Insect Ecology - An Ecosystem Approach 2nd ed - Chapter 1 docx

This integration substantially broadens the scope ofinsect ecology and contributes to prediction and resolution of the effects of current envi-ronmental changes as these affect and are a

T his second edition provides an updated and expanded synthesis of feedbacks and

interactions between insects and their environment A number of recent studies haveadvanced understanding of feedbacks or provided useful examples of principles Mo-lecular methods have provided new tools for addressing dispersal and interactions amongorganisms and have clarified mechanisms of feedback between insect effects on, andresponses to, environmental changes Recent studies of factors controlling energy and nutri-ent fluxes have advanced understanding and prediction of interactions among organisms andabiotic nutrient pools

The traditional focus of insect ecology has provided valuable examples of adaptation toenvironmental conditions and evolution of interactions with other organisms By contrast,research at the ecosystem level in the last 3 decades has addressed the integral role of her-bivores and detritivores in shaping ecosystem conditions and contributing to energy andmatter fluxes that influence global processes This text is intended to provide a modern per-spective of insect ecology that integrates these two traditions to approach the study of insectadaptations from an ecosystem context This integration substantially broadens the scope ofinsect ecology and contributes to prediction and resolution of the effects of current envi-ronmental changes as these affect and are affected by insects

This text demonstrates how evolutionary and ecosystem approaches complement eachother, and is intended to stimulate further integration of these approaches in experimentsthat address insect roles in ecosystems Both approaches are necessary to understand andpredict the consequences of environmental changes, including anthropogenic changes, forinsects and their contributions to ecosystem structure and processes (such as primary pro-ductivity, biogeochemical cycling, carbon flux, and community dynamics) Effective manage-ment of ecosystem resources depends on evaluation of the complex, often complementary,effects of insects on ecosystem conditions as well as insect responses to changing conditions.Two emerging needs require the integration of traditional and emerging perspectives ofinsect roles in ecosystems First, we are becoming increasingly aware that global environ-

PREFACE

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mental changes must be addressed from a global (rather than local) perspective,with emphasis on integrating ecological processes at various levels of resolutionand across regional landscapes Insect population structure, interactions withother species, and effects on ecosystem processes are integral to explaining andmitigating global changes Second, the changing goals of natural resource man-agement require a shift in emphasis from the traditional focus on insect–plantinteractions and crop “protection” to an integration of ecosystem componentsand processes that affect sustainability of ecosystem conditions and products.Integrated pest management (IPM) is founded on such ecological principles.The hierarchical model, familiar to ecosystem ecologists and used in this text,focuses on linkages and feedbacks among individual, population, community, andecosystem properties This model contributes to integration of evolutionary andecosystem approaches by illustrating how properties at higher levels of resolu-tion (e.g., the community or ecosystem) contribute to the environment perceived

at lower levels (e.g., populations and individuals) and how responses at lowerlevels contribute to properties at higher levels of this hierarchy Some overlapamong sections and chapters is necessary to emphasize linkages among levels.Where possible, overlap is minimized through cross-referencing

A number of colleagues have contributed enormously to my perspectives oninsect and ecosystem ecology I am especially grateful to J T Callahan, J.-T Chao,

S L Collins, R N Coulson, D A Crossley, Jr., R Dame, D A Distler, L R Fox,

J F Franklin, F B Golley, J R Gosz, M D Hunter, F Kozár, M D Lowman, G

L Lovett, H.-K Luh, J C Moore, E P Odum, H T Odum, D W Roubik, T R.Seastedt, D J Shure, P Turchin, R B Waide, W G Whitford, R G Wiegert, M

R Willig, and W.-J Wu for sharing ideas, data, and encouragement I also havebenefited from collaboration with colleagues at Louisiana State University andOregon State University and associated with U.S Long Term EcologicalResearch (LTER) sites, International LTER projects in Hungary and Taiwan, theSmithsonian Tropical Research Institute, Wind River Canopy Crane ResearchFacility, Teakettle Experimental Forest, USDA Forest Service Demonstration ofEcosystem Management Options (DEMO) Project, USDA Western RegionalProject on Bark Beetle-Pathogen Interactions, and the National Science Foun-dation L R Fox, T R Seastedt, and M R Willig reviewed drafts of the previousedition Several anonymous reviewers provided useful comments addressed inthis edition I also am indebted to C Schowalter for encouragement and feed-back K Sonnack, B Siebert and H Furrow at Elsevier provided valuable edito-rial assistance I am, of course, solely responsible for the selection andorganization of material in this book

Overview

I Scope of Insect Ecology

II Ecosystem Ecology

A Ecosystem Complexity

B The Hierarchy of Subsystems

C Regulation

III Environmental Change and Disturbance

IV Ecosystem Approach to Insect Ecology

V Scope of This Book

INSECTS ARE THE DOMINANT GROUP OF ORGANISMS ON EARTH, IN terms of both taxonomic diversity (>50% of all described species) and ecologicalfunction (E Wilson 1992) (Fig 1.1) Insects represent the vast majority of species

in terrestrial and freshwater ecosystems and are important components of shore marine ecosystems as well This diversity of insect species represents anequivalent variety of adaptations to variable environmental conditions Insectsaffect other species (including humans) and ecosystem parameters in a variety ofways The capacity for rapid response to environmental change makes insectsuseful indicators of change, major engineers and potential regulators of ecosys-tem conditions, and frequent competitors with human demands for ecosystemresources or vectors of human and animal diseases

near-Insects also play critical roles in ecosystem function They represent importantfood resources or disease vectors for many other organisms, including humans,and they have the capacity to alter rates and directions of energy and matter fluxes (e.g., as herbivores, pollinators, detritivores, and predators) in ways thatpotentially affect global processes In some ecosystems, insects and other arthro-pods represent the dominant pathways of energy and matter flow, and their bio-mass may exceed that of the more conspicuous vertebrates (e.g., Whitford 1986)

Some species are capable of removing virtually all vegetation from a site Theyaffect, and are affected by, environmental issues as diverse as ecosystem health,air and water quality, genetically modified crops, disease epidemiology, frequen-

cy and severity of fire and other disturbances, control of invasive exotic species,land use, and climate change Environmental changes, especially those resultingfrom anthropogenic activities, affect abundances of many species in ways thatalter ecosystem and, perhaps, global processes

A primary challenge for insect ecologists is to place insect ecology in anecosystem context that represents insect effects on ecosystem properties, as well

1

as the diversity of their adaptations and responses to environmental conditions.Until relatively recently, insect ecologists have focused on the evolutionary sig-nificance of insect life histories and interactions with other species, especially aspollinators, herbivores, and predators (Price 1997) This focus has yielded muchvaluable information about the ecology of individual species and species associ-ations and provides the basis for pest management or recovery of threatened andendangered species However, relatively little attention has been given to theimportant role of insects as ecosystem engineers, other than to their effects onvegetation (especially commercial crop) or animal (especially human and live-stock) dynamics.

Ecosystem ecology has advanced rapidly during the past 50 years Majorstrides have been made in understanding how species interactions and environ-mental conditions affect rates of energy and nutrient fluxes in different types ofecosystems, how these provide free services (such as air and water filtration), andhow environmental conditions both affect and reflect community structure (e.g.,

Costanza et al 1997, Daily 1997, H Odum 1996) Interpreting the responses of a

diverse community to multiple interacting environmental factors in integratedecosystems requires new approaches, such as multivariate statistical analysis and

modeling (e.g., Gutierrez 1996, Liebhold et al 1993, Marcot et al 2001, Parton

et al 1993) Such approaches may involve loss of detail, such as combination

of species into phylogenetic or functional groupings However, an ecosystemapproach provides a framework for integrating insect ecology with the changingpatterns of ecosystem structure and function and for applying insect ecology tounderstanding of ecosystem, landscape, and global issues, such as climate change

or sustainability of ecosystem resources Unfortunately, few ecosystem studieshave involved insect ecologists and, therefore, have tended to underrepresentinsect responses and contributions to ecosystem changes

800,000 700,000 600,000 500,000 400,000 300,000 200,000 100,000 0

Insects 751,000

Plants 275,300

Other arthropods 123,400

Other invertebrates 106,300 Fungi 69,000 Chordates42,300 Protozoa30,800

Viruses and bacteria 5,800

FIG 1.1 Distribution of described species within major taxonomic groups Species numbers for insects, bacteria, and fungi likely will increase greatly as these groups become better known Data from E O Wilson (1992).

I SCOPE OF INSECT ECOLOGY

Insect ecology is the study of interactions between insects and their environment

Ecology is, by its nature, integrative, requiring the contributions of biologists,chemists, geologists, climatologists, soil scientists, geographers, mathematicians,and others to understand how the environment affects organisms, populations,and communities and is affected by their activities through a variety of feedbackloops (Fig 1.2) Insect ecology has both basic and applied goals Basic goals are

to understand and model these interactions and feedbacks (e.g., Price 1997)

Applied goals are to evaluate the extent to which insect responses to mental changes, including those resulting from anthropogenic activities, mitigate

environ-or exacerbate ecosystem change (e.g., Croft and Gutierrez 1991, Kogan 1998),especially in managed ecosystems

Research on insects and associated arthropods (e.g., spiders, mites, centipedes,millipedes, crustaceans) has been critical to development of the fundamentalprinciples of ecology, such as evolution of social organization (Haldane 1932,Hamilton 1964, E Wilson 1973); population dynamics (Coulson 1979, Morris

1969, Nicholson 1958, Varley and Gradwell 1970, Varley et al 1973, Wellington et

al 1975); competition (Park 1948, 1954); predator–prey interaction (Nicholson

and Bailey 1935); mutualism (Batra 1966, Bronstein 1998, Janzen 1966, Morgan

1968, Rickson 1971, 1977); island biogeography (Darlington 1943, MacArthurand Wilson 1967, Simberloff 1969, 1978); metapopulation ecology (Hanski 1989); and regulation of ecosystem processes, such as primary productivity, nutri-

ent cycling, and succession (Mattson and Addy 1975, J Moore et al 1988,

Schowalter 1981, Seastedt 1984) Insects and other arthropods are small and

ECOSYSTEM CONDITIONS Climate

Substrate Disturbances Vertical and horizontal structure Energy flow

Biogeochemical cycling

INDIVIDUAL TRAITS Tolerance ranges Mobility

Resource acquisition Resource allocation

COMMUNITY STRUCTURE Diversity

Species interactions Functional organization Ecological succession

POPULATION SYSTEM Density

Distribution Genetic structure Natality

Mortality Dispersal

FIG 1.2 Diagrammatic representation of feedbacks between various levels of ecological organization Size of arrows is proportional to strength of interaction Note that individual traits have a declining direct effect on higher organizational levels but are affected strongly by feedback from all higher levels.

easily manipulated subjects Their rapid numeric responses to environmentalchanges facilitate statistical discrimination of responses and make them particu-larly useful models for experimental study Insects also have been recognized fortheir capacity to engineer ecosystem change, making them ecologically and eco-nomically important.

Insects fill a variety of important ecological (functional) roles Many speciesare key pollinators Pollinators and plants have adapted a variety of mechanismsfor ensuring transfer of pollen, especially in tropical ecosystems where sparse dis-tributions of many plant species require a high degree of pollinator fidelity toensure pollination among conspecific plants (Feinsinger 1983) Other species areimportant agents for dispersal of plant seeds, fungal spores, bacteria, viruses, orother invertebrates (Moser 1985, Nault and Ammar 1989, Sallabanks andCourtney 1992) Herbivorous species are particularly well-known as agriculturaland forestry “pests,” but their ecological roles are far more complex, often stim-ulating plant growth, affecting nutrient fluxes, or altering the rate and direction

of ecological succession (MacMahon 1981, Maschinski and Whitham 1989,

Mattson and Addy 1975, Schowalter and Lowman 1999, Schowalter et al 1986, Trumble et al 1993) Insects and associated arthropods are instrumental in pro-

cessing of organic detritus in terrestrial and aquatic ecosystems and influence soil

fertility and water quality (Kitchell et al 1979, Seastedt and Crossley 1984).

Woody litter decomposition usually is delayed until insects penetrate the barkbarrier and inoculate the wood with saprophytic fungi and other microorganisms(Ausmus 1977, Dowding 1984, Swift 1977) Insects are important resources for avariety of fish, amphibians, reptiles, birds, and mammals, as well as for other inver-tebrate predators and parasites In addition, some insects are important vectors

of plant and animal diseases, including diseases such as malaria and plague, thathave affected human and wildlife population dynamics

The significant economic and medical or veterinary importance of many insectspecies is the reason for distinct entomology programs in various universities andgovernment agencies Damage to agricultural crops and transmission of humanand livestock diseases has stimulated interest in, and support for, study of factorsinfluencing abundance and effects of these insect species Much of this researchhas focused on evolution of life history strategies, interaction with host plantchemistry, and predator–prey interactions as these contribute to our understand-ing of “pest” population dynamics, especially population regulation by biotic andabiotic factors However, failure to understand these aspects of insect ecologywithin an ecosystem context undermines our ability to predict and manage insectpopulations and ecosystem resources effectively (Kogan 1998) Suppressionefforts may be counterproductive to the extent that insect outbreaks representecosystem-level regulation of critical processes in some ecosystems

II ECOSYSTEM ECOLOGY

The ecosystem is a fundamental unit of ecological organization, although itsboundaries are not easily defined An ecosystem generally is considered to rep-resent the integration of a more or less discrete community of organisms and the