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Silica defences in grasses have recently been suggested to be a potential driver of vole population dynamics. However, the ability of grasses to induce silica in response to herbivory has not been tested in northern ecosystems where small rodents are important herbivores. We conducted a large-scale field experiment in subarctic tundra using three river catchments differing in herbivore densities, and examined the effects of small rodent and/or reindeer exclusion on leaf silica levels in five grass species (Avenella flexuosa, Anthoxanthum nipponicum, Calamagrostis phragmitoides, Deschampsia cespitosa and Phleum alpinum). We also conducted a greenhouse experiment using three of these species (A. flexuosa, A. nipponicum and D. cespitosa) and Festuca ovina to determine whether intraspecific genotypic variation affects baseline silica concentrations and the capacity to induce silica in response to simulated grazing. Baseline leaf silica concentrations and silica induction varied with plant species in both experiments, with catchment in the field experiment and with genotype in the greenhouse experiment. These findings show that the allocation to silica defences in grasses is highly variable, and suggest that the combined effects of grazing pressure, plant species and intraspecific genotypic differences are likely to determine the circumstances under which silica induction may be an optimal defence strategy. A better understanding of the interplay between grazing and other factors influencing silica induction is necessary to interpret the role of silica in plant–herbivore interactions.

Nutrient inputs to plant communities are often spatially heterogeneous, for example those deriving from the dung and urine of large grazing animals. The effect of such localised elevation of nutrients on plant growth and composition has been shown to be modified by the grazing of large herbivores. However, there has been little work on interactions between small mammalian herbivores and such patchy nutrient inputs, even though these interactions are potentially of major significance for plant performance and community structure. We examined the effect of simulated cattle urine deposition on the vegetation structure, above-ground biomass and species composition of chalk grassland within enriched patches. Short-term exclosures were used to determine whether a small herbivore (rabbit) would preferentially graze the vegetation in enriched patches and what impact this interaction would have on the performance of plants in such patches. Rabbit grazing pressure determined whether nutrient inputs had a negative or positive effect on plant biomass. Nutrients increased plant biomass in the absence of grazing, but when exposed to grazing, plants in nutrient-rich patches had more biomass consumed by herbivores than neighbouring plants. Further, nutrients increased the relative palatability of a less preferred forage species (Brachypodium pinnatum), contributing to changes in plant community composition. We conclude that a small herbivore can drive plant responses to patchily distributed nutrients.

There is increasing concern over the potential impact of anthropogenic factors (e.g. increasing nutrient inputs, global climate change) on the rate of loss of diversity in ecosystems. Such losses may affect ecosystem processes. In addition, a change in diversity of one group of organisms may influence the diversity of species of the next trophic level. We examined the extent to which plant species richness influences that of insect herbivores in two systems: a long-term field experiment on heather moorland and a model community in the Ecotron controlled environment facility. We examined the response of these two plant communities to environmental change, specifically increased levels of nutrients, grazing and atmospheric CO 2 . We measured the indirect effects of changes in these factors on insect herbivores, both above- and below-ground. In the moorland system, grazing was the largest influence on plant community structure. The community was dominated by one species, Calluna vulgaris, and loss of cover under heavy grazing allowed competing species to invade. However, grazing regime was not a major influence on the species richness of the insect herbivore community. Site was more important: there were a greater number of Hemiptera species on sites with more mineral soils than on peat sites, possibly because a greater variety of grass and herb species was present on the former sites. In the Ecotron, below-ground factors were also important drivers of community change: elevated C0 2 increased carbon availability in the soil and there were simultaneous changes in the community composition of soil biota. Above-ground, some plant species increased in abundance and others decreased, leading to interaction-specific effects on the insect herbivores. In two very different studies of the effects of environmental change on the interactions between plants and their herbivores, several similar conclusions can be drawn: (1) effects are likely to be site- and interaction-specific; (2) outcomes are likely to be strongly dependent on the initial state and the dominant species of the plant community; and (3) indirect effects, often mediated by below-ground factors, may have a bigger influence on insect-plant interactions than more direct effects of above-ground factors.

The winter moth, Operophtera brumata, shows varying population dynamics in different host plant habitats. Populations in Sitka spruce, Picea sitchensis, plantations and in Scottish moorlands have a tendency to outbreak that is not shown by winter moth in lowland oak woods. Since pupal predators have previously been identified as being important for the regulation of winter moth in a lowland oak wood it was hypothesized that invertebrate pupal predators were failing to control winter moth in outbreak populations. This hypothesis was tested by comparing the abundance of invertebrate predators and patterns of spatially density dependent pupal predation across habitats. Several results supported this hypothesis. Carabid predators of winter moth were one or two orders of magnitude more abundant in oak woods than in moorland or spruce habitats. Staphylinid predators were also more abundant in high winter moth density oak woods than in any other habitat. Beetle predation of tagged cocoons in the field was inversely density dependent in Highland moors in experiments in 1999 and 2000, and in Sitka spruce in 1999. However, in opposition to our hypothesis, pupal predation was also inversely density dependent in oak woods in 2000, although this result may be explained by the low range of winter moth densities in the field that year. These results are discussed in relation to the role of natural enemies in regulating winter moth populations and the differences in life-history of the beetle predators in different habitats.

A general model of insect-pathogen dynamics is presented which includes explicit host and pathogen dispersal. Four distinctive, wave-like, patterns of dispersal are produced which can be categorised by two universal parameters – the speed of advance, and the position of the leading edge, of the wave of dispersal of the host relative to that of the disease. These patterns are (1) the pathogen becomes extinct, allowing the host to disperse at the carrying capacity across the land surface, (2) the host disperses more rapidly than the pathogen, producing host densities at the carrying capacity in a region behind the leading edge of the wave, with these densities reduced due to interaction with the pathogen in the wave interior, (3) the host and pathogen disperse at the same speed but the leading edge of the host extends beyond that of the pathogen, allowing the host to reach ‘high’ density at the leading edge only, and (4) the host and pathogen disperse at the same speed but the leading edge of the pathogen extends beyond that of the host, producing ‘low’ density host dispersal across the land surface. A biological description explaining the causes of these patterns has important consequences regarding the use of pathogens for biological control of insect pests. The model is modified to represent a specific insect-pathogen system, the winter moth, Operophtera brumata, and its nuclear polyhedrosis virus. The same patterns, categorised by the same universal parameters, are observed. Thus, it is suggested that the strength of infection and the relative dispersal rates of the host and pathogen are influential in determining the patterns of host outbreaks observed in this insect.

We present a Protein Competition Model (PCM) for predicting total phenolic allocation and concentration in leaves of terrestrial higher plants. In contrast to predictions based on the carbon composition of end products, the PCM is based on metabolic origins of pathway constituents, alternative fates of pathway precursors, and biochemical regulatory mechanisms. Protein and phenolic synthesis compete for the common, limiting resource phenylalanine, so protein and phenolic allocation are inversely correlated. Phenolic allocation can be predicted from the effects of development, inherent growth rate and environment on leaf functions that create competing demands for proteins or phenolics. We present the model general principles. We predict phenolic concentrations as leaves develop; in inherently fast versus slow growing species; and in response to the environment (nitrogen, light, phosphorus, heat shock, herbivore and pathogen injury, and carbon dioxide). Because predictions generally fit observed patterns, we argue that, for phenylalanine-derived phenolics, the mechanistically distinctive PCM complements the Growth Differentiation and Resource Availability Hypotheses, and is a viable, testable alternative to the Carbon Nutrient Balance Hypothesis.

Plants are subject to attack by a range of insect herbivores including those producing galls. Gall formation is a remarkable process in which the insect becomes the new organiser of plant development: normal cellular differentiation is inhibited, and the growth of the tissue is altered to produce characteristic and often bizarre structures (Meyer 1987, Shorthouse and Rohfritsch 1992). In addition to gall-forming insects, there are other organisms which attack plants causing tumour production: the bacteria from the Rhizobium group, which cause nodules on plant roots, and the bacterium Agrobacterium tumefaciens, which causes crown gall, are examples. Molecular techniques have allowed the mechanisms by which these organisms induce growth deformations in plants to be elucidated, and phenolic compounds have been found to be a fundamental component of the tumour-forming process in both systems. Traditionally phenolic compounds have been viewed as being important in defending plants against attack; in the case of attack by parasitic tumour-forming agents, however, phenolics appear to have a rather different role. In these systems, phenolic compounds are used by the invading organisms as indicators of susceptible host plants, and they also mediate the manipulation of plant growth by the attacking organism because they are able to affect plant gene expression and to alter plant hormone activity. Galls are particularly close associations between an attacking insect and a plant; the gall insect appears to overwhelm the plant's defences in a unique way. Gall insects may be more similar, in terms of their effects on plant defences such as phenolic compounds, to other tumour-forming organisms than they are to other insects. Here I examine the idea that since phenolic compounds are important in mediating the interactions between plants and tumour-forming bacteria and may have a role in the regulation of plant growth, the same physiological mechanisms may be operating in gall formation. The essential question is: Do gall-formers use phenolic compounds to control plant growth and development as other tumour-forming organisms have been shown to do? Evidence to support this idea is indirect. Very little work has been carried out on the mechanisms of gall formation at the molecular level. It is, however, worth attempting to gather such evidence since consideration of the important advances in understanding of other plant tumour-inducing processes made by plant physiologists could give entomologists some clues to the changes in plant physiology and gene expression required to initiate galls. Although many entomologists and ecologists are familiar with the idea of phenolic compounds being involved in the defence of plants against attack by insects (and other organisms), fewer have considered these compounds as signals which mediate interactions between plants and invading organisms, including insects. Perhaps most entomologists are unlikely to have regarded gall insects as large rhizobia! At this point it is important to clarify that whilst Rhizobium/plant associations are mutualistic, no implication is made here that galling is a mutualism, merely that gall insects may have adopted a similar physiological methodology to control plant growth as rhizobia have.

This study investigated the effect of environmental factors on plant allocation to secondary compounds, and whether differences in these allocations between individual plants explained the selection behaviour of mammalian herbivores. Sitka spruce (Picea sitchensis) seedlings and heather (Calluna vulgaris) plants were subjected to fertiliser and shading treatments which altered their nutrient and secondary compound content. The response of Orkney voles (Microtus arvalis orcadensis) to these treatments was tested in laboratory feeding trials. Foliage from the spruce seedlings which had received both shading and fertiliser were the most palatable to voles. This preference correlated with the effects of the treatments on seedling nitrogen and phenolic levels, but not with monoterpene content. Heather which had been fertilised was also the most preferred, but in this case the chemical basis for palatability was nitrogen and fibre content as the phenolic content of the plants was not altered by the treatments. This study shows that predictions of the carbon/nutrient balance hypothesis for allocation to plant defences are not applicable to all groups of secondary compounds, or to all plant species, and that voles are capable of using a variety of chemical cues to detect environmentally induced differences in the nutrient quality of individual plants within a species