Weighted individual-resource networks in prey–predator systems: the role of prey availability on the emergence of modular structures

Ecological networks,usually depicting interactions among species,have been recently down-scaled to the individual level,permitting description of patterns of inter-individual resource variation that are usually hindered at the species level.Optimal diet theory(ODT)models,applied to prey–predator systems,predict different patterns of nestedness and modularity in the network,depending on the available resources and intra-specific competition.The effect of resource availability on the emergence of networks structures,and ODT framework,has not yet fully been clarified.Here,we analyzed the structural patterns of individual-resource networks in 3 species of Mediterranean salamanders,in relation to changes in prey availability.We used weighted individual-resource network metrics to interpret the observed patterns,according to 3 ODT models.We found significant nestedness recurring in our study system,indicating that both selective and opportunistic individuals occur in the same population.Prey diversity,rather than abundance,was apparently related to inter-individual resource variation and promoted the emergence of significant modularity within all networks.The observed patterns of nestedness and modularity,together with the variation in resource diversity and intra-specific competition,are in agreement with the distinct preferences model of ODT.These findings suggest that in the focal prey–predator systems,individuals were able to perceive changes in prey diversity and to exploit in different ways the variations in composition of available resources,shifting their diet assembly rules accordingly.Our findings also confirm that the use of weighted individual-resource networks,in prey–predator systems,allows to disclose dynamics that are masked at the species or population level.

Frequent and prolonged nocturnal occupation of port areas by Yangtze finless porpoises (Neophocaena asiaeorientalis): Forced choice for feeding?

During the Yangtze Freshwater Dolphin Expedition 2012,Yangtze finless porpoises(Neophocaena asiaeorientalis)were acoustically monitored in 9 port areas at night.During 6566 min of nocturnal monitoring,porpoise sonar was detected for 488 min(7.43%of the total time).Of all 81 encounters,the longest echolocation span obtained was 102.9 min,suggesting frequent and prolonged porpoise occupation of the port areas.A combined total of 2091 click trains were recorded,with 129(6.2%)containing minimum inter-click intervals(ICIs)below 10 ms(termed a buzz).Buzzes with a decrease in ICIs and search and approach phases that resembled feeding echolocation signals accounted for 44.2%(N=52)of all buzzes.Buzzes with an increase in ICIs,suggesting a mirrored prey capture phase,accounted for 20.2%(N=26)and could reflect attempts to locate escaped prey because they were followed by approach-phase feeding buzzes.Anecdotal evidence of porpoises fleeing the proximity of vessels was observed.The recordings indicating clusters of porpoises feeding near the port areas suggest a forced choice for feeding due to the relatively higher prey availability in the port areas compared to other areas in the Yangtze River that are probably overfished.