Food source quality and ant dominance hierarchy influence the outcomes of ant-plant interactions in an arid environment

Plant species with larger extrafloral nectaries produce better quality nectar when needed and interact with the best ant partners

Few studies have explored the phenotypic plasticity of nectar production on plant attractiveness to ants. Here, we investigate the role of extrafloral nectary (EFN) size on the productivity of extrafloral nectar in three sympatric legume species. We hypothesized that plant species with larger EFNs (i) have higher induced nectar secretion after herbivory events, and (ii) are more likely to interact with more protective (i.e. dominant) ant partners. We target 90 plants of three Chamaecrista species in the field. We estimated EFN size and conducted field experiments to evaluate any differences in nectar traits before and after leaf damage to investigate the phenotypic plasticity of nectar production across species. We conducted multiple censuses of ant species feeding on EFNs over time. Plant species increased nectar descriptors after leaf damage, but in different ways. Supporting our hypothesis, C. duckeana, with the largest EFN size, increased all nectar descriptors, with most intense post-herbivory-induced response, taking its place as the most attractive to ants, including dominant species. EFN size variation was an excellent indicator of nectar productivity across species. The higher control over reward production in plants with larger sized EFNs reflects an induction mechanism under damage that reduces costs and increases the potential benefits of indirect biotic defences.

The changing dynamics of ant-tree cholla mutualisms along a desert urbanization gradient

Urbanization, among the most widespread and multifaceted anthropogenic change drivers, exerts strong influences on a diversity of ecological communities worldwide. We have begun to understand how urbanization affects species diversity, yet we still have limited knowledge about the ways that species interactions are altered by urbanization. We have an especially poor understanding of how urbanization influences stress-buffering mutualisms, despite the high levels of multivariate stress that urban organisms must overcome and the importance of these interactions to the fitness of many organisms. In this study, we investigated the effects of urbanization on a mutualism between tree cholla cacti (Cylindropuntia imbricata) and visiting ants. We first examined how plant size, ant species composition, and ant activity varied on C. imbricata across an urbanization gradient (urban, suburban, wild) in and around Albuquerque, NM. Ant species composition and activity varied significantly across the urbanization gradient, with ant communities from wildlands having the highest activity and the most dissimilar species composition compared to both suburban and urban sites. In contrast, plant size remained constant regardless of site type. We then experimentally assessed how nectar levels influenced ant aggressive encounters with proxy prey (Drosophila melanogaster larvae) on C. imbricata across urban and wild sites. Ants were more likely to discover, attack, and remove proxy prey in wild sites compared to urban sites; they also performed these behaviors more quickly in wild sites. Nectar supplementation had weaker effects on ant aggression than urbanization, but consistently increased the speed at which aggressive behaviors occurred. Future studies that examine nectar quality and herbivorous arthropod abundance may help explain why this strong difference in ant composition and aggression was not associated with lower plant fitness proxies (i.e. size traits). Nevertheless, this study provides unique insight into the growing body of work demonstrating that mutualisms vary significantly across urbanization gradients.

Extrafloral nectar production induced by simulated herbivory does not improve ant bodyguard attendance and ultimately plant defence

Highly competitive and aggressive ant species are efficient bodyguards that monopolize the more attractive plants bearing extrafloral nectaries. Given that herbivory often increases the quality of extrafloral nectar, we hypothesized that plants damaged by herbivory would be more prone to interact with high-quality ant bodyguards and be better defended against herbivores. We performed an experiment with Chamaecrista nictitans plants. We induced anti-herbivore responses by applying jasmonic acid to a group of plants while keeping another group unmanaged. We measured extrafloral nectar production, censused ants visiting extrafloral nectaries and, subsequently, added herbivore mimics to measure the efficiency of ant anti-herbivore defence in both conditions. Induction increased the volume of extrafloral nectar and the mass of sugar per nectary without affecting the sugar concentration or the patterns of plant attendance and defence by ants. Thus, we found no evidence that defence-induced C. nictitans plants are more prone to interact with high-quality bodyguards or to receive better anti-herbivore defence. These findings highlight that increases in extrafloral nectar production are not always rewarded with increases in the biotic defences; instead, these rewards might be dependent on the traits of the nectar induced by herbivory events and/or on the ecological context in which the interaction is embedded. Consequently, herbivory might increase the costs of this induced biotic defence to plants bearing extrafloral nectaries when the induced defence does not increase the attractiveness of the plants to ants.

Ant social foraging strategies along a Neotropical gradient of urbanization

During the last decades, urbanization has been highlighted as one of the main causes of biodiversity loss worldwide. Among organisms commonly associated with urban environments, ants occupy urbanized green areas and can live both inside and around human settlements. However, despite the increasing number of studies on the ecological dynamics of ant species developed mainly in temperate urban ecosystems, there is still little knowledge about the behavioral strategies that allow ant species to live and even thrive within cities. In this study, we evaluated the role of urbanization in shaping ant communities, including their social foraging, considering built cover as a gradually changing variable that describes an urban gradient. Specifically, we assessed whether species richness, composition, and the proportion of exotic ant species are related to an urban gradient in a medium-sized Neotropical city immersed in a cloud forest context in Mexico. Moreover, we evaluated the social foraging strategies that could promote ant species coexistence in an urban environment. In general, and contrary to our hypothesis, we found no evidence that the built cover gradient affected the richness, composition, or proportion of exotic ant species foraging on food resources, indicating a filtering and simplification of ant communities given by urbanization. Moreover, we show for the first time that urban ant species exhibited a “discovery-defense strategy”, whereby the ant species with the greatest capacity to discover new food resources were those that showed the greatest ability to monopolize it after 120 min of observation, regardless of the type of resource (i.e., tuna or honey bait). Our findings have a direct impact on the knowledge about how urbanization shapes ant communities and behavior, by showing the foraging strategies of ant species that feed on similar food resources present that allows them to coexist in urban environments.

Aggressive bodyguards are not always the best: Preferential interaction with more aggressive ant species reduces reproductive success of plant bearing extrafloral nectaries

Variation in partner species and frequency of interaction between species pairs are potential drivers of the net outcome of generalized mutualisms. In ant-plant mutualisms, the quality of defence provided by ants is related to ant aggressiveness. Hence, we hypothesize that the performance of plants bearing extrafloral nectaries will be higher when they interact more frequently with more aggressive ant species. We estimated ant aggressiveness in the field by observing their behaviour towards soil baits. Afterwards, we observed the frequency with which individuals from these ant species visited plants through an entire reproductive cycle. We measured the production and persistence of plants reproductive structures through this period and the total seed production. Increasing in the interaction frequency with highly aggressive ants reduced the number of floral buds and seeds produced. Increased visitation frequency by less aggressive ants increased the number of floral buds and seeds per branch. The inverse relationship between ant aggressiveness and seed production may be influenced by the costs imposed by different mutualistic partners. Thus, frequent interaction with highly aggressive ants may lead to a higher accumulation of costs through time, resulting in a negative net outcome for the plants. Our results bring new evidence highlighting the importance to incorporate temporal aspects in the study of mutualistic interactions. We suggests that the quality of mutualistic partners must be understood as a function of its per-interaction benefit and their cumulative costs to their partner over time, what puts in check our current classification regarding partner quality in mutualistic systems.