The resource‐mediated modular structure of a non‐symbiotic ant–plant mutualism

Abstract

1. Plant–animal mutualisms are key processes that influence community structure, dynamics, and function. They reflect several neutral and niche‐based mechanisms related to plant–animal interactions.2. However, the strength with which these processes influence community structure depends on functional traits that influence the interactions between mutualistic partners. In mutualisms involving plants and ants, nectar is the most common reward, and traits such as quantity and quality can affect ant species' responses by influencing their recruitment rates and aggressiveness.3. In this study, nectar traits that mediate ant–plant defensive mutualisms were manipulated to test whether resource quantity and quality affect the structure of ant–plant interaction networks. A downscaling approach was used to investigate the interaction network between ant species and individual plants of the extrafloral nectary‐bearing terrestrial orchid Epidendrum secundum.4. We found a short‐term reorganization of the ant assemblage that caused the interaction networks to become more specialised and modular in response to a more rewarding nectar gradient. Furthermore, the ant species tended to narrow their foraging range by limiting their associations to one or a few individual plants.5. This study shows that ant species' responses to variable resource traits play an important role in the structure of the ant–plant interaction network. We suggest that more rewarding nectar enhanced aggressiveness and a massive recruitment of some ant species, leading to lower niche overlap and thus a less connected and more specialised network.

Positive species interactions shape species' range limits

“…Communities such as plant–pollinator, seed–dispersal or host–parasitoid systems are often modular (Olesen et al 2007, Fortuna et al 2010, Schleuning et al 2014). At local scales, these modules are likely to reflect eco‐evolutionary pressures shaping who interacts with whom (Donatti et al 2011, Ballarin et al 2020); however, habitat filtering can also cluster species with similar habitat requirements, leading to modular structures at larger scales. Indeed, dynamical models accounting for niche relationships and distributions of species uncovered a naturally emerging modular structure in mutualistic systems, one in which each module presented in turn an internal nested structure (Cai et al 2020).…”

Time course of inducibility of indirect responses in an ant‐defended plant

“…We found that ant abundance closely followed extrafloral nectar production over the time course of our study. Numerous studies have shown that ant abundance and aggressiveness are directly related to nectar volume and composition (Anjos et al, 2017; Ballarin et al, 2020; Bixenmann et al, 2011; Calixto, Lange, Bronstein, et al, 2021; Calixto, Lange, Moreira, & Del‐Claro, 2021; Lange et al, 2017; Pacelhe et al, 2019). In the Brazilian Cerrado, EFN activity is most concentrated in the rainy season, from October to March (Calixto, Novaes, dos Santos, et al, 2021), which corresponds to the period with the highest ant activity and colony growth (Calixto, Lange, & Del‐Claro, 2021).…”

Extrafloral nectaries of an Atlantic Forest conservation area in Southeastern Brazil

“…A novel research avenue that could also be explored in future studies is the investigations of the functional attributes of EFN in plant communities. For example, two recent studies suggest that nectar volume and concentration in EFN are strongly related to ant-plant community structure and defensive behavior in ants that visit EFNs (Ballarin et al 2019;Calixto et al 2021). These studies point out that other EFN traits (e.g., nectar volume and concentration) could be relevant for future surveys since they are closely related to ecological processes and patterns.…”

“…Tocoyena formosa has a nectar gland surrounding the apex of the ovary during blooming and, after pollination, the nectariferous disk on young Acta Botanica Brasilica, 2022, 36: e2021abb0187 fruits are fully functional and produce an exudate which is attractive for ants. Epidendrum denticulatum produces nectar on the apex of the peduncle on buds, flowers, and fruits that, by attracting ants, provide an efficient protection against herbivory (Calvo 1990;1993;Almeida & Figueiredo 2003;Ballarin et al 2019).…”