Insect Ecology - An Ecosystem Approach 2nd ed - Chapter 11

SECTION IV ECOSYSTEM LEVEL THE ECOSYSTEM LEVEL OF ORGANIZATION INTEGRATES species interactions and community structure with their responses to, and effects on, the abiotic environment. Interactions among organisms are the mechanisms governing energy and nutrient fluxes through ecosystems. The rates and spatial patterns in which individual organisms and populations acquire and allocate energy and nutrients determine the rate and direction of these fluxes (see Chapters 4 and 8). Communities vary in their ability to modify their abiotic environment. The relative abundance of various nutrient resources affects the efficiency with which they are cycled and retained within the ecosystem. Increasing biomass confers increased storage capacity and buffering against changes in resource availability. Community structure also can modify climatic conditions by controlling albedo and hydric fluxes, buffering individuals against changing environmental conditions. A major issue at the ecosystem level is the extent to which communities are organized to maintain optimal conditions for the persistence of the community. Species interactions and community structures may represent adaptive attributes at the supraorganismal level that stabilize ecosystem properties near optimal levels for the various species. If so, anthropogenic interference with community organization (e.g., species redistribution, pest control, overgrazing, deforestation) may disrupt stabilizing mechanisms and contribute to ecosystem degradation. Insects affect virtually all ecosystem properties, especially through their effects on vegetation, detritus, and soils. Insects clearly affect primary productivity, hence the capture and flux of energy and nutrients. In fact, insects are the dominant pathway for energy and nutrient flow in many aquatic and terrestrial ecosystems. They affect vegetation density and porosity, hence albedo and the penetration of light, wind, and precipitation. They affect accumulation and decomposition of litter and mixing and porosity of soil and litter, thereby affecting soil fertility and water flux. They often determine disturbance frequency, succession, and associated changes in efficiency of ecosystem processes over time. Their small size and rapid and dramatic responses to environmental changes are ideal attributes for regulators of ecosystem processes, through positive and negative feedback mechanisms. Ironically, effects of detritivores (largely ignored by insect ecologists) on decomposition have been addressed by ecosystem ecologists, whereas effects of herbivorous insects (the focus of insect ecologists) on ecosystem processes have been all but ignored by ecosystem ecologists until recently. Chapter 11 summarizes key aspects of ecosystem structure and function, including energy flow, biogeochemical cycling, and climate modification. Chapters 12–14 cover the variety of ways in which insects affect ecosystem structure and function. The varied effects of herbivores are addressed in Chapter 12. Although not often viewed from an ecosystem perspective, pollination and seed predation affect patterns of plant recruitment and primary production as described in Chapter 13. The important effects of detritivores on organic matter turnover and soil development are the focus of Chapter 14. Finally, the potential roles of these organisms as regulators of ecosystem processes are explored in Chapter 15. 11 Ecosystem Structure and Function I. Ecosystem Structure A. Trophic Structure B. Spatial Variability II. Energy Flow A. Primary Productivity B. Secondary Productivity C. Energy Budgets III. Biogeochemical Cycling A. Abiotic and Biotic Pools B. Major Cycles C. Factors Influencing Cycling Processes IV. Climate Modification V. Ecosystem Modeling VI. Summary TANSLEY (1935) COINED THE TERM “ECOSYSTEM” TO RECOGNIZE THE integration of the biotic community and its physical environment as a funda-mental unit of ecology within a hierarchy of physical systems that span the range from atom to universe.Shortly thereafter,Lindeman’s (1942) study of energy flow through an aquatic ecosystem introduced the modern concept of an ecosystem as a feedback system capable of redirecting and reallocating energy and matter fluxes. More recently, during the 1950s through the 1970s, concern over the fate of radioactive isotopes from nuclear fallout generated considerable research on biological control of elemental movement through ecosystems (Golley 1993). Recognition of anthropogenic effects on atmospheric conditions, especially greenhouse gas and pollutant concentrations,has renewed interest in how natural and altered communities control fluxes of energy and matter and modify abiotic conditions. Delineation of ecosystem boundaries can be problematic. Ecosystems can be described at various scales.At one extreme, the diverse flora and fauna living on the backs of rainforest beetles (Gressitt et al. 1965, 1968) or the aquatic commu-nities in water-holding plant structures (Richardson et al. 2000a, b) (Fig. 11.1) constitute an ecosystem.At the other extreme,the interconnected terrestrial and marine ecosystems constitute a global ecosystem (Golley 1993, J. Lovelock 1988, Tansley 1935). Generally, ecosystems have been described at the level of the 315 316 11. ECOSYSTEM STRUCTURE AND FUNCTION Fig. 11.1 The community of aquatic organisms, including microflora and invertebrates, that develops in water-holding structures of plants, such as Heliconia flowers, represents a small-scale ecosystem with measurable inputs of energy and matter, species interactions that determine fluxes and cycling of energy and matter, and outputs of energy and matter. landscape patch composed of a relatively distinct community type. However, increasing attention has been given to the interconnections among patches that compose a broader landscape-level or watershed-level ecosystem (e.g., O’Neill 2001, Polis et al. 1997a,Vannote et al. 1980). Ecosystems can be characterized by their structure and function. Structure reflects the way in which the ecosystem is organized (e.g., species composition, distribution of energy, and matter [biomass], and trophic or functional organiza-tion in space). Function reflects the biological modification of abiotic conditions, including energy flow, biogeochemical cycling, and soil and climate modification. This chapter describes the major structural and functional parameters of ecosys-tems to provide the basis for description of insect effects on these parameters in Chapters 12–14. Insects affect ecosystem structure and function in a number of ways and are primary pathways for energy and nutrient fluxes. I. ECOSYSTEM STRUCTURE Ecosystem structure represents the various pools (both sources and sinks) of energy and matter and their relationships to each other (i.e.,directions of matter or information flow; e.g., Fig. 1.3).The size of these pools (i.e., storage capacity) I. ECOSYSTEM STRUCTURE 317 determines the buffering capacity of the system. Ecosystems can be compared on the basis of the sizes and relationships of various biotic and abiotic com-partments for storage of energy and matter. Major characteristics for comparing ecosystems are their trophic or functional group structure, biomass distribution, or spatial and temporal variability in structure. A. Trophic Structure Trophic structure represents the various feeding levels in the community.Organ-isms generally can be classified as autotrophs (or primary producers), which synthesize organic compounds from abiotic materials, and heterotrophs (or sec-ondary producers), including insects, which ultimately derive their energy and resources from autotrophs (Fig. 11.2). Autotrophs are those organisms capable of fixing (acquiring and storing) inor-ganic resources in organic molecules. Photosynthetic plants, responsible for fixa-tion of abiotic carbon into carbohydrates, are the sources of organic molecules. This chemical synthesis is powered by solar energy. Free-living and symbiotic N-fixing bacteria and cyanobacteria are an important means of converting inorganic N2 into ammonia,the source of most nitrogen available to plants.Other chemoau-totrophic bacteria oxidize ammonia into nitrite or nitrate (the form of nitrogen available to most green plants) or oxidize inorganic sulfur into organic com-pounds. Production of autotrophic tissues must be sufficient to compensate for amounts consumed by heterotrophs. Heterotrophs can be divided into several trophic levels depending on their source of food.Primary consumers (herbivores) eat plant tissues.Secondary con-sumers eat primary consumers, tertiary consumers eat secondary consumers, and so on. Omnivores feed on more than one trophic level. Finally, reducers Fig. 11.2 Biomass pyramid for the Silver Springs ecosystem. P, primary producers; H, herbivores; C, predators;TC, top predators; D, decomposers. From H. Odum (1957) with permission from the Ecological Society of America. ... - tailieumienphi.vn