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We have tested the hypothesis that a plant may detect the ptesence of a neighbour, befote being shaded by it, thtough the petception of the spectt-al composition of reflected sunlight. Within seedlitig canopies the red : far-red t-atio (R : FR) of the light received by a sensor with a geotnett-y appt-oxitnating that of a stetn was significantly teduced by selective reflection. This effect was observed befote any reduction in the atnount of photosytithetic light energy received by an individual seedling could be detected. Stnall gt-een fences of grass, east-west orientated, alteted the specttal distribution of the light on the north (sunlit) side of thetn. Fully illuminated seedlings of Sinapis alba giown on the north side of these gt-een fences produced longer internodes and had a lower leaf: stetn dry weight ratio than those grown in ftont of fences of bleached grasses. A sitnilar tedistribution of gtowth was elicited in seedlings of Chettopodinm album. Datura ferox and S. alba growing iti full sunlight by exposing plants to additional small quantities of fat-red reliected by seleetive tnirtot-s. These t-esults suggest that the change in the R: FR ratio serves as an early warning signal of oncotning cotnpetition.

Symbiotic microorganisms that live intimately associated with terrestrial plants affect both the quantity and quality of resources, and thus the energy supply to consumer populations at higher levels in the food chain. Empirical evidence on resource limitation of food webs points to primary productivity as a major determinant of consumer abundance and trophic structure. Prey quality plays a critical role in community regulation. Plants infected by endophytic fungi are known to be chemically protected against herbivore consumption. However, the influence of this microbe-plant association on multi-trophic interactions remains largely unexplored. Here we present the effects of fungal endophytes on insect food webs that reflect limited energy transfer to consumers as a result of low plant quality, rather than low productivity. Herbivore-parasite webs on endophyte-free grasses show enhanced insect abundance at alternate trophic levels, higher rates of parasitism, and increased dominance by a few trophic links. These results mirror predicted effects of increased productivity on food-web dynamics. Thus 'hidden' microbial symbionts can have community-wide impacts on the pattern and strength of resource-consumer interactions.

Persistence and ubiquity of vertically transmitted Neotyphodium endophytes in grass populations is puzzling because infected plants do not consistently exhibit increased fitness. Using an annual grass population model, we show that the problems for matching endophyte infection and mutualism are likely to arise from difficulties in detecting small mutualistic effects, variability in endophyte transmission efficiency and an apparent prevalence of non-equilibrium in the dynamics of infection. Although endophytes would ultimately persist only if the infection confers some fitness increase to the host plants, such an increase can be very small, as long as the transmission efficiency is sufficiently high. In addition, imperfect transmission limits effectively the equilibrium infection level if the infected plants exhibit small or large reproductive advantage. Under frequent natural conditions, the equilibrium infection level is very sensitive to small changes in transmission efficiency and host reproductive advantage, while convergence to such an equilibrium is slow. As a consequence, seed immigration and environmental fluctuation are likely to keep local infection levels away from equilibrium. Transient dynamics analysis suggests that, when driven by environmental fluctuation, infection frequency increases would often be larger than decreases. By contrast, when due to immigration, overrepresentation of infected individuals tends to vanish faster than equivalent overrepresentation of noninfected individuals.

P. 2004. Do foliar endophytes affect grass litter decomposition? A microcosm approach using Lolium multiflorum . Á/ Oikos 104: 581 Á/590.Symbiotic infection with fungal endophytes has been shown to decrease herbivory in several temperate grasses. We tested the hypothesis that foliar endophytes of grasses may also affect below-ground processes upon their host death, by altering the litter quality for detritivores or the microenvironment for decomposition. Microcosm Á/ litterbag experiments were used to assess decay rates for litter produced by endophyte (Neotyphodium sp.) infected vs uninfected Lolium multiflorum plants, and to examine endophyte-mediated effects of prior site occupants on current litter decomposition. We found that litter from endophyte-infected L. multiflorum decomposed more slowly than litter from endophyte-free conspecifics and from a naturally uninfected grass, Bromus unioloides. Furthermore, the endophyte Á/grass association modified the decomposition environment, so that B. unioloides litter decomposed faster when placed underneath a thick layer of endophyte-free L. multiflorum litter. Litter decay rates increased with the amount of root debris remaining in situ from the previous season, but were not affected by the infection status of prior site occupants. The lower decomposability of litter from infected L. multiflorum plants persisted across a range of microenvironments, as determined by different amounts of above-ground litter and soil moisture conditions. Endophyte infection tended to reduce the N content of decaying litter; however, litter N and C/N ratio mainly accounted for interspecific differences in decomposition. Our results imply that fungal endophytes not only can affect herbivory food chains, but also soil organisms and the ecosystem processes they regulate. This study suggests a novel role for symbiotic foliar endophytes in linking above-ground and below-ground sub-systems.

Variation exists in the frequency of obligate, vertically transmitted symbiotic organisms within and among host populations; however, these patterns have not been adequately explained by variable fi tness eff ects of symbionts on their hosts. In this forum, we call attention to another equally important, but overlooked mechanism to maintain variation in the frequency of symbioses in nature: the rate of vertical transmission. On ecological time scales, vertical transmission can aff ect the equilibrium frequencies of symbionts in host populations, with potential consequences for population and community dynamics. In addition, vertical transmission has the potential to infl uence the evolution of symbiosis, by aff ecting the probability of fi xation of symbiosis (and therefore the evolution of complexity) and by allowing hosts to sanction against costly symbionts. Here we use grass -epichloae symbioses as a model system to explore the causes and consequences of variation in vertical transmission rates. We identify critical points for symbiont transmission that emerge from considering the host growth cycle devoted to reproduction (asexual vs sexual) and the host capability to maintain homeostasis. We also use information on the process of transmission to predict the environmental factors that would most likely aff ect transmission rates. Altogether, we aim to highlight the vertical transmission rate as an important process for understanding the ecology and evolution of symbiosis, using grass -epichloae interactions as a case study.

2004.Litter quality and nutrient cycling affected by grazing-induced species replacements along a precipitation gradient. Á/ Oikos 107: 148 Á/160.One of the potential mechanisms for the impact of herbivores on nutrient cycling is the effect of selective grazing on litter quality through changes in species composition. However, the scarce evidence collected on this mechanism is controversial and seemingly influenced by site-specific variables. In this paper, we explored the consequences of grazing-induced changes in species composition on litter quality and nitrogen cycling with a regional perspective. Along a 900-mm of mean annual rainfall gradient, we selected species promoted and diminished by grazing from three natural rangelands of Argentina, analyzed their litter quality, and determined their decomposition and nutrient release kinetics under common greenhouse conditions. Litter quality and decomposition rates were strongly associated with plant response to grazing. However, the magnitude and direction of these differences depended on the ecosystem considered. In the wettest site, the species promoted by grazing (forbs) had higher nitrogen and phosphorus contents, faster decomposition rates, and higher release of nitrogen to the soil than species diminished by grazing (C 3 and C 4 grasses). In the intermediate and dry sites, species promoted by grazing had lower nitrogen and phosphorus contents, and slower decomposition rates than those diminished by grazing (C 3 grasses in both cases). Decomposition of the entire group of species was not correlated with mean annual rainfall, but when litter of the species diminished by grazing was analyzed, it was negatively correlated with precipitation. Nitrogen was immobilized more often than mineralized, even after one year of incubation. Immobilization was negatively correlated with precipitation. All these results indicate that grazing may significantly alter nutrient cycling by affecting litter quality through changes in species composition. These effects seem to be larger when species replacements induced by grazing either involve functional groups, as it was the case in our wettest site, or change root to shoot ratios. Therefore, the functional groups involved in the replacement of species as well as shifts between belowground and aboveground allocation should play a key role in grazing-induced changes on nitrogen cycling.