Basic background material

(a) Chronology of the life of the Prophet

c. 570 Birth of Muhammad (his father died a few months earlier).

576 Death of his mother Amina.

595 Marriage to Khadija.

c. 610 Beginning of Call.

615 Flight of his followers to Ethiopia.

619 Death of Khadija.

620 Muhammad's reputed ‘Night Journey’ from Mecca to Jerusalem, and from there to the Seventh Heaven.

622 The Hijra (Flight or Migration) of Muhammad and his followers to Medina, and the beginning of the Muslim Era.

624 Battle of Badr: the Quraysh defeated by the Muslims.

625 Battle of Uhud: the Muslims defeated.

626 The Jewish tribe of al-Nadhir crushed and expelled.

627 ‘The War of the Ditch’ – the Meccans' expedition against the Muslims in Medina. Attackers driven off.

627 The Jewish tribe of Qurayza raided by Muhammad; some 800 men beheaded (only one Jew abjuring his religion to save his life) and all the women and children sold as slaves.

628 The Treaty of Hudaybiyya: truce with the Quraysh, who recognise Muhammad's right to proselytise without hindrance.

629 The Jews of Khaybar put to the sword.

630 Truce broken by the Quraysh. Mecca taken by Muhammad–the entire population converted, and the Ka'ba established as the religious centre of Islam.

631 ‘The Year of Embassies’ – Islam accepted by Arabian tribes.

632 Muhammad's Farewell Pilgrimage to Mecca.

632, 8 June Death of Muhammad, three months after his return to Medina.

The different grades of co-evolution

Ehrlich and Raven first introduced the term co-evolution in 1964 in a paper on the relationship between butterflies and the host food plants of the larval stage. They were concerned in particular with the ‘patterns of interaction between two major groups of organism with a close and evident ecological relationship’ (p. 586). Within the paper they also introduced the term community evolution: ‘evolutionary interactions found among different kinds of organisms where exchange of genetic information among the kinds is assumed to be minimal or absent’ (p. 586). Their definition of community evolution is broad, and they seem to consider co-evolution as a smaller subset of community evolution, but their application of the term has been interpreted in a number of ways by ecologists since then.

The most restricted use of the term co-evolution is applied to the evolutionary relationships between two species which are, ecologically, very closely associated. This is often called pairwise or one-on-one co-evolution. Usually this implies that as one species evolves particular characteristics, the other species also evolves in response to this; the first species then evolves in response to the response of the second, and so on. Examples of pairwise co-evolution include ant species living on acacia trees (see Section 19.2.2), butterfly caterpillars and their host plants, and bees with the plants they pollinate (see Section 19.4.3).

Populations and population change

Organisms do not live solitary lives in isolation from other members of their species. Usually they live in groups or, if normally single, they interact with other members of their species at various times in their lives. A group of organisms of the same species which live together in one geographical area at the same time is called a population. Within a population, all the individuals capable of reproduction have the opportunity to reproduce with other mature members of the group.

Although it is quite easy to define a population on paper, it is often difficult to study one in its natural environment (Crawley, 1990). First of all, the species to be investigated has to be identified. This is often easy, but may be difficult in some taxa or in particular circumstances. For example, insects often require microscopic scrutiny, one species in a mixed flock of birds may be difficult to pick out and count accurately, and plants sometimes hybridise with other species or occur as difficult taxonomic aggregates (see Section 6.2.5). The second problem is to decide what is an individual in the population. The bracken fronds in Figure 3.1, for example, are all members of one clone, that is one genetic individual, although they may look like a lot of separate plants (see also Section 4.7).

The realities of attempting conservation

In Chapter 20 we looked at the principles of conservation. In this, our final, chapter, we look at conservation in practice. We shall examine why conservation sometimes succeeds and why it all too frequently fails, and ask what needs to be done in the future. This chapter is not, therefore, only about biology. It includes elements of economics, psychology and politics. The reason for this is that successful conservation demands much more than a good understanding of biology. It requires determination, political clout and an understanding of people and economic systems. Without such an understanding, conservationists risk being naive do-gooders unlikely ever to achieve much of lasting significance.

To help structure this chapter we will follow the same pattern as in Chapter 20, looking first at the conservation of species, then at ecosystems and lastly at the biosphere. It needs to be recognised, though, that the boundaries between these three levels of conservation are somewhat fluid. Effective species conservation nearly always involves habitat conservation and the distinction between habitat and ecosystem conservation is mainly one of scale.

The importance of genetics to ecology

The source of variation

Every organism has a collection of genes consisting of lengths of DNA (deoxyribonucleic acid). This DNA can be read like a book and is used by the organism to construct proteins including enzymes. Each cell has a copy of this genetic material stored in the nucleus. The genes are arranged in a set order on several chromosomes. Most cells have two sets of chromosomes and, usually, one set comes from each of the two parents of the individual. The genes are responsible for the growth, development, maturation and reproduction of the individual.

In this book we are not concerned with how genes are constructed or exactly how the cell uses them. Ecological genetics is the study of how the genes an organism possesses affect its life and the way it interacts with its environment. The study is also concerned with how genes are passed on to new individuals in the population.

A single gene can have many different forms called alleles. If a gene has more than one allele, it is said to be polymorphic. Because many genes are polymorphic considerable variation is possible between different organisms of the same species. The set of alleles an individual possesses on its chromosomes is called its genotype. Different genotypes result in individuals with different appearances. An obvious instance of this is hair colour and pattern in cats, or shapes and sizes of dogs.

What is ecology?

Ecology is the study of organisms in relation to the surroundings in which they live. These surroundings are called the environment of the organism. This environment is made up of many different components, including other living organisms and their effects, and purely physical features such as the climate and soil type.

Ecologists, those who study ecology, are always aiming to understand how an organism fits into its environment. The environment is of supreme importance to an organism and its ability to exist in the environment where it lives will determine its success or failure as an individual. This means that much of this book is really about the study of environments from the point of view of various organisms (but see Chapter 9 which describes in detail what makes up and affects an environment).

There are several definitions of ecology. Many workers have produced their own description of this branch of biology. The word ecology was first used by a German called Ernst Haeckel in 1869. It comes from two Greek words oikos meaning home and logos meaning understanding. Haeckel described ecology as ‘the domestic side of organic life’ and ‘the knowledge of the sum of the relations of organisms to the surrounding outer world, to organic and inorganic conditions of existence’. This ‘surrounding outer world’ is another way of saying the environment. In 1927 Charles Elton wrote that ecology is ‘the study of animals and plants in relation to their habits and habitats’.

Ecology: principles and applications provides a clear and up-to-date introduction to ecology for students studying at first-year undergraduate level or at advanced level. We outline the principles of ecology and show how relevant applications follow from these principles. The chapters follow a sequence of ascending scale beginning with individual organisms and proceeding through communities and ecosystems to global considerations of biogeography, co-evolution and conservation. Human ecology and applied topics are considered throughout the text wherever appropriate and a wealth of examples are drawn from all five kingdoms and around the world. Each chapter contains a summary. Most chapters also include extra, somewhat peripheral, material, presented as boxed text. These boxes may contain a particular case history, an historical perspective, a worked example of a quantitative concept or the outline of a controversial theory. Sometimes they contain material that is more advanced than the main text.

Key terms are printed in bold throughout the text and are defined in a glossary at the back of the book. There is an extensive bibliography at the end of the book which allows particular aspects of ecology to be pursued in more detail.

The second edition shows a number of changes over the first. The number of diagrams and photographs has been increased and some sections are now in colour. New material has been added throughout the text and examples are now taken from a greater number of countries. The most extensive changes have been to the sections on nutrient cycling, pollution, endangered species, biodiversity and conservation.

The pattern of nutrient transfer and its connection with pollution

As we discussed in Chapter 12, organisms need a supply of energy in order to survive, grow and reproduce. They also, of course, need to obtain the elements of which they are composed (Figure 13.1). Energy transfer can be represented as a pyramid where each trophic level obtains less energy than the trophic level beneath it (Section 12.8). This is because at each trophic level of a food web most of the energy is lost from the community as the heat of respiration. This heat ends up in the physical environment and is rarely of direct use to organisms. This means that energy transfer is linear. Energy passes from the Sun to primary producers and so forth up the food web.

The fundamental difference between energy transfer and nutrient transfer is that the pattern of nutrient transfer is basically circular or cyclical. The elements which form the molecules of which organisms are made are unalterable in natural conditions on Earth, so they remain in circulation when molecules pass from one trophic level to another.

The community concept

Definitions

The use of a collective term to describe organisms of several species found in association together was first applied to plants. Many definitions of community have been botanical ones. Hence a community has been defined as ‘an aggregate of living plants having mutual relations among themselves and to the environment’ (Oosting, 1956), or more recently, as ‘a collection of plant populations found in one habitat type in one area, and integrated to a degree by competition, complementarity and dependence’ (Grubb, 1987b). The key points about communities are that they are collections of species which occur together in some common environment or habitat and that the organisms making up the community are somehow integrated or interact as a society.

Of course, communities are not constructed only of plants. Some communities are mainly animals, like the association of fish and invertebrates which make up coral reef communities. Most communities are composed of a mixture of plants, animals, fungi, prokaryotes and protoctists.

Recognition of communities

Communities are usually recognised in two ways. One way a community can be identified is from the form of the environment or habitat in which it occurs. Some communities take their names from physical features; this is the case for rock pools, lakes and sand dunes. Other communities are recognised by the dominant species in the association. These are usually the largest or most abundant plant species present, hence deciduous oak wood, cypress swamp, grassland and sphagnum bog communities.

The meaning of autecology

Autecology is the name given to ecological studies which concentrate on one species (autos is the Greek for self). An autecological study aims to answer the questions asked in Chapter 1 and to understand the processes described in Chapter 2 for a chosen species. To find out all there is to know about a species is an enormous task which requires considerable time, a great deal of observation and numerous experiments. Usually different ecologists are involved over many years in finding out about one species. Often they are working on specimens from different parts of the world. Several fields of biology are usually involved including genetics and biochemistry as well as the more traditional forms of ecology.

There are an estimated 30–40 million species in existence today. Very few of these have as yet been studied sufficiently for us to understand much of their autecology. This chapter gives two examples of species which have been investigated fairly extensively. One, the bracken fern, is an autotrophic plant, and the other, the European starling, is a heterotrophic vertebrate. These examples will give you an idea of how the results of many autecological studies can be combined to help us to understand the whole life cycle of a species. Much more is known about each of these species than is mentioned here, but you can get a flavour of the scope of autecology from these accounts.

Vegetation changes

Vegetation is not static and unchanging: it can be altered in many ways. For example, wetland ecosystems, like those described in Section 15.3, are often drained by humans. When this happens the species in the community which are adapted to living in the waterlogged habitats die out and are replaced by other species which are more characteristic of drier conditions. Drained land can be ploughed and planted with crops or used for other agricultural purposes. If such drained land is abandoned it is quickly invaded by other species and turns into grassland, scrub or, given time, woodland. Often vegetation changes occur which are not caused by human intervention. In Section 15.3.5, we saw how swamp cypress needed a brief phase of drying out to regenerate and how, if the area remained under water for hundreds of years, swamp could turn into shallow lake. The reverse can also occur: a shallow lake which dries out occasionally can be invaded by Taxodium and become swampland.

These are examples of vegetation changes, but not all can be called succession. Succession is a natural change in the structure and species composition of a community. Changes caused by the direct influence of humans in clearing and replanting land have never been included in the definition of succession. If human influence declines the changes which then take place are usually classified as succession.

Energy and disorder

Living organisms are highly organised. In order to survive and maintain this internal order organisms need supplies of the relevant nutrients, a source of energy and the ability to create a large amount of disorder outside themselves. This last requirement may sound rather odd, but the second law of thermodynamics states that the amount of disorder in a closed system, such as the universe, increases with time. For organisms to create order, as they do when they make new cells, they need energy and/or the ability to create disorder outside themselves. Respiration both releases energy and creates disorder as relatively large molecules, such as glucose, are broken down to smaller and therefore less ordered molecules, i.e. carbon dioxide and water. The trophic levels at which species feed have been considered in Chapter 11, and Chapter 13 will look at how organisms obtain the nutrients they need. This chapter aims to look quantitatively at how organisms obtain their energy and how they pass energy up a food chain.

Because the vast majority of primary production in the world is the result of photosynthesis rather than chemosynthesis, we will first examine the environmental factors that determine the amount of photosynthesis in different communities. We will then see whether there are any ecological rules governing the transfer, or movement, of this energy up a food web through the trophic levels.

Population growth

Population growth without regulation – exponential growth

As mentioned in Chapter 4, populations are dynamic: they are always changing. Organisms are born, die, immigrate or emigrate. But what would happen if a population grew in size without stopping? Darwin considered this situation and wrote ‘The elephant is reckoned to be the slowest breeder of all known animals … it breeds when thirty years old, and goes on breeding till 90 years old, bringing forth three pairs of young in this interval; if this be so, at the end of the fifth century there would be alive fifteen million elephants, descended from the first pair.’ (Darwin, 1859, Chapter 3.) But this does not happen: we are not crowded out by elephants, mice or any other species. In other words populations do not usually increase to such an extent that they are out of balance with the rest of the environment. There are exceptions, where species suddenly increase in numbers and a plague follows, but numbers usually return to lower levels quite quickly (see Sections 5.3 and 19.5).

What stops elephant populations, or any other species, from increasing continuously? The sizes of populations are often regulated by environmental factors. The main factors known to be involved in population regulation are described in Section 5.2. To understand the effects of population regulation we must first see what happens to a population when no controlling influence exists.

Habitats

The word habitat is used extensively in ecology when describing where an organism lives. Unfortunately, it is difficult to give a precise definition of the term habitat. The word is a Latin one and literally means ‘it inhabits’ or ‘it dwells’. It was first used in eighteenth-century floras or faunas to describe the natural place of growth or occurrence of a species. These guides to the plants or animals of a region used always to be written in Latin, hence the Latin word habitat. When floras and faunas began to be written in modern languages, the term ‘habitat’ remained untranslated and began to be used as a technical term.

It is easy to give the habitats of some species. For instance, the lowland gorilla (Gorilla gorilla) has as its habitat lowland tropical secondary forest; the fungus Hericium abietis is found on coniferous logs and trees in the Pacific north-west of the USA (Figure 10.1). Some species, though, have several habitats. Those of the tiger (Panthera tigris) include tropical rainforest, snow-covered coniferous and deciduous forests and mangrove swamps (Sunquist, 1985).

Why look at individuals in ecology?

Although ecologists are often interested in the complex interactions between species, it is worth remembering that it is individual organisms that are the products of natural selection (Chapter 8). This chapter deals with the biology of individuals. Individuals are the fundamental units of populations, communities, ecosystems and biomes which are discussed in later chapters. In this chapter we will look at individuals from an ecological perspective. We will start with the essentials of how they obtain their energy and nutrients, and then consider how these are allocated to maintenance, growth and reproduction.

Autotrophs and heterotrophs

All organisms need energy to live and different organisms obtain this energy in different ways. There are many approaches to classifying the ways in which individuals obtain their food. A useful one is to divide organisms into autotrophs and heterotrophs. Autotrophs obtain only the simplest inorganic substances from their environment. Green plants are the most obvious autotrophs. These need only visible light, water, carbon dioxide and inorganic ions such as nitrate (NO3) to survive, grow and reproduce. The process of photosynthesis enables most plants, the photosynthetic bacteria and some protoctists to synthesise all the complex organic molecules that they require from these simple building blocks. Because these organisms use light as their energy source, they are called photoautotrophs.

What is behavioural ecology?

In the past ecologists often took little account of the behaviour of organisms they studied. Increasingly it has been realised that just as knowledge of genetics is important for an understanding of ecology (see Chapter 6), so is a knowledge of behaviour. An individual needs to do many things in order to survive and reproduce. Some of these are simply bodily functions, like those described in Chapter 2, such as ingestion and excretion, but around these functions is a pattern of activity, such as food searching, resource defence, mate location and parental care, which can be described as behaviour.

Behavioural ecology investigates how the behaviour organisms show is related to their ecology. For example, if we want to understand how an individual's feeding behaviour helps it to survive and reproduce, we need to know a lot about its ecology. We need information about the habitat in which it lives and the food on which it feeds. We also need to know something about the other organisms, if there are any, that feed on the same food.

Much of the research done on behavioural ecology looks at three problems that organisms face: obtaining food; avoiding being eaten; and reproducing (Krebs & Davies, 1993). In recent years some ecologists have begun to use the language of behavioural ecology to see how plants solve these problems. In this sense even plants and fungi can be said to behave.