Context-dependence in plant-herbivoreinteractions

Proceedings of a workshop held at Ekena� s, Sweden, inOctober, 2001

Sponsored by European Science Foundation and by Oskar and Lilli Lamm’s Foundation

Edited byChrister Bjo� rkman and Peter Hamba� ck

OIKOS 101: 3–5, 2003

Introduction to the symposium

Christer Bjorkman, Dept of Entomology, Swedish Uni�. of Agricultural Sciences, P.O. Box 7044, SE-750 07Uppsala, Sweden. – Peter A. Hamback, Dept of Ecology and Crop Production Science, Swedish Uni�. ofAgricultural Sciences, P.O. Box 7043, SE-750 07 Uppsala, Sweden.

Interactions between species are always embeddedwithin a larger community, and the identity of speciesin this community may have important effects on theoutcome of the species interaction. These context-de-pendent effects may arise either as direct effects on theinteracting species densities (density-mediated), or aseffects on the behaviour of individuals (trait-mediated).The role of context is a basic premise in several cur-rent plant–herbivore research topics; e.g. on the possi-ble connection between biodiversity and ecologicalfunction, and on the geographic-mosaic hypothesisabout co-evolution in plant–herbivore systems. A ma-jor question in these topics is whether the context-de-pendent effects are due to biodiversity per se or if theyarise due to the actions of a smaller set of species (e.g.keystones or dominants). In order to answer this ques-tion, it is necessary to examine the underlying mecha-nisms, as some mechanisms can be expected to bemore generalistic than others. In October 2001, therole of context for plant–herbivore interaction wasdiscussed at a workshop sponsored by the EuropeanScience Foundation (Program: LINKECOL) and byOskar and Lilli Lamm’s foundation, whose generousfinancial support we gratefully acknowledge. The goalof the workshop was to discuss plant–herbivore inter-actions in both ecological and evolutionary time, ex-amining how this interaction is affected by otherspecies in the community. For this purpose, we invitedecologists from divergent disciplines; population, com-munity, ecosystem and evolutionary ecology, but witha common interest in plant–herbivore interactions.The papers in this special feature are based on presen-tations at the workshop.

Above vs below ground processes

Several recent studies suggest that the presence andidentity of soil organisms may affect the interactionsbetween plants and herbivores, both above- and be-

low-ground. This possibility is illustrated by Hartleyand Jones (Plant diversity and insect herbivores: effectsof environmental change in contrasting model sys-tems). They show, for two quite different systems, thateffects of environmental change on insect–plant inter-actions are often manifested through indirect effectsmediated by below-ground factors. In an ecotronstudy, that elevated CO2 to increase carbon availabilityin the soil, it was found that above-ground changeswere connected to a change in the soil fauna commu-nity. Similarly, in a field study of a moorland system,they found that species richness in the community ofherbivorous insects was less influenced by above-ground grazing than by soil properties.

Spatial scales

Plant–herbivore interactions may be affected by bothclose and distant neighbours, much depending on themovement characteristics of the mediator (e.g. the in-sect). This problem is addressed by several papers inthe special feature. First, the paper by Thies andcoworkers (Effects of landscape context on herbivoryand parasitism at different spatial scales) examines thecharacteristic spatial scales involved in the tri-trophicinteraction between plants, herbivores and parasitoids.They find that the characteristic spatial scale is notlarger for the parasitoids–herbivore interaction thanfor the herbivore–plant interaction, contrary to theo-retical predictions. They suggest that this discrepancymay be associated with the similarity in size betweenherbivores and parasitoids. Second, the paper by Ham-back and Beckerman (Herbivory and plant resourcecompetition: a review of two interacting interactions)reviews studies on the interactive effect of herbivoryand plant competition for plant growth and abun-dance. Their starting point is that effects of plantneighbours on the interaction between a focal plant

Accepted 11 October 2002

Copyright © OIKOS 2003ISSN 0030-1299

OIKOS 101:1 (2003)4

and its herbivores can vary with the spatial scale. Aproblem is that most studies that examined the effectson plant growth have not considered scaling effects inthe design of their experiment. Moreover, there is nostudy that has, for the same system, examined both themechanisms underlying the interactive effect and theimportance for plant growth. This raises a number ofquestions for the development of a general theory oninteractive effects of plant neighbours and herbivoreson plant growth and abundance. Third, Dicke andcoworkers (Mixed blends of herbivore-induced plantvolatiles and foraging success of carnivorousarthropods) examined if the effect of herbivore-inducedvolatiles is limited to controlled laboratory conditionsor if the effect could also occur in the field. Theargument has been that these effects may disappear inthe complex odour environment occurring under fieldconditions. They found, however, no indication thatmixed blends would disrupt the ability of predators totrack herbivore-induced plant volatiles. This findingsupports the suggestion that olfactory cues are seldomdisrupted by habitat complexity (Hamback and Becker-man). Fourth, the paper by Holt and Barfield (Impactsof temporal variation on apparent competition andcoexistence in open ecosystems) examines consequencesfrom the flow of organisms between habitats on localorganismal densities. Previous studies have shown, fororganisms with stable densities, that such flow maycreate spatial apparent competition and species exclu-sions. Holt and Barfield show that temporal variabilitymay affect this indirect effect in a predictable way,depending on the mechanism underlying the temporalvariability, and propose a general protocol for examin-ing the combined effect of temporal and spatial hetero-geneities on species interactions. Finally, the paper byOnzo and coworkers (Dynamics of refuge use: diurnal,vertical migration by predatory and herbivorous miteswithin cassava plants) examines how heterogeneitywithin plants affects species interactions. They show

how the dynamic use by predatory mites of plant–pro-vided refuges affects both the behaviour and the distri-bution of herbivorous mites.

Evolutionary aspects

The context dependence of plant–herbivore interac-tions on ecological time scales may also have conse-quences for the evolution of plant, or herbivorecharacters, e.g. on host plant specialisation. The workby Ballabeni and coworkers (local adaptation and eco-logical genetics of host–plant specialization in a leafbeetle) provide a nice empirical example showing howthe context may influence the evolution of local adapta-tion. The context is in this case the presence of alterna-tive host plants at a site. They found that specialisationin single-host populations also increased performanceon the alternative, not naturally encountered, host as aconsequence of positive genetic correlation for using thetwo hosts. The authors conclude that the genetic archi-tecture of the performance traits involved may limit thepossibilities for divergence.

It is apparent that to understand several aspects ofplant–herbivore interactions we have to adopt abroader perspective than the simple interaction. Evi-dence is accumulating that the strength and importanceof herbivore effects on plants is often strongly affectedby both the structure of the surrounding plant commu-nity, the structure of the local food web and processesbelow-ground. It would therefore come as no surprise ifnovel insights in plant–herbivore studies will arise atthe intersection of disciplines such as ecosystem ecol-ogy, population ecology and evolutionary ecology. Forinstance, the question of plant responses to herbivory isstudied in all these disciplines. Yet, very few studieshave addressed ecosystem aspects when examining, forinstance, the evolution of plant tolerance.

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