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.

Variation of parental and chick diet in opportunistic insectivorous European Bee-eaters

Insectivorous Palaearctic bird species associated with open habitats rely on high prey abundances,which are currently declining due to habitat loss and intensive agricultural practices.The European Bee-eater(Merops apiaster)is an opportunistic insectivore of open habitats,preying mainly on medium to large-sized flying insects.Its diet composition received some attention in the past,but the current variation in diet composition of birds breeding across different habitats,and between adults and chicks remains poorly known.In this study,we determine variation in bee-eaters’diet in colonies located in five common habitats at the Iberian Peninsula.We also assess differences in the diet composition of chicks and adults and investigate seasonal diet selectivity of adults.Finally,we explore the variability in the size of prey provided to chicks throughout their growth period.Hymenoptera and Coleoptera were the most important groups for bee-eaters,with adults and chicks consuming 58.8%and 64.1%of hymenopterans and 37.6%and 28.6%of coleopterans,respectively.The proportion of Hymenoptera(42.3–55.7%)and Coleoptera(43.3–53.5%)in the diet was similar in colonies in pasture and oak habitats,but Hymenoptera dominated(83.8%and 95.7%)in meadow and mixed forest colonies.Despite being a generally opportunistic predator,adult bee-eaters provide their progeny with an increasing proportion of larger insects through chick development.Moreover,they equally take Hymenoptera and Coleoptera for themselves and their chicks,even when the abundance of these insects decreases seasonally.Overall,these results suggest that local prey availability associated with specific habitats influences diet composition and that regional declines in certain groups may,therefore,affect insectivore species differently according to their dietary and habitat preferences.

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.