Jonathan Z. Shik

More food, less habitat: how necromass and leaf litter decomposition combine to regulate a litter ant community

1. In brown food webs of the forest floor, necromass (e.g. insect carcasses and frass) falling from the canopy feeds both microbes and ants, with the former decomposing the homes of the latter. In a tropical litter ant community, we added necromass to 1 m2 plots, testing if it added as a net food (increasing ant colony growth and recruitment) or destroyer of habitat (by decomposing leaf litter).

Towards a nutritional ecology of invasive establishment: aphid mutualists provide better fuel for incipient Argentine ant colonies than insect prey

Many potential species invasions fail before establishment. This is likely especially true for invasive Argentine ants that must overcome a severe founding bottleneck and transition from propagules that rely on protein-rich prey to massive supercolonies that dominate by consuming carbohydrate-rich honeydew from hemipteran mutualists. While this dietary shift supports the classic idea that protein fuels early colony development and carbohydrates maintain adult workers, recent evidence suggests that carbohydrates can govern initial colony establishment. In this study, we use lab experiments to show that resources from aphid mutualists had greater benefits for Argentine ant propagule survival, maintenance, growth, and worker activity rates than did prey items. These effects persisted at low aphid densities, and when colonies were otherwise starved. Moreover, prey-starved colonies did not appear to consume aphids, suggesting that carbohydrate-rich honeydew is a mechanism that facilitates colony establishment. Combined, these results support a hypothesis that the dietary shift from prey to honeydew is driven more by increased access to hemipterans after establishment, than by specific benefits of prey early in colony development. The results highlight the important role of nutritional ecology for studying invasive establishment, linking propagule success not only to the supply of food resources, but also to their quality.

The metabolic costs of building ant colonies from variably sized subunits

Ant colonies are superorganisms with emergent traits that, for some species, reflect the combined activity of physically distinct worker castes. Although larger castes have high production costs, they are thought to save their colonies energy by efficiently performing specialized tasks. However, because workers are generally idle until sensing specific stimuli, their maintenance costs may be an important component of colony-level investment. I used metabolic scaling to examine the maintenance costs of dimorphic major and minor Pheidole castes across levels of colony organization (e.g., individual, group, and colony). Majors from three species had lower mass-specific metabolic rates than minors because of allometries at both individual and group levels and subsequently lived longer when starved. Thus, large major castes may offset their production costs in both their idle and active states. The slope scaling metabolic rate from incipient to reproductive colonies of Pheidole dentata (∼colony mass0.89) fell between the slopes for minor groups (∼group mass1.04) and major groups (∼group mass0.79) and appears to reflect developmental shifts in subunit mass and number and their offsetting effects on per capita energy demands. These results highlight how metabolic scaling may help visualize the energetic correlates of emergent behavior and unravel the mechanisms governing colony organization.

Towards a general life-history model of the superorganism: predicting the survival, growth and reproduction of ant societies.

Social insect societies dominate many terrestrial ecosystems across the planet. Colony members cooperate to capture and use resources to maximize survival and reproduction. Yet, when compared with solitary organisms, we understand relatively little about the factors responsible for differences in the rates of survival, growth and reproduction among colonies. To explain these differences, we present a mathematical model that predicts these three rates for ant colonies based on the body sizes and metabolic rates of colony members. Specifically, the model predicts that smaller colonies tend to use more energy per gram of biomass, live faster and die younger. Model predictions are supported with data from whole colonies for a diversity of species, with much of the variation in colony-level life history explained based on physiological traits of individual ants. The theory and data presented here provide a first step towards a more general theory of colony life history that applies across species and environments.

The life history continuum hypothesis links traits of male ants with life outside the nest

An ant society, headed by a mated queen, can live for decades. Male ants, in contrast, are generally assumed to be ephemeral sperm delivery vessels programmed to die hours after leaving the nest to mate. However, the events from dispersal to mate location have rarely been studied, and the links between male traits and the ecological demands of diverse mating systems remain poorly understood. Here, we propose that interspecific variation in the length of mating flights has generated a life history continuum for male ants, and that the previously proposed ‘male aggregating’ and ‘female calling’ mating syndromes represent the endpoints. We also provide the first evidence for systematic divergence in pre‐mating traits between males that attract females to brief nuptial swarms (Male aggregation syndrome) and those that must survive while searching for patchily distributed females that signal with pheromones (Female calling syndrome). Specifically, female‐calling males tend to have larger eyes and mandibles, but the length of the basal antennal segment (scape) appears relatively constant across body sizes. After exploring these patterns, we review evidence that key components of fitness like mating frequency vary across a male life history continuum, and then explore links between male traits and a colonys per capita reproductive investment. Systematic variation in pre‐flight provisioning of males relative to mating systems may have important ecological implications, given that ants are dominant consumers on a global scale, and colonies ultimately use large fractions of harvested resources to fuel reproduction.

Scaling community structure: how bacteria, fungi, and ant taxocenes differentiate along a tropical forest floor

Taxa with smaller individuals tend to have shorter generation times and higher local abundance and diversity. The scaled specialization hypothesis (SSH) posits that taxocenes of smaller individuals should differentiate more rapidly and thoroughly along physiochemical gradients of a given age and extent. In a Panama rainforest, we evaluated how bacteria, fungi, and ants responded to two such gradients: one topographic and the other arising from nine years of NPK fertilization. Terminal restriction fragment length polymorphism (T-RFLP) delineated bacteria and fungi operational taxonomic units (OTUs); traditional taxonomy delineated the ants. Bacteria had higher local species richness than fungi and ants (averaging 48 vs. 30 vs. 6 OTUs in < 0.25 m2). Bacteria OTUs were also more widely distributed (17% of OTUs were found on > or = 50% of sample plots compared to 3% for fungi and ants). Consistent with SSH, bacterial composition differed across short-term (+N and +P) and long-term (topographic) gradients; fungal taxocenes differed only along the long-term gradient; and ant taxocenes were homogenous across both. Body size can help predict community responses to a changing environment.

Metabolism and the Rise of Fungus Cultivation by Ants

Most ant colonies are comprised of workers that cooperate to harvest resources and feed developing larvae. Around 50 million years ago (MYA), ants of the attine lineage adopted an alternative strategy, harvesting resources used as compost to produce fungal gardens. While fungus cultivation is considered a major breakthrough in ant evolution, the associated ecological consequences remain poorly understood. Here, we compare the energetics of attine colony-farms and ancestral hunter-gatherer colonies using metabolic scaling principles within a phylogenetic context. We find two major energetic transitions. First, the earliest lower-attine farmers transitioned to lower mass-specific metabolic rates while shifting significant fractions of biomass from ant tissue to fungus gardens. Second, a transition 20 MYA to specialized cultivars in the higher-attine clade was associated with increased colony metabolism (without changes in garden fungal content) and with metabolic scaling nearly identical to hypometry observed in hunter-gatherer ants, although only the hunter-gatherer slope was distinguishable from isometry. Based on these evolutionary transitions, we propose that shifting living-tissue storage from ants to fungal mutualists provided energetic storage advantages contributing to attine diversification and outline critical assumptions that, when tested, will help link metabolism, farming efficiency, and colony fitness.

Diet specialization in an extreme omnivore: nutritional regulation in glucose-averse German cockroaches

Organisms have diverse adaptations for balancing dietary nutrients, but often face trade‐offs between ingesting nutrients and toxins in food. While extremely omnivorous cockroaches would seem excluded from such dietary trade‐offs, German cockroaches (Blattella germanica) in multiple populations have rapidly evolved a unique dietary specialization – an aversion to glucose, the phagostimulant in toxic baits used for pest control. We used factorial feeding experiments within the geometric framework to test whether glucose‐averse (GA) cockroaches with limited access to this critical metabolic fuel have compensatory behavioural and physiological strategies for meeting nutritional requirements. GA cockroaches had severely constrained intake, fat and N mass, and performance on glucose‐based diets relative to wild‐type (WT) cockroaches and did not appear to exhibit digestive strategies for retaining undereaten nutrients. However, a GA × WT ‘hybrid’ had lower glucose aversion than GA and greater access to macronutrients within glucose‐based diets – while still having lower intake and survival than WT. Given these intermediate foraging constraints, hybrids may be a reservoir for this maladaptive trait in the absence of positive selection and may account for the rapid evolution of this trait following bait application.

Effect of Scattered and Discrete Hydramethylnon Bait Placement on the Asian Needle Ant

ABSTRACT The Asian needle ant (Pachycondyla chinensis Emery) is invading natural and disturbed habitats across the eastern United States. While recent studies document the impact of P. chinensis on native ecosystems and human health, effective control measures remain unknown. Thus, we evaluated the field performance of a hydramethylnon granular bait, Maxforce Complete Granular Insect Bait, dispersed in clumps or scattered against P. chinensis. We also measured the effect of this bait on P. chinensis outside of the treatment zone. Surprisingly, unlike reports for other ant species, we achieved nearly complete P. chinensis population reductions 1 d after treatments were applied. Significant ant reductions were achieved until the end of our study at 28 d. No difference was recorded between clumped and scattered application methods. We found no overall difference in ant reductions from the edge out to 5 m beyond the treatment zone. Other local ant species appeared to be unaffected by the bait and foraging activity increased slightly after P. chinensis removal from treated areas. We suggest that Maxforce Complete Granular Insect Bait can be effective in an Asian needle ant treatment program.

Preliminary Assessment of Metabolic Costs of the Nematode Myrmeconema neotropicum on its Host, the Tropical Ant Cephalotes atratus

abstract:  The parasitic nematode Myrmeconema neotropicum infects workers of the neotropical arboreal ant Cephalotes atratus. Infected ants exhibit altered behavior, e.g., reduced aggression and slower tempo, as well as physical traits, e.g., gaster changes from shiny black to bright red. These changes are thought to induce fruit mimicry and attract frugivorous birds, which are the presumed paratenic hosts for the nematodes. We used respirometry to measure the energetic costs of nematode infection, testing the prediction of higher metabolic rates for infected workers maintaining both ant and nematode biomass. Contrary to this prediction, infected workers had lower mass-specific metabolic rates than uninfected workers. Parasites are limited to the gasters (abdomens) of adult ants, and infected gasters had 57% more mass, but 37% lower metabolic rates, compared to uninfected gasters. These results use a metabolic currency to measure, in vivo, the energetic costs of parasitism, and they shed light on the complex co-evolutionary relationship between host and parasite.