There have numerous studies that set out to find how the interspecific interactions between ants and plants and how these relations affect their diversity and abundance. However, there is a theory that suggests that the long-term effects of co-evolution between two species, such as an ant and a plant, may occur over large geographical scales rather just within local populations. Ants can be involved in many interspecific interactions with plants whether via seed dispersal, leaf-cutting, or through seed-harvesting. But these interactions may occur in one geographic area but be absent in another. This geographic mosaic theory is based on the thought that some species may adapt and therefore become specialized to another species in a different way than if it were in another region. This model suggests that the main factor of coevolution involving groups of species and their interactions where selection pressures favor certain genes that then spread out into other populations (in this case ants and plants) often occurs on a large geographical scale.
This theory is also based upon three ecological observations proposed by John Thompson, ecology profess at UC Santa Cruz. First, the majority of species are collections of genetically divergent populations and one must understand how this property of species effects local adaptations and specialization in interacting ants and plants. The second observation Is that the outcomes of the interactions between these organisms varies among communities. Therefore it is important to understand how the geographic and even the temporal dynamics of interspecific interactions shape the coevolution between two species. Lastly, the two interacting species will also differ in their geographic ranges and as a result form networks of interactions among various species rather in pairs. (Site Thompson's website or his book).
The amount of research conducted in the area of interspecific ant-plant interactions that take the geographic mosaic model of coevolution into regard is small. However, the importance of this model lies in the fact that it often influences the nature of coevoulationary process and the evolutionary interactions between these two species. Due to the fact that there are so many different types of interactions between countless species of ants and plants, there is no wonder that through natural selection the geographic differences of both species are integrated into a mosaic (reference book). We will look at some of the studies that examine the interspecific interactions between various species of ants and plants and how they take the geographic mosaic model into account.
Geographical Mosaic Co-evolution in a Social Parasite System
According to Fischer & Foitzik (2004), this certain theory of coevolution can predict the divergence of the coevolutionary developments between local communities based on their composition and the strength of the selection pressures brought on by resource availability and competition. In this study, they wanted to examine the local co-adaptation in different populations of a social parasite. That is, they looked at the raiding efficiency between a range of populations of a slave-making ant and the defense abilities of local hosts. There thoughts, according to the theory, were that the outcome of these interactions and the amount of specialization between the ant and its hosts will vary among the physical and biotic environments. The outcome of this study would also depend on the environment and the community in which the interaction takes place. (fischer and foitzik)
Because there are few studies related to the interactions between social insect parasites and their hosts, the authors wanted to propose a new model system for the wonder of social parasitism. Where a eusocial species is oppressed by another such species is common in ants, there is no reason to think that the closely related host and parasite could have equal degrees of specialization or adaptation even in different communities. Therefore, Fischer & Foitzik set out to find how close this degree of specialization was between different populations of a small European slave-making ant Harpagozenus sublaevis and its main host species Leptothoraz acervorum by concentrating on their interactions in varying communities (i.e. Germany, Italy, and England host populations with German and Italy slave-maker populations).
One of the main results found in their study was that the outcome of slave raids strongly depended on the effects of the interactions between the parasite and host populations. Host populations differed in how often they attacked intruding slave-makers with the English host colonies being most aggressive against slave-makers from both sites. As seen in their Fig. 1, the number of attacks against the intruding slave-makers was greater on the host population that was foreign.
Overall, their study confirmed that the interactions during the slave raids of the social parasite H. sublaevis against its main host L. acervorum significantly depend on the origin of the host and the slave-maker. Their results further demonstrated how geographical variation in the coevolutionary interactions between parasite and host, supporting the geographical mosaic theory of coevolution. One hypothesis they propose to help explain their results would be due to the differences in the genetic structure and gene flow of the host and parasite populations.
Ant Mosaics in Tropical Rainforests
In 2007, Bluthgen and Stork observed that two or more species of ants were usually much more abundant than any other species and that the areas of activity of these "dominant" ant species in the trees did not overlap. Their study was conducted in the lowland rainforests of North-East Queensland, Australia. Such dominant species sustain large colonies and were characterized by their high abundance and activity density and the patchwork distribution of these ants formed was termed 'ant mosaic'. An ant mosaic is used to describe the structure of arboreal ant communities in plantations and other forest systems. This ant's numerical dominance is sometimes related with behavioral dominance in terms of intraspecific aggression and potential to defend resources and territories against other ants. Therefore, ant mosaics can reflect the current or past competition between dominant ants involved, which would possibly lead to common exclusion of the dominant ants' territories. And the presence or absence of these types of mosaics may be vital to understand the role of interspecific competition in communities with variable rates of disturbance and complexity due to the fact that they play an essential role for the structure of the arthropod community as a whole and resulting ecosystem functions.
Their results found that social insects such as ants usually exhibit a highly patchy distribution when they are mapped at a small spatial scale, particularly when nests or resources with mass recruitment are involved (to resist competition with other organisms). In their case study, they proposed that the consequences of competition between ants in species-rich communities may need to be investigated with an appropriate spatial scale and stronger statistical evidence. However, they conclude that an 'ant mosaic' implies a merely static spatial pattern that can be based on ecological or behavioral process and may be reasonably dynamic. But Bluthgen and Stork go on to say that spatial distribution is only an indirect measure or outcome of a complex array of competitive interactions and may not inevitably reveal underlying structural mechanisms of competitive hierarchies in ant communities.