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.

Biology is not enough

This chapter is about the principles of conservation. It looks at the philosophical basis of conservation by addressing the question ‘Why should we conserve?’, and at the biology that underpins conservation. However, philosophy and biology are not enough. In Chapter 21 we will look at conservation in practice: at reasons why conservation sometimes works and at reasons why it often doesn't, even if the philosophy is sound and the biology correct.

The need for conservation

The pressure on wildlife

The number of people alive and the demands we make on the Earth's resources continue to increase. As the human population grows in size (see Box 4, p. 35), more and more land is brought under direct human control for agriculture and housing. In fact, around 40% of all the world's photosynthesis ends up being used by us. In consequence, the amount of natural vegetation diminishes and with it the space available for the species which live in such habitats. Although it is difficult to quantify with any precision the effects of human disturbance on the world's biomes (Hannah et al, 1994), it is hard, if not impossible now, to find any habitat which is not affected by some change in species composition or balance due to introductions, disturbance or pollution. The vast expanses of tropical forest have become increasingly threatened in recent decades as large commercial companies back clearance schemes for cattle ranching and timber exploitation.

Why study trophic levels?

There are many approaches to studying the ecology of organisms in their natural surroundings. We have already seen several of these: changes in population size, the occupation of niches, and so on. Another way is for ecologists to study organisms from the point of view of their feeding relationships – what they feed on and what feeds on them. There are a number of reasons why this is a good approach. First, one of the major problems faced by all organisms is how to obtain enough energy and nutrients to survive, grow and reproduce. Second, if the feeding relationships of a group of organisms can be unravelled, then we get a clearer understanding of how those organisms interact. Finally, there simply is not enough time, nor sufficient funds, to study every species in detail. Ecologists therefore often cannot always look at each and every species, one by one. Rather, they study the autotrophs, the herbivores, the carnivores, the decomposers and so on, lumping together the species that have a similar role in the community.

In this chapter we will look at the arrangement of organisms into these community groups or trophic levels. Trophic literally means feeding, so trophic levels are the levels or positions at which species feed. Examples of trophic levels include ‘herbivores’ and ‘decomposers’. In this chapter we will look at the characteristics of organisms in each of the trophic levels. We will then examine approaches to the study of the feeding relationships of organisms.

What is the environment?

The astronaut in Figure 9.1 is surrounded by very hostile conditions on the Moon. There is no breathable atmosphere, no running water, very high temperatures in the glare of the Sun and very low ones on the dark side of the Moon, no soil, just dust and rocks and the risk of meteors. In fact the Moon is probably unable to sustain any form of life, although biologists live in hope that some microorganisms may be found there. This is why the astronaut has to wear a spacesuit to carry his oxygen supply and maintain his temperature correctly.

All these characteristics of temperature, light, aridity and ground structure make up the environment on the Moon. It is a very unusual environment, by Earth standards, because there are no living organisms; in other words the environment on the Moon is abiotic: it is made up only of physical conditions. On Earth the abiotic environment of an organism is composed of physical variables such as temperature, rain- or snowfall, nutrient and toxic content of the soil, the power of wave action and wind speed. Unlike on the Moon, on Earth an organism also experiences the influence of other organisms, for example through competition, predation, herbivory, pollination and seed dispersal. The effects of such organisms forms the biotic part of the environment.

Species distribution – where and why?

So far we have looked at the distribution of organisms in communities and have seen how particular communities are typical of certain environmental conditions. On a larger, global scale a particular species may occur only on one continent or island, only in the tropics or on a single mountain range. Most of us, for example, would recognise Figure 18.2 as African savannah. Often the same ecosystem occurs in several parts of the world, but the species or genera in the community are different in different regions. The study of this geographical distribution of species is called biogeography. Biogeographers investigate the location of species and the reasons for the distributions they find. Thus biogeography links ecology with geology, earth history, evolution, climatology and geography.

If you look back at Figure 17.2 you can see the distribution of the major terrestrial biomes of the world. The same biome often occurs on several continents or marine regions. For example, tundra and boreal forest occur in northern America and Europe; temperate grasslands are found in the dry centres of most continental landmasses. If you compare Figure 17.2 with the major biogeographic regions of species distribution (Figure 18.1) you will see a very different pattern. In this case the terrestrial regions are associated with landmasses such as major continents, not with climatic zones.

How many biomes are there?

Many ecologiste have tried to classify the principal vegetation types into which the world is divided (Mueller-Dombois, 1984). The world vegetation types have come to be known as biomes and aquatic biomes have also been recognised. It is impossible in a single chapter to look at all the biomes in any detail. Instead, we will try to give a flavour of what life must be like for some of the organisms found there. We will also consider why particular biomes are found where they are. Some biomes have already been considered in earlier chapters and we will refer to these again only briefly.

No two ecologists seem to agree on how many biomes there are. This is hardly surprising as a biome is not a natural unit. If individual ecosystems are the ‘species’ of ecology, then biomes are the phyla or divisions. No two taxonomists seem to agree on a system of classification, so it is hardly surprising that a definitive list of biomes cannot be produced. This does not mean, however, that the concept of the biome has no use. Biomes provide a convenient shorthand for describing the world's flora and fauna. Traditionally biomes have been defined mainly in terms of their vegetation. However, in our descriptions below we will look at the animals in them (and sometimes the fungi, protoctists and prokaryotes too) as well as at the plants.