Abstract
BackgroundEcological communities of interacting species analyzed as complex networks have shown that species dependence on their counterparts is more complex than expected at random. As for other potentially mutualistic interactions, ant-plant networks mediated by extrafloral nectar show a nested (asymmetric) structure with a core of generalist species dominating the interaction pattern. Proposed factors structuring ecological networks include encounter probability (e.g., species abundances and habitat heterogeneity), behavior, phylogeny, and body size. While the importance of underlying factors that influence the structure of ant-plant networks have been separately explored, the simultaneous contribution of several biological and ecological attributes inherent to the species, guild or habitat level has not been addressed.MethodsFor a tropical seasonal site we recorded (in 48 censuses) the frequency of pairwise ant-plant interactions mediated by extrafloral nectaries (EFN) on different habitats and studied the resultant network structure. We addressed for the first time the role of mechanistic versus neutral determinants at the ‘fine-grain’ structure (pairwise interactions) of ant-plant networks. We explore the simultaneous contribution of several attributes of plant and ant species (i.e., EFN abundance and distribution, ant head length, behavioral dominance and invasive status), and habitat attributes (i.e., vegetation structure) in prevailing interactions as well as in overall network topology (community).ResultsOur studied network was highly-nested and non-modular, with core species having high species strengths (higher strength values for ants than plants) and low specialization. Plants had higher dependences on ants than vice versa. We found that habitat heterogeneity in vegetation structure (open vs. shaded habitats) was the main factor explaining network and fine-grain structure, with no evidence of neutral (abundance) effects.DiscussionCore ant species are relevant to most plants species at the network showing adaptations to nectar consumption and deterrent behavior. Thus larger ants interact with more plant species which, together with higher dependence of plants on ants, suggests potential biotic defense at a community scale. In our study site, heterogeneity in the ant-plant interactions among habitats is so prevalent that it emerges at community-level structural properties. High frequency of morphologically diverse and temporarily-active EFNs in all habitats suggests the relevance and seasonality of plant biotic defense provided by ants. The robust survey of ecological interactions and their biological/ecological correlates that we addressed provides insight of the interplay between adaptive-value traits and neutral effects in ecological networks.
Highlights
Interspecific interactions have been recently analyzed at the ecological community level with a complex network perspective, where important attention have been paid at network organization and its underlying factors both for mutualisms or antagonisms
In particular we addressed the following questions: 1) Which is the network structure of this ant-plant community mediated by extrafloral nectaries?; 2) Which is the detailed “finegrain” structure resulting from the frequency of pairwise interactions?; and 3) Which is the relative contribution of biological or ecological correlates in rendering the pair-wise “fine-grain” and overall network structure?
Eight plant species and four ant species constituted the central core of this network, the remaining species were peripherical; plant core species were: Cordia spinescens, Cedrela odorata, Callicarpa acuminata, Turnera ulmifolia, Canavalia rosea Crotalaria incana, Calopogonium caerulium and Mansoa hymenaea, while ant core species were: Camponotus planatus, Camponotus mucronatus, Crematogaster brevispinosa, Paratrechina longicornis
Summary
Interspecific interactions have been recently analyzed at the ecological community level with a complex network perspective, where important attention have been paid at network organization and its underlying factors both for mutualisms or antagonisms. For a tropical seasonal site we recorded frequency of pairwise ant-plant interactions mediated by extrafloral nectaries, attributes of interacting species, habitat attributes, cover of plants with EFNs, and studied the resultant network structure. Proposed factors affecting network organization include encounter probability (species abundances, habitat heterogeneity), behavior, phylogeny and body size. While the importance of underlying factors that influence structure of ant-plant networks have been separately explored, simultaneous contribution of several biological and ecological attributes inherent to the species, guild or habitat level have not been addressed
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