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Cambridge University Press 978-0-521-83488-9 - Insect Ecology: Behavior, Populations and Communities Peter W. Price, Robert F. Denno, Micky D. Eubanks, Deborah L. Finke and Ian Kaplan Excerpt More information

Part I Introduction

CONTENTS Chapter 1 The scope of insect ecology

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 Part II of this book. With millions of species of insects comes the question of how so many can evolve and coexist, subjects addressed in this chapter and other parts of the book. Also, we consider the roles that insects play in ecosystems, and the scientiﬁc method employed in their study. These introductory considerations set the stage for expanding many themes in subsequent parts and chapters. Part II is devoted to behavioral ecology, Part III to species interactions and Part IV to population ecology. Moving to larger arrays of interacting species we devote Part V to food webs and communities, and Part VI to patterns and processes over the Earth’s surface. We generally are innately fascinated by insects and other arthropods at a young age, but cultural defects tend to diminish this enjoyment, while enhancing dread and avoidance. With more understanding provided by insect ecology we can recover a sense of wonder, and a knowledge of belonging with insects on this planet.

© in this web service Cambridge University Press www.cambridge.org Cambridge University Press 978-0-521-83488-9 - Insect Ecology: Behavior, Populations and Communities Peter W. Price, Robert F. Denno, Micky D. Eubanks, Deborah L. Finke and Ian Kaplan Excerpt More information

1 The scope of insect ecology

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. When considering the features of insects that make them remarkable, many attributes come to mind; their diversity of numbers, shapes, colors and habits are incredible. Their potential for future evolutionary change is unimaginably rich. The ecological interactions that insects enter into are diverse and important, involving consumption of plants, including crops and forest trees, predation on other insects and ecosystem processes, such as cycling of nutrients and decomposition. Some insects are highly beneﬁcial for humans, while others are harmful. Thus, insect ecology serves the needs of both the desire to understand nature as a basic contribution to knowledge, and the need to solve the practical problems posed by insect pests concerning human hygiene, animal husbandry, agriculture, forestry, horticulture and the urban environment. We will elaborate on these features of insects, and the need to study and understand them, in the following sections of this chapter, and in the remainder of the book. First we discuss the evolution and design of insects, before looking at the richness of the insect fauna and their relationships. In the later part of the chapter we turn our attention to how insects have become so numerous and diverse through adaptive radiation, and their roles in ecosystem processes.

4 The scope of insect ecology

1.1 Fascination with insects evidence of insects feeding on plants, including generalized foliage feeding, boring internally, and While developing a fascination with insects we piercing and sucking types, and by 250 million years necessarily enter into the whole realm of Nature, ago most types of insect feeding were evident and because insects interact with almost all other living most types of plant parts were fed upon by insects. In species in one way or another. By studying insects for the tree fern swamp forests of the late Carboniferous their own enjoyment, “with good fortune” we may (308 MYA) the first evidence of gall-inducing insects perform good science. “Nature first, then theory” was has been described, and seed predation on seed ferns the order advised by Wilson (1994, p. 191). was evident. As plants diversified, so too did the The fascination with insects derives from many of insects, providing a rich paleoecological background their characteristics. Their charms and annoyances for studies of plant and herbivore interactions. are multifaceted. Insects have inspired art, design and Insects shared the land with other arthropods such literature, they act as a significant source of food in as centipedes, millipedes and spiders through the some parts of the world, they afflict millions with Devonian and Carboniferous, their distributions bites and infections while providing essential being widespread and presumably with high services in pollination and ecosystem processes. Their abundance. Their lives were uncomplicated by the interactions with humans, agricultural crops, forests, presence of vertebrate predators for perhaps 20–25 livestock and other domesticated animals make million years. But in the late Devonian, amphibians insects of ubiquitous concern. Seldom will a day pass made a partial entry into the terrestrial fauna, while without seeing or interacting with insects. They are still breeding in water. Both on land and in water so common locally, and widespread geographically, amphibians were no doubt preying on insects, that virtually all humans experience their presence. although impact was probably small, and 20–25 But, in spite of their commonness, many people million years is a pretty good run without vertebrate misunderstand insects, regarding them as vermin, predators for the insects. and are even frightened by some. However, the study The evolution of flight in insects constituted a of insect ecology can only contribute to our breakthrough to an extraordinary adaptive radiation fascination with them, and our admiration for the in the late Carboniferous – an adaptive radiation roles they play in nature and in environments never equaled on this Earth. This was about 150 modified by humans. million years before pterosaurs, birds and bats flew. The conquest of the air, so early among terrestrial animals, was no doubt of prime 1.2 Antiquity of insects importance in the spread of insects across the globe. Add to this the sheer age of insects, and the The earliest insect fossils date back to about time for diversification, and we can begin to 400 million years ago (Kukalova´-Peck 1991, understand why insect species are so numerous, Labandeira 2002, Grimaldi and Engel 2005), deep in and in some ways dominant on Earth. the Devonian Period and Palaeozoic Era. Plants diversified during the Siluro-Devonian “explosion” (420–360 MYA), followed by a rapid radiation of 1.3 Insect body plan insects during the Carboniferous, with an extraordinary emergence of flying insect taxa by The body plan features of insects have, without a 300 million years ago. We can see from Figure 1.1 doubt, contributed in key ways to the impressive that already in the Devonian (400 MYA) there is fossil radiation of the group. Two characters in particular

© in this web service Cambridge University Press www.cambridge.org © in in © More Excerpt Peter W. Price, RobertF. Denno, MickyD. Eubanks,DeborahL. Finke andIanKaplan - - 978-0-521-83488-9 U Cambridge information this web External foliage feeding Feeding on internal tissues Surface fluid Pollination Aquatic

Seed nive Functional General- Margin Hole Skeleton- Free Bud Piercing Boring Leaf Galling feeding syndromes feeding service feeding group ized feeding feeding ization feeding feeding and sucking mining predation ty rsity yrs) 6 Insect Ecology:Behavior, PopulationsandCommunities Cambr Press Ma (×10 Period or Period subperiod Era 0 Neo- gene Univer idge

50 Caenozoic Palaeogene

sity 100 Cretaceous Press

150 Mesozoic

Jurassic Angiosperm 200 revolution

Tree fern swamp forests 250 Permian Triassic 300 Penn. Palaeozoic

350 Miss. Carboniferous

400 Devonian

Figure 1.1 The fossil record of insects associated with plants according to the functional feeding groups of insects. Shaded columns represent the geochronological duration of each feeding group, with horizontal lines in columns showing actual records. External feeding and internal feeding categories are grouped. Major features of host plants www.cambridge include tree fern swamp forests and the adaptive radiation of the angiosperms, shown as horizontal shaded bars. Abbreviations: Miss. ¼ Mississipian; Penn. ¼ Pennsylvanian. From Labandeira 2002. Reprinted with permission from Blackwell Science. .org Cambridge University Press 978-0-521-83488-9 - Insect Ecology: Behavior, Populations and Communities Peter W. Price, Robert F. Denno, Micky D. Eubanks, Deborah L. Finke and Ian Kaplan Excerpt More information

6 The scope of insect ecology

permitted a series of novelties which resulted in their which larvae are very different from adults, as in the richness on Earth: they were primitively terrestrial caterpillar and butterfly. About 90% of insect species and they had an exoskeletal integumentary system. are holometabolous, including all the very large Together, these traits facilitated a tracheal system of orders – Coleoptera, Hymenoptera, Lepidoptera, and integumentary invaginations dividing through the Diptera (Figure 1.2b), with the origin of body, providing air almost to the cells at work. With holometaboly about 300 MYA (Kukalova´-Peck 1991, a permeability constant for oxygen through air of Grimaldi and Engel 2005). This leaves about 1% of 660 cm2/atm.h compared to a permeability through insects that are primitively wingless, including water of 2 1013 cm2/atm.h, tracheae were hexapods now classified outside the Class Insecta extraordinarily efficient, thereby allowing insects (Protura, Collembola and Diplura), although they are to be highly active (Alexander 1971, see also commonly included in ecological studies on insects. Chapman 1998). However, every adaptation, such These percentages are based on numbers of described as an exoskeleton, has its own constraints: a hard species per insect order provided in Triplehorn and exoskeleton limited size. This was because, to Johnson (2005). They illustrate a remarkable grow, an insect needed to cast its integument, breakthrough in animal design through while replacing it with a larger one. But this left metamorphosis, especially the holometabolic form. the individual briefly vulnerable and subject to Larval forms and habits are enormously varied, from bodily collapse since the supporting exoskeleton maggots (vermiform larvae), to grubs (scarabaeiform had been abandoned. Nevertheless, the tough larvae), to caterpillars (eruciform larvae), to active integument, tracheal system and small size and often predatory campodeiform larvae, and long provided extraordinary potential for evolutionary and slender elateriform larvae (cf. Triplehorn and innovation, and we look at each of these in turn Johnson 2005). Maggots may squirm through mud, in this section. or animal bodies, mine in plants or live in water, while caterpillars are active as external foliage feeders, wood borers, leaf miners and gall inducers. 1.3.1 Metamorphosis Certainly, metamorphosis has contributed The constraint of the exoskeleton and the significantly to the adaptive radiation of the insects. requirement for molting also provided a new opportunity in insect design: an insect could change 1.3.2 Exoskeleton and flight its shape from one molt to another. This metamorphosis resulted in larvae evolving with The exoskeleton provided great strength in small shapes and habits very different from adults. The structures, a strong skeleton to support heavy-duty caterpillar of a moth, butterfly or sawfly spends its muscular contractions and considerable protection life feeding and differs remarkably in design from against many enemies. Small size meant that gravity the adult, while adults may or may not feed, but are exerted a relatively weak force, equivalent to involved mainly with reproduction: courting, mating adhesion and cohesion for insects around 1 mm in and ovipositing. All insects with winged adults length (Went 1968), enabling adhesion to leaves, change from immature wingless forms to winged walls and ceilings, but making a drop of water a adults. About 9% of insects are hemimetabolous, disabling hazard. Small size, the exoskeleton and the with incomplete metamorphosis, for the immature tracheal system also contributed to the evolution of nymphs are similar to adults except that they lack the first flying animals on Earth. With gravity as a wings. But the large majority of insects are relatively weak force, gliding may well have been a holometabolous, with complete metamorphosis,in possibility, with hardly any particular special

1.3 Insect design 7

(b) Insects

Lepidoptera 160 000 Diptera 125 000 Hymenoptera 150 000 Hemiptera 112 000

Orthoptera (a) Named organisms Coleoptera 350 000 20 000 Thysanoptera 5000 Insects Other orders 990 000 51 000

Mammals 4600 Other animals 307 000 Plants 265 000 (c) Plants

Fungi Protoctists 70 000 Bacteria 80 000 >4000 Angiosperms 235 000

Gymnosperms 620 Minor groups Ferns Bryophytes 7300 11 000 10 000

Figure 1.2 The estimated numbers of named species on Earth (a), the insects (b) and the plants (c). Note the large proportion of angiosperm plants (the ﬂowering plants), and the large size of insect orders composed of many herbivores, many of which depend on ﬂowering plants for food. From Price 2002a. Reprinted with permission from Blackwell Science.

8 The scope of insect ecology

adaptation for flight. Any extension of the cuticle National Park, in brine lakes and deep in caves would improve the possibility of effective gliding, (e.g. Culver 1982, Fincham 1997). and articulation would result in controlled flight. The “success” of insects is often noted, although Alternatively, articulated flaps acted as gills or gill usually without defining what success actually covers in aquatic insects, that secondarily became means. Clearly the term is subjective, so that modified for flying. No matter which processes were quantifiable criteria are best employed to express the involved in the evolution of flight, and there are impressive adaptive radiation of the group. Most many hypotheses (e.g. Kukalova´-Peck 1978, 1983, often used is the sheer number of species, but other 1987, 1991, Gullen and Cranston 2005, Grimaldi and factors need consideration, such as the diversity of Engel 2005), flying became a reality for the insects, morphology, lifestyles, their biomass per unit area, and a major breakthrough in their adaptive radiation. the rich relationships with each other and among Approximately 99% of insects can fly in the adult species, and their many roles in natural and managed stage, or are derived from a lineage with flight systems. We will examine some of these criteria in (Price 2002a). The primitively wingless insects and the following paragraphs. other hexapods have remained a relatively depauperate group. 1.4 Richness of the insect fauna 1.3.3 Small size Figure 1.2 shows the estimated number of named Small size, combined with flight, results in insects’ species on Earth. As we can see from Figure 1.2b the ability to exploit small and scattered resources, with named insects number about 1 million species narrowly defined ecological niches, or places to live. (Foottit and Adler 2009). However, most insects have As a consequence they have specialized in the not been named, and their numbers are unknown, but colonization of dung, carrion, tree holes, rotting logs, debated none the less. Some estimates converge on birds and their nests, other insects, pools, temporary approximately 5–10 million species, or about 90% streams and endless other microhabitats (see Gullan of all terrestrial animal species (e.g. Gaston 1991, and Cranston 2005). Additional resources include 1992, degaard 2000), but others, based on sampling pollen and nectar, blood, fungi, plant sap, fruits, in the tropics, consider 30 million species more seeds and other plant parts, often available in small likely (Erwin 1982, 1988). Among the described quantities and briefly in the year. Just these few insects, 90% are members of orders including examples highlight insects as living almost many herbivorous species, with very large orders everywhere, each species specialized for a particular represented: Coleoptera, Hymenoptera, Lepidoptera, way of life utilizing a particular resource. Even on Diptera, Hemiptera, Orthoptera and Thysanoptera small, remote, subarctic islands on which no (Figure 1.2). indigenous terrestrial mammals and few birds are The number of insect species is vastly greater found, hundreds of invertebrates exist. About 120 than any vertebrate group, and even all invertebrate species have been recorded from Marion vertebrates combined: fish, amphibians, reptiles, Island (46S) alone, with 17 families in 7 orders of birds and mammals, which add up to about 45 000 insects present (Mercer et al. 2001). (Marion Island species. Bird species number something a little less lies at 2300 km southeast of Cape Town, South than 9000 species and mammals about 4600 Africa, with intervening islands, and a surface area species; depauperate groups indeed. (“Other of about 290 km2 [Chown 1992].) Insects are also animals” in Figure 1.2a includes vertebrates other found in thermal pools at 25–40 C in Yellowstone than mammals, and the many invertebrate

1.5 Richness of relationships 9

groups such as sponges, corals, flatworms, crabs, 1994, 2005, Bossart and Carlton 2002, and see also spiders, snails and worms.) Compare these the Journal of Insect Conservation). Protection of numbers for whole classes of vertebrates with the breeding sites for aquatic insects like dragonflies, species numbers for single families of insects. restoration of habitat for others and monitoring of For the ants, family Formicidae, 8804 species were populations are all part of the conservation strategy described by 1990, but quite likely 20 000 species and insect ecology. Naturally, much ecological exist on Earth (Ho˝lldobler and Wilson 1990). In research is needed to understand the status of species the parasitoid wasp family Ichneumonidae about and populations, and the risks to which they are 15 000 species have been described, with a likely exposed, so conservation biology will be discussed 60 000 species in the world fauna (Townes 1969). repeatedly in this book. Townes notes than an ichneumonid genus is more or less equivalent taxonomically to a bird family. 1.5 Richness of relationships Insect biomass is equally impressive, with just ants and termites representing an estimated 33% Needless to say, the richness of insect species means of all animal biomass in the Amazonian terra firme that they are involved with an even richer set of rain forest (Ho˝lldobler and Wilson 1990). “Insects, at interactions, for each individual species interacts an estimated weight of 27 billion tons, outweigh the with a multitude of others. Table 1.1 shows us that human population by about six times. In terms of many species exploit plants in one way or another, biological mass, or biomass, insects are by far exhibiting a wide range of resource exploitation, for the dominant animal life form on Earth” every part of a plant may be utilized by one insect (Grissell 2001, p. 35). species or another. For example, a large oak tree may The numbers of individuals per species are also support several hundred species of insect, with staggering in some cases. In an outbreak of forest almost all parts vulnerable to attack, including all caterpillars, numbers may reach 104–105 stages from seed, to seedling, to the mature plant individuals per 100 m2.Antsmaynumber (Figure 1.3). Here, we discuss direct and indirect 20 million individuals per hectare in the tropics, relationships by looking at feeding links and and some driver ant colonies may even contain community interactions. 20 million workers. Early naturalist explorers were amazed at the mass dispersal of Lepidoptera, like “snowing butterflies.” Even in our own gardens and 1.5.1 Feeding links and types landscaping around our houses, probably more than a hundred species live, represented by Moving up the feeding links, or food chain, the thousands of individuals. herbivores are in turn fed upon by carnivores, both Such biodiversity of insects needs protection as insects and other animals such as reptiles and birds. much as any other group. All species are impacted Feeding on animals such as insects involves three by reduced habitat through deforestation, main types: predators, parasitoids and parasites. expansion of agriculture and urbanization, and the Predators generally kill their prey and consume most fragmentation of habitat into smaller and smaller or all of the dead body. Parasitoids are parasitic in parcels, rendering populations at greater risk of local the larval stage but free-living as an adult, with a extinction. Thus, the conservation of insect species female parasitoid that searches for hosts in or on and populations is attracting greater attention and which to lay an egg. Thus, parasitoid species can be stronger resolve (e.g. Gaston et al. 1993, Samways regarded as mainly parasitic because of usually long

10 The scope of insect ecology

Table 1.1 The resources provided by plants association with an insect host, or predatory because utilized by insects, and their kinds of a female adult searches like a predator, usually exploitation dooming the host to death. This is because of larval feeding in or on the host and, as a consequence, the Resources provided Name of exploiter parasitoid acts like a predator in relation to the Living plant or Herbivore, phytophage population dynamics of the prey. Parasites of insects plant parts in include microorganisms (bacteria, protozoa, fungi), general mites, nematodes and the larval stages of parasitoids. A parasite can be deﬁned as an organism that lives Eating plants and Omnivore in or on another living organism, which obtains part animals or all of its food from that organism, which is usually Shoots None adapted by morphology, physiology and behavior to living with its host and which causes some Leaves Folivore measurable damage to its host (see Price 1980, Bush Buds None et al. 2001). Flowers Florivore These categories of herbivores and carnivores cover only a small selection of relationships among Nectar Nectarivore insects, and their food sources (Table 1.2). Some Anthers and/or Anthophage interactions may be beneﬁcial to each species, pollen constituting a mutualism, such as in pollination. Carpel and/or fruit Frugivore Mutualists may live intimately with each other, like the protozoa in the paunches of termites, with the Seeds Granivore protozoa and termites forming a symbiosis. Spores None Symbioses are not restricted to mutually positive relationships, as they also include parasites and their Cones None hosts in close association. Competition may be Wood/xylem Xylophage observed among any organisms that exploit the same Cambium None resource in limited supply. And competition was thought to be most likely among members of the Bark/cortex/ None same guild: species that exploit the same resource in periderm a similar manner (Figure 1.4, Root 1973). Such Roots/rhizomes Rhizophage competition may be very one-sided – asymmetric competition – with one species hardly affected, Tubers, corms, None but the other negatively impacted (a 0 interaction), bulbs and competition now appears to be frequent outside Sap (phloem and None guild membership (e.g. Kaplan and Denno 2007, xylem) Denno and Kaplan 2007, see Chapter 5 on Exudates/oozes None Competition).

Dead plant material Saprophage, detritivore, decomposer 1.5.2 Community interactions Slightly modiﬁed from Price 2002a. Reprinted with Clearly, these interactions are very rich in any one permission. locality. Such interacting species comprise a