The white halo fungus thus acts as a “carnivore” consuming both rust fungus and scale insects. The parasitoids of the scale insects also act as “carnivores,” competing with the white halo fungus for the scale resources. The ant inserts an interaction completely independent of energy transfer but behaviorally altering the trait of parasitism. The food web would make little sense without understanding this key trait-mediated effect.In coffee agroecosystems throughout the Neotropics one of the key interactions is the mutualism between arboreal ants in the genus Azteca and the green coffee scale, C. viridis. Although the ants nest within shade trees, they forage and tend scales on coffee plants near their nests. Extensive research has revealed how a complex ecological network centered on the Azteca-green coffee scale mutualism is structured. This network, in addition to the normal energy transferring connections, includes competition, mutualism, predation, parasitism, and disease-host relationships and includes several key trait-mediated interactions. The network also plays a role in the ecosystem service of pest control . One of the main predators of the green coffee scale that the ants defend against isAzya orbigera, a coccinellid beetle . As in other myrmecophilous coccinellids , the larvae of the beetle are protected against ant attack, in this case by waxy filaments . Although the larvae are protected, container size for blueberries unprotected adult females must be able to lay unprotected eggs where ants are actively patrolling but must place the eggs where ants cannot find them.

One of the main safe sites for oviposition is directly underneath the adult-scale insects , where ants cannot patrol. But how can the gravid female beetles search for safe sites to lay eggs when under continual attack by the ants? We return to this question later. Another key player in the system is a parasitic fly . In the coffee system the phorid flies that attack Azteca are common, and they dramatically change the foraging behavior of the ants , reducing foragingactivity by as much as 50% . Indeed, the ants display an unusual behavior when under attack by the phorids, either rushing back to the closest nest entrance or assuming a catatonic posture seemingly protecting the anterior thorax—the oviposition site of the fly . Furthermore, this characteristic behavior is generalized over a rather large area , indicating a chemical communication among ant individuals to warn of imminent phorid attack. Adult beetle residence time on a coffee branch is, not surprisingly, strongly correlated with ant activity . Beetles that would otherwise be restricted in residence time are partly released from that restriction when the phorids arrive , thus resolving the problem of locating safe oviposition sites . Phorid flies locate concentrations of foraging ants by recognizing an ant pheromone that the ants produce when disturbed. Once near the ants, phorids cannot oviposit unless they see movement on the part of an individual ant , suggesting the adaptive significance of the ant’s catatonic behavior. Yet it is not clear how the beetles find these high-quality patches and then manage to know that the ants have adopted their protective position. A series of olfactometer experiments demonstrated that both female and male beetles are attracted to the green coffee scale, but only female beetles are attracted to ant pheromones .

More importantly, female beetles are attracted to ants only when the ants are under attack by phorid flies, and the attraction to ant pheromones is manifested only when females are gravid . Because beetle larvae have restricted movements and are attacked by several parasitoids , there is clear pressure for female beetles to oviposit in ant-tended areas, where high prey density and low risk of parasite attack are secured. However, the aggressive behavior of ants apparently renders female beetles incapable of ovipositing in these high-quality areas. The olfactometer experiments described above suggest that female beetles avoid this problem by being able to detect some kind of “phorid alert” pheromone released by Azteca ants, thus allowing them to take advantage of low ant activity periods to search for safe oviposition sites. This system is an example of cascading trait-mediated indirect interactions facilitated by the ant’s chemical communication system . In addition to attacking adult coccinellid beetles, Azteca ants remove any herbivores from where they are actively foraging and also attack coffee berry borers , coffee’s most damaging insect pest , making it difficult for the borers to make their way into coffee fruits to lay eggs. However, when Azteca ants are under attack from phorid flies, and in the catatonic state, coffee berry borers can invade just as many coffee fruits as when no ants are protecting coffee plants . The effects of phorids are not entirely negative, however, when other ants, such as arboreal twig-nesting ants, are present. Azteca ants have negative impacts on other species of ants; yet, when they are being attacked by phorids, other ant species have significantly greater access to food resources . This extends to biological control of the coffee berry borer, as demonstrated with lab experiments.

Pairs or trios of functionally redundant ant species have synergistic protective effects for the coffee by preventing attacks by the borer; however, these synergistic effects are only evident where phorids are present . In effect, as Azteca behavior is limited to a great extent by the phorids, other ant species, normally attacked by the Azteca, increase their predatory activities, thus maximizing negative effects on this globally important coffee pest. In sum, the trait-mediated effects result in complex networks of interactions that provide pest control and maintain a diversity of organisms in shaded coffee farms.For coffee agroforests, the first subject to receive a large amount of attention in the area of ecology and conservation was birds. Several studies have reviewed the impacts of coffee intensification on bird abundance, richness, and community composition . Many have documented changes in resource use, foraging behavior, and roost selection based on differences in availability and stability of fruit and floral resources , epiphytes , and distance to forest fragments . Coffee farms have been used as laboratories for understanding how sex and age influence bird foraging behavior and how foraging strata influence infestation of birds by mite parasites . Furthermore, coffee agroforests have been used to examine differences in foraging behavior in solitary foragers versus members of mixed-species flocks and to test whether flock-related foraging shifts differ with habitat type . Here, we focus on summarizing what has been learned about competition and foraging in bird communities via studies in coffee agroforests by specifically examining work on competition between migrant and resident birds, aggression, and home range, as well as competition between avian insectivores and ants. Shaded coffee farms are easily divided into two distinct foraging strata, raspberry grow in pots the understory layer that is primarily composed of coffee plants and the shade-tree layer that can sometimes be a single layer dominated by a nitrogen-fixing legume or a multistrata canopy. Understanding how birds use these different foraging strata and interactions between bird species in different vegetation layers has been an important focus of bird studies in coffee, especially because of the relative ease with which foraging maneuvers can be observed in coffee compared with more complex forest habitats. For example, in Chiapas, Mexico, Jedlicka et al. examined the foraging behavior of one resident bird species during both the wet season , when migratory birds are absent from coffee farms, and the dry season , during which hundreds of migratory birds forage in shaded coffee farms.

They found that these insectivorous warblers spent a similar amount of time foraging in the canopy and understory in the summer but had higher capture rates in the canopy. However, in the winter, migratory birds dominated the canopy layer while the warblers shifted foraging such that >80% of maneuvers were in the understory. Furthermore, the warblers were less successful at capturing prey in both the canopy and understory during the winter. There were significant changes in the abundance of canopy arthropods during the wet and dry seasons with significantly fewer large arthropods available in the canopy during the dry season. This change in arthropod abundance may haveprecipitated the change in foraging by the rufous-capped warblers. Yet this study may also provide evidence that there is a niche shift on the part of this warbler species to avoid competition with the neotropical migrants that dominate the shade-tree canopy. A recent study conducted in coffee farms in Jamaica tested whether habitat type, prey availability, and tree strata influenced the degree of aggression from insectivorous black-throated blue warblers . Smith et al. radio-tagged several warblers and then tested their response to decoy intruders in coffee farms and in nearby forests . They documented differences in aggression based on habitat type and strata, with the highest aggression being displayed by black-throated blues against the conspecific decoy in the shade-tree canopy, followed by those in the forest understory and coffee layer. Arthropod abundance in the habitat types and strata followed the same pattern, with highest insect abundance being found in the shade-tree canopy, followed by the forest understory and coffee layer. In addition, they found that several explanatory variables influenced the degree of aggression of the black-throated blue warblers—including sex, home-range size, proximity to the center of the home range, habitat, food supply, and presence of another aggressive bird in the area. Importantly, this study used natural variation in the abundance of insects in different strata within coffee farms to demonstrate that the black-throated blue warblers more aggressively defend areas with greater food supply. Coffee farms have also been used as model systems for examining competitive interactions between insectivorous birds and ants that forage on canopy trees. The same ant species described in detail above, Azteca sericeasur, is aggressive not only to insects but also to other organisms. Philpott et al. examined how much the presence of A. sericeasur on coffee shade trees would influence the foraging of birds, including insectivores. They compared bird foraging between two species of shade trees with and without nests of A. sericeasur. They found that the presence of ants on shade trees did not affect the number of bird individuals or species that visited shade trees; however, ant presence did shorten the length of visits for all birds, and specifically for insectivores, but only on Alchornea latifolia shade trees. Two interesting patterns result from this finding. It appears that A. sericeasur is not directly aggressive to birds, deterring them from visiting shade trees, and therefore ants likely do not influence birds via interference competition. However, the reduction in time spent foraging on some trees with A. sericeasur, and the specific response of insectivores indicates that A. sericeasur may affect bird foraging via exploitative competition, reducing the arthropod abundance sufficiently to lower the time birds spend foraging on certain shade trees. The response on A. latifolia trees, but not the Inga spp. shade trees examined, is likely because ant colonies are larger and stronger on those trees .Ant community assembly and ant diversity are strongly impacted by several characteristics, including abiotic factors associated with changes in vegetation , temperature or moisture conditions , changes in nest availability , and changes in food availability and distribution . In addition, species interactions may drive assembly, as competition from native and non-native ants can affect nest recruitment and resource acquisition . Ants may be affected by environmental requirements or filters such that certain ant species thrive only in certain environments. Differences in shade quality or temperatures, for example,may limit ant foraging or survival, as exemplified in an experimental manipulation of both shade and leaf litter that dramatically affected the nature of competition interactions among ground foraging species in a Costa Rican shade coffee system . Similarly, as discussed further below, availability of appropriate nesting sites is at least occasionally limiting. For example, the diversity of tree species from which hollow twigs were derived was a major determinant of the species diversity of the ants occupying them in the leaf litter of a shaded coffee farm in Colombia. Although such environmental filters are well known to be important, it is nevertheless a major assumption of most ant biologists that competitive interactions among ant species frequently determine community structure .