Extensive studies of community assembly of ants in the coffee agroecosystem reveal that both environmental filtering, which is frequently associated with nesting opportunities , and competitive interactions are likely involved . Here we divide that literature into two general groupings, ants that generally nest in the ground or leaf litter, broadly construed, versus ants that nest in hollow cavities, generally referred to as twig-nesting ants. The first, we suggest, are structured largely through competition for feeding resources, whereas the second are structured more through competition for nesting sites.The general spatial patterns seen with ground-foraging ants are frequently thought to result from interspecific competition . Part of the evident spatial structure is the existence of mosaics in which patches of dominant species are arranged in space something like a jigsaw puzzle. Ground-foraging ants on coffee farms seem also to form this mosaic spatial pattern; various species form patches that are relatively discrete, with each species occupying its own space and the patches fitting together almost as if they were pieces in a jigsaw puzzle . Studies in coffee farms suggest that much of the spatial pattern of ground-foraging ants is indeed a consequence of the underlying structure of competitive interactions among the various species , although that structure is not simple.For both arboreal ants that nest in hollow trees or shrub twigs and leaf litter ants that inhabit hollow twigs on the ground, the size, number, or diversity of available twigs may be an important determinant of community structure. For arboreal ants in coffee agroforests, hollow coffee twigs and shade-tree twigs and branches comprise the vast majority of available nest sites, and there can be considerable variation in the size of twigs available. However, in the coffee plants themselves, most ants prefer to nest in similar-sized twigs . Although one larger ant species, Procryptocerus hylaeus, occupied twigs with larger cavities in the field and selected larger twigs in choice experiments in the lab, 30 planter pot nine other species examined showed no difference in the mean nest size occupied .

Nest availability also strongly varies, and competition for nesting resources may be an extremely important determinant of community structure and contribute to community assembly processes in tropical communities. Work in coffee farms has enabled learning the importance of diversity, quality, and size of nesting resources for community assembly. Ants compete for nesting resources, and among indications of nest limitation in ants are the occupation of a large fraction of available resources, takeovers of nests by different colonies or species, and occupation of artificial nests . Coffee agroforests that differ in shade structure,and therefore in the overall availability of nest sites, have been used to demonstrate the importance of nest-site limitation for ants, and the fact that nest-site limitation can contribute to a loss of ant diversity . In a comparison of two different coffee farms varying in the amount of shade, nest-site limitation was stronger in farms with less shade. In addition, increasing the number of artificial nests in high-shade coffee farms resulted in a large increase in the number of species colonizing twig nests, but there were less dramatic increases in richness in low-shade sites . Thus nest limitation, but also other processes must be important drivers of community assembly in large, intensively managed farms. In coffee agroforests in Colombia, nest-site limitation is important for twignesting ants both on coffee plants and in the leaf litter. With a similar technique, Armbrecht et al. added artificial nests to coffee plants and added artificial nests and leaf litter in forests and in three coffee systems varying in shade management to examine the bottom-up influences of resources on ant assembly. Both litter addition and twig addition increased the number of ant colonies, indicating that ant colonies in these habitats are litter- and nest-limited. The numbers of ant species colonizing resources did not increase with litter or twig addition but did increase with increasing shade in coffee farms, a similar result to that found in Mexico . Nest diversity may also be important in determining the numbers of species that can colonize and survive in tropical environments. In Colombia, Armbrecht et al. tested whether the diversity of nesting resources affected colonization by litter-dwelling ants.

They collected branches of eight species of shade trees and drilled holes in the branches so that they would resemble natural hollow twigs found in the leaf litter. They then placed the nests in bags and added the bags to the leaf litter on the ground in either single-species bags, containing eight twigs of the same tree species, or mixed bags, containing one twig of each of eight tree species. They found that the mixed bags were colonized by a significantly greater diversity of ants but not for the reasons expected. Whereas it was expected that different species of ants would select different species of twigs, leading to an increase in ant diversity in mixed bags, they instead found that most ant species preferred the mixed bags, but not due to strong tree-species and ant-species associations. Thus, diversity in a nesting resource greatly influenced ant assembly and led to a more diverse ant community. Ecological interactions with other species, such as dominant ants or parasites, may also contribute to the structure and maintenance of ant communities. The presence of aggressive, dominant ants may influence the foraging by arboreal ants and may also limit their abilities to colonize new areas as well as limit their access to food resources and perhaps survival once they are established. But little experimental work has documented the importance of arrival and survival processes for ant communities . Two interesting studies in coffee farms have documented the importance of aggression from canopy-dominant ants on twig-nesting ants that belong to a different nesting guild. A. sericeasur ants are aggressive toward many types of arthropods, including even birds. In a manipulative field experiment, Philpott added artificial nests on coffee plants with A. sericeasur foragers and on coffee plants without A. sericeasur foragers to assess their impact on nest colonization. Fewer nests were colonized by twig-nesting ant species when A. sericeasur were present, and six out of ten common species occupied significantly more artificial nests placed on plants without A. sericeasur. Nest colonization increased with tree density , indicating that both abiotic and biotic factors are contributing to assembly processes for this community.

In a clever experiment to test the influences of the A. sericeasur–C. viridis or ant-scale mutualism on foraging by A. sericeasur and other arboreally foraging ant species, MacDonald et al. sprayed scale insect populations near clusters of A. sericeasur nests with an aqueous solution of L. lecanii, a fungal parasite of the scale insects. TheL. lecanii killed significant numbers of scales and resulted in increases in the foraging of other ant species proportional to the reduction in the numbers of A. sericeasur foraging on coffee plants. Thus A. sericeasur likely affects not only arrival but also survival of the twig-nesting ant colonies in coffee farms. As an aside, plastic growers pots the same sorts of reductions in foraging by A. sericeasur are also experienced frequently when they are under attack by Pseudacteon spp. phorid flies, resulting in similar increases in resource access for twig-nesting ant species . Such manipulative experiments have rarely been conducted in natural habitats, highlighting the role of coffee farms as model systems for ecological tests. Coffee farms have also provided a natural laboratory for simultaneously testing multiple hypotheses for community assembly. Livingston & Philpott used twig-nesting ants to investigate drivers of community assembly, including nest-site preferences; environmental filters, such as nest-site abundance and disturbance by A. sericeasur; influences of competition among ant species; and stochastic dispersal. They examined co-occurrence patterns of 25 twig-nesting ant species and used a “core/satellite” approach to address dispersal heterogeneity among species in the community . Species were divided into four groups: core species that are common at both local and regional scales, regional dominants that are found globally and dominant locally, local dominants that are dominant locally but not well distributed globally, and satellite species that are neither dominant locally nor well distributed. According to theory, competition among core species should result in overdispersed co-occurrence patterns , whereas dispersal effects should result in random or aggregated co-occurrence for satellite species. Several patterns emerged from the field analysis. First, >60% of twigs were occupied, demonstrating strong nest-site limitation. Second, most ant species inhabited similar-sized twigs, all species examined had proportionally similar responses to gradients in nest-site abundance, and A. sericeasur presence had little influence on richness or composition of ants occupying coffee twigs. Thus, Livingston & Philpott concluded that species sorting through environmental filters plays a minor role. Third, all ant species showed random patterns of co-occurrence with respect to each other in the absence of the “core” species, Pseudomyrmex simplex. But in the presence of P. simplex, regional dominants, local dominants, and satellite species showed aggregated patterns of co-occurrence with respect to one another. Thus, P. simplex likely plays an important role in “assembling” the community by providing a core meta population into which the other species fall.

Fourth, there was evidence for priority effects, as two ant species of equal competitive rank rarely co-occurred. Taken together, these data provide support for a competitive meta-community structure and suggest that competition for nest sites should be the dominant species interaction in this community. In a follow up paper, Livingston et al. used an expanded data set, coupled with data on dispersing alates , to examine evidence for species sorting and mass effects . They used data on twig-nesting ants from natural twig nests in five microhabitat types in a shaded coffee agroecosystem and coupled this with data on artificial nest occupation and numbers of alates caught in light traps. They then used community similarity and a variance decomposition to partition community variance into spatial and environmental components. Twig-nesting ant communities were distinct in each of the five microhabitats, and dissimilarity among communities was largely driven by changes in relative abundance of dominant and subdominant species but also by extensive turnover in the rare species. Space and environment together predicted 24.5% of the variation in the ant community, and space and environment explained unique fractions of the variance, indicating that both species sorting and mass effects are likely important drivers in this community. Alate abundancefor a species was correlated with colony abundance of species, indicating that microhabitats are dominated by internal dispersal. Finally, ant richness in artificial nests was higher than that in natural nests in coffee; however, natural nests had higher richness than artificial nests in shade trees and in the forest. Thus, abundances of the dominant and subdominant species are predominant in community dynamics, and dispersal of rare species from the canopy or adjacent forest patches may support mass effects into coffee microhabitats. To summarize, evidence to date for this twig-nesting ant community suggests that environmental filtering at the level of the nest may not occur but that some differences in habitat may impact ant assembly. Furthermore, dispersal and competition appear to be highly important in this community.Azteca sericeasur nests in trees and is common in the Mesoamerican tropics, where it is frequently encountered on casual walks in the forest. However, discerning any spatial pattern of its colonies in a tropical forest is inevitably obstructed by the heterogeneity of the habitat. In a 45-ha plot regular surveys revealed that Azteca colonies were distributed neither randomly nor uniformly within the plantation. Rather, they were strongly clustered . Examining a variety of environmental factors, such as slope, identity of nesting tree, and size of nesting tree, showed that nothing external to the ant population was correlated with the clusters, suggesting that the underlying biology of the ant was potentially the source of a self-organized pattern. The basic biology of the ant is not unusual for a species that has multiple queens and may be quite general for species that acquire most of their energy from hemipterans . After a queen establishes a colony in a tree, the colony may grow to the point that new nests are established in neighboring trees, evidently one part of the mechanism whereby patchiness is generated. However, unabated new nest formation would obviously result in a continuous expansion of colonies, which means that some force must limit this expansion. Turing’s concept of diffusive instability is a useful metaphor , with the tendency of the ants to disperse to neighboring trees equivalent to the “activator” of the system. However, there must also be some “suppressor,” otherwise the ants would simply disperse over the whole farm.