Multitrophic effects of nutrient addition in upland grassland.

Although the effects of nutrient enhancement on aquatic systems are well documented, the consequences of nutritional supplements on soil food webs are poorly understood, and results of past research examining bottom-up effects are often conflicting. In addition, many studies have failed to separate the effects of nutrient enrichment and the physical effects of adding organic matter. In this field study, we hypothesised that the addition of nitrogen to soil would result in a trophic cascade, through detritivores (Collembola) to predators (spiders), increasing invertebrate numbers and diversity. Nitrogen and lime were added to plots in an upland grassland in a randomised block design. Populations of Collembola and spiders were sampled by means of pitfall traps and identified to species. Seventeen species of Collembola were identified from the nitrogen plus lime (N+L) and control plots. Species assemblage, diversity, richness, evenness and total number were not affected by nutrient additions. However, there was an increase in the number of Isotomidae juveniles and Parisotoma anglicana trapped in the N+L plots. Of the 44 spider species identified, over 80% were Linyphiidae. An effect on species assemblage from the addition of N+L to the plots was observed on two of the four sampling dates (July 2002 and June 2003). The linyphiid, Oedothorax retusus, was the only species significantly affected by the treatments and was more likely to be trapped in the control plots.The increased number of juvenile Collembola, and change in community composition of spiders, were consequences of the bottom-up effect caused by nutrient inputs. However, despite efforts to eliminate the indirect effects of nutrient inputs, a reduction in soil moisture in the N+L plots cannot be eliminated as a cause of the invertebrate population changes observed. Even so, this experiment was not confounded by the physical effects of habitat structure reported in most previous studies. It provides evidence of moderate bottom-up influences of epigeic soil invertebrate food webs and distinguishes between nutrient addition and plant physical structure effects. It also emphasises the importance of understanding the effects of soil management practices on soil biodiversity, which is under increasing pressure from land development and food production.

Rapid and recent changes in fungal fruiting patterns.

Information on responses of higher organisms to climate change is dominated by events in spring. Far less is known about autumnal events and virtually nothing about communities of microorganisms. We analyzed autumnal fruiting patterns of macrofungi over the past 56 years and found that average first fruiting date of 315 species is earlier, while last fruiting date is later. Fruiting of mycorrhizal species that associate with both deciduous and coniferous trees is delayed in deciduous, but not in coniferous, forests. Many species are now fruiting twice a year, indicating increased mycelial activity and possibly greater decay rates in ecosystems.

Impacts of soil faunal community composition on model grassland ecosystems.

Human impacts, including global change, may alter the composition of soil faunal communities, but consequences for ecosystem functioning are poorly understood. We constructed model grassland systems in the Ecotron controlled environment facility and manipulated soil community composition through assemblages of different animal body sizes. Plant community composition, microbial and root biomass, decomposition rate, and mycorrhizal colonization were all markedly affected. However, two key ecosystem processes, aboveground net primary productivity and net ecosystem productivity, were surprisingly resistant to these changes. We hypothesize that positive and negative faunal-mediated effects in soil communities cancel each other out, causing no net ecosystem effects.

Interactions between arbuscular mycorrhizal fungi and foliar-feeding insects in Plantago lanceolata L.

A field experiment was conducted to investigate whether infection by arbuscular mycorrhizal (AM) fungi has any effect on herbivory by foliar-feeding insects. Plants of PI ant ago laureolata L. were grown in a randomized block design and natural levels of mycorrhizal infection reduced by the application of the granular fungicide iprodione. Plant growth responses were examined and herbivore bioassays performed by rearing both a chewing and sucking insect on the leaves of mycorrhizal and fungicide-treated plants. Fungicide application successfully reduced mycorrhizal infection, and this led to reductions in foliar biomass, caused by lower leaf number. However, fungicide-treated plants suffered consistently higher levels of damage by centralist chewing and leaf-mining insects, which colonized the plants. The chewing insect bioassay confirmed the field results, in that larvae of Arttia caja L. (Lepidoptera: Arctiidae) consumed more leaf material from plants in which infection was reduced. There was no evidence that AM fungi altered food quality for the chewing insect. Instead, infection caused an increase in the carbon/nutrient balance, which in turn led to increased levels of the carbon-based feeding deterrents, aucubin and catalpol, The sucking insect, Mvzus perskae (Sulzer) reacted in an opposite fashion to the ehewtr. with performance being greater on mycorrhizal plants. Again, there was no evidence that an alteration in food Quality was the cause, and in this case infection may result in changes in leaf morphology which benefit the insect. We suggest that under conditions of high light and low nutrient availability. AM infection can alter the carbon/nutrient balance of plants, leading to an increased allocation to carbon-based defences. This can have important consequences for insect herbivore performance and the patterns of herbivory in field situations.

Mechanisms of seedling mortality by subterranean insect herbivores.

The nature of the mortality of germinating seeds of Vicia sativa, the common vetch, was investigated in a pot trial under controlled conditions. Chafer and tipulid larvae were restricted by nylon mesh partitions to enable radicle and/or hypocotyl herbivory to occur. The effects of the two insects were very similar. In control situations, an average of 88% of the viable seed sown recruited successfully. Hypocotyl and radicle herbivory had similar effects on seedling mortality, with recruitment of viable seed sown being reduced to 52%. When both modes of attack occurred together, successful recruitment was only 34%. Feeding on both plant parts resulted in an average post-emergence mortality of 14% of the seed sown, but the effect on pre-emergence mortality was of greater importance, amounting to 41%. The mechanisms by which soil-dwelling herbivores may cause seedling mortality in the field are discussed.

Effects of root herbivory by an insect on a foliar-feeding species, mediated through changes in the host plant.

The effects of root herbivory by larvae of the scarabaeid, Phyllopertha horticola, on the growth of Capsella bursa-pastoris were examined. Individuals of Aphis fabae were reared on the leaves to determine what effect, if any, root feeding has on the performance of this insect. The experiment was conducted under two watering regimes ('low' and 'high'). Low watering and root feeding caused water stress in the plants and this was reflected in a reduction in vegetative biomass and an increase in the proportion of material allocated to reproduction. Supplying plants with ample water in the 'high' treatment enabled the water stress caused by root herbivory to be offset, but not completely overcome. Low watering and root feeding caused an increase in aphid weight and growth rate, while root feeding also increased fecundity and adult longevity. These effects are attributed to an improvement in food quality, measured by total soluble nitrogen, and caused by amino acid mobilization due to the water stress. The implications of these results in agricultural and ecological situations are discussed.