An Overview of Insect Ecology
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Speight-The Insects 9781405131148_4_001 Page Proof page 1 20.11.2007 6:44pm Chapter 1 AN OVERVIEW OF INSECT ECOLOGY 1.1 INTRODUCTION encompasses a whole variety of disciplines, both quali- tative and quantitative, whole organism, cellular and In this first chapter we provide a brief overview of molecular, from behavior and physiology, to evolution the major concepts in insect ecology, and attempt to and interactions within and between populations. present a taste of what is to come in the 11 detailed In 1933, Elton suggested that ecology represented, chapters that follow. Unavoidably, a little repetition partly at least, the application of scientific method to may therefore occur, in that topics briefly discussed natural history. As a science, Elton felt that ecology in this chapter will appear again in more detail else- depended on three methods of approach: field obser- where in the book. This overlap is entirely intentional vations, systematic techniques, and experimental on our part, and we would encourage the random work both in the laboratory and in the field. These reader to explore this introductory chapter first, and three basic systems still form the framework of ecology then turn to the details in whichever subject and later today, and it is the appropriate integration of these chapter takes his or her fancy. Readers who know systems, which provides our best estimates of the what they are looking for can, of course, proceed associations of living organisms with themselves and directly to the relevant chapter(s). their environment, which we call ecology. We have assumed that most readers will have Insects have dominated the interests of zoologists some knowledge of insect taxonomy, and we have for centuries. Those who could drag themselves away not attempted to provide an in-depth coverage of from the charismatic but species-poor vertebrates this topic. Some information is, however, provided soon found the ecology of insects to be a complex to set the scene. Readers who wish to know more and rich discipline, which has fascinated researchers are recommended to obtain one of various excellent for at least 200 years. At first, the early texts were entomology textbooks such as that by Gullan and mainly descriptive of the wonders of the insect world. Cranston (2005). Starting in 1822, Kirby and Spence published their four volumes of An Introduction to Entomology. This was a copious account of the lives of insects, written 1.2 HISTORY OF ECOLOGY AND over three decades before Darwin first produced On the ENTOMOLOGY Origin of Species in 1859. The fact that Kirby and Spence did not have the benefit of explaining their The science of ecology is broad ranging and difficult to observations on the myriad interactions of insects define. Most of us think we know what it means, and and their environment as results of evolution did indeed it can imply different things to different people. not detract from the clear fascination that the world It is best considered as a description of interactions of insects provided. This fascination has withstood between organisms and their environment, and its the test of time. Half a century or more after Kirby basic philosophy is to account for the abundance and Spence, Fabre published insightful and exciting and distribution of these organisms. In fact, ecology accounts of the lives of insects, starting before the Speight-The Insects 9781405131148_4_001 Page Proof page 2 20.11.2007 6:44pm 2 Chapter 1 __________________________________________________________________________ Table 1.1 Summary of associations in ecology. Type Interaction level Insect example Competition Intraspecific Individuals within a population of one species Lepidoptera larvae on trees Interspecific Individuals within populations of two different species Bark beetles in tree bark Herbivory, or phytophagy Between autotroph and primary consumer Aphids on roses Symbiosis Mutualism Between individuals of two species Ants and fungi Sociality Between individuals of one species Termites Predation Between primary and secondary consumer Mantids and flies trophic levels Parasitism Between primary and secondary consumer trophic Ichneumonid wasps and levels sawflies turn of the 20th century (Fabre 1882), wherein at a population level, or over an entire community. he presented in great detail observations made over Table 1.1 summarizes some of these basic associ- many years of the ecologies of dung beetles, spider- ations. With the exception of primary production, it hunting wasps, cicadas, and praying mantids. These should be possible to discover insect examples for books epitomized the first of Elton’s three approaches all other kinds of ecological interactions. The fact to ecology, that of field observations. Later work began that insects are so widespread and diverse in their to enhance the third approach, that of experimental ecological associations is not surprising when it is insect ecology, summarized in Insect Population Ecology considered just how many of them there are, at least by Varley et al. (1973). This later book was probably on land and in fresh water, how adaptable they are to the first and certainly most influential, concise ac- changing and novel environmental conditions, and count of insect ecology derived from quantitative, how long they have existed in geological time. scientific research. 1.4 THE INSECTA 1.3 ECOLOGICAL ASSOCIATIONS 1.4.1 Structure The term ‘‘association’’ used by Elton covers a great many types of interaction, including those between This is not a book about insect morphology or physio- individuals of the same species, between species in logy in the main, but both are certainly worthy of the same trophic level (a trophic level is a position in some consideration, as the ability of an organism to a food web occupied by organisms having the same succeed in its environment is dictated by form and functional way of gaining energy), and between differ- function. Here, we merely provide a few basic details. ent trophic levels. These associations may be mutually The basic structure of a typical adult insect is beneficial, or, alternatively, involve the advancement shown in Figure 1.1 (CSIRO 1979). Comparing the or enhancement of fitness of certain individuals at figure with a photograph of a grasshopper (Figure 1.2) the expense of others. Of course, insects can live in will enable any new student of entomology to quickly association with other organisms without having any realize how insects evolved a highly technologically influence on them, or being influenced by them; such efficient set of specialized body parts and appendages. associations are ‘‘neutral’’. The three basic sections (called tagmata) of an insect’s Ecological associations among insects operate at body are admirably adapted for different purposes. one of three levels: at the level of individual organisms, The head specializes in sensory reception and food Speight-The Insects 9781405131148_4_001 Page Proof page 3 20.11.2007 6:44pm _____________________________________________________ An overview of insect ecology 3 (a) (b) Foreleg Antenna Tegmen (forewing) Eye Maxillary palp Remigium Labial palp Pronotum Mesepisternum Anal lobe Mesepimeron Remigium S1 Metathoracic T1 T1 S2 spiracle Metepisternum T2 S3 Anal lobe Metepimeron T3 S4 Midleg T4 Abdomen Thorax Head S5 Tympanal T5 S6 organ T6 Hindwing S7 T7 T8 Hindleg T8 S8 T9 T10 Terminalia Hindwing 10 mm Tegmen Figure 1.1 The external structure of an adult insect, illustrating general features of a non-specialized species. (From CSIRO 1979.) gathering, the thorax in locomotion, and the abdomen other body structures in terms of their evolution and in digestion and reproduction. All but a minimum ecology. This basic plan is of course highly variable, number of appendages have been lost when compared and the most specialized insects, for example, blowfly with ancestors, leaving a set of highly adapted mouth- larvae (the fisherman’s maggot), bear little or no parts and a pair of immensely stable tripods, the legs. superficial similarity to this plan. Throughout the book, we shall refer to these and 1.4.2 Taxonomy We hope that the reader does not get too bogged down with taxonomic nomenclature or classification. We are concerned with what insects do rather than what their names are, and although it is essential that detailed taxonomy is eventually carried out as part of an ecological investigation, in this book we have pro- vided scientific names more as a reference system for comparison with other published work than as some- thing people should automatically learn. However, some introduction to the taxonomic relationships of the insects will help to set the scene. Figure 1.2 Side view of grasshopper showing general The class Insecta belongs within the superclass structure. Hexapoda, within the phylum Arthropoda. These in Speight-The Insects 9781405131148_4_001 Page Proof page 4 20.11.2007 6:44pm 4 Chapter 1 __________________________________________________________________________ turn belong to a huge group or clade of invertebrates the Myriapods are separate, probably much more known as the Ecdysozoa – animals that grow by shed- closely allied to the Chelicerates (Schultz & Regier ding their cuticles (molting) (Mallatt & Giribet 2006). 2000). The Pancrustacea makes a lot of sense eco- The Ecdysozoa bring together perhaps unlikely rela- logically, since insects and crustaceans have many tives, the nematodes and the arthropods for instance. similar lifestyles, niches, and so on; one just