Showing papers by "James D. Bever published in 2007"

Coevolution of symbiotic mutualists and parasites in a community context.

Abstract: Recent advances in our knowledge of parasitic and mutualistic associations have confirmed the central role of coevolutionary interactions in population and community ecology. Here, we discuss the potential coevolutionary interdependence of the strength and specificity of symbiotic interactions with the complexity and productivity of their environment. We predict that interactions become less beneficial with increasing environmental quality and that the association of productivity with symbiont specificity depends on the relative strengths of tradeoffs between host range and other life-history parameters. However, as biotic complexity increases, pathogen specificity is predicted to decline, whereas mutualist specificity will increase. Testing these predictions on a geographical scale would contribute significantly to the predictive science of coevolution, and to our ability to manage biological interactions embedded in increasingly fragmented landscapes.

Direct and interactive effects of enemies and mutualists on plant performance: a meta-analysis

Abstract: Plants engage in multiple, simultaneous interactions with other species; some (enemies) reduce and others (mutualists) enhance plant performance. Moreover, effects of different species may not be independent of one another; for example, enemies may compete, reducing their negative impact on a plant. The magnitudes of positive and negative effects, as well as the frequency of interactive effects and whether they tend to enhance or depress plant performance, have never been comprehensively assessed across the many published studies on plant-enemy and plant-mutualist interactions. We performed a meta-analysis of experiments in which two enemies, two mutualists, or an enemy and a mutualist were manipulated factorially. Specifically, we performed a factorial meta-analysis using the log response ratio. We found that the magnitude of (negative) enemy effects was greater than that of (positive) mutualist effects in isolation, but in the presence of other species, the two effects were of comparable magnitude. Hence studies evaluating single-species effects of mutualists may underestimate the true effects found in natural settings, where multiple interactions are the norm and indirect effects are possible. Enemies did not on average influence the effects on plant performance of other enemies, nor did mutualists influence the effects of mutualists. However, these averages mask significant and large, but positive or negative, interactions in individual studies. In contrast, mutualists ameliorated the negative effects of enemies in a manner that benefited plants; this overall effect was driven by interactions between pathogens and belowground mutualists (bacteria and mycorrhizal fungi). The high frequency of significant interactive effects suggests a widespread potential for diffuse rather than pairwise coevolutionary interactions between plants and their enemies and mutualists. Pollinators and mycorrhizal fungi enhanced plant performance more than did bacterial mutualists. In the greenhouse (but not the field), pathogens reduced plant performance more than did herbivores, pathogens were more damaging to herbaceous than to woody plants, and herbivores were more damaging to crop than to non-crop plants (suggesting evolutionary change in plants or herbivores following crop domestication). We discuss how observed differences in effect size might be confounded with methodological differences among studies.

Mycorrhizal species differentially alter plant growth and response to herbivory.

Abstract: Plants simultaneously interact with multiple organisms which can both positively and negatively affect their growth. Herbivores can reduce plant growth through loss of plant biomass and photosynthetic area, while plant mutualists, such as mycorrhizal fungi, can increase plant growth through uptake of essential nutrients. This is the first study examining whether species-specific associations with mycorrhizal fungi alter plant tolerance to herbivory. We grew Plantago lanceolata plants with three species of mycorrhizal fungi previously shown to have differential impacts on plant growth and subjected them to herbivory by the specialist lepidopteran herbivore, Junonia coenia. Association with mycorrhizal fungus Glomus white provided the greatest growth benefit but did not alter plant response to herbivory. Alternatively, association with Archaeospora trappei provided less growth promotion but did lead to tolerance to herbivory in the form of an increased growth rate. Finally, an association with the fungus Scutellospora calospora led to neither plant growth promotion nor tolerance to herbivory. In fact, an association with S. calospora appeared to reduce plant tolerance to herbivory. An association with all three species of mycorrhizae resulted in a pattern of growth similar to that of plants grown only with Glomus white, suggesting that growth promotion by multiple mycorrhizal species is driven by the inclusion of a "super fungus," in this case, Glomus white. This work illustrates that plant response to herbivory depends upon the mycorrhizal fungal mutualist with which a plant is associated.

Isolation, culture, and detection of arbuscular mycorrhizal fungi.

Abstract: This chapter summarizes methods to (i) isolate and estimate numbers of soilborne propagules of arbuscular mycorrhizal (AM) fungi, (ii) propagate AM fungi by traditional and innovative methods, and (iii) detect and assess properties of these fungi by using recent biochemical and molecular technology. Soilborne propagules of AM fungi may include chlamydospores or azygospores, colonized roots, and hyphae. Isolates of special interest should be given a unique isolate code and then classified to the species level at a later date. To observe AM structures within the root, it is necessary to clear cortical cells of cytoplasm and phenolic compounds and then to differentially stain the fungal tissue. The most commonly used methods to obtain an estimate of the total number of propagules are the most probable number (MPN) and infectivity assays. The culture of AM fungi on plants in disinfested soil, using spores, roots, or infested soil as inocula, has been the most frequently used technique for increasing propagule numbers. Conducive environmental conditions for cultures of AM fungi are a balance of high light intensity, adequate moisture, and moderate soil temperature without detrimental additions of fertilizers or pesticides. Biochemical methods have been used to improve the means of detection and quantification of AM fungi in the environment. Although current molecular methods improve our ability to detect AM fungi in the field, monitoring of the abundance and distribution of individual fungal species remains laborious and expensive.