Historical contingency in species interactions: towards niche-based predictions.

The way species affect one another in ecological communities often depends on the order of species arrival. The magnitude of such historical contingency, known as priority effects, varies across species and environments, but this variation has proven difficult to predict, presenting a major challenge in understanding species interactions and consequences for community structure and function. Here, we argue that improved predictions can be achieved by decomposing species' niches into three components: overlap, impact and requirement. Based on classic theories of community assembly, three hypotheses that emphasise related, but distinct influences of the niche components are proposed: priority effects are stronger among species with higher resource use overlap; species that impact the environment to a greater extent exert stronger priority effects; and species whose growth rate is more sensitive to changes in the environment experience stronger priority effects. Using nectar-inhabiting microorganisms as a model system, we present evidence that these hypotheses complement the conventional hypothesis that focuses on the role of environmental harshness, and show that niches can be twice as predictive when separated into components. Taken together, our hypotheses provide a basis for developing a general framework within which the magnitude of historical contingency in species interactions can be predicted

Forager bees (Apis mellifera) highly express immune and detoxification genes in tissues associated with nectar processing.

Pollinators, including honey bees, routinely encounter potentially harmful microorganisms and phytochemicals during foraging. However, the mechanisms by which honey bees manage these potential threats are poorly understood. In this study, we examine the expression of antimicrobial, immune and detoxification genes in Apis mellifera and compare between forager and nurse bees using tissue-specific RNA-seq and qPCR. Our analysis revealed extensive tissue-specific expression of antimicrobial, immune signaling, and detoxification genes. Variation in gene expression between worker stages was pronounced in the mandibular and hypopharyngeal gland (HPG), where foragers were enriched in transcripts that encode antimicrobial peptides (AMPs) and immune response. Additionally, forager HPGs and mandibular glands were enriched in transcripts encoding detoxification enzymes, including some associated with xenobiotic metabolism. Using qPCR on an independent dataset, we verified differential expression of three AMP and three P450 genes between foragers and nurses. High expression of AMP genes in nectar-processing tissues suggests that these peptides may contribute to antimicrobial properties of honey or to honey bee defense against environmentally-acquired microorganisms. Together, these results suggest that worker role and tissue-specific expression of AMPs, and immune and detoxification enzymes may contribute to defense against microorganisms and xenobiotic compounds acquired while foraging

Whose Phenotype is it Anyway? The Complex Role of Species Interactions and Resource Availability in Determining Plant Defense Phenotype and Community Consequences.

The expression of plant defense is influenced by resource availability and biotic interactions, with consequences for herbivores and plant fitness. While the majority of plants associate with mycorrhizal fungi, which dramatically affect plant resource status, the role of these belowground interactions in shaping the expression of plant defense is poorly understood. In addition, plant-herbivore interactions affect plant growth and defense, but their effects on mycorrhizal interactions can vary dramatically. I hypothesized that changes in plant resource status and subsequent defense expression may mediate the interactions between mycorrhizal fungi and aboveground herbivores. Drawing from current knowledge of resource mutualisms, I hypothesized that the carbon costs and nutrient benefits of hosting mycorrhizal fungi would predict a nonlinear effect of mycorrhizae on the expression of plant defense. An experimental manipulation of the abundance and identity of mycorrhizal fungi associating with Asclepias syriaca revealed mycorrhizal colonization nonlinearly affected the expression of plant defense, although the shape of the response to increasing fungal colonization depended on the plant trait examined. In particular, traits (eg. trichomes, plant biomass) that increased with the concentration of phosphorus responded unimodally to mycorrhizal colonization as predicted, while those traits that were putatively carbon-limited (eg. latex and toughness) declined with fungal colonization. I also manipulated carbon available to plants and examined changes in plant defense and the effects of herbivores on mycorrhizal fungi. Growth under elevated CO2 increased plant biomass by 15% and toughness by 40%, but decreased cardenolide concentration by 20% and had little effect on trichome density. Herbivory by either aphids or caterpillars had no effect on mycorrhizal colonization when plants were grown in ambient CO2, but herbivory dramatically increased mycorrhizal colonization under elevated CO2. Taken together, these results indicate that fungi and aboveground herbivores interact through changes in plant resource status and defense phenotype and exert strong influence on the expression of plant defense phenotype. In addition, these experiments revealed substantial genetic variation within a single population of A. syriaca in the expression of plant defense and in response to mycorrhizal colonization and carbon addition, indicating the potential for evolutionary adaptation to changing environmental conditions.Ph.D.Ecology and Evolutionary BiologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/86373/1/raleva_1.pd

Mycorrhizal abundance affects the expression of plant resistance traits and herbivore performance

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98805/1/jec12111.pd

Orthogonal fitness benefits of nitrogen and ants for nitrogen‐limited plants in the presence of herbivores

Predictable effects of resource availability on plant growth‐defense strategies provide a unifying theme in theories of direct anti‐herbivore defense, but it is less clear how resource availability modulates plant indirect defense. Ant‐plant‐hemipteran interactions produce mutualistic trophic cascades when hemipteran‐tending ants reduce total herbivory, and these interactions are a key component of plant indirect defense in most terrestrial ecosystems. Here we conducted an experiment to test how ant‐plant‐hemipteran interactions depend on nitrogen (N) availability by manipulating the presence of ants and aphids under different N fertilization treatments. Ants increased plant flowering success by decreasing the densities of herbivores, and the effects of ants on folivores were positively related to the density of aphids. Unexpectedly, N fertilization produced no changes in plant N concentrations. Plants grown in higher N grew and flowered more, but aphid honeydew chemistry stayed the same, and neither the density of aphids nor the rate of ant attraction per aphid changed with N addition. The positive effects of ants and N addition on plant fitness were thus independent of one another. We conclude that N was the plant’s limiting nutrient and propose that addition of the limiting nutrient is unlikely to alter the strength of mutualistic trophic cascades.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/139927/1/ecy2013_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/139927/2/ecy2013-sup-0002-AppendixS2.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/139927/3/ecy2013-sup-0003-AppendixS3.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/139927/4/ecy2013-sup-0001-AppendixS1.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/139927/5/ecy2013-sup-0006-AppendixS6.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/139927/6/ecy2013.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/139927/7/ecy2013-sup-0007-AppendixS7.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/139927/8/ecy2013-sup-0004-AppendixS4.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/139927/9/ecy2013-sup-0005-AppendixS5.pd

Plant‐derived differences in the composition of aphid honeydew and their effects on colonies of aphid‐tending ants

In plant–ant–hemipteran interactions, ants visit plants to consume the honeydew produced by phloem‐feeding hemipterans. If genetically based differences in plant phloem chemistry change the chemical composition of hemipteran honeydew, then the plant's genetic constitution could have indirect effects on ants via the hemipterans. If such effects change ant behavior, they could feed back to affect the plant itself. We compared the chemical composition of honeydews produced by Aphis nerii aphid clones on two milkweed congeners, Asclepias curassavica and Asclepias incarnata , and we measured the responses of experimental Linepithema humile ant colonies to these honeydews. The compositions of secondary metabolites, sugars, and amino acids differed significantly in the honeydews from the two plant species. Ant colonies feeding on honeydew derived from A. incarnata recruited in higher numbers to artificial diet, maintained higher queen and worker dry weight, and sustained marginally more workers than ants feeding on honeydew derived from A. curassavica . Ants feeding on honeydew from A. incarnata were also more exploratory in behavioral assays than ants feeding from A. curassavica . Despite performing better when feeding on the A. incarnata honeydew, ant workers marginally preferred honeydew from A. curassavica to honeydew from A. incarnata when given a choice. Our results demonstrate that plant congeners can exert strong indirect effects on ant colonies by means of plant‐species‐specific differences in aphid honeydew chemistry. Moreover, these effects changed ant behavior and thus could feed back to affect plant performance in the field. The role of indirect effects in trait evolution remains poorly understood. We show that plant chemical traits indirectly affect ant colony fitness and behavior via direct interactions with aphids. These plant‐derived effects on ant behavior could feed back to affect plant fitness in the field.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/109587/1/ece31277.pd