Non-trophic key players in aquatic ecosystems: a mesocosm experiment

The trophic network (TN) has been well stablished, and recently knowledge concerning non-trophic relationships (NTRs) is receiving increasing attention. Although NTRs can influence trophic ones, network models, including both types of interactions (multi-interaction network, IN) and changes in the role of nodes when NTRs are added to TN, are scarce. To evaluate the role of NTRs in freshwater shallow ecosystems, where these interactions are relevant mainly in the benthic habitat, we constructed, from the same communities, the two mentioned networks and compared them focusing on the nodes’ topological roles. Our approach is based on empirical data from a mesocosm experiment where aquatic communities inhabited coupled habitats (pelagic, within-meadow and benthic) under three environmental scenarios: warming, increased ultraviolet radiation, plus control conditions. The experiment allowed us to assess: the topological roles of the nodes from different habitats when NTRs were added to the TN, and the relative impact of adding NTRs according to environmental scenarios. We calculated a set of node indices by considering both direct and indirect connections up to an ecologically meaningful number of steps. Our results highlight significant differences in the nodes’ roles between both network versions. When NTRs were added: 1) pelagic nodes lost relevance in the network; 2) the number of within-meadow relevant nodes increased and 3) the large benthic consumers in TN were substituted by charophytes, plus a chain of small within-meadow predators/preys, as the most relevant to the IN. Furthermore, the scenarios modulated changes in the nodes’ roles when including NTRs. The warming scenario promotes the central position of some nodes (e.g. charophytes) and harms others (e.g. benthic cladocerans), and UVR modulates changes in benthic filamentous primary producers’ roles. Therefore, the inclusion of NTRs in ecological models seems crucial to better understand the functioning of complex communities and their response to environmental disturbances.