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Theory predicts that bottom-up ecological forces can affect community dynamics, but whether this extends to the effects of heritable plant variation on tritrophic communities is poorly understood. In a field experiment, I contrasted the effects of plant genotype (28 genotypes; 1064 plants), aphid density, and the presence/absence of mutualistic ants in affecting the per capita population growth of a specialist aphid herbivore, as well as the effects of plant genotype on the third trophic level. Plant genotype strongly affected aphid population growth rate, explaining 29% of the total variation in growth rate, whereas aphid density and ant-aphid interactions explained substantially less variation (

In an experiment that directly manipulated grassland plant species richness and composition, decreased plant species richness ('diversity') increased pathogen load (the percentage of leaf area infected by species-specific foliar pathogens across the plant community) in 1998. Pathogen load was almost three times greater in the average monoculture than in the average plot planted with 24 grassland plant species, an approximately natural diversity. Eleven individual diseases increased in severity (percentage of leaf area infected by a single disease) at lower plant species richness, and severity of only one disease was positively correlated with diversity. For 10 of the 11 diseases whose severity was negatively related to diversity, disease severity was positively correlated with host abundance, and in six of these cases, species diversity had no effect on disease severity after controlling for the effects of host abundance. These results suggest that increased abundances of individual host species at lower species diversity increased disease transmission and severity. In 1996 and 1997, similar results for a smaller number of diseases sampled were found in this experiment and another similar one. Although the effect of diversity on disease was highly significant, considerable variance in pathogen load remained among plots of a given diversity level. Much of this residual variance was explained by community characteristics that were a function of the species composition of the communities (the identity of species present vs. those lost). Specifically, communities that lost less disease-prone species had higher pathogen loads; this effect explained more variance in pathogen load than,did diversity. Also, communities that lost the species dominant at high diversity had higher pathogen loads, presumably because relaxed competition allowed greater increases in host abundances, but this effect was weak. Among plant species, disease proneness appeared to be determined more by regional than local processes, because it was better correlated with frequency of the plant species' populations across the region than with local abundance or frequency across the state. In total, our results support the hypothesis that decreased species diversity will increase foliar pathogen load if this increases host abundance and, therefore, disease transmission. Additionally, changes in community characteristics determined by species composition will strongly influence pathogen load. Key words: biodiversity and ecosystem functioning; functional composition; fungal pathogens; global change; habitat simplification; host density; macroecology; parasites; plant community; plant pathogens; polyculture vs. monoculture; species richness.

Populations of feral donkeys, Equus asinus, in northern Australia are limited by density-dependent mortality during the first 6 mo of individuals' lives. A female's ability to raise her offspring successfully increases with age and is dependent on her maintaining high levels of stored mineral nutrients (calcium, phosphorus, and sodium). A low proportion of offspring was successfully raised in a population at carrying capacity where females had low levels of stored mineral nutrients. Females in a growing population that had been reduced to 45% of carrying capacity had high levels of stored minerals and successfully raised a higher proportion of offspring. Females in the population at carrying capacity ingested a species-poor diet (predominantly a grass, Sehima nervosa) containing low levels of nitrogen and mineral nutrients, and high levels of crude fibre. Females in the growing population ingested a more species-rich diet with higher levels of nitrogen and minerals, and less crude fibre. Levels of calcium, phosphorus, and potassium in the faeces of females in the population at carrying capacity were higher than those in the food ingested. The reverse was true for females in the growing population. At the time of sampling (early dry season), 17% of females in the population at carrying capacity and 0% of females in the growing population were eating or had recently eaten clay. Increased salivary secretion and gastrointestinal irritation caused by high-fibre diets (even when lubricated by clay) may be responsible for mineral depletion of females at carrying capacity, and hence population limitation through the inability of females to raise offspring.

The escape hypothesis predicts that isolated individuals of a given plant species will escape their specialized herbivores and so survive better than individuals growing in clumps. This phenomenon could promote plant coexistence. I examined assumptions underlying the hypothesis by considering insect herbivores associated with neotropical Passiflora vines. I found that the hypothesis was probably of only limited value because plants were attacked by a range of herbivore species, most of which were not really specialized. Furthermore, some oligophagous herbivore species colonized isolated P. pittieri more readily than they colonized plants that were near to other P. pittieri plants: isolated plants did not escape herbivory. Because of the activities of herbivores that feed on > 1 plant species, [greater than or equal to] 2 plant species that share herbivores may not show independent patterns of herbivory or independent local distributions.

Interactions between plants and soil affect plant-plant interactions and community composition. Festuca arundinacea (tall fescue) is often infected with the mutualistic fungal endophyte Neotyphodium coenophialum. Infected (E+) F. arundinacea has been reported to inhibit soil pathogens, nematodes, and mycorrhizal fungi, and to have greater mineral uptake from the soil relative to uninfected (E-) F. arundinacea. In addition, endophyte presence may indirectly influence the soil by altering the plant-plant interactions and the composition of communities. Recent studies have shown that local plant species diversity was reduced in experimental field plots with E+ F. arundinacea compared to plots with E- plants. Endophyte-induced changes, direct or indirect, in biotic or abiotic soil properties could therefore affect subsequent plant growth. We investigated soil-mediated effects of E+ F. arundinacea on E+ and E- F. arundinacea, Trifolium repens, and Plantago lanceolata by growing each in soil collected from the aforementioned field plots seeded with either E+ or E- F. arundinacea. Infection status of F. arundinacea plots had no direct effect on any measured characteristic of plants grown in soil from those plots. However, responses of experimental plants suggest that, by inducing changes in plant community composition, E+ F. arundinacea may indirectly affect the soil. Most notable was an increase in E+ F. arundinacea total biomass when grown in soil previously dominated by Poa pratensis and other grass species compared to biomass when grown in soil previously dominated by E+ F. arundinacea, indicative of an indirect negative feedback on E+ F. arundinacea growth. In addition, Plantago and Trifolium root biomass was significantly higher when grown in soil that previously supported a high proportion of P. pratensis relative to other plant species. Such plant-soil dynamics may have important community-level implications in areas dominated by F. arundinacea. Key words: community structure; endophyte; Festuca arundinacea; negative feedback; Neotyphodium coenophialum; Plantago lanceolata; plant-soil interaction; Poa pratensis; soil community; Trifolium repens.

The hypothesis that individual plants undergo predictable changes in their resistance to herbivory as they mature was experimentally tested. The performance of two species of insects transferred onto different-aged trees of the same naturally occurring clones of narrowleaf cottonwood, Populus angustifolia, showed opposing and significant changes in host resistance as a function of tree age. The gall-formin aphid Pemphigus betae was 70 times as common on mature trees as on juvenile trees. Transfer experiments demonstrated that this pattern is adaptive; survivorship on mature trees was 50% higher than on juvenile trees. The leaf-feeding battle Chrysomela confluens exhibited opposite distribution on host, with densities 400 times as high on juvenile as on mature trees. This is also adaptive; larvae transferred to mature hosts had 50% lower survival and took 25% longer to reach adulthood than those transferred to juvenile trees. These survivorship and performance data and our inability to support the hypotheses of other mechanisms that would produce the observed distributions suggest that the distributions result from the insects' preferences for different-aged hosts. Several basic implications emerge. (1) Development changes in resistance and susceptibility of hosts are important components in determining the distribution of herbivores. (2) A single plant can change rapidly in its resistance traits with age such that a 10-fold change in resistance can occur over a 2-yr period. (3) As trees mature, increased and decreased resistance to different herbivores can produce nonoverlapping herbivore distributions that could be misinterpreted as being the result of competition (e.g., in this study, beetle abundance was strongly, negatively correlated with aphid abundance). (4) While host maturation processes have large effects on within- and between-tree variation in resistance to herbivores, the implications for herbivore population dynamics and community structuring have not been generally appreciated.