Community structure and ecological specialization in plant-ant interactions

Background/Question/Methods

The composition and diversity of natural communities are fundamental ecological factors influencing species interactions, and specialization. Species composition varies geographically creating mosaics of different interaction outcomes. To understand the dynamics and persistence of interactions we must study them along their distributional gradients. This study expands our knowledge of how community structure and specialization in obligate mutualisms vary geographically.

Interaction patterns, community organization and species roles (e.g. generalist, specialist) can be described using network theory. Symbiotic plant-ant associations are of particular interest in studies of specialization and are used to study network structure. Specific questions include:

1. What is the network topology of plant-ant associations?
2. Is that structure maintained among geographic areas? 
3. Does plant-ant specialization vary among communities? 
4. Does species specialization vary among geographic areas?

Ten individuals, separated by >100 m, were collected for each host species. Plant-ant communities were sampled at Yasuni, Ecuador, Los Amigos, Peru, and La Selva, Costa Rica. Resident ants were collected into 95% ethanol, mounted and identified. Plant species were identified in situ or from pictures. Data were analyzed with commonly used qualitative and quantitative metrics. 

Results/Conclusions

Networks produced in the three locales were compartmentalized. The link distribution of networks showed that most species are connected by < 10 links and in Ecuador the Azteca alfari complex was connected to hosts by 57 links. Compartments had five species in Ecuador and four in Peru and Costa Rica.

Ecuador had the lowest connectance while Peru and Costa Rica had similar proportions of realized interactions. Nestedness was not significantly different from random in Ecuador and Costa Rican networks (P > 0.05) and it was marginally significant in Peru (P=0.051). Interactions were more diverse in Ecuador than in Peru or Costa Rica. Specialization for the entire networks was significantly different from random (P < 0.001) in Ecuador and Costa Rica, and P < 0.05 in Peru. These plant-ant interactions are highly specialized and very symmetrical (P = 1).

Species specialization differed geographically. Among ant species, the alfari complex is present at the three locales and the highest level of specialization (di’=0.96) is reached in Costa Rica. Plant species such as Cecropia herthae and Toccoca guianensis had high levels of specialization (di’ = 0.74-1).

Network compartmentalization does not change geographically. However, ant species with wide distributions play somewhat different roles along distributional gradients.