Multitrophic interactions are those that link several (i.e., more than two) trophic levels, including plants (first trophic level), herbivores (second trophic level), and predators, parasitoids or pathogens (third trophic level and beyond; hereafter referred to as “enemies”). These types of multispecies consumer-resource dynamics are also referred to as tritrophic interactions when they specifically involve three trophic levels. Because food webs typically possess more than three trophic levels, with higher-order predators preying on intermediate predators and hyperparasitoids attacking primary parasitoids, we use “multitrophic” as an all-encompassing term that includes tritrophic interactions, but allows for the consideration of more complex food-web dynamics.
Fortunately, the individual components that comprise multitrophic interactions have already been described in some detail throughout earlier sections of this book, especially Chapters 4 through 8 (Part III: Species interactions), which address plant-herbivore interactions, competition, mutualism, predator–prey and host–parasite interactions, respectively. Also, Chapter 2 provided a conceptual foundation for the role of semiochemicals in plant–insect associations, a critical issue that we will revisit often in this chapter as insects live in a chemically mediated world. The purpose then of this chapter is to assemble these isolated units to form an integrated understanding of how plants, herbivores and enemies interact, with the explicit recognition that plants exert direct effects on the ecology of predators and parasitoids, and that indirect effects linking plants with higher trophic levels are commonplace. This has not always been the prevailing view. Historically, plant–herbivore and enemy–prey interactions were treated as two separate fields with no unifying theme. Seminal publications by Hairston et al. (1960), Price et al. (1980) and Oksanen et al. (1981), however, provided the theoretical framework that was lacking and spawned an emergence of studies over subsequent decades. As a result, the field of multitrophic interactions is among the most productive, innovative and exciting realms of insect ecology.
Populations illustrate characteristics such as growth and decline, birth and death, immigration and emigration, life histories adapted to the environment, and dynamical behavior. These are the topics in this part of the book. They are of great interest to the insect ecologist because they provide ways of understanding species in nature and in managed systems. We need these approaches in order to predict population trends and to plan pest-management strategies. Emphasis on population phenomena also provides an intermediary level of understanding between species interactions (Part III), community organization (Part V) and broader patterns over the landscape (Part VI). Naturally, species interactions have population consequences, such as natural enemies impacting prey populations, so comparable to what we did with species interactions, moving up the trophic levels, we now approach by moving up a complexity gradient from individuals (Part II), to interactions (Part III) and now to their consequences at the population level.
When considering the word “parasite” various kinds of organisms may spring to mind, depending upon the audience. Veterinarians may think of scabies, mange mites, warble flies, helminthes such as liver flukes, bird lice and disease organisms that cause rabies, plague, West Nile Virus and bird flu. Medical entomologists may be more concerned about malaria, sleeping sickness and vectors of disease like mosquitoes and blackflies. Agriculturalists would perhaps worry about plant pathogenic diseases and nematode parasites. Those involved with the biological control of weeds or insect herbivores may well think of diseases of plants and insects, and parasitoid wasps and flies. But springing to mind among only a few insect ecologists, perhaps, would be the insect herbivores themselves: caterpillars of the moths, butterflies and sawflies, larvae of chrysomelid beetles, bark beetles, weevils, grubs of root-feeding insects such as rootworms, and wood-boring larvae, such as cerambycid and buprestid beetles, and clearwing and carpenter moths. And yet, when the definition of the term parasite is appreciated, we can begin to recognize the vast convergence of many lineages toward the parasitic way of life. In common usage is the term “host plant” for insect herbivores, with the clear implication that plants are hosts to parasites. Explicit recognition of herbivorous insects as parasites is provided in the definition of a gall: “Galls are abnormal growths formed from tissues of a plant or other host, due to the parasitic activity of another organism” (Redfern and Shirley 2002, p. 207). This broad view of what is a parasite is somewhat controversial because of the ingrained emphasis on parasites of animals, particularly in the field of parasitology. However, plant pathology treats parasitic diseases extensively and we can observe life-history convergence in parasites on animals and plants (Section 8.6), so we should recognize the common features of all kinds of parasites.
In this chapter we emphasize the diversity of insect parasites, as well as the convergence of many groups toward similar life cycles, their main characteristics and how they relate to host species. We compare parasites with other ways of life such as predation, emphasizing that intimacy and duration of relationships between parasite and host have major importance in understanding evolutionary pathways in parasite lineages: life-history convergence, adaptive radiation and phylogenetic tracking of host lineages. Kinds of damage inflicted on hosts are discussed. Host responses in the form of defenses, how parasites can modify host behavior, and the population dynamics of host and parasite all contribute to a general appreciation of the parasite's ecological and evolutionary roles.
We have built the conceptual basis for this part of the book through our earlier treatment of behavior, then interactions among species at the same and different trophic levels. We have also discussed population ecology in Part IV, so that we are aware of the kinds of interaction and variation that species may contribute to the communities they live in. Now, in Part V, we explore concepts and evidence on how species fit together into communities, how communities are organized and the interactions promoting order and predictability.
The largest scales on which we can examine insect relationships with their environments are time and space: time throughout the fossil history of insects, and the space over the globe in which topography, land masses and climate are constantly in flux. Planet Earth provides the relevant scale. At this dimension we can examine the paleobiological record and how past events have influenced insect diversification, the current impacts of climate change and global patterns in ecological relationships. Breaking the planet down into smaller units reveals the role of insects in ecosystems, their importance as invaders of continents and ecosystems, and the necessity for conservation of habitats and their denizens. These are the subjects discussed in this chapter.
The paleobiological record
The history of life provides clues, or predictions, about what the future may bring. Using families of insects as a measure of richness in the fossil record provides a stable estimate: family richness is high enough to capture diversity changes, and family size is large enough to provide a reliable signal (Labandeira and Sepkoski 1993). In the Carboniferous period numbers of families increased rapidly, it declined during the Permo-Triassic extinctions about 245 million years ago, and then increased at a steady rate for the next 220 million years (Figure 15.1). The jump in richness in the mid-Tertiary resulted from rich fossiliferous deposits from that time, including Baltic amber, and the Florissant shales of Colorado. At the end of the Paleozoic era and the Permo-Triassic boundary, whole orders of insects went extinct, including the Palaeodictyoptera and related clades (Figure 15.2). These were endowed with piercing and sucking mouthparts for feeding on plants, and presumably were impacted by major losses in plant diversity during the extinctions. After the end of the Paleozoic, four major groups of insects, extant today, expanded exponentially through the Mesozoic and Cenozoic eras: the Hemiptera, Coleoptera, Diptera and Hymenoptera. The Lepidoptera were late arrivals in the fossil record, undergoing expansive radiation in the upper Cenozoic (Figure 15.2).
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.
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.
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.
We build from on behavioral ecology, which is devoted to interactions among individuals and within social groups, to interactions between species and trophic levels. Gradually we move up the trophic system, starting with plant and herbivore associations, and then to interactions among herbivores involving competition, strong asymmetric interactions and facilitative interactions. After this, in the next three chapters (6–8), we return to interactions between trophic levels, treating mutualistic relationships, the interplay of prey and predator, and of host with parasite. In aggregate these kinds of relationships constitute the main forms of interactions to be found on any landscape.
Behavior can be defined as anything that an individual does during its life, involving action in response to a stimulus. Eating behavior is stimulated by hunger; sleeping or resting behavior is in response to fatigue; escape is a response to attack and reproductive behavior is in response to physiological urges and stimulation by members of the opposite sex. Throughout the life of an individual insect it is behaving constantly in one way or another, making behavior a large and important subject.
Many behaviors are in response to external stimuli, part of the environment, making them ecologically relevant, and behavioral ecology is an important part of ecological understanding. Understanding much of behavior results from the study of how species are adapted to the problems of survival and reproduction, and how natural selection shapes the trajectory of a lineage through the costs and benefits, the opportunities and constraints, of any particular genetic and phenotypic change in that lineage.
Social insects are major components of most ecosystems and are key players in communities. We will see in this chapter that their biomass is impressive, their activities as ecosystem engineers – making nests, trails and moving soil – are massive, and their impacts on other community members are widespread.
Social insects stimulate immense fascination among their human observers because of their ubiquity, their diurnal activity and their complex social structure involving many sophisticated behavioral interactions. They also pose the problem of how such societies evolved: under which ecological conditions would selection favor the banding together of related individuals into dense populations distinct from most species whose individuals disperse widely from others? The interplay of life-history evolution, behavior, ecology and phylogeny in the emergence of social insects offers an excellent example of how these biological processes are inevitably meshed together and how we need to address them with an integrated-biology approach.
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).
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