A study in natural ecosystems comparing forest and savanna found species richness to be affected by habitat and strata ; the two environments clearly differentiated in terms of their species composition . In our study, canopy vegetation was not a strong driver for the community of twig-nesting ants since our best models did not include a VCI. However, species compositional differences observed across both vegetation layers could be an effect of microhabitat diversity and canopy connectivity . Providing complex vegetation not only promotes ant diversity but also other organisms that facilitate ant colonization into new twigs. Presumably ants often nest in hollow branches of trees that have been previously dwelled or inhabited by beetles . Moreover diversity of trees might also provide nesting resources that are different in terms of how difficult or attractive they are to dig cavities, for example studies have found that tropical woods can be different in terms of their structure, chemistry and biology ; this could suggest important drivers in the differentiation of ants that inhabit them. We found a large number of arboreal twig-nesting ant species in this coffee agroecosystem study supporting the notion that managed ecosystems, such as agroforestry systems in the tropics, blueberries in containers have the potential to host a great diversity of species. A number of previous studies have provided evidence that ant diversity increases control of pests and fungal diseases . We document here that increases in nest entrance size diversity on an individual tree relates to increases in ant diversity on trees. This may thus have important implications for promoting ants as biological control agents in agroforestry systems.
We conclude that the availability of a variety of nesting options and vegetation strata are important drivers of species diversity and support the idea that niche partitioning drives species coexistence . Future studies should further investigate the competitive hierarchies of the species colonizing twigs if we want to understand how species using similar resources interact with each other; and evaluate colony fitness in face of multiple resource use, as has been done in the past for colonies of Cephalotes persimilis . Since ants often engage in interactions that deliver ecosystem services future studies should focus on evaluating roles of different ant combinations using a diverse array of twig entrance sizes in agricultural pest control. Furthermore, we have learned from this study that the structuring of ant communities is multi-factorial and that local as well as regional factors should be considered when explaining species assemblages in the tropics. Habitat complexity is critical for the functioning of ecological communities in both terrestrial and aquatic systems. Processes such as resource foraging, colonization, and species interactions often depend on the level of heterogeneity in the configuration of physical elements in a habitat . Vegetation connectivity and structure are important components of habitat complexity and can influence species interactions and community patterns at local scales. In aquatic systems, more complex habitats made up of macrophytes support communities that are more diverse and abundant, and allow for greater food capture than systems without vegetation . In terrestrial systems, vegetation structure– such as the biomass of foliage and the variety of plant architectures– generally influences species composition, and increases species richness and abundance of numerous taxa . Additionally, vegetation structure can influence mobility and foraging success of vertebrates and invertebrates . In tropical ecosystems, ants are among the most abundant and bio-diverse of taxonomic groups , and are considered important predators, herbivores, and seed dispersers .
Ants are cursorial central-place foragers – organisms that forage from a central place to which they return with food to feed with the colony . Therefore, foraging and discovery of food resources is strongly constrained by the need to construct and follow trails along vegetation . This is particularly relevant for ants using the arboreal stratum as their primary foraging space . For instance, the availability of vegetation connections can maximize ants’ foraging efficiency, locomotion, and velocity , as well as contribute to changes in community composition and species richness . The availability of such resources can ultimately lead to differences in resource utilization by ant communities . In tropical agricultural systems, especially agroforests, ants play important ecological roles , and management practices can strongly influence ant behavior and their potential for providing biological pest control services . Indeed, one of the oldest known records of the use of ants for pest control dates to 304 A.D in citrus plantations in China. In these systems artificial connections made of bamboo were used by farmers to facilitate foraging by the Weaver Ant to suppress damaging phytophagous insects . In that same study, Huang and Yan report anecdotal evidence that suggests equal yields in orchards that use chemicals vs. orchards that use ant bridges to control for pests. Similarly, Peng et al. , report lower levels of fruit damage in cashew with the presence of weaver ants. However, as vegetation complexity declines in agroecosystems, tree density and diversity may also decrease , as well as the possibility to generate connections between the arboreal vegetation, which might impact arthropod populations . The lack of connectivity between trees in managed systems can have a significant impact on the mobility of worker ants and their ability to control resources. This impact may be particularly marked at greater distances from the nest, where ant dominance may be lower . This in turn may influence the ecosystem services provided by ants, particularly the suppression of pest outbreaks .
Shaded coffee plantations, which maintain high levels of shade and structural complexity can sustain complex networks of organisms, which can result in biological pest control . In coffee systems, ants are a functionally diverse and abundant group of ground and arboreal-nesting arthropods and are considered important biological control agents . Ants are predators of the most devastating coffee pest, the coffee berry borer , a beetle that drills cavities in coffee berries and severely damages the seed . Several species of arboreal ants, with nests attached to or inside tree trunks, branches, or twigs, control adult and immature stages of this pest either through direct predation or deterrence . Ants of the genus Azteca are numerically dominant in shaded coffee plantations. These ants forage intensively on coffee plants , and deter CBB adults by removing them from the coffee plant, therefore lowering fruit damage . In shaded coffee plantations, Azteca sericeasur ants nest on shade trees and access adjacent coffee plants through the leaf litter or available pathways, such as fallen branches, vines, and other vegetation , matching the description by Longino for this species in forest habitats. In more intensively managed coffee systems, with fewer and more distant nesting trees, connectivity may be sparse or absent and artificial connections might buffer against this loss. Vegetation structure and arboreal characteristics in coffee plantations are likely to be important factors influencing ant foraging behavior and nesting in arboreal ants . However, the influence of vegetation connectivity on the foraging of this dominant arboreal ant, and its effect on pest removal in coffee plantations has not yet been studied. Previous work has documented the importance of arboreal connections for ants and biological control in agricultural systems. For example, various studies and farmers’ manuals suggest that connecting nests to adjacent trees using bamboo strips enables weaver ants to colonize new trees, which increases ants’ efficiency in removing pests, including the pentatomid insect Tesserarotoma papillosa . However, there is little evidence about the effect of increasing arboreal connectivity on biological control using experimental data. We report an experiment testing the influence of adding connections between shade trees and coffee plants and its effects on CBB removal on coffee plants. To our knowledge, planting blueberries in pots this is the first study providing experimental data on the effect of adding connectivity on ant activity and pest removal in coffee agroecosystems. Specifically, we tested one hypothesis: connectivity affects CBB removal in this system by increasing recruitment rates of A. sericeasur ants to prey items; we predicted that 1) A. sericeasur ants use artificial connections between nesting trees and coffee plants; 2) plants with connectivity have higher ant activity than isolated plants; 3) plants with connections have grater removal rates of CBB by A. sericeasur ants; and 4) A. sericeasur activity and CBB removal rates by A. sericeasur ants decrease with increased distance from A. sericeasur nests. Within the farm, we haphazardly selected 20 non-overlapping sites located at least 10 m away from each other with one Inga micheliana tree containing an A. sericeasur carton nest on the tree trunk .
A. sericeasur is a polydomous, arboreal ant species , which occurs in ~13% of trees at our study site , and forages on coffee plants . Trees were selected only if ant nests were noticeably active. In each site, we quantified ant activity on the nest tree as the number of ants crossing a single point on the main trunk during one minute. This methodology has been used in previous studies to measure overall ant activity of a nest . We then selected the six coffee plants nearest to the nesting tree, making sure they were not directly touching each other or the tree by removing branches and vines . We then randomly assigned three of the coffee plants at each site to aconnection treatment and three as controls without connections, then measured ant activity on the plants by counting the number of ants passing a point on the central trunk for one minute. We connected treatment coffee plants to the nesting tree using jute string . Strings remained in the field for three days to allow for ant acclimation to disturbance and for ants to establish new foraging pathways. After three days, we returned to the sites and remeasured ant activity on the nesting tree and coffee plants. Observations took place between 10 am and 1 pm, and were immediately stopped as soon as it started raining, as this drastically decreases ant activity.To test how connectivity impacts potential biological control provided by ants, we added dead adult CBB onto connected and control coffee plants to directly assess ant removal rates. We collected CBB infested coffee berries from the field, dissected them, extracted female adult CBB individuals , and placed them in the freezer for up to 24 h, after which beetles were dead. Three days after placing strings and after reassessing ant activity, we placed 10 dead CBB adults on a small piece of white card on each coffee plant near the center of the trunk, left cards for 30 min, and then counted the number of CBB remaining. Restricting movement of sentinel prey, either by gluing them to observation sites or freezing them is a common technique for assessing predator behavior . We used frozen sentinel prey to increase the availability and similarity of beetles on cards and to reduce the potential for live prey to escape from the arena. To assess whether CBB removal was due to ant activity, we monitored cards across the plot over a period of 30 minutes and recorded any arthropods present. Only ants were observed on the cards, indicating that these were responsible for removing the CBB. Although we acknowledge that the use of dead prey may alter ant behavior, it is already well established that A. sericeasur both antagonizes and predates live CBB in the field, and reduces CBB infestation on plants . We used dead prey in this experiment to more readily assess ant removal rates and infer that these changes translate to changes in the bio-control efficiency of this ant on live prey. Immediately following each experiment, we characterized the vegetation in each site because several different environmental factors are known to influence ant foraging in coffee systems . We measured the percentage of canopy cover , coffee plant height, and distance from each coffee plant to the central Inga nest-tree.To test for statistical differences in ant activity on coffee plants before and after establishing connections we used a GLMM. We included time , treatment , coffee plant distance to nest tree, the interaction between time and treatment, and the interaction between time and distance as fixed effects . We also included coffee plant height and ant activity on nest tree as covariates.