NEW PAPER OUT! Thermal tolerance regulates foraging behaviour of ants

2nd pub for 2022 just came out in Ecological Entomology! This work was led by Dr. Diane Roeder and explores how temperature and physiological tolerances regulate foraging behaviour of harvester ants. We started this work many years ago as one of our first collaborations and I am excited to see this first paper out in press. Our abstract sums up our results nicely…

  1. Theory suggests that performance increases with temperature up to an optimization point before rapidly decreasing as an animal approaches its upper thermal limit. Here, we use the red harvester ant, Pogonomyrmex barbatus, to test predictions about how daily temperature fluctuations and thermal tolerance combine to influence one metric of performance—foraging.
  2. Over 2 years, we tracked 322 foraging trips from 15 colonies in a mixed grass prairie of southwestern Oklahoma. During each trip, we measured surface temperature, distance, time, worker mass, seed mass, and foraging tempo (i.e., running speed). To assess P. barbatus heat tolerance, we measured CTmax and knock-down resistance of field-collected workers in the lab.
  3. Trip time, but not distance, decreased with increasing temperature, resulting in an increased foraging tempo as ants neared their CTmax of 50°C. Knock-down resistance trials confirmed that 50°C is an upper thermal limit, as individuals showed decreasing survival from 100% at 45°C to 0% at 50°C. Worker size and collected seed size were unrelated to temperature.
  4. Our results highlight how daily temperature fluctuations drive activity, not only by limiting foraging but also by increasing foraging rates near the thermal limit. If temperatures continue to increase, the foraging ability of this and similar species may be restricted to an ever-narrowing window with effects potentially extending to the surrounding community.

You can find the full manuscript by [CLICKING HERE]. Also be on the lookout for at least two additional papers in the near future on harvester ant biology!

NEW PAPER OUT! Testing the role of body size and litter depth on invertebrate diversity across six forests in North America

Probably the last pub for 2021 or our first for 2022. This paper in Ecology is technically my last “official” dissertation chapter and it feels fantastic to get it out. To provide some background, I started working on this project in the summer of 2014 and spent almost a year measuring >40000 mites, springtails, and spiders from six forests in North America. Using the data from these measurements and species identifications from Brittany, we tested three hypothesis about body size, litter depth, abundance and species richness. Our abstract sums up our results nicely…

“Ecologists search for rules by which traits dictate the abundance and distribution of species. Here we search for rules that apply across three common taxa of litter invertebrates in six North American forests from Panama to Oregon. We use image analysis to quantify the abundance and body size distributions of mites, springtails, and spiders in 21-m2 plots per forest. We contrast three hypotheses: two of which focus on trait-abundance relationships and a third linking abundance to species richness. Despite three orders of magnitude variation in size, the predicted negative relationship between mean body size and abundance per m2 occurred in only 18% of cases—never for large bodied taxa like spiders. We likewise found only 18% of tests supported our prediction that increasing litter depth allows for high abundance; 2/3 of which occurred at a single deciduous forest in Massachusetts. In contrast, invertebrate abundance constrained species richness 76% of the time. Our results suggest that body size and habitat volume in brown food webs are rarely good predictors of variation in abundance, but that variation in diversity is generally well predicted by abundance.

You can find an open access version of our paper by [CLICKING HERE]

NEW PAPER OUT! Thermal traits predict the winners and losers under climate change: an example from North American ant communities

Our next pub for 2021 was just published in Ecosphere. This paper marks one of many papers from our group from our resampling events across the United States that took place in 2017 and 2018. In collaboration with Jelena Bujan, Kirsten de Beurs, Michael Weiser, and Michael Kaspari, we asked which ant genera are increasing or decreasing over the last ~20 years. Here is our abstract that sums up this work nicely…

“Across the globe, temperatures are predicted to increase with consequences for many taxonomic groups. Arthropods are particularly at risk as temperature imposes physiological constraints on growth, survival, and reproduction. Given that arthropods may be disproportionately affected in a warmer climate—the question becomes which taxa are vulnerable and can we predict the supposed winners and losers of climate change? To address this question, we resurveyed 33 ant communities, quantifying 20-yr differences in the incidence of 28 genera. Each North American ant community was surveyed with 30 1-m2 plots, and the incidence of each genus across the 30 plots was used to estimate change. From the original surveys in 1994–1997 to the resurveys in 2016–2017, temperature increased on average 1°C (range, −0.4°C to 2.5°C) and ~64% of ant genera increased in more than half of the sampled communities. To test Thermal Performance Theory’s prediction that genera with higher average thermal limits will tend to accumulate at the expense of those with lower limits, we quantified critical thermal maxima (CTmax: the high temperatures at which they lose muscle control) and minima (CTmin: the low temperatures at which ants first become inactive) for common genera at each site. Consistent with prediction, we found a positive decelerating relationship between CTmax and the proportion of sites in which a genus had increased. CTmin, by contrast, was not a useful predictor of change. There was a strong positive correlation (r = 0.85) between the proportion of sites where a genus was found with higher incidence after 20 yr and the average difference in number of plots occupied per site, suggesting genera with high CTmax values tended to occupy more plots at more sites after 20 yr. Thermal functional traits like CTmax have thus proved useful in predicting patterns of long-term community change in a dominant, diverse insect taxon.”

You can find an open access version of our paper by [CLICKING HERE]

NEW PAPER OUT! Ant thermal tolerance: a review of methods, hypotheses, and sources of variation

Our 2nd pub for 2021 was just published in Annals of the Entomological Society of America. The idea for this review article started years ago at an annual meeting and we have been talking about topics to include ever since. In collaboration with Diane Roeder and Jelena Bujan, we put those ideas on paper. We spent about half a year going through published literature on ant thermal tolerance and found some interesting trends.

Over the past 30 years, there has been exponential growth in the number of ant thermal papers published. Many of these discuss 5 common metrics: critical thermal limits, lethal thermal limits, knock-down resistance, chill-coma recovery, and supercooling. We break down what we think are interesting patterns and hypotheses of thermal tolerance along spatial and temporal temperature gradients, focusing on relationships with latitude, elevation, urbanization, microclimate and ways ants cope with different temperatures like seasonal plasticity and acclimation. We further discuss other sources of variation including evolutionary history, body size, age, castes, and nutrition. To move the field further we highlight several topics that we think are interesting but currently lacking, ranging in scope from methods development to the impacts of climate change.

You can find an open access version of our paper by [CLICKING HERE]

NEW PAPER OUT! Testing effects of invasive fire ants and disturbance on ant communities of the longleaf pine ecosystem

1st pub for 2021 was just published in Ecological Entomology. This manuscript was a few years in the making as Julian Resasco and I originally talked about this back in ~2018. After a few job changes and moves, I am happy to say we finally finished it up!

Broadly, we were interested in how disturbance and invasive fire ant removal affected native ant communities. We took advantage of a framework proposed by MacDougal and Turkington that posited different ways invasive species could be characterized, either as “drivers” or “passengers” of change. The ‘driver’ model posits that species interact strongly and that native species are limited or excluded by competition with invasive species. Under this scenario, removal of an invasive species should increase native species richness and abundance. In contrast, the ‘passenger’ model posits that communities are primarily structured by factors other than interactions with invasive species (e.g. habitat disturbance) and that those factors are more beneficial to invasive species than native species. Under the ‘passenger’ model, removal of invasive species should have relatively little impact on native species.

But we also added a twist, an “interacting driver” model that was proposed as an alternative to the strict interpretations of “driver” and “passenger models” (Didham et al. 2005). The “interacting driver” model suggests additive or synergistic effects of habitat disturbance and invasive species that combine to reduce native species richness and abundance. Under this model, removal or reduction in abundance of invasive species should result in partial recovery of some native species.

To test these different hypotheses, we randomly assigned treatments of (1) unmanipulated, (2) soil disturbance, (3) fire ant removal and (4) soil disturbance + fire ant removal to experimental blocks and measured how ant communities changed over two years in thirty-six 15-m2 plots. In doing so we found some interesting results. First, fire ant abundance in removal plots averaged 42% lower in pitfall traps and 95% lower on baits compared to unmanipulated, control plots (Fig. 1). No difference was found between disturbance and control plots though.

Second, species richness of co-occurring ants also decreased 42% in removal plots (Fig. 2), with significant changes in community composition. And again, no difference was found between disturbance and control plots.

Third, fire ant diet breadth—measured using carbon and nitrogen stable isotopes—increased up to 4.7‐fold in soil disturbance + removal plots. This last result is intriguing as it may suggest fire ants in removal plots consumed a greater variety of prey items and likely competed for resources with more co-occurring species.

Combined, our results suggest fire ants follow an alternative ‘interacting drivers’ model in which partial recovery of some species occurs when populations of an invasive species are reduced. However, further recovery of native ants may be limited by persisting, landscape‐level effects of fire ants suppressing co‐occurring ants below historical levels.

[CLICK HERE FOR PAPER]

NEW PAPER OUT! Bioenergy landscapes drive trophic shifts in generalist ants

The 6th and likely last pub for 2020 was just published in Journal of Animal Ecology. Led by Jackson Helms IV, this paper investigates isotopic and trophic relationships of ants in corn, switchgrass, and restored prairies in Michigan. A summary from Jackson in JAE…

  1. Changes in trophic niche—the pathways through which an organism obtains energy and nutrients—are a fundamental way in which organisms respond to environmental conditions. But the capacity for species to alter their trophic niches in response to global change, and the ways they do so when able, remain largely unknown.
  2. Here we examine food webs in three long‐term and large‐scale experiments to test how resource availability and nutritional requirements interact to determine an organism’s trophic niche in the context of one of the largest global trends in land use—the rise in bioenergy production.
  3. We use carbon and nitrogen stable isotope analyses to characterize arthropod food webs across three biofuel crops representing a gradient in plant resource richness (corn monocultures, fields dominated by native switchgrass and restored prairie), and to quantify changes in the trophic niche of a widespread generalist ant species across habitats. In doing so, we measure the effects of basal resource richness on food chain length, niche breadth and trophic position. We frame our results in the context of two hypotheses that explain variation in trophic niche—the niche variation hypothesis which emphasizes the importance of resource diversity and ecological opportunity, and the optimal diet hypothesis which emphasizes dietary constraints and the availability of optimal resources.
  4. Increasing plant richness lengthened food chains by 10%–20% compared to monocultures. Niche breadths of generalist ants did not vary with resource richness, suggesting they were limited by optimal diet requirements and constraints rather than by ecological opportunity. The ants instead responded to changes in plant richness by shifting their estimated trophic position. In resource‐poor monocultures, the ants were top predators, sharing a trophic position with predatory spiders. In resource‐rich environments, in contrast, the ants were omnivores, relying on a mix of animal prey and plant‐based resources.
  5. In addition to highlighting novel ecosystem impacts of alternate bioenergy landscapes, our results suggest that niche breadth and trophic diversification depend more on the presence of optimal resources than on ecological opportunity alone.

Link: [CLICK HERE TO SEE THE PAPER]

Welcome Jesse!

After a brief delay, we are back. Please welcome Jesse Daniels as the 1st member of our research team! We are incredibly excited to have him join us in Brookings and are looking forward to what the future will hold.

Jesse joins us from the Appalachian Mountains. His prior research experience involved analyzing plant-pollinator interactions and trophic networks to better understand invasive species’ effects on flowering plant communities. Previously, Jesse worked as an adjunct faculty member at Mountain Empire Community College in Big Stone Gap, VA and an environmental interpreter at Natural Tunnel State Park in Duffield, VA. As a family man, Jesse is enthusiastic about sharing new adventures and opportunities with his expecting wife and one year old son.

Website under construction…

Lots of big, exciting changes happening right now! Once we get settled more details will follow, but I think you can guess based on some of the updates here that I have taken a job with USDA as a Research Entomologist. Unfortunately, it may be a while till I get the website formatted to the way I like it. More hopefully soon!

-Karl

Image designed by starline / Freepik (http://www.freepik.com)

NEW PAPER OUT! Thermal diversity of North American ant communities: Cold tolerance but not heat tolerance tracks ecosystem temperature

Jelena

Our second NEON ants and 5th pub for 2020 was just published in Global Ecology and Biogeography. Led by Jelena Bujan, this is our first paper linking physiological traits like critical thermal max and min (CTmax and CTmin ) to different environmental abiotic conditions at a large geographical scale. A summary from Jelena in GEB…

In ectotherms, gradients of environmental temperature can regulate metabolism, development and ultimately fitness. The thermal adaptation hypothesis assumes that thermoregulation is costly and predicts that more thermally variable environments favour organisms with wider thermal ranges and thermal limits (i.e., critical thermal minima and maxima, CTmin and CTmax) which track environmental temperatures. We test the thermal adaptation hypothesis at two biological levels of organization, the community and species level. We used ramping assays to measure CTmax and CTmin for 132 species of North American ants across 31 communities spanning 15.7° of latitude. 

Ants were cold tolerant in cooler environments particularly at the community level where CTmin was positively correlated with the maximum monthly temperature (CTmin = 0.24Tmax − 0.4; R 2 = .39, p  < .001). In contrast, most ant communities included some highly thermophilic species, with the result that CTmax did not covary with environmental temperature means or extremes. Consequently, we found no evidence that thermally variable environments supported ant communities with broader thermal ranges. We found a strong phylogenetic signal in CTmax but not CTmin. Species level responses paralleled community data, where maximum monthly temperatures positively correlated with species CTmin but not CTmax, which was significantly lower in subterranean species.

Our results suggest a large fraction of continental trait diversity in CTmax and CTmin can be found in a given ant community, with species with high CTmax widely distributed regardless of environmental temperature. Species level analyses found the importance of local microclimate and seasonality in explaining thermal tolerances. Frequent invariance in CTmax of insects at a large scale might be caused by (a) local adaptations to a site’s microclimates and (b) species acclimation potential, both of which cannot be accounted for with mean annual temperatures.”

Link: [CLICK HERE TO SEE THE PAPER]

NEW PAPER OUT! Seasonal plasticity of thermal tolerance in ants

Ant Sampling

Great to see another publication out from one of my favorite places to do field work, the University of Oklahoma Biological Station. This is the 4th pub for 2020, which is shaping up to be quite the productive year! Recently published in Ecology, Jelena and I were interested in seeing if thermal tolerance, an important trait for understanding activity patterns of ants, changed over the seasons with temperature. We collected ants across three seasons in Oklahoma across a range of temperatures finding some pretty interesting results. A summary from Jelena…

“Analyses of heat tolerance in insects often suggest that this trait is relatively invariant, leading to the use of fixed thermal maxima in models predicting future distribution of species in a warming world. Seasonal environments expose populations to a wide annual temperature variation. To evaluate the simplifying assumption of invariant thermal maxima, we quantified heat tolerance of 26 ant species across three seasons that vary two‐fold in mean temperature. Our ultimate goal was to test the hypothesis that heat tolerance tracks monthly temperature. Ant foragers tested at the end of the summer, in September, had higher average CTmax compared to those in March and December. Four out of five seasonal generalists—species actively foraging in all three focal months—had, on average, 6°C higher CTmax in September. The invasive fire ant, Solenopsis invicta, was among the thermally plastic species, but the native thermal specialists still maintained higher CTmax than S. invicta. Our study shows that heat tolerance can be plastic, and this should be considered when examining species‐level adaptations. Moreover, the plasticity of thermal traits, while potentially costly, may also generate a competitive advantage over species with fixed traits and promote resilience to climate change.”

Link: [CLICK HERE TO SEE THE PAPER]