Tropical forests experience a relatively stable climate, but are not thermally uniform. The tropical forest canopy is hotter and thermally more variable than the understory. Heat stress in the canopy ...is expected to increase with global warming, potentially threatening its inhabitants. Here, we assess the impact of heating on the most abundant tropical canopy arthropods—ants. While foragers can escape hot branches, brood and workers inside twig nests might be unable to avoid heat stress. We examined nest choice and absconding behavior—nest evacuation in response to heat stress—of four common twig-nesting ant genera. We found that genera nesting almost exclusively in the canopy occupy smaller cavities compared to
Camponotus
and
Crematogaster
that nest across all forest strata.
Crematogaster
ants absconded at the lowest temperatures in heating experiments with both natural and artificial nests.
Cephalotes
workers were overall less likely to abscond from their nests. This is the first test of behavioral thermoregulation in tropical forest canopy ants, and it highlights different strategies and sensitivities to heat stress. Behavioral avoidance is the first line of defense against heat stress and will be crucial for small ectotherms facing increasing regional and local temperatures.
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•We evaluated the effect of sucrose consumption on insect critical thermal maxima.•Starvation reduced and sucrose diet increased upper thermal limit of a canopy ant.•These changes ...were unrelated to the trophic position and total body phosphorus.•Dietary carbohydrates enable thermal plasticity of some ants.•Insects relying on a carbohydrate diet might have an advantage in a warming world.
While adaptive responses to climate gradients are increasingly documented, little is known about how individuals alter their upper thermal tolerances. Long-term increases in dietary carbohydrates can elevate upper thermal tolerances in insects. We explored how the nutritional state of a Neotropical canopy ant governs its CTmax – the temperature at which individuals lose muscle control. We predicted that Azteca chartifex workers recently fed a carbohydrate-rich diet, such as honeydew and extrafloral nectar, would use that energy to increase their CTmax. Moreover, if a carbohydrate-rich diet increases CTmax, then we predicted that ants from colonies with high CTmaxs feed at a lower trophic level, and thus have a higher carbon:nitrogen ratio.
We used A. chartifex colonies from a long-term fertilization experiment where phosphorus addition increased A. chartifex foraging activity with respect to controls. As foraging activity can be governed by resource availability, we first measured CTmax of field collected colonies. In freshly collected field colonies, CTmax was 2°C higher in control plots. This difference disappeared when ants were provided with only water for 10h. Ants were then provided with a 10% sucrose solution ad lib which increased CTmax by 5°C. We thus support the hypothesis that enhanced carbohydrate nutrition enables higher thermal tolerance, but this does not appear to be linked to colony trophic status, higher carbon:nitrogen ratios, or higher total body phosphorus.
This short-term thermal plasticity linked to carbohydrate nutrition demonstrates the importance of ant diet in shaping their physiological traits. It is especially relevant to ant species that maintain high abundance by feeding on plant exudates. In a rapidly warming world, carbohydrate availability and use may represent a new element for predicting population and community responses of herbivorous insects.
Urban green spaces can have an important role in biodiversity conservation. However, they are not often a focus of biodiversity studies, although their global area is raising. We investigated the ...impact of habitat characteristics of urban green spaces on an ecologically important group of insects—ants (Hymenoptera: Formicidae). We tested three questions: 1) does ant species richness positively correlate with park size and the extent of wooded areas? 2) Is ground temperature the best predictor of ant activity? 3) Will communities be dominated by thermophilic, dry-tolerant, generalist species? Using pitfall traps, hand collecting, and baiting we sampled 7595 ants belonging to 30 species, across eight localities in urban protected areas of the city of Zagreb, Croatia. Parks with larger wooded areas had high species richness, but park size was not a good predictor of species richness. Ant activity was largely influenced by temperature. Five out of eight localities had similar ant community composition. The remaining three localities each had unique ant communities with no overlap with any other locality. Overall, in addition to typical urban species, we recorded species of conservation interest. Thus, small urban green spaces have the capacity to support and conserve diverse ant communities, but their habitat characteristics need to be considered in urban planning.
We present a meta-analysis of plant responses to fertilization experiments conducted in lowland, species-rich, tropical forests. We also update a key result and present the first species-level ...analyses of tree growth rates for a 15-yr factorial nitrogen (N), phosphorus (P), and potassium (K) experiment conducted in central Panama. The update concerns community-level tree growth rates, which responded significantly to the addition of N and K together after 10 yr of fertilization but not after 15 yr. Our experimental soils are infertile for the region, and species whose regional distributions are strongly associated with low soil P availability dominate the local tree flora. Under these circumstances, we expect muted responses to fertilization, and we predicted species associated with low-P soils would respond most slowly. The data did not support this prediction, species-level tree growth responses to P addition were unrelated to species-level soil P associations. The meta-analysis demonstrated that nutrient limitation is widespread in lowland tropical forests and evaluated two directional hypotheses concerning plant responses to N addition and to P addition. The meta-analysis supported the hypothesis that tree (or biomass) growth rate responses to fertilization are weaker in old growth forests and stronger in secondary forests, where rapid biomass accumulation provides a nutrient sink. The meta-analysis found no support for the long-standing hypothesis that plant responses are stronger for P addition and weaker for N addition. We do not advocate discarding the latter hypothesis. There are only 14 fertilization experiments from lowland, species-rich, tropical forests, 13 of the 14 experiments added nutrients for five or fewer years, and responses vary widely among experiments. Potential fertilization responses should be muted when the species present are well adapted to nutrient-poor soils, as is the case in our experiment, and when pest pressure increases with fertilization, as it does in our experiment. The statistical power and especially the duration of fertilization experiments conducted in old growth, tropical forests might be insufficient to detect the slow, modest growth responses that are to be expected.
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.
Abstract Human activities are rapidly changing biogeochemistry across the globe, yet little is known about biogeochemical impacts on higher‐level consumers. In a Panamanian rainforest, we measured ...the effects of chronic nitrogen, phosphorus, and potassium fertilization on ants: hyper‐abundant terrestrial arthropods and ecosystem engineers. We tested two compatible hypotheses: the nutrient limitation hypothesis—where adding a limiting nutrient increases ant activity and abundance; and the community homogenization hypothesis—where adding a limiting nutrient decreases ant diversity. Lowland tropical rainforests are expected to be phosphorus‐limited, so we predicted higher ant activity but lower diversity on phosphorus plots. In each fertilization plot, we baited trees and lianas to attract both canopy and ground nesting ants. After controlling for temperature, which accounted for roughly 20% of the variation in ant foraging activity, ant activity remained higher on phosphorus addition plots than on any other fertilization treatment. Genus level diversity was 16% lower on plots receiving phosphorus than the control, consistent with the paradox of enrichment frequently observed in plant communities. This pattern, however, did not hold for species level diversity. The community‐level response was largely driven by the most abundant genus, Azteca , which increased foraging activity and abundance across phosphorus plots. The high activity and low diversity of ants on experimental phosphorus plots point to the potentially strong influence of biogeochemistry on these ubiquitous insects with potential ramifications for the forest food web. This is, to our knowledge, the first study relating biogeochemistry of macronutrients to foraging activity, diversity, and abundance of consumers, implicating strong bottom‐up structuring of the ant community in one of the world's most diverse ecosystems.
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 critical thermal maximum (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.
Desiccation resistance, the ability of an organism to reduce water loss, is an essential trait in arid habitats. Drought frequency in tropical regions is predicted to increase with climate change, ...and small ectotherms are often under a strong desiccation risk. We tested hypotheses regarding the underexplored desiccation potential of tropical insects. We measured desiccation resistance in 82 ant species from a Panama rainforest by recording the time ants can survive desiccation stress. Species' desiccation resistance ranged from 0.7 h to 97.9 h. We tested the desiccation adaptation hypothesis, which predicts higher desiccation resistance in habitats with higher vapor pressure deficit (VPD) – the drying power of the air. In a Panama rainforest, canopy microclimates averaged a VPD of 0.43 kPa, compared to a VPD of 0.05 kPa in the understory. Canopy ants averaged desiccation resistances 2.8 times higher than the understory ants. We tested a number of mechanisms to account for desiccation resistance. Smaller insects should desiccate faster given their higher surface area to volume ratio. Desiccation resistance increased with ant mass, and canopy ants averaged 16% heavier than the understory ants. A second way to increase desiccation resistance is to carry more water. Water content was on average 2.5% higher in canopy ants, but total water content was not a good predictor of ant desiccation resistance or critical thermal maximum (CTmax), a measure of an ant's thermal tolerance. In canopy ants, desiccation resistance and CTmax were inversely related, suggesting a tradeoff, while the two were positively correlated in understory ants. This is the first community level test of desiccation adaptation hypothesis in tropical insects. Tropical forests do contain desiccation‐resistant species, and while we cannot predict those simply based on their body size, high levels of desiccation resistance are always associated with the tropical canopy.
This is the first community level test of desiccation resistance for tropical ants from microhabitats that varied in potential desiccation stress. Desiccation resistance did not scale as predicted with body size. Canopy ants have higher desiccation resistance than understory ants, but experience a tradeoff between desiccation resistance and CTmax.
Ants (Hymenoptera: Formicidae) are a conspicuous group of ectotherms whose behavior, distribution, physiology, and fitness are regulated by temperature. Consequently, interest in traits like thermal ...tolerance that enable ants to survive and thrive in variable climates has increased exponentially over the past few decades. Here, we synthesize the published literature on the thermal tolerance of ants. We begin our review with discussion of common metrics: critical thermal limits, lethal thermal limits, knock-down resistance, chill-coma recovery, and supercooling. In particular, we highlight the ways each thermal metric is quantified and offer a set of methodological caveats for consideration. We next describe patterns and hypotheses for ant thermal tolerance along spatial and temporal temperature gradients. Spatially, we focus on relationships with latitude, elevation, urbanization, and microclimate. Temporally, we focus on seasonal plasticity, daily variation, dominance-thermal tolerance tradeoffs, and acclimation. We further discuss other sources of variation including evolutionary history, body size, age, castes, and nutrition. Finally, we highlight several topics of interest to ant thermal biologists, ranging in scope from methods development to the impacts of climate change.
Aim
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.
Location
Continental USA.
Time period
May–August 2016 and May–August 2017.
Major taxa studied
Ants (Hymenoptera:Formicidae).
Methods
We used ramping assays to measure CTmax and CTmin for 132 species of North American ants across 31 communities spanning 15.7° of latitude.
Results
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; R2 = .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.
Main conclusions
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.
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