Biology subjects

Halsch, C. A.

Publications and source records attributed to Halsch, C. A..

8 recordsLinked to original sources

bioRxiv2025

Allochronic isolation between sympatric populations of an alpine butterfly

The role of isolation by distance in contributing to population structure across space is well understood, and the same principles should operate over time. Allochrony, the isolation of populations due to separation in time, can be a source of genetic differentiation, but remains infrequently documented between yearly asynchronous populations. In this study, we investigate divergence between two putative sympatric populations of the alpine butterfly Oeneis chryxus ivallda at Castle Peak, CA, USA. We find clear genetic differentiation between butterflies collected in odd and even years, with limited instances of admixture. The observed GST of 0.05 between the two populations is approximately equivalent to 26 km based on pairwise GST values observed between O. chryxus populations across space. These results demonstrate a potential source of population differentiation in systems that promote prolonged development, often found in high-elevation and high-latitude environments.

evolutionary biology↗

bioRxiv2025

Dispersal mediates metapopulation response to local and regional stressors

Many populations face multiple Anthropogenic threats simultaneously, and as a result, we have observed loss of biodiversity worldwide. Moreover, population stressors act at different spatial scales, and the understanding of variation in outcomes for species will depend on their dispersal ability. In this study, we developed a spatially explicit metapopulation simulation to investigate how stressors that act at different spatial scales interact with landscape composition and dispersal behavior to drive patterns of metapopulation extirpations. We are particularly interested in gaining insight into the decline of not only range-limited species, but also widespread butterflies that have been reported in recent years, contrary to conventional wisdom about traits that make species more at risk of population decline. We found that stressors acting at different spatial scales interact with dispersal, especially in highly developed landscapes. On average, being less dispersive produces worse outcomes because more dispersive species benefit from semi-natural habitats, which can strengthen connections between source populations. At the same time, it is the degradation of these types of land in particular that may disproportionately impact dispersive species. These findings enhance our understanding of insect biodiversity loss and demonstrate that the conservation of widespread insects will likely require consideration of larger-scale landscape connectivity.

ecology↗

bioRxiv2025

Temporal occupancy distributions reveal multifaceted and heterogeneous effects of climatic variation on montane butterflies

Climate change has substantially altered the phenology of many organisms, with profound implications for biodiversity and species interactions. However, phenological studies of insects have often focused on shifts over time, without explicitly examining how climate drives these changes. In addition, the metrics used to measure changes in phenology often capture only limited information about the flight period. In this study, we analyzed multidecadal observational and climate data using a hierarchical Bayesian framework to model the annual probability of occurrence distributions for 135 butterfly species across five montane sites along an elevational gradient. Our analysis used polynomial models that account for shifts in abundance and the timing and length of flight periods to investigate the effect of climate on butterfly phenology. We found that spring maximum and minimum temperatures, as well as winter precipitation, are important predictors of butterfly phenology. High winter precipitation delayed phenology at high-elevation sites where substantial snowfall occurs, while increased spring maximum temperatures generally advanced phenology across all elevations. Even modest increases in spring minimum temperatures caused substantial shift in phenology. We documented variability in the effect of climate on phenology across sites, among species within a site and among populations of the same species across different sites with variability among species within a site being especially pronounced. We also found that climate influences different aspects of the flight period differently (e.g., timing versus duration), underscoring the need to move beyond single metrics such as day of first flight. These findings highlight the importance of examining the entire flight period and considering the interplay of species-specific traits to improve predictions of how climate change impacts phenology. Such approaches might be essential for designing more targeted and effective conservation strategies in response to ongoing climate change.

ecology↗

bioRxiv2022

Assessing risk for butterflies in the context of climate change, demographic uncertainty, and heterogenous data sources

AbstractOngoing declines in insect populations have led to substantial concern and calls for conservation action. However, even for relatively well-studied groups, like butterflies, information relevant to species-specific status and risk is scattered across field guides, the scientific literature, and agency reports. Consequently, attention and resources have been spent on a miniscule fraction of insect diversity, including a few well-studied butterflies. Here we bring together heterogenous sources of information for 396 butterfly species to provide the first regional assessment of butterflies for the 11 western US states. For 184 species, we use monitoring data to characterize historical and projected trends in population abundance. For another 212 species (for which monitoring data are not available, but other types of information can be collected), we use exposure to climate change, development, geographic range, number of host plants, and other factors to rank species for conservation concern. A phylogenetic signal is apparent, with concentrations of declining and at-risk species in the families Lycaenidae and Hesperiidae. A geographic bias exists in that many species that lack monitoring data occur in more southern states where we expect that impacts of warming and drying trends will be most severe. Legal protection is rare among the taxa with the highest risk values: of the top 100 species, one is listed as threatened under the US Endangered Species Act and one is a candidate for listing. Among the many taxa not currently protected, we highlight a short list of species in decline, including Vanessa annabella, Thorybes mexicanus, Euchloe ausonides, and Pholisora catullus. Notably, many of these species have broad geographic ranges, which perhaps highlights a new era of insect conservation in which small or fragmented ranges will not be the only red flags that attract conservation attention.

ecology↗

bioRxiv2022

Lethal and sub-lethal concentrations of pesticides detected in milkweeds purchased from nurseries across the United States

The decline of monarch butterflies in both the eastern and western United States has garnered widespread public interest. Planting milkweeds, their larval host plants, has been promoted as one action individuals can take, but little is known with respect to potential pesticide contamination of store-bought milkweeds. In this study, we collected leaf samples from 235 milkweed plants purchased at 33 retail nurseries across the US to screen for pesticides. Across all samples, we detected 61 different pesticides with an average of 12.2 ({+/-}5.0) compounds per plant. While only 9 of these compounds have been experimentally tested on monarch caterpillars, 38% of samples contained a pesticide above a concentration shown to have a sub-lethal effect for monarchs. We detected only a modest predictive ability of retailer size and milkweed species; and plants with labels advertising their value for wildlife did not have fewer pesticides at concentrations known to have a negative effect on monarchs. These results demonstrate the extensiveness of pesticide exposure within nursery milkweeds and the potential impacts on monarchs and other insects exposed to store-bought plants. HighlightsO_LIMilkweeds were collected from stores in the United States and screened for pesticides. C_LIO_LIWe detected multiple pesticides in every milkweed plant sampled. C_LIO_LIOver one third of samples contained a pesticide at a known harmful concentration for monarchs. C_LIO_LIPlants labeled as wildlife-friendly did not have fewer potentially harmful compounds. C_LI

ecology↗

bioRxiv2020

Insects and recent climate change

Insects have diversified through 400 million years of Earths changeable climate, yet recent and ongoing shifts in patterns of temperature and precipitation pose novel challenges as they combine with decades of other anthropogenic stressors including the conversion and degradation of land. Here we consider how insects are responding to recent climate change, while summarizing the literature on long-term monitoring of insect populations in the context of climatic fluctuations. Results to date suggest that climate change impacts on insects have the potential to be considerable, even when compared to changes in land use. The importance of climate is illustrated with a case study from the butterflies of Northern California, where we find that population declines have been severe in high-elevation areas removed from the most immediate effects of habitat loss. These results shed light on the complexity of montane-adapted insects responding to changing abiotic conditions and raise questions about the utility of temperate mountains as refugia during the Anthropocene. We consider methodological issues that would improve syntheses of results across long-term insect datasets and highlight directions for future empirical work. Significance statementAnthropogenic climate change poses multiple threats to society and biodiversity, and challenges our understanding of the resilience of the natural world. We discuss recent ideas and evidence on this issue and conclude that the impacts of climate change on insects in particular have the potential to be more severe than might have been expected a decade ago. Finally, we suggest practical measures that include the protection of diverse portfolios of species, not just those inhabiting what are currently the most pristine areas.

ecology↗

bioRxiv2020

Pesticide contamination of milkweeds across the agricultural, urban, and open spaces of low elevation Northern California

Monarch butterflies (Danaus plexippus) are in decline in the western United States and are encountering a range of anthropogenic stressors. Pesticides are among the factors that likely contribute to this decline, though the concentrations of these chemicals in non-crop plants is not well documented, especially in complex landscapes with a diversity of crop types and land uses. In this study, we collected 227 milkweed (Asclepias spp.) leaf samples from 19 sites representing different land use types across the Central Valley of California. We also sampled plants purchased from two stores that sell to home gardeners. We found 64 pesticides (25 insecticides, 27 fungicides, and 11 herbicides, as well as 1 adjuvant) out of a possible 262 in our screen. Pesticides were detected in every sample, even at sites with little or no pesticide use based on information from landowners. On average, approximately 9 compounds were detected per plant across all sites, with a range of 1 to 25 compounds in any one sample. For the vast majority of pesticides detected, we do not know the biological effects on monarch caterpillars that consume these plants, however we did detect a few compounds for which effects on monarchs have been experimentally investigated. Chlorantraniliprole in particular was identified in 91% of our samples and found to exceed a tested LD50 for monarchs in 58 out of 227 samples. Our primary conclusion is the ubiquity of pesticide presence in milkweeds in an early-summer window of time that monarch larvae are likely to be present in the area. Thus, these results are consistent with the hypothesis that pesticide exposure could be a contributing factor to monarch declines in the western United States. This both highlights the need for a greater understanding of the lethal and sublethal effects of these compounds (individually, additively, and synergistically) and suggests the urgent need for strategies that reduce pesticide use and movement on the landscape. Contribution to the FieldInsects are facing multifaceted stressors in the Anthropocene and are in decline in many parts of the world. The widespread use of pesticides is believed to be an important part of the problem. In particular, the monarch butterfly is in sharp decline in the western United States. Here we show that milkweeds in the Central Valley of California, a large urban and agricultural landscape that is part of the monarch breeding and migration route, are contaminated with a diverse array of pesticides. We found a few in high concentrations and many in trace amounts. We do not know how these compounds act together and with other large-scale stressors to cause declines, but it is clear that monarchs and other non-target insects are encountering these pesticides. These results provide critical insight into the growing literature on the impact of pesticides on butterflies specifically and non-target insects more broadly. We hope these field realistic concentrations will aid in the design of further experiments in the field and the lab.

ecology↗

bioRxiv2019

A winner in the Anthropocene: changing host plant distribution explains geographic range expansion in the gulf fritillary butterfly

O_LIThe changing climate is altering the geographic distributions of species around the world with consequences for population dynamics, resulting in winners and losers in the Anthropocene.\nC_LIO_LIAgraulis vanillae, the gulf fritillary butterfly, has expanded its range in the past one hundred years in the western United States. We combine time series analysis with species distribution modeling to investigate factors limiting the distribution of A. vanillae and to predict future shifts under warming scenarios.\nC_LIO_LIIn the western US, where we have time series and geographic data, urban development has a positive influence (the host plant is an ornamental in gardens), being associated with year of colonization. Colonization was also associated to a lesser extent with winter maximum temperatures, while a negative impact of minimum temperatures and precipitation was apparent on population growth rates.\nC_LIO_LIAcross the country, the butterfly is primarily limited by host availability and positively affected by human presence. Perhaps counter-intuitively for a largely tropical ectotherm, current effects of a warming climate (in the years post-colonization) are either negative (on population growth rate) or indirect, likely mediated through availability of areas that can support the host.\nC_LIO_LIUnder future climate scenarios, conditions are predicted to become more suitable for Passiflora in many urban areas, which would likely result in further expansion of A. vanillae during the dispersive season. These results illustrate the value of combining time series with spatial modeling to understand and predict shifting geographic ranges in the Anthropocene.\nC_LI

ecology↗