The study of life histories involves the full span of life, from egg or sperm through reproduction and death. The ways in which individuals maximize progeny production and survival are about as diverse as the populations and species themselves. Research interest in life history is therefore strongly comparative, looking at the variation in life-history characteristics within populations, within species and among related species. As with ecology in general, we search for patterns in nature and develop hypotheses and theories, which account for the trends that we observe, and prompt new observations in a cycle of efforts to refine knowledge.

Understanding of life histories is fundamental to insect ecology, and we have covered many examples of life-history studies already in this book, and more will follow. Behavioral traits that promote fitness were covered in Chapter 2, and the evolution of life histories of social insects was discussed in Chapter 3. Chapter 4 included discussion of wing polymorphism in planthoppers (Figure 4.5), and jousting and territoriality of Pemphigus aphids (Figure 4.11). Density effects on planthopper fecundity and wing polymorphism were involved with competition (Chapter 5, Figure 5.8), and egg-laying schedules of a fruit fly were described in Chapter 9 (Figure 9.10). We noted also in Chapter 9 the differences between pro-ovigenic and synovigenic egg production, and the marked differences in survival of progeny portrayed in survivorship curves. In Chapter 6 different strategies of yucca moths were noted among true pollinators and two classes of cheaters (Figure 6.14). We also saw divergence of life-history types in scarab beetles (Figure 6.16) and the constraints on ovariole number and fecundity of parasitoid wasps and flies (Figure 8.4). Indeed, the study of life-history traits runs throughout ecology, and this is how it should be, because understanding the evolution of whole life histories is at the heart of understanding insects, and the evolutionary pathways along which they have traveled. This point is taken up again in the last section of this chapter on applications.

Everybody is conscious of insects, and even concerned about them. In fact, we each have an ecological relationship with their kind. We share our houses and gardens with them, our walks and picnics, and our adventures. So should we not understand them? Their richness in species and interactions, their beauty and behavioral intricacy, all enrich our lives if we understand who they are, and what they are doing. Therefore, the ecology of insects is for everybody.

Eisner (2003, p. 1), in his latest book, For Love of Insects, starts by writing that “This book is about the thrill of discovery.” And, Wilson (1994, p. 191), in his autobiographical, Naturalist, advised, “Love the organisms for themselves first, then strain for general explanations, and, with good fortune, discoveries will follow. If they don't, the love and the pleasure will have been enough.” Here is sound advice from two of the greatest practitioners of entomology and ecology, for discovery is thrilling, and the deeper the fascination one develops, the greater will be the discoveries that follow.

Contents

This book concludes with discussions on the broadest biological patterns we can observe on this Earth, and the reasons for their existence. We also examine smaller scales of variation that would be seen on the landscape and ecosystem levels. In doing so we pick up various topics discussed in previous chapters, such as the roles of time and space as influences on species richness, and expand this view to the global level, showing that the same factors remain important as we scale up our perspective to interactions on Earth. We also look at the paleobiological record again, as we did in Chapter 1 to note the long evolutionary history of insects, but in this section we examine the record for clues on what might be expected as global changes occur, and if predictions are possible.

All natural populations fluctuate: they are dynamic. introduced the concepts of population growth and regulation. How and why populations change are the subjects of this chapter. Population dynamics has been of major concern in insect ecology for at least two reasons. First, ecology has been defined as the study of the distribution and abundance of species (Andrewartha and Birch 1954, Krebs 1972). Since the study of population dynamics must include both changes in numbers over time and over the landscape, the subject acts as a central theme in ecology: a unifying concept that permeates the science. It is therefore critical to the conceptual development of ecology. Second, the subject is directly applicable to problems in managing plants as resources for humans, in agriculture, horticulture and forestry. In fact, the need to monitor and understand insect damage to crops and forests was a major motivation for the beginnings of ecology and the study of population dynamics. Other applications include the study of vectors of diseases, such as mosquitoes, pests of cattle such as ticks and screwworm, and vectors of plant pathogens. For these reasons the field of insect population dynamics has played a prominent role in the development of basic ecology and in the understanding and management of serious pests over the landscape.

First we will examine major patterns in populations and then move on to mechanisms that may be driving these patterns, including abiotic and biotic influences, and complex interactions involving both. We also consider the question of how common eruptive species are, how frequently eruptions occur, and whether outbreaks result from human interference with natural dynamics. We note that long-term studies are essential in deciphering the reasons for population change, but many potential influences need to be investigated. Bottom-up effects from plants, top-down effects from natural enemies and lateral effects all need attention. Spatial distribution of populations is also important in a fragmented landscape, covered by the field of metapopulation dynamics. Population dynamics is a field of wide application for understanding pest species, epidemiology, biological control and conservation, all requiring information for planning and management.

Contents

Behavior links the organism to its environment. At every moment of the day and night behavioral responses to environmental cues, and internal drives, guide an individual through its often perilous existence. Behavior determines success or failure in the efforts to reproduce. Discovering the many cues that insects use to inform them of their environment, and their responses to these stimuli, is an ongoing pursuit of behavioral ecologists. This is because insects can be much more specialized than humans in the cues employed to locate food, and cues that stimulate feeding. Indeed, the ways in which they relate to their surroundings is often notably different from we humans. As a result, behavioral ecology is a broad and fascinating subject which reaches well beyond this section of the book, into on species interactions and other parts of the book.

In the preceding chapter, we focused on interactions between phytophagous insects and their host plants and saw how species occupying different trophic levels can influence each other's abundance, distribution and evolution. Other important intertrophic level relationships include predator–prey and host–parasitoid interactions, and these will be dealt with in forthcoming chapters (see Chapters 7 and 8). Here we consider lateral interactions, those that occur among individuals feeding at the same trophic level, and how such interactions (e.g., competition, amensalism, facilitation and mutualism) can affect species' abundance, distribution and community structure. Because lateral interactions, and in particular competition, have been studied so extensively using herbivorous insects, we begin our consideration of the topic focusing on this group of consumers, deferring our treatment of lateral interactions in other insect consumer groups (e.g., detritivores, scavengers, predators and parasitoids) to a bit later in the chapter.

Lateral interactions between insect herbivores can be negative (competition and amensalism), neutral or positive (facilitation and mutualism) (Damman 1993, Denno et al. 1995, Denno and Kaplan 2007, Kaplan and Denno 2007). In competitive interactions, both participants (either conspecifics or heterospecifics) are negatively affected (−, −), whereas in cases of amensalism one of the players suffers from the interaction but the other remains unaffected (−, 0). Positive interactions include facilitation when at least one organism benefits from the interaction (+, 0) and mutualistic interactions in which both participants benefit (+, +) (Bruno et al. 2003, Bourtzis and Miller 2006). Moreover, mutualisms can involve tightly coevolved obligate interactions, such as aphids and their bacterial symbionts, or they can entail much looser facultative relationships, such as generalist pollinators and their nectar source plants. Because of the complexity and often intertrophic nature of mutualisms (e.g., protectionist ants and plants that offer rewards), we devote a whole chapter to this fascinating topic (Chapter 6). There we discuss only positive interactions between organisms feeding at the same trophic level, although the strength of such interactions (and negative ones as well) are often mediated by basal resources (plants) and natural enemies (Denno et al. 1995, Denno and Kaplan 2007, Kaplan and Denno 2007).

Biological diversity, or biodiversity, is the number and variety of species and other taxa in any locality, ecosystem, region or the biosphere. The commonest measure of biodiversity is the number of species present in the area of interest. Species number, species richness and species diversity are used almost interchangeably in the ecological literature, although diversity may be employed when considering both the species number and their abundances in a sample (see below) (Calow 1998, 1999).

Learning how many species there are in a particular locality has formed a major basis of enquiry in ecology. Once descriptive data are available another basic question becomes why numbers of species differ in different localities, on different plant species, or in different lakes? With increasing knowledge we can begin to ask questions about the total number of species on this earth, and on the patterns of their distribution in space and time. Thus, the subject of biological diversity reaches the local, to regional, to global scale, encompassing species richness in component communities to the biogeography of species and interactions among species and the physical environment.

In Chapter 4 we learned how plants and herbivores can influence each other's abundance, distribution and evolution. Here we consider another important inter trophic level relationship that can have widespread ecological and evolutionary effects on biological communities, the interaction between prey and their predators. In an ecological sense, predators can dramatically affect the abundance and distribution of their prey populations, and reciprocal effects of prey on their predators are also inevitable, as prey are obviously an important food source for predators. Likewise, the diverse feeding habits of predators form linkages that are responsible for the flow of energy through food webs. Predation can also be a powerful evolutionary force with natural selection favoring more effective predators and less vulnerable prey. Thus, it is imperative that we understand the process of predation and its complex effects on species interactions, and population and community dynamics. In the sections that follow, we explore critical elements of prey–predator interactions, namely how prey and predators interact to affect each other's long-term population dynamics, what factors stabilize prey–predator interactions and promote their persistence, how multispecies interactions influence the role of predation in complex food webs, the contribution of behavior to a predator's total impact on prey populations, and how predators and prey have reciprocally influenced each other's evolution.

What is a predator?

In a very general sense, predation can be viewed as the consumption of one living organism (the prey) by another organism (the predator). Usually the whole prey item is killed and eaten. If the prey organism in question is a plant, then this general definition of predation includes herbivory. However, whole plants are usually only killed and eaten by a single predator when the plant is in the seed or seedling stage. Hence the terms seed predator and seedling predator are in common usage. In the context of this chapter, however, we restrict our definition of predation to acts of carnivory in which animals consume other animals. We define predators as animals that kill and consume all or parts of their prey, and require many prey items to reach maturity. This definition distinguishes predators from parasitoids, such as some small wasps and flies, which require and eat only one prey item during their life span. Parasitoids are free living as adults, and lay their eggs in or on a host. Larvae hatch from the eggs and live parasitically in or on the host, eventually killing it. For conceptual simplicity we discuss predators and parasitoids as representing distinct biological groups; however, the line distinguishing predators from parasitoids is often blurred, with biological reality perhaps better represented as a continuum rather than discrete categories.

Contents

We introduce insect ecology by looking at the many remarkable features of the insects: their long evolutionary history, important design characteristics, including wings and flight, and the prodigious numbers of species and numbers of individuals per species. Inevitably, such vast richness entails many kinds of interaction, the basis for the study of insect ecology, because individuals and species provide part of the environment which any insect experiences. Ecology is the science of relationships of organisms to their environment: the physical and the biotic components with which they interact. How they relate depends on their design and their behavior, the latter aspect forming of this book.