthesis. As they do so, the plants remove more carbon dioxide from the atmosphere so that as long as there are sufficient plants, the system can be rebalanced. A French proverb has been used to summarise the process of negative feedback: Plus ça change, plus c’est la même chose (the more things change, the more they stay the same).
Self-assessment questions
System state
(c) corrective action deviation from set point
Stress or disturbance changes the internal environment.
return toward set point
Change is detected by receptors. set point (optimal conditions)
stress corrective action
Corrective measures are activated. Corrective measures counteract the change back toward set point.
Time
Figure 1.9 Negative feedback is a regulating mechanism in which a change in a variable results in a correction. If body temperature rises, the body activates cooling processes until the temperature returns to normal.
1
2 3 4 5
How does the first law of thermodynamics explain how energy moves through an ecosystem? What is meant by ‘entropy’ and how does it relate to a natural system? Outline the difference between a steady-state equilibrium and a static equilibrium. Why does positive feedback lead to increasing change in a system? Research idea: The human population is growing at an exponential rate. Research the possible consequences of this example of positive feedback.
1.1 Systems and models
7
Case study Predator—prey relationship
• Outline how negative feedback allows a system to maintain a steady-state.
Transfers and transformations •
Transfers involve flow through a system and involve a change in location.
•
Transformations lead to an interaction within a system and the formation of a new end product or they may involve a change of state.
Transfers Both matter and energy pass through ecosystems and if their movement does not involve any change of form (or state), the movement is called a transfer. Transfers usually involve a flow through a system and involve changes in location. A trophic level is a group of organisms which are all the same number of energy transfers from the producer (plant) in a food chain or food web (Topic 2, page 13). Energy flows through an ecosystem as biomass found in the bodies of organisms in one trophic level are eaten, so biomass and energy pass to the next trophic level.
rabbit
Population size
In an ecosystem, predation is a mechanism of population control which involves negative feedback. If we consider the relationship between a prey species such as a rabbit and a predator species such as a fox, we can predict that when the number of foxes is low, the number of rabbits should rise (Figure 1.10). An increase in the number of rabbits means that the foxes have more food and can produce more offspring and may be able change their hunting habits. As the number of foxes increases, the number of rabbits declines as more are caught and eaten. This results in food scarcity for foxes so that more starve and die or fail to reproduce. Negative feedback balances out the two populations and the cycle can begin again (Topic 2, page 20).
Time
Figure 1.10 When the numbers of predators are low, the prey species is able to increase in number. As more prey animals become available, predators increase in number and the population of prey falls.
•
transfer of energy as wind carries heat energy from one part of the world to another
•
transfer of matter as water flows from a river to the sea.
Transformations A transformation occurs when a flow in a system involves a change of form or state, or leads to an interaction within the system. The evaporation of water from a river is an example of a transformation because water is changed in form from a liquid to a vapour in the atmosphere. In ecosystems, energy is transformed from sunlight into chemical energy in the bonds of molecules in plants during the process of photosynthesis. As organisms respire, chemical energy is transformed into heat and kinetic energy. Some examples of transformations include:
•
energy to energy – light energy to electrical energy in a solar panel (photovoltaic cell)
•
matter to matter – decomposition of leaf litter into inorganic materials
•
matter to energy – burning coal to produce heat and light
•
energy to matter – light energy converted by photosynthesis to produce glucose molecules.
Some examples of transfers include:
•
8
transfer of matter through a food chain as one animal eats another
Systems and models
fox
Flows (inputs and outputs) and storages (stock) in a system Energy and matter are the inputs and outputs which flow through ecosystems but they are also stored within the system as storages (or stock). In an ecosystem, the energy input is sunlight which is transformed into chemical energy in the bonds of glucose formed during photosynthesis. Energy flows from one part of an ecosystem to another as one organism eats another. Some energy is used to drive the life processes of these organisms and energy leaves the system in the form of heat which is released as a result of respiration. Matter flows from one trophic level to the next as plants or animals are eaten. Eventually, matter is recycled through the decomposition and decay of dead organisms and of their waste products. In any ecosystem, there are storages linked by different flows. Carbon and nitrogen are two elements which are cycled around an ecosystem and pass between storages in living organisms, the atmosphere, and the land (Topic 2, page 49–50). Diagrams are used to show the flow and storage of matter and energy in a system. If the storages are represented by boxes, the boxes can be drawn to be proportional to the size of the storage. Likewise, arrows which indicate flows between storages can be drawn so that their width is in proportion to the size of the flow (Figures 2.19–2.21, page 45– 47).
Models involving flow and storages in a system Environmental scientists use models to show the flows, storages and links within an ecosystem. A model, in this case, is a diagram that uses different symbols to represent each part of the system. Arrows are used to represent the flow of energy or materials and different boxes are often used to represent producers, consumers and so on. Figure 1.11 illustrates the general flows in an ecosystem. In Figure 1.12, the flow of energy is shown by the red arrows. The blue arrows show the cycling of nutrients and the boxes represent storages.
atmospheric gases
respiration weathering feeding
elements locked in rocks
erosion
plants absorb elements from soil
uptake
rock formation
animals absorb elements from plants
death and decomposition
gases
minerals in soils
Figure 1.11 The biogeochemical cycle showing flows (arrows) and storages (boxes).
Sun
heat
heat
producers
consumers
inorganic nutrient pool
decomposers
heat
Figure 1.12 Energy flow and nutrient cycling in a system.
1.1 Systems and models
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Models like these enable environmental scientists to draw comparisons between different ecosystems by representing the different inputs, outputs and storages in proportion to their sizes.
Advantages
Disadvantages
• allows complex systems to be simplified
• models may be oversimplified so that accuracy is lost
Evaluation of models’ strengths and weaknesses
• allows predictions to be made about future events
• models and predictions depend on the skills and experience of the people making them
Models are drawn to represent situations found in real systems but in reality they can only be approximations and predictions in most cases. Computer modelling and simulations are used to predict outcomes such as the pattern of the weather and the likely course of climate change.
• different scenarios can be considered by changing inputs and calculating likely outcomes
• models may be interpreted differently by different scientists • different models may predict different outcomes
• can form the basis for discussion and consultation with others who are interested in the system being modelled
• data may not be accurate and models can be manipulated for financial or political gain
Models have many strengths and weaknesses and it is important to bear these in mind when models are used. Computer modelling of climate change is a good example of how modelling can lead to controversy as well as consensus. Not everyone agrees on the scale of projected inputs and outputs, or on the interpretation of the models. Table 1.2 shows the advantages and disadvantages of modelling.
Table 1.2 Modelling – the pros and cons.
Self-assessment questions 1 2 3
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What is the difference between transfer and transformation in an ecosystem? Give one example of each of the following in an ecosystem: an input, an output, a storage. Give three advantages to drawing a model of climate change and suggest three weaknesses of the modelling process.
Systems and models
4
Discussion point: Why do you think that scientists are keen to use models to communicate their ideas to the general public and politicians? What are the merits of presenting information in this way?
Case study Comparing agricultural and natural ecosystems Figure 1.13 compares the inputs and outputs of an agricultural ecosystem and an urban ecosystem. Notice that the arrows have been drawn so that their sizes represent the sizes of the inputs and outputs of the two systems. Draw your own diagrams to compare the inputs and outputs in an intensive agricultural system which produces wheat with those of a natural ecosystem such as a woodland or forest.
human inputs
natural inputs
waste
human inputs
products urban ecosystem
Use the following information to help you. • Agricultural ecosystems use selected, domesticated plants or animals to produce food, fibre or fuel for human consumption.
products agricultural ecosystem
natural inputs
waste
• Some agricultural ecosystems differ greatly from natural ecosystems; others do not. Pasture ecosystems with grazing animals require fewer human inputs than crop ecosystems because pasture is similar to a natural ecosystem.
Figure 1.13 Comparing inputs and outputs of a small agricultural and an urban ecosystem.
• Modern agricultural ecosystems need inputs from farm machinery, chemical fertilisers, pesticides and irrigation.
• Intensive inputs to agricultural ecosystems depend on large amounts of petroleum energy to produce fertilisers and pesticides and for transport.
• Intensive inputs increase the conversion of sunlight energy to human food energy during photosynthesis by (a) providing the best conditions for plant growth (e.g. ample water and nutrients from fertilisers) and (b) excluding plants and animals such as weeds or pests that compete for the ecosystem’s production. • Agricultural ecosystems contain non-living things made by people such as irrigation ditches and farm equipment which require maintenance.
• Water is a key input and modern irrigation methods need large amounts of water, sometimes from hundreds of kilometres away. • High inputs in agriculture produce high outputs and high yields of crops or animals. • Outputs of modern agricultural ecosystems include waste such as fertilisers and pesticides that can damage nearby ecosystems.
1.1 Systems and models
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End-of-topic questions 1 a Describe what is meant by a model of a system. [2] b Outline how models are used to make predictions about: i
changes in the climate based on carbon emissions
ii the effect of measures taken to reduce carbon emissions. [6] 2 a Outline the difference between negative feedback and positive feedback.
Calculate the value of output X and suggest where this output will go [2]
ii Output Y will be lost to the ecosystem. State the form of this energy. [1] 4 Complete the systems diagram below to show three inputs, processes and outputs for an intensive arable farming system that produces wheat.
c Suggest why most ecosystems are negative feedback systems. State the first law of thermodynamics.
input 1 ............................................
process 1 .......................................
input 2 ............................................
process 2 .......................................
input 3 ............................................
process 3 .......................................
[2] [1]
ii Calculate the amount of output X in the system shown below. Show your working. [2] input
loss to atmosphere
10 000 kJ
10%
% of remainder stored 15%
remainder to output
10 units / energy lost to decomposers
trophic level 2
X
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Systems and models
output 1 ........................................ output 2 ........................................ output 3 ........................................
100 units/ energy absorbed from previous trophic level 65 units
arable farm producing wheat
X
b The diagram below represents the energy (in arbitrary units) which enters and leaves a trophic level in a food chain.
Y
[9]
[2]
b Draw a diagram to show how a positive feedback process involving methane may affect the rate of global warming. [4]
3 a i
i