Joshua G. Harrison

Increasing neonicotinoid use and the declining butterfly fauna of lowland California

The butterfly fauna of lowland Northern California has exhibited a marked decline in recent years that previous studies have attributed in part to altered climatic conditions and changes in land use. Here, we ask if a shift in insecticide use towards neonicotinoids is associated with butterfly declines at four sites in the region that have been monitored for four decades. A negative association between butterfly populations and increasing neonicotinoid application is detectable while controlling for land use and other factors, and appears to be more severe for smaller-bodied species. These results suggest that neonicotinoids could influence non-target insect populations occurring in proximity to application locations, and highlights the need for mechanistic work to complement long-term observational data.

Species with more volatile population dynamics are differentially impacted by weather

Climatic variation has been invoked as an explanation of population dynamics for a variety of taxa. Much work investigating the link between climatic forcings and population fluctuation uses single-taxon case studies. Here, we conduct comparative analyses of a multi-decadal dataset describing population dynamics of 50 co-occurring butterfly species at 10 sites in Northern California. Specifically, we explore the potential commonality of response to weather among species that encompass a gradient of population dynamics via a hierarchical Bayesian modelling framework. Results of this analysis demonstrate that certain weather conditions impact volatile, or irruptive, species differently as compared with relatively stable species. Notably, precipitation-related variables, including indices of the El Niño Southern Oscillation, have a more pronounced impact on the most volatile species. We hypothesize that these variables influence vegetation resource availability, and thus indirectly influence population dynamics of volatile taxa. As one of the first studies to show a common influence of weather among taxa with similar population dynamics, the results presented here suggest new lines of research in the field of biotic–abiotic interactions.

The Many Dimensions of Diet Breadth: Phytochemical, Genetic, Behavioral, and Physiological Perspectives on the Interaction between a Native Herbivore and an Exotic Host.

From the perspective of an herbivorous insect, conspecific host plants are not identical, and intraspecific variation in host nutritional quality or defensive capacity might mediate spatially variable outcomes in plant-insect interactions. Here we explore this possibility in the context of an ongoing host breadth expansion of a native butterfly (the Melissa blue, Lycaeides melissa) onto an exotic host plant (alfalfa, Medicago sativa). We examine variation among seven alfalfa populations that differed in terms of colonization by L. melissa; specifically, we examined variation in phytochemistry, foliar protein, and plant population genetic structure, as well as responses of caterpillars and adult butterflies to foliage from the same populations. Regional patterns of alfalfa colonization by L. melissa were well predicted by phytochemical variation, and colonized patches of alfalfa showed a similar level of inter-individual phytochemical diversity. However, phytochemical variation was a poor predictor of larval performance, despite the fact that survival and weight gain differed dramatically among caterpillars reared on plants from different alfalfa populations. Moreover, we observed a mismatch between alfalfa supporting the best larval performance and alfalfa favored by ovipositing females. Thus, the axes of plant variation that mediate interactions with L. melissa depend upon herbivore life history stage, which raises important issues for our understanding of adaptation to novel resources by an organism with a complex life history.

Beyond annual and seasonal averages: using temporal patterns of precipitation to predict butterfly richness across an elevational gradient

1. Ecologists often make predictions about community richness and diversity using climate variables that include seasonal precipitation totals and mean daily temperatures. While means and totals can be effective predictors to a certain extent, the complexities of faunal–climate relationships might be over‐simplified through the use of coarse‐grained variables.

Vertical stratification of the foliar fungal community in the world’s tallest trees

PREMISE OF THE STUDY The aboveground tissues of plants host numerous, ecologically important fungi, yet patterns in the spatial distribution of these fungi remain little known. Forest canopies in particular are vast reservoirs of fungal diversity, but intracrown variation in fungal communities has rarely been explored. Knowledge of how fungi are distributed throughout tree crowns will contribute to our understanding of interactions between fungi and their host trees and is a first step toward investigating drivers of community assembly for plant-associated fungi. Here we describe spatial patterns in fungal diversity within crowns of the worlds tallest trees, coast redwoods (Sequoia sempervirens). METHODS We took a culture-independent approach, using the Illumina MiSeq platform, to characterize the fungal assemblage at multiple heights within the crown across the geographical range of the coast redwood. KEY RESULTS Within each tree surveyed, we uncovered evidence for vertical stratification in the fungal community; different portions of the tree crown harbored different assemblages of fungi. We also report between-tree variation in the fungal community within redwoods. CONCLUSIONS Our results suggest the potential for vertical stratification of fungal communities in the crowns of other tall tree species and should prompt future study of the factors giving rise to this stratification.

Modern approaches to study plant–insect interactions in chemical ecology

Phytochemical variation among plant species is one of the most fascinating and perplexing features of the natural world and has implications for both human health and the functioning of ecosystems. A key area of research on phytochemical variation has focused on insects that feed on plants and the enormous diversity of plant-derived compounds that reduce or deter damage by insects. Empirical studies on the ecology and evolution of these chemically mediated plant–insect interactions have been guided by a long history of theoretical development. However, until recently, such theory was substantially limited by inadequate data, a situation that is rapidly changing as ecologists partner with chemists utilizing the latest technological advances. In this Review, we aim to facilitate the union of ecological theory with modern chemistry by discussing important theoretical frameworks for studying chemical ecology and outlining the steps by which hypotheses on insect–phytochemical interactions can be advanced using current methodologies and statistical approaches. We highlight unique approaches to isolation, synthesis, spectroscopy, metabolomics and genomics relevant to chemical ecology and describe future areas for research that will bring an unprecedented understanding of phytochemical variation.The union of theory in chemical ecology with modern methods in chemistry has enhanced our understanding of phytochemical variation among and within plants. This Review outlines these theoretical frameworks and approaches for hypothesis testing, with a focus on chemically mediated plant–insect interactions.

Host plant associated enhancement of immunity and survival in virus infected caterpillars

Understanding the interaction between host plant chemistry, the immune response, and insect pathogens can shed light on host plant use by insect herbivores. In this study, we focused on how interactions between the insect immune response and plant secondary metabolites affect the response to a viral pathogen. Based upon prior research, we asked whether the buckeye caterpillar, Junonia coenia (Nymphalidae), which specializes on plants containing iridoid glycosides (IGs), is less able to resist the pathogenic effects of a densovirus infection when feeding on plants with high concentrations of IGs. In a fully factorial design, individuals were randomly assigned to three treatments, each of which had two levels: (1) exposed to the densovirus versus control, (2) placed on a plant species with high concentrations of IGs (Plantago lanceolata, Plantaginaceae) versus low concentrations of IGs (P. major), and (3) control versus surface sterilized to exclude surface microbes that may contribute to viral resistance. We measured phenoloxidase (PO) activity, hemocyte counts, and gut bacterial diversity (16S ribosomal RNA) during the fourth larval instar, as well as development time, pupal weight, and survival to adult. Individuals infected with the virus were immune-suppressed (as measured by PO response and hemocyte count) and developed significantly faster than virus-free individuals. Contrary to our predictions,mortality was significantly less for virus challengedindividuals reared on the high IG plant compared to the low IG plant.This suggests that plant secondary metabolites can influence survival from viral infection and may be associated with activation of PO. Removing egg microbes did not affect the immune response or survival of the larvae. In summary, these results suggest that plant secondary metabolites are important for survival against a viral pathogen. Even though the PO response was better on the high IG plant, the extent to which this result contributes to survival against the virus needs further investigation.

Synchronous population dynamics in California butterflies explained by climatic forcing

A long-standing challenge for population biology has been to understand why some species are characterized by populations that fluctuate in size independently, while populations of other species fluctuate synchronously across space. The effects of climatic variation and dispersal have been invoked to explain synchronous population dynamics, however an understanding of the relative influence of these drivers in natural populations is lacking. Here we compare support for dispersal- versus climate-driven models of interspecific variation in synchrony using 27 years of observations of 65 butterfly species at 10 sites spanning 2750 m of elevation in Northern California. The degree of spatial synchrony exhibited by each butterfly species was used as a response in a unique approach that allowed us to investigate whether interspecific variation in response to climate or dispersal propensity was most predictive of interspecific variation in synchrony. We report that variation in sensitivity to climate explained 50% of interspecific variation in synchrony, whereas variation in dispersal propensity explained 23%. Sensitivity to the El Niño Southern Oscillation, a primary driver of regional climate, was the best predictor of synchrony. Combining sensitivity to climate and dispersal propensity into a single model did not greatly increase model performance, confirming the primacy of climatic sensitivity for driving spatial synchrony in butterflies. Finally, we uncovered a relationship between spatial synchrony and population decline that is consistent with theory, but small in magnitude, which suggests that the degree to which populations fluctuate in synchrony is of limited use for understanding the ongoing decline of the Northern California butterfly fauna.

Draft alfalfa genome assembly

FASTA file contains the draft alfalfa genome described in the paper, which is organized in scaffolds. The assembly was generated with ALLPATHS-LG. The shell script used to run the assembly is included as a ReadMe file.