THE ROLE OF BIOGEOCHEMISTRY AND CLIMATE IN A NEOTROPICAL ANT COMMUNITY

Human activities are rapidly changing nutrient availability and climate across the globe, and this trend is predicted to continue. The effect of biogeochemistry on consumer communities is underexplored in the world’s most diverse ecosystems – the tropics. Nitrogen (N) and phosphorus (P) are leading candidates in limiting the fitness of arthropods because they are required in substantial quantities as the building blocks of numerous macromolecules, but are relatively scarce in most foods consumed by herbivorous insects. Thus, nutrient limitation of plants may negatively impact herbivores. In addition to changes in nutrient availability, climate is changing. These changes include global increases in temperature and alteration in precipitation regimes across the globe and Neotropical forests are predicted to suffer severe droughts. My work examines the role of both biogeochemistry and climate in Neotropical ants. In Chapter One I investigate the effect of nutrient addition on the ant community of a tropical lowland forest in Panama. I used the Gigante fertilization experiment (GFP), which includes 32, 1600m2 plots that have been fertilized annually with N, P, and potassium (K) since 1998. Plants in tropical lowland rainforests grow on old, weathered soils, and tropical plant growth is typically P limited. Plots fertilized with P fertilization had higher forging activity—number of baits occupied by ants—supporting the nutrient limitation hypothesis. The same plots, however, had lower genus level diversity, consistent with the paradox of enrichment frequently observed in plant communities. Azteca was the only ant genus whose activity was higher on P plots, a pattern largely driven by one species, Azteca chartifex. Chapter Two investigates the potential of Neotropical ants to survive water loss by comparing ant communities in two habitats with different microclimates—the canopy and the litter. The Neotropical canopy was 1° C warmer, and 9 times drier than the litter, and canopy ants had 3 times higher desiccation resistance – the ability to reduce water loss. By exploring mechanisms for the observed difference, I found that smaller ants desiccated faster given their higher surface area to volume ratio, as desiccation resistance increased with ant mass, and canopy ants were, on average, 16% heavier than the understory ants. A second way to increase desiccation resistance is to carry more water. Although water content in canopy ants was on average 2.5% higher, it was not a good predictor of desiccation resistance. Animals experiencing dry conditions are likely experiencing warm conditions as well, so I examined if critical thermal maximum (CTmax), a measure of an ant’s thermal tolerance positively co-varies with desiccation resistance. In canopy ants, desiccation resistance and CTmax were inversely related, suggesting a tradeoff, while the two were positively correlated in litter ants. Chapter Three combines the results from the first two chapters to ask why is A. chartifex successful and dominant in this community, particularly on P plots, and yet has low desiccation resistance and CTmax for a canopy ant. The diet of A. chartifex largely includes honeydew and extra-floral nectar secretions. I predicted that workers of A. chartifex which had recently fed on this carbohydrate-rich diet, will use that energy to increase the upper limits of thermal tolerance. I used A. chartifex colonies from control and P plots from GFP where I had previously recorded higher foraging activity. As foraging activity can be governed by resource availability, I measured CTmax of field collected colonies and found that CTmax was 2° C higher in control plots than P. This difference disappeared when ants were starved. After providing colonies with a 10% sucrose solution, their CTmax increased by 5° C, supporting the hypothesis that carbohydrate nutrition allows higher thermal tolerance. This does not appear to be linked to colony trophic status, higher C:N ratios, or higher total body P. This short-term thermal plasticity linked to carbohydrate nutrition demonstrates the importance of diet in shaping physiological traits. Chapter Four investigates the effect of biogeochemistry on A. chartifex nesting patterns across GFP. I found that N suppressed Azteca nest density, as plots which received N had 48% lower number of nests. Adding P did not affect colony abundance. The addition of both N and P significantly increased number of A. chartifex nests as well as the size of nests. The best predictor of colony size was tree size as larger trees supported larger nests. Azteca chartifex nests were non-randomly distributed, and certain tree species were preferred, despite their low frequency across the forest, while some abundant trees were avoided. Our study suggests that both nutrient availability and forest composition act in concert and help A. chartifex dominate this tropical forest. My research finds that “bottom-up” forces shape this ant community by indirectly affecting the nest density and activity of a numerically and behaviorally dominant ant. In a rapidly warming world, carbohydrate availability and use may represent a fundamental predictor of the population and community responses of herbivorous insects in a changing world

Jedan rod i tri vrste mrava (Hymenoptera : Formicidae) prvi nalazi za Hrvatsku

From the last review of the ant fauna of Croatia (Bračko, 2006), three species and one genus of ants have been recorded for Croatia: Camponotus tergestinus Müller, 1921; Lasius citrinus Emery, 1922; and Proformica pilosiscapa Dlussky, 1969. Finding of the latter represents not just the first record of the genus Proformica Ruzsky, 1902 for Croatia, but also for the broader area of the neighboring countries. Here, we present data on the distribution and biology of all three species. The number of recorded species for Croatia is now 143.Od zadnje revizije vrsta mrava za Hrvatsku (Bračko, 2006), otkrivene su tri nove vrste i jedan novi rod mrava, a to su: Camponotus tergestinus Müller, 1921; Lasius citrinus Emery, 1922; i Proformica pilosiscapa Dlussky, 1969. godine. Pronalazak posljednjeg prvi je nalaz roda Proformica Ruzsky, 1902., za Hrvatsku, ali i za šire područje okolnih zemalja. Za sve tri vrste navodimo podatke o rasprostranjenosti i biologiji. Do sada su za Hrvatsku zabilježene 143 vrste

Plant responses to fertilization experiments in lowland, species-rich, tropical forests.

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

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

Abstract 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

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

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

Species energy and Thermal Performance Theory predict 20-yr changes in ant community abundance and richness

In an era of rapid climate change, and with it concern over insect declines, we used two theories to predict 20-yr changes in 34 North American ant communities. The ecosystems, from deserts to hardwood forests, were first surveyed in the 1990s. When resurveyed in 2016–2017, they averaged 1°C warmer with 200 g C·m−2·yr−1 higher plant productivity. Ant colony abundance changed from −49% to +61%. Consistent with Thermal Performance Theory, colony abundance increased with temperature increases < 1°C, then decreased as a site's mean monthly temperature change increased up to +2.4°C. Consistent with Species Energy Theory, (1) ant abundance tracked changes in a measure of energy availability (net aboveground productivity, g C·m−2·yr−1) and (2) increases in colony abundance drove increases in local plot- and transect-level species richness but not that of Chao 2, an estimate of the size of the species pool. Even after accounting for these drivers, local species richness was still higher ~20 yr after the original surveys, likely due to the increased activity of ant workers. These results suggest community changes are predictable using theory from geographical ecology, and that warming can first enhance but may ultimately decrease the abundance of this important insect taxon

Thermal traits predict the winners and losers under climate change : An example from North American ant communities

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

Can thermoregulatory traits and evolutionary history predict climatic niches of thermal specialists?

Aim: Predictions of future species distributions rest on the assumption that climatic conditions in the current range reflect fundamental niche requirements. So far, it remains unclear to what extent this is true. We tested if three important factors determining fundamental niche—ecophysiology, morphology and evolutionary history—can predict the realized niche, using thermal specialist ants. They are suitable model organisms because their body temperature, metabolism and fitness are closely tied to the habitat temperatures. Location: Iberian Peninsula and Maghreb. Time period: 2013–2015. Major taxa studied: Ants (Hymenoptera:Formicidae). Methods: We measured heat tolerance, chill coma recovery, body size and phylogenetic relationships in 19 desert specialist ants in the genus Cataglyphis to test if these important determinants of fundamental niches are good predictors of species realized niches. We modelled species climatic niches using 19 bioclimatic variables from WorldClim for recorded occurrence of each species. Results: None of the determinants of the species' fundamental niche were linked to their realized climatic niche, modelled using species distribution models. However, both heat tolerance and chill coma recovery were highly correlated with body size and all three thermoregulatory traits were phylogenetically constrained, suggesting they reflect fundamental requirements of each species. Main conclusions: Our results challenge the basic assumption of climatic niche modelling, that the realized niche can be used as a proxy for determining fundamental niche requirements. These findings are particularly concerning for studies that use the species' current realized niche to predict their responses to climate change.Programme de la Famille Sandoz-Monique de Meuron pour la relève universitaire; Spanish Ministry of Economy and Competitiveness and FEDER, Grant/ Award Number: CGL2015-65807; Centre National de la Recherche ScientifiquePeer reviewe