This book is organized hierarchically to emphasize feedbacks among individual, population, and community levels and the ecosystems they represent. Four questions have been used to develop this text:
1. How do insects respond to variation in environmental conditions, especially gradients in abiotic factors and resource availability?
2. How do interactions among individuals affect the structure and function of populations and communities?
3. How do insect-induced changes in ecosystem properties affect the gradients in environmental conditions to which individuals respond?
4. How can this information be incorporated into management decisions and environmental policy?
Chapter and topic organization are intended to address these questions by emphasizing key spatial and temporal patterns and processes at each level and their integration among levels. Environmental policy and management decisions (Section V) depend on evaluation of insect effects on ecosystem parameters and their responses to environmental change. The evaluation of insect effects on ecosystem parameters and their responses to environmental change (Section IV) depends on understanding of species diversity, interactions, and community organization (Section III) that, in turn, depends on understanding of population dynamics and biogeography (Section II), that depends on understanding of indi vidual physiological and behavioral responses to environmental variation (Section I).
Three themes integrate these ecological levels. First, spatial and temporal patterns of environmental variability and disturbance determine survival and reproduction of individuals and patterns of population, community, and ecosystem structure and dynamics. Individual acquisition and allocation of resources, population distribution and colonization and extinction rates, community patterns and successional processes, and ecosystem structure and function reflect environmental conditions. Second, energy and nutrients move through individuals, populations and communities, and abiotic pools. The net foraging success and resource use by individuals determines energy and nutrient fluxes at the population level. Trophic interactions among populations determine energy and nutrient fluxes at the community and ecosystem levels. Third, regulatory mechanisms at each level serve to balance resource demands with resource availability (carrying capacity) or to dampen responses to environmental changes. Regulation results from a balance between negative feedback that reduces population size or process rates and positive feedback that increases population size or process rates. Regulation of population sizes and process rates tends to stabilize ecosystem conditions within ranges favorable to most members. The capacity to regulate environmental conditions increases from individual to ecosystem levels (see Fig. 1.2). If feedbacks within or among levels contribute to ecosystem stability, then human influences on ecosystem structure and function could enhance or seriously impair this function.
Section I (Chapters 2-4) addresses the physiological and behavioral ecology of insects. Physiology and behavior represent the means by which organisms interact with their environment. Physiology represents "fixed" adaptations to predictable variation in environmental conditions, whereas behavior represents a more flexible means of adjusting to unpredictable variation. Chapter 2 summarizes insect responses to variable habitat conditions, especially gradients in climate, water, and chemical conditions. Chapter 3 describes physiological and behavioral mechanisms for acquiring energy and matter resources, and Chapter 4 addresses the allocation of assimilated resources to various metabolic and behavioral pathways. These chapters provide a basis for understanding distribution patterns and movement of energy and matter through populations and communities.
Section II (Chapters 5-7) deals with population ecology. Populations of organisms integrate variation in adaptive strategies and foraging patterns among individuals. Chapter 5 outlines population systems, including population structure and the processes of reproduction, mortality, and dispersal. Chapter 6 addresses processes and models of population change; Chapter 7 describes biogeography, processes and models of colonization and extinction, and metapopulation dynamics over landscapes. These population parameters determine population effects on ecological processes through time in various patches across regional landscapes.
Section III (Chapters 8-10) addresses community ecology. Species populations interact with other species in a variety of ways that determine changes in community structure through time and space. Chapter 8 describes species interactions (e.g., competition, predation, symbioses). Chapter 9 addresses measures of diversity and community structure and spatial patterns in community structure. Chapter 10 addresses changes in community structure over time, especially community responses to environmental change. These community characteristics determine spatial and temporal patterns of energy and nutrient storage and flux through ecosystems.
Section IV (Chapters 11-15) focuses on ecosystems and is the unique contribution of this text to graduate education in insect ecology. Chapter 11 addresses general aspects of ecosystem structure and function, especially processes of energy and matter storage and flux that determine resource availability. Chapter
12 describes patterns of herbivory and effects on ecosystem parameters; Chapter
13 describes patterns and effects of pollination, seed predation, and seed dispersal; and Chapter 14 describes patterns and effects of detritivory and burrowing on ecosystem processes. Chapter 15 addresses the developing concept of ecosystem self-regulation and mechanisms, including species diversity and insect effects, that may contribute to ecosystem stability.
Section V (Chapter 16) provides synthesis and application. Chapter 16 summarizes and synthesizes major concepts. This chapter also provides examples of applications and suggests future directions and data necessary to improve understanding of linkages and feedbacks among hierarchical levels. Solutions to environmental problems require consideration of insect ecology at ecosystem, landscape, and global levels. Although the focus of this book clearly is on insects, examples from studies of other organisms are used where appropriate to illustrate concepts.
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