Section I Introduction
1 The Various Effects of Insects on Ecosystem Functioning ..... 3
W.W. Weisser and E. Siemann
1.1 Summary ..................................................... 3
1.2 Introduction ................................................ 3
1.3 A Brief Overview of Insect Effects on Ecosystem Function .... 8
1.3.1 Insect Effects on Ecosystem Function Via
Interactions with Plants .............................. 8
1.3.1.1 Herbivory ..................................... 8
1.3.1.2 Plant-Insect Mutualisms ...................... 14
1.3.2 Other Direct and Indirect Effects of Insects on
Ecosystem Function ................................... 14
1.4 The Aim and Structure of this Book ......................... 15
References ..................................................... 19
Section II Insects and the Belowground System
2 Insect Herbivores, Nutrient Cycling and Plant
Productivity ............................................... 27
S.E. Hartley and Т.Н. Jones
2.1 Summary .................................................... 27
2.2 Introduction ............................................... 28
2.3 Decomposition .............................................. 28
2.3.1 The Resources Available .............................. 28
2.3.2 Effects of Insect Herbivory on Decomposition ......... 31
2.3.2.1 Herbivory and Litter Quality ................. 31
2.3.2.2 Herbivory, Root Exudation and Root Biomass ... 32
2.4 Nutrient Cycling and Plant Productivity .................... 33
2.4.1 Effects on Carbon and Nitrogen Cycling ............... 34
2.4.1.1 Methane and Carbon Dioxide ................... 34
2.4.1.2 Nitrogen and Phosphorus ...................... 35
2.4.1.3 Inputs from Aboveground Herbivores ........... 36
2.4.1.4 The Importance of Belowground Biota:
Evidence from Controlled Environment
Studies ...................................... 39
2.4.1.5 Insect Herbivory and Spatial Variation in
Nutrient Availability ........................ 40
2.4.2 Herbivory and Plant Biomass .......................... 41
2.5 Conclusions ................................................ 45
References ..................................................... 46
3 Indirect Effects of Invertebrate Herbivory on
the Decomposer Subsystem ................................... 53
D.A. Wardle and R.D. Bardgett
3.1 Summary .................................................... 53
3.2 Introduction ............................................... 54
3.3 Mechanistic Bases of Invertebrate Herbivore Effects ........ 54
3.3.1 Immediate Effects on Resource Quantity ............... 56
3.3.2 Longer-Term Effects on Resource Quantity ............. 56
3.3.3 Effects of Changed Litter Quality .................... 57
3.3.4 Return of Invertebrate Waste Products ................ 58
3.3.5 Effects of Changes in Vegetation Composition ......... 59
3.3.6 Feedbacks and Aboveground Consequences ............... 61
3.4 Significance of Invertebrate Herbivore Outbreaks ........... 61
3.5 Multiple Species Herbivore Communities ..................... 62
3.6 Comparisons of Ecosystems .................................. 64
3.7 Conclusions ................................................ 65
References ..................................................... 66
4 Biotic Interactions in the Rhizosphere: Effects on Plant
Growth and Herbivore Development ........................... 71
M. Bonkowski and S. Scheu
4.1 Summary .................................................... 71
4.2 The Rhizosphere - Interface of Intense Microbial and
Faunal Interactions ........................................ 72
4.2.1 Plants as Drivers of Rhizosphere Interactions ........ 73
4.3 Belowground Interactions and the Herbivore System .......... 74
4.3.1 Effects of Mycorrhiza and Rhizobacteria on
Aboveground Herbivores ............................... 76
4.3.2 Interactions with the Micro-Decomposer Food Web ...... 77
4.3.2.1 The Bacterial Loop and Herbivore
Performance .................................. 78
4.3.2.2 The Fungal Food Chain and Herbivore
Performance .................................. 79
4.3.2.3 Ecosystem Engineers and Herbivore
Performance .................................. 81
4.4 Top-Down Effects by Subsidizing Generalist Predators ....... 83
References ..................................................... 85
5 Belowground Herbivores and Ecosystem Processes ............. 93
G.J. Masters
5.1 Summary .................................................... 93
5.2 Introduction ............................................... 94
5.3 Experimenting with Belowground Insect Herbivores ........... 94
5.4 Belowground Herbivory and Plant Productivity: Allocation
and Biomass ................................................ 97
5.5 Implications of Belowground Herbivory for Nutrient
Cycling ................................................... 101
5.6 Implications of Belowground Herbivory for Multitrophic
Interactions .............................................. 104
5.7 Conclusion ................................................ 109
References .................................................... 109
Section III Plant-Insect Interactions and Ecosystem Processes
6 Bottom-Up Effects and Feedbacks in Simple and Diverse
Experimental Grassland Communities ........................ 115
J. Joshi, S.J. Otway, J. Koricheva, A.B. Pfisterer,
J. Alphei, В.А. Roy, M. Scherer-Lorenzen, B. Schmid,
E. Spehn and A. Hector
6.1 Summary ................................................... 115
6.2 Introduction .............................................. 116
6.3 Effects of Plant Diversity on Herbivorous Insects
Feeding Above Ground ...................................... 117
6.3.1 Hypotheses Predicting the Response of Herbivores
to Higher Plant Diversity ........................... 117
6.3.2 Responses of Specialist and Generalist Herbivores
in Plant Diversity Experiments ...................... 119
6.3.3 Concomitant Responses of Natural Enemies of
Herbivores .......................................... 123
6.3.4 Insect Herbivores as Drivers of Ecosystem
Processes ........................................... 124
6.4 Effects of Plant Diversity on Pathogens ................... 125
6.5 Belowground Food Web ...................................... 126
6.5.1 Plant Biomass and Microbial Response ................ 126
6.5.2 Soil Animals that Feed on Microbes .................. 128
6.6 Conclusions ............................................... 129
References .................................................... 130
7 The Potential of Phytophagous Insects in Restoring
Invaded Ecosystems: Examples from Biological Weed
Control ................................................... 135
H. Zwölfer and H. Zimmermann
7.1 Summary ................................................... 135
7.2 Introduction .............................................. 136
7.3 Success Rates and Successes in Biological Weed Control .... 137
7.4 Weed Characteristics and Positive Traits of Insects in
Biological Control ........................................ 138
7.4.1 Weed Species ........................................ 139
7.4.2 Insect Species ...................................... 139
7.5 Three Examples of Successful Weed Control ................. 140
7.5.1 Rhinocyllus conicus on Carduus nutans ............... 140
7.5.2 Interactions Between Three Weevil Species in
the Biocontrol of the Invader Sesbania punicea in
South Africa ........................................ 143
7.5.2.1 The Seed-Destroying Agents: Trichapion
lativentre and Rhyssomatus marginatus ....... 144
7.5.2.2 The Stem-Borer:
Neodiplogrammus quadrivittatus .............. 145
7.5.3 Aquatic Weeds ....................................... 146
7.6 Discussion and Conclusions ................................ 147
References .................................................... 150
8 Plant-Insect-Pathogen Interactions on Local and Regional
Scales .................................................... 155
A. Kruess, S. Eber, S. Kluth and T. Tscharntke
8.1 Summary ................................................... 155
8.2 Introduction .............................................. 156
8.3 Biological Weed Control, Interactions and Ecosystem
Processes ................................................. 157
8.3.1 Classical Biological Control ........................ 157
8.3.2 Plant-Pathogen-Herbivore Interactions ............... 158
8.4 Creeping Thistle, Insects, Pathogens and Processes ........ 160
8.4.1 The Creeping Thistle (Cirsium arvense) .............. 160
8.4.2 Interactions Between Pathogens and Insect Vectors
on a Local Scale .................................... 161
8.4.3 Regional Dynamics of Cirsium arvense and
an Associated Herbivore ............................. 163
8.4.4 The Influence of Landscape Context at Different
Spatial Scales ...................................... 165
8.5 Conclusions and Future Outlook ............................ 168
References .................................................... 169
9 Food Web Interactions and Ecosystem Processes ............. 175
A. Janssen and M.W. Sabelis
9.1 Summary ................................................... 175
9.2 Introduction .............................................. 175
9.3 Interactions Among Entire Trophic Levels .................. 178
9.4 Effects of Diversity Within Trophic Levels ................ 179
9.4.1 Apparent Competition ................................ 180
9.4.2 Omnivory ............................................ 180
9.4.3 Intraguild Predation ................................ 181
9.4.4 Plant-Mediated Indirect Interactions Between
Herbivores .......................................... 181
9.4.5 Indirect Plant Defences ............................. 182
9.4.6 Interactions Among Plants ........................... 183
9.4.7 Behavioural Effects ................................. 184
9.5 Conclusions and Perspectives .............................. 184
References .................................................... 186
10 A General Rule for Predicting When Insects Will Have
Strong Тор-Down Effects on Plant Communities:
On the Relationship Between Insect Outbreaks and Host
Concentration ............................................ 193
W.P. Carson, J. Patrick Cronin and Z.T. Long
10.1 Summary .................................................. 193
10.2 Introduction ............................................. 193
10.3 The Significance of Insect Outbreaks ..................... 194
10.3.1 Insect Outbreaks Are Common in Numerous
Community-Types Worldwide ......................... 195
10.3.2 Insect Outbreaks Are More Common and More
Devastating per Host in Large, Dense and
Continuous Host Stands ............................ 199
10.3.3 Native Outbreaking Insects Function as Keystone
Species by Reducing the Abundance of
the Dominant Species and Increasing Diversity ..... 200
10.3.4 Insect Outbreaks Are Common Relative to Host
Life Span Yet May Often Go Unnoticed .............. 201
10.3.5 Chyrsomelid Beetles and Lepidoptera Seem to be
Responsible for the Majority of Outbreaks ......... 201
10.4 The Host Concentration Model May Predict Insect Impact
on Plant Communities at Multiple Spatial Scales Better
Than Resource Supply Theory .............................. 202
10.4.1 Resource Supply Theory ............................ 202
10.4.2 The Host Concentration Model (HCM) ................ 203
10.4.3 Distinguishing Between the Two Models ............. 204
10.5 Relationship to Other Related Processes Proposed to
Promote Diversity ........................................ 204
10.5.1 Does Pathogen Impact Increase with Host
Concentration? .................................... 205
References .................................................... 205
11 The Ecology Driving Nutrient Fluxes in Forests ........... 213
B. Stadler, E. Mühlenberg and B. Michalzik
11.1 Summary .................................................. 213
11.2 Introduction ............................................. 214
11.3 Life Histories of Canopy Insects ......................... 215
11.3.1 Aphids ............................................ 215
11.3.2 Scale Insects ..................................... 215
11.3.3 Lepidopterous Larvae .............................. 216
11.4 Population Ecological Background of Nutrient Fluxes ...... 217
11.4.1 Sites and Experimental Setup ...................... 219
11.4.2 Results ........................................... 220
11.5 Trophic Effects and Organic Pathways ..................... 224
11.6 Herbivore-Mediated Changes in Quality and Quantity of
Nutrient Fluxes .......................................... 226
11.7 Synthesis and Conclusions ................................ 230
11.7.1 Understanding the Temporal Dynamics of Energy
and Nutrient Fluxes ............................... 230
11.7.2 Understanding the Spatial Variability in Fluxes ... 231
11.7.3 Understanding the Mechanics that Regulate
Fluxes ............................................ 232
11.7.4 Generating Testable Hypotheses .................... 233
References .................................................... 235
Section IV Methods: Reducing, Enhancing and Simulating
Insect Herbivory
12 Simulating Herbivory: Problems and Possibilities ......... 243
J. Hjältén
12.1 Summary .................................................. 243
12.2 Introduction to the Problem .............................. 244
12.3 Advantages of Simulated Herbivory ........................ 245
12.4 Disadvantages of Simulated Herbivory ..................... 247
12.4.1 Simple Biotic Interactions ........................ 247
12.4.2 Complex Biotic Interactions ....................... 249
12.4.3 Basic Ecosystem Processes ......................... 250
12.5 Conclusions and Suggestions for the Future ............... 251
References .................................................... 253
13 The Use and Usefulness of Artificial Herbivory in
Plant-Herbivory Studies .................................. 257
K. Lehtilä and E. Boalt
13.1 Summary .................................................. 257
13.2 Introduction ............................................. 258
13.3 Material and Methods ..................................... 258
13.4 Commonness of Differences Between Natural and
Artificial Herbivory ..................................... 260
13.5 Strength of the Effect of Natural and Artificial
Damage ................................................... 266
13.6 Responses of Different Types of Response Traits to
Artificial and Natural Damage ............................ 267
13.7 Simulations of Mammalian and Invertebrate Herbivory ...... 269
13.8 Attempts of Exact Simulation ............................. 270
13.9 Conclusions .............................................. 271
References .................................................... 273
14 From Mesocosms to the Field: The Role and Value of Cage
Experiments in Understanding Top-Down Effects in
Ecosystems ............................................... 277
O.J. Schmitz
14.1 Summary .................................................. 277
14.2 Introduction ............................................. 278
14.3 Research Approach ........................................ 281
14.4 In-Ecosystem Investigation Using Enclosure Experiments ... 282
14.4.1 Natural History: Knowing the Players in
the System ........................................ 282
14.4.2 Enclosure Cages: Design and Biophysical
Properties ........................................ 285
14.4.3 Considerations for the Design of Cage
Experiments ....................................... 288
14.4.3.1 Artificial Complements of Populations
or Communities in Enclosure Cages Are
Not Realistic ............................ 288
14.4.3.2 Experimental Outcome Could Be
an Artifact of the Venue ................. 288
14.4.3.3 Enclosures Unrealistically Constrain
Movement of Species ...................... 289
14.4.3.4 Time Scale of Enclosure Experiments
Exclude or Distort Important Features
of Communities and Ecosystems ............ 290
14.4.4 Mechanistic Insights from Enclosure Cage
Experiments ....................................... 290
14.4.4 Identifying the Potential for Top-Down Control .... 291
14.4.5 Of-Ecosystem Studies: Testing the Reliability of
Mechanistic Insights from Cage Experiments ........ 297
14.4.5.1 Direct and Indirect Effects of Top
Predators ................................ 299
14.4.5.2 Top Predator Effects on Plant Diversity
and Productivity ......................... 300
References .................................................... 300
15 Reducing Herbivory Using Insecticides .................... 303
E. Siemann, W.P Carson, W.E. Rogers and W.W. Weisser
15.1 Summary .................................................. 303
15.2 Basic Concepts ........................................... 303
15.3 Using Insecticides to Infer the Role of Herbivores ....... 304
15.4 Ghost of Herbivory Past .................................. 307
15.5 Artifacts of Method May Masquerade as Release from
Herbivory ................................................ 308
15.5.1 What Types of Artifacts Are a Concern? ........... 308
15.5.2 Overview of Published Studies .................... 309
15.5.3 Quantification of Herbivore Damage ............... 310
15.5.4 Phytotoxic Effects ............................... 311
15.5.5 Insecticides May Be Toxic to Several Groups of
Insects .......................................... 313
15.5.6 Effects of Insecticides on Non-Arthropods ........ 314
15.5.7 Effects of Insecticides on Soil Organisms ........ 314
15.5.8 Nutrient Inputs May Facilitate Plant Growth ...... 315
15.5.9 Insect-Vectored Diseases ......................... 317
15.5.10 Community-Level Artifacts ........................ 318
15.6 Are There Better Types of Insecticides? .................. 318
15.7 Conclusions .............................................. 319
Appendix: Results of Surveyed Studies ......................... 320
References .................................................... 324
16 The Role of Herbivores in Exotic Plant Invasions:
Insights Using a Combination of Methods to Enhance or
Reduce Herbivory ......................................... 329
W.E. Rogers and E. Siemann
16.1 Summary .................................................. 329
16.2 Introduction ............................................. 329
16.3 The Role of Herbivores in Exotic Plant Invasions ......... 330
16.4 Focal Plant Species ...................................... 331
16.5 Experimental Methods for Assessing Herbivory Effects ..... 331
16.5.1 Common Garden/Reciprocal Transplant Studies ....... 332
16.5.2 Reducing Herbivory on Target Plants Using
Insecticide Sprays ................................ 336
16.5.3 Reducing Herbivory on Community Assemblages
Using Insecticide Sprays .......................... 337
16.5.4 Factorial Manipulations of Herbivory, Resources
and Competition ................................... 338
16.5.5 Simulating Herbivory Via Mechanical Leaf Damage ... 339
16.5.6 Simulating Herbivory Via Mechanical Root Damage ... 341
16.5.7 Simulating Herbivory Using Herbicide Sprays ....... 342
16.5.8 Assessing Herbivore Damage Using Exclosures and
Enclosures ........................................ 344
16.6 Implications and Potential Significance .................. 347
References .................................................... 349
17 Herbivore-Specific Transcriptional Responses and Their
Research Potential for Ecosystem Studies ................. 357
C. Voelckel and I.T. Baldwin
17.1 Summary .................................................. 357
17.2 The Subtle Effects of Insects on Ecosystem Function ...... 357
17.3 Transcriptional Regulation of Plant Responses ............ 358
17.4 Insect-Induced Transcriptional Changes ................... 362
17.5 How a Molecular Understanding of Plant-Insect
Interactions Can Help Elucidate Ecosystem Function ....... 371
References .................................................... 375
Section V Synthesis
18 Testing the Role of Insects in Ecosystem Functioning ..... 383
E. Siemann and W.W. Weisser
18.1 Summary .................................................. 383
18.2 Introduction ............................................. 384
18.3 Simple Models of Niche Space ............................. 385
18.3.1 Reduced Vigour Model .............................. 385
18.3.2 Reduced Range of Tolerance Model .................. 387
18.3.3 Specialist Herbivores ............................. 388
18.4 Effects of Herbivores in Resource Competition Models ..... 389
18.4.1 Specialist Herbivores in Resource Competition
Models ............................................ 391
18.4.2 Generalist Herbivores in Resource Competition
Models ............................................ 395
18.5 Differential Impacts on Plants with Different Traits ..... 396
18.6 Conclusions from the Modelling Work ...................... 396
18.7 Suggestions for Future Studies ........................... 397
18.7.1 Exploring Below- and Aboveground Interactions in
More Detail ....................................... 397
18.7.2 Measuring Herbivory Effects at Nominal Levels as
Well as in Outbreak Situations .................... 398
18.7.3 Quantifying the Effects of Plant Resource
Allocation Under Herbivory for Ecosystem
Functioning ....................................... 399
18.7.4 Combining Various Methodologies to Achieve
an Understanding of Insect Effects on Ecosystem
Function .......................................... 399
References .................................................... 400
Subject Index ................................................. 403
Taxonomic Index ............................................... 409
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