THE PREVIOUS FOUR SECTIONS HAVE ADDRESSED insect ecology at the individual, population, community, and ecosystem levels of organization. Resource acquisition and allocation by individuals (Section I) can be seen to depend on population (Section II), community (Section III), and ecosystem (Section IV) conditions that the individual also influences. Insects are involved in a particularly rich variety of feedbacks between individual, population, community, and ecosystem levels as a consequence of their dominance and diversity in terrestrial and freshwater ecosystems and their sensitivity and dramatic responses to environmental changes. The hypothesis that insects are major regulatory mechanisms in homeostatic ecosystems has important ecological and management implications and warrants critical testing. The importance of temporal and spatial scales is evident at each level of the ecological hierarchy. Individuals have a period and range of occurrence, populations are characterized by temporal dynamics and dispersion patterns, and communities and ecosystems are represented over temporal and spatial scales. In particular, ecosystem stability and its effect on component individuals traditionally has been evaluated at relatively small scales, in time and space, but larger scales are more appropriate. The dynamic mosaic of ecosystem types at the landscape or biome level is conditionally stable in its proportional representation of ecosystem types. This concluding chapter summarizes and synthesizes the study of insect ecology. The focus will be on important aspects of insect ecology, major applications, and intriguing questions for future study. V SECTION SYNTHESIS 016-P088772.qxd 1/24/06 11:07 AM Page 463 16 Synthesis I. Summary II. Synthesis III. Applications A. Management of Crop, Forest, and Urban “Pests” B. Conservation/Restoration Ecology C. Indicators of Environmental Conditions D. Ecosystem Engineering IV. Critical Issues V. Conclusions THE STUDY OF INSECT ECOLOGY TRADITIONALLY ADDRESSED INSECT adaptations to their environment, including interactions with other organisms, and effects on plant growth and vegetation structure. Insects represent the full scope of heterotrophic strategies, from sessile species whose ecological strategies resemble those of plants to social insects whose range of behavioral attributes is more like that of advanced vertebrates. The variety of insect interactions with other species spans the range of ecological complexity and often brings them to the attention of natural resource managers as pests, biological control agents, or key pollinators or seed dispersers of endangered plants. Three of the four sec- tions in this book emphasize this traditional approach to the study of insect ecology. However, this traditional focus on species adaptations and community inter- actions does not portray the full scope of insect ecology.Whereas the evolution- ary perspective emphasizes insect responses to environmental conditions, as demonstrated by adaptive physiology, behavior, and interspecific interactions, the ecosystem perspective emphasizes feedbacks between organisms and their environment. Insects, as well as other organisms, influence their environment in complex, and often dramatic, ways. The foraging pattern of any organism affects its interactions with other organisms and the resulting distribution of resources. Population outbreaks of some herbivorous insects can reshape vegetation structure and alter biogeochemical cycles and local or regional climate. Natural selection represents a major feedback between ecosystem conditions and indi- vidual attributes that affect ecosystem parameters. Other feedback mechanisms between individuals, populations, and communities can stabilize or destabilize ecosystem, landscape, and global processes. Understanding these feedbacks is critical to prediction of ecosystem responses to environmental changes. Phy- tophages dramatically alter the structure of landscapes and potentially stabilize 465 016-P088772.qxd 1/24/06 11:07 AM Page 465 primary production and other processes affecting global climate and biogeo- chemistry (Chapter 12). Termites account for substantial portions of carbon flux in some ecosystems (Chapter 14). Section IV, dealing with feedbacks between insects and ecosystem properties, is the unique contribution of this book. This chapter summarizes key ecological issues, synthesizes key integrating variables, describes applications, and identifies critical issues for future study. I. SUMMARY The hierarchical organization (see Fig. 1.2 or Table 1.1) of this text emphasizes linkages and feedbacks among levels of ecological organization. Linkages and feedbacks are strongest between neighboring levels but are significant even between individual and ecosystem levels of the hierarchy. Physiological and behavioral responses to environmental variation are under genetic control and determine individual fitness, but they also affect the rate and geographic pattern of resource acquisition and allocation that control climate and energy and bio- geochemical fluxes at the ecosystem level.These feedbacks are an important and largely neglected aspect of insect ecology that affect ecosystem stability and global processes. The geographic distribution of individual species generally reflects the envi- ronmental template established by continental history, latitude, mountain ranges, and global atmospheric and oceanic circulation patterns. The great diversity of insects reflects their rapid adaptation, conferred by small size, short life spans, and rapid reproductive rates, to environmental variation. These attributes have facilitated speciation at multiple scales: among geographic regions, habitats, and resources and at microscales on or within resources (e.g., individual leaves). However, within the potential geographic range of a species, the spatial and tem- poral patterns of abundance reflect disturbance dynamics, resource distribution, and interactions with other species that affect individual fitnesses and enhance or limit colonization and population growth. Energy and resource budgets (see Fig. 4.1) are key aspects of individual fitness, population persistence, and community interactions. All organisms require energy to accumulate resources, necessary for growth and reproduction, against resource concentration gradients and thereby maintain the thermodynamic disequilibrium characteristic of life. Where resources are more concentrated, relative to individual needs, less energy is required for acquisition. Interactions among organisms often may be controlled by mass balances of multiple nutri- ents. Resource use requires adaptations to acquire necessary limiting nutrients, such as nitrogen, while avoiding or circumventing toxic or defensive chemicals as well as overabundant nutrients. Much research has addressed plant defenses against feeding by insects and other herbivores. Insect herbivores have evolved a variety of mechanisms for avoiding, detoxifying, or inhibiting expression of plant defenses. All species have mobile stages adapted to find new resources before current resources are depleted or destroyed. The early evolution of flight among insects greatly facili- tated foraging, escape from unsuitable environmental or resource conditions, and 466 16. SYNTHESIS 016-P088772.qxd 1/24/06 11:07 AM Page 466 discovery of more optimal conditions. Individuals or populations that fail to acquire sufficient energy and nutrients to grow and reproduce do not survive. Adaptations for detecting and acquiring resources are highly developed among insects. Many insects can detect the presence and location of resources from chemical cues carried at low concentrations on wind or water currents. The diversity of strategies among insect species for acquiring resources has perhaps drawn the most ecological attention. These strategies range from ambush to active foraging; often demonstrate considerable learning ability (especially among social insects); and involve insects in all types of interactions with other organisms, including competition (e.g., for food, shelter, and oviposition site resources), predation and parasitism (on plant, invertebrate, and vertebrate prey or hosts and as prey or hosts), and mutualism (e.g., for protection, pollination, and seed dispersal). Spatial and temporal variation in population and community structure reflects net effects of environmental conditions. Changes in population and community structure also constrain survival and reproduction of associated species. Population density and competitive, predatory, and mutualistic interactions affect foraging behavior and energy and nutrient balances of individuals. Individuals forced to move constantly to avoid intraspecific or interspecific competitors or predators will be unable to forage sufficiently for energy and nutrient resources. However, energy and nutrient balances can be improved through mutualistic inter- actions that enhance the efficiency of resource acquisition. The relative contribu- tions of intraspecific and interspecific interactions to individual survival and reproduction remain a central theme of ecology but have been poorly integrated with ecosystem conditions. Debate over the importance of bottom-up versus top- down controls of populations perhaps reflects variation in the contributions of these factors among species as well as spatial and temporal variation in their effect. Ecosystems represent the level at which complex feedbacks among abiotic and biotic processes are integrated. Ecosystems can be viewed as dynamic energy- and nutrient-processing engines that modify global energy and nutrient fluxes. Cycling and storage processes controlled by organisms reduce variation in abiotic conditions and resource availability. Although ecosystem properties are largely determined by vegetation structure and composition, insects and other animals modify ecosystem conditions, often dramatically, through effects on primary pro- duction, decomposition and mineralization, and pedogenesis. Insect herbivore effects on vegetation structure affect albedo, evapotranspiration, and wind abate- ment. Changes in decomposition processes affect fluxes of carbon and trace gases as well as soil structure and fertility. Insect roles as ecosystem engineers mitigate or exacerbate environmental changes resulting from anthropogenic activities. Resolution of environmental issues requires attention to these roles of insects as well as to their responses to environmental changes. II. SYNTHESIS Insect ecology addresses an astounding variety of interactions between insects and their environment. However, key aspects of insect ecology involve feedback between insect responses to changes in environmental conditions, especially II. SYNTHESIS 467 016-P088772.qxd 1/24/06 11:07 AM Page 467 resource supply, and their capacity to modify, and potentially stabilize, energy and nutrient fluxes. As shown throughout this text, each level of hierarchical organization can be described in terms of characteristic structure, function, and feedback regulation. Feedback integration among hierarchical levels occurs primarily through responses to, and modification of, variation in environ- mental conditions (see Fig. 1.2). Insect behavioral and physiological attributes that affect their interactions with the environment are under genetic control. Evolution represents feedback on individual attributes that affect higher levels of organization. The importance of environmental change and disturbance as a central theme in insect ecology has been recognized only recently. Disturbance, in particular, provides a context for understanding and predicting individual adaptations, pop- ulation strategies, organization and succession of community types, and rates and regulation of ecosystem processes. Environmental changes or disturbances kill individuals or affect their activity and reproduction. Some populations are reduced to local extinction, but others exploit the altered conditions. Population strategies and interactions with other species also affect ecosystem properties in ways that increase the probability of disturbance (or other changes) or that mit- igate environmental changes and favor persistence of species less tolerant to change. Insects contribute greatly to feedback between ecosystem properties and environmental variation. This aspect of insect ecology has important conse- quences for ecosystem responses to global changes resulting from anthropogenic activities. Energy and biogeochemical fluxes integrate individuals, populations, and com- munities with their abiotic environment. Energy flow and biogeochemical cycling processes determine rates and spatial patterns of resource availability. Many, perhaps most, species attributes can be shown to represent tradeoffs between maximizing resource acquisition and optimizing resource allocation among metabolic pathways (e.g., foraging activity, defensive strategies, growth, and reproduction). The patterns of energy and nutrient acquisition and allocation by individuals determine the patterns of storage and fluxes among populations; fluxes among species at the community level; and storage and flux at the ecosystem level that, in turn, determine resource availability for individuals, populations, and communities. Resource availability is fundamental to ecosystem productivity and diversity. Resource limitation, including reduced availability resulting from inhibition of water and nutrient fluxes, is a key factor affecting species interactions. Herbivore and predator populations grow when increasing numbers of hosts or prey are available or incapable of escape or defense because of insufficient resource acquisition or poor food quality. Regulatory mechanisms emerge at all levels of the ecological hierarchy. Negative feedback and reciprocal cooperation are apparent at population, community, and ecosystem levels. Cooperation benefits individuals by improving ability to acquire limiting resources.This positive feedback balances the negative feedbacks that limit population density, growth, and ecological processes.At the population level, positive and negative feedbacks maintain density within narrower ranges than occur when populations are released from regulatory 468 16. SYNTHESIS 016-P088772.qxd 1/24/06 11:07 AM Page 468 mechanisms. The responsiveness of insect herbivores to changes in plant density and condition, especially resulting from crop management, introduction into new habitats, and land use, bring some species into conflict with human interests. However, insect outbreaks in natural ecosystems appear to be restricted in time and space and function to (1) maintain net primary production (NPP) within relatively narrow ranges imposed by the carrying capacity of the ecosys- tem and (2) facilitate replacement of plant species that are poorly adapted to current conditions by species that are better adapted to these conditions. Regu- latory capacity appears to reflect selection for recognition of cues that signal changes in host density or condition that affect long-term carrying capacity of the ecosystem. The issue of ecosystem self-regulation is a key concept that significantly broadens the scope of insect ecology. Although this idea remains controversial, accumulating evidence supports a view that insect outbreaks function to reduce long-term deviation in NPP, at least in some ecosystems. Although outbreaks appear to increase short-term variation in some ecosystem parameters, reversal of unsustainable increases in NPP could reduce long-term variation in ecosystem conditions. Models of group selection predict that stabilizing interactions are most likely in ecosystems where pairs of organisms interact consistently. Hence, selection for stabilizing interactions might be least likely in ecosystems where such interac- tions are inconsistent, such as in harsh or frequently disturbed environments. However, selection for stabilizing interactions also might be less direct in pro- ductive, highly diverse ecosystems with little variation in abiotic conditions or resource availability, such as tropical rainforest ecosystems. Stabilizing interac- tions are most likely in ecosystems where selection would favor interactions that reduce moderate levels of variation in abiotic conditions or resource availability. Insects play key roles in regulation of primary and secondary production. Their large numbers, rapid reproduction, and mobility may maximize their inter- actions with other organisms and the rate at which they evolve reciprocal cooperation. III. APPLICATIONS Insect ecology represents the intersection between basic understanding of how insects interact with their environment and necessary applications for pest man- agement, ecosystem restoration, and other aspects of ecosystem management. Understanding feedbacks between insects and their environment provides useful information for understanding insects in the broader context of ecosystem and global processes. Although insect outbreaks occur in natural ecosystems when conditions are favorable, anthropogenic changes in ecosystem conditions often promote population growth of species that are viewed as “pests.” These changes often can be reversed or mitigated with adequate ecological information. Insect ecology also addresses the variety of insect effects on ecosystem conditions. Such information is necessary to determine when suppression of outbreaks may be warranted to meet specific management goals. III. APPLICATIONS 469 016-P088772.qxd 1/24/06 11:07 AM Page 469 A. Management of Crop, Forest, and Urban “Pests” Management of crop, forest, and urban “pests” has been a major application of insect ecology. Insect roles in ecosystems may conflict with crop and livestock production and human health and habitation when conditions favor insect pop- ulation growth. For example, densely planted monocultures of crop species, often bred to reduce bitter (defensive) flavors, provide ideal conditions for population growth of herbivorous species (see Chapter 6). Similarly, buildings provide pro- tected habitats for ants, termites, cockroaches, and other species, especially when moisture and unsealed food create ideal conditions. Insects become viewed as pests when their activities conflict with human values. Traditional views of herbivorous and detritivorous insects as destructive, or at least nuisances, and ecological communities as nonintegrated, random assem- blages of species supported harsh control measures. Early approaches to insect control included arsenicals, although much classic research on population regu- lation by predators and parasites also occurred prior to World War II. With the advent of broad-spectrum,long-lived, chlorinated hydrocarbons and organophos- phates, developed as nerve toxins and used for control of disease vectors in combat zones during World War II, management of insects seemed assured. However, reliance on these insecticides exposed many target species to intense selection over successive generations and led to rapid development of resistant populations of many species (Soderlund and Bloomquist 1990). Concurrently, movement of the toxins through food webs resulted in adverse environmental consequences that became widely known in the 1960s through publication of Rachel Carson’s Silent Spring (1962). The last legal use of DDT (dichlorodiphenyltrichloroethane) in the United States, against the Douglas-fir tussock moth, Orgyia pseudotsugata, in 1974 during an outbreak in Oregon and Washington required emergency authorization by the U.S. Environmental Protection Agency, which had canceled use of DDT in the United States in 1972 (Brookes et al. 1978). This emergency authorization, based on apparent lack of practical alternatives, mandated intensified research on alternative methods of control. Although the importance of nuclear polyhe- drosis virus, Baculovirus spp., in terminating tussock moth outbreaks had been known since the 1960s, applications of DDT or other chemicals reduced larval densities to levels incapable of supporting epizootics (Brookes et al. 1978) and masked the importance of natural regulatory mechanisms. Subsequent research has demonstrated that enhancement of epizootics by application of technical-grade viral preparation to first instar larvae can cause population collapse within the same year; this currently is the preferred means of control. Accumulating evidence indicates that the Douglas-fir tussock moth may be an important regulator of forest conditions (see Chapter 15): compensatory timber production following outbreaks offsets economic losses (Alfaro and Shepherd 1991, Wickman 1980). Much subsequent research has addressed the effects of pesticide residues on nontarget organisms and has led to cancellation of registration for chemicals with adverse environmental effects and to development and use of more specific 470 16. SYNTHESIS 016-P088772.qxd 1/24/06 11:07 AM Page 470 chemicals, including insect growth regulators (IGRs) and chitin sythesis inhibitors (CSIs), with shorter half-lives in the environment. Research results also have led to greater use of microbial pathogens, including nuclear polyhedrosis viruses (NPV) and Bacillus thuringiensis (Bt). Effectiveness of these tools can be enhanced by attention to ecological factors. For example, invasive ants and termites, which often are inaccessible to broadcast application of toxins, can be controlled effectively by attracting foragers to a bait containing nonre- pellent, slow-acting toxin, IGR, or CSI that is shared with nestmates through trophyllaxis, accomplishing population reduction with minimal effect on nontarget species. Much ecological research also has demonstrated the importance of using mul- tiple tactics, including elimination of conducive conditions, enhanced plant defenses, insect growth regulators, pheromones, predators, and parasites, that constitute an integrated pest management (IPM) approach (e.g., Barbosa 1998, Huffaker and Messenger 1976, Kogan 1998, Lowrance et al. 1984, Rabb et al. 1984, Reay-Jones et al. 2003, Rickson and Rickson 1998, Risch 1980, 1981). An eco- logical approach emphasizes multiple tactics representing the combination of bottom-up, top-down, and lateral factors that regulate natural populations. For example, increased tree spacing can interrupt bark beetle and defoliator out- breaks in forests, reducing the likelihood of outbreaks and need for pesticides. Agroforestry and multiple-cropping systems that increase crop diversity also can interrupt spread of insect populations (Fig. 16.1). In addition, elicitors of induced defenses, such as jasmonic acid, could be used to elevate resistance to pests in crop plants and stimulate biological control at appropriate times (M. Stout et al. 2002). Because of the delay in expression of induced defenses, this approach would be most effective when infestations can be reliably anticipated and economic thresholds are high. Augmentation or introduction of predator and parasite populations for biological control requires retention of necessary habitat, such as native vegetation in hedgerows, or alternative resources, such as floral nectar sources (Hassell et al. 1992, Landis et al. 2000, Marino and Landis 1996, Thies and Tscharntke 1999). Implementation of control measures should be based on predictive models that indicate when the insect population is expected to exceed a calculated threshold, based on net cost–benefit of insect effect and control, above which intolerable loss of economic or environmental values would occur if the population is not controlled (Rabb et al. 1984). Herbivorous insects also have been used to control invasive plant species. Introducing biological control agents from the pest’s region of origin requires consideration of their ability to become established in the new community and their effects on nontarget species, as well as on the costs and benefits of invasive plant persistence and insect introduction. Many crop species have been genetically engineered to express novel defenses, such as Bt toxins. However, reliance on such strategies threatens to undermine their long-term effectiveness, given insect ability to evolve resistance. Therefore, a high-dose-with-refuge strategy is recommended to prevent survival of pests on the Bt crop and maintain a large, nonadapted population in non-Bt refuges (Alstad and Andow 1995, Carriére et al. 2003). Management of resistance III. APPLICATIONS 471 016-P088772.qxd 1/24/06 11:07 AM Page 471 development to transgenic crops could be undermined if pollen contamination of nontransgenic refuges or native vegetation leads to variable Bt concentrations and effects on nontarget species in the landscape (Chilcutt and Tabashnik 2004, Zangerl et al. 2001). This requires attention to the landscape structure of Bt and non-Bt crops (especially for insects with broad host ranges that might include multiple transgenic crops) and cooperation among scientists, growers, and gov- ernment agencies (Carrière et al. 2001a). Another promising new tool includes 472 16. SYNTHESIS FIG. 16.1 Examples of multiple cropping to hinder spread of insect species over agricultural landscape in northeastern China. A: Embedded intercropping within rows. B: Multiple crop species arranged in strips. 016-P088772.qxd 1/24/06 11:07 AM Page 472 use of chemicals, such as jasmonic acid, to elicit expression of targeted defenses by crop plants (e.g., M. Stout et al. 2002, Thaler 1999b, Thaler et al. 2001). However, expression of defenses by plants depends on adequate resources. Advances in understanding of insect effects on a variety of plant and ecosys- tem attributes also has influenced evaluation of the need for insect management. Furthermore, management goals for natural ecosystems has become more complex in many regions, as societal needs have changed from a focus on extrac- tive uses (e.g., fiber, timber, or livestock production) to include protection of water yield and quality, fisheries, recreational values, biodiversity, and ecosystem integrity. In many cases, insect outbreaks now are viewed as contributing to, rather than detracting from, management goals for natural or seminatural ecosys- tems. Recognition that low levels of herbivory stimulate primary production by many plants, including crop species (Pedigo et al. 1986, Trumble et al. 1993, S. Williamson et al. 1989), and may affect soil structure, infiltration, fertility, and climate requires evaluation of the integrated effects, or net cost–benefit, of changes in insect abundance or activity. Many serious human diseases, such as malaria, yellow fever, bubonic plague, and equine encephalitis, are vectored by arthropods among humans and other animal species, especially rodents and livestock. Rodents are reservoirs for several important human diseases, but horses and cattle also are sources of inocu- lum. West Nile virus has a particularly broad reservoir of hosts, including birds, small mammals, and reptiles. The rapid spread of this disease across North America between 1999 and 2004 reflected a combination of insect transmission of the virus among multiple hosts and rapid bird movement across the continent (Marra et al. 2004). The importance of these diseases to human population dynamics, including the success of military campaigns, underscores the impor- tance of understanding human roles in ecological interactions. Increasing human intrusion into previously unoccupied ecosystems has exposed humans to novel animal diseases that may involve insect vectors.Transmission frequency increases with density of human, reservoir, or vector populations. Management must involve a combination of approaches that augment natural controls and reduce exotic breeding habitat for vectors (e.g., tires, flower pots, roadside ditches) or reservoir hosts as well as inoculation of humans who may be exposed. Termites, carpenter ants, and wood-boring beetles often threaten wooden structures. Considerable investment has been made in research to reduce damage, especially in historically important buildings. Again, management requires multiple approaches, including chemical barriers to make buildings less attractive to these insects; removal or treatment of infested building material, nearby wood waste, or infested trees; pheromone disruption of foraging behav- ior; nonrepellent termiticides that can be transferred in lethal doses to other colony members through trophyllaxis; and microbial toxins to inhibit gut flora and fauna (J. K. Grace and Su 2001, Shelton and Grace 2003). Other urban “pests” include nuisances and health hazards, such as exotic ants, biting or swarm- ing flies, and even winter aggregations of ladybird beetles, that may be promoted by proximity of lawns, gardens, and ornamental pools. Frequent pesticide appli- cation or elimination of native vegetation in urban settings often reduces the III. 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