The bat-bird-bug battle: daily flight activity of insects and their predators over a rice field revealed by high-resolution Scheimpflug Lidar.

Elin Malmqvist,Mikkel Brydegaard,Katarina Svanberg,Sune Svanberg,Shiming Zhu,Jens Rydell,Joakim Bood,Wansha Li,Ziwei Song,Susanne Åkesson,Samuel Jansson

doi:10.1098/rsos.172303

Abstract

We present the results of, to our knowledge, the first Lidar study applied to continuous and simultaneous monitoring of aerial insects, bats and birds. It illustrates how common patterns of flight activity, e.g. insect swarming around twilight, depend on predation risk and other constraints acting on the faunal components. Flight activity was monitored over a rice field in China during one week in July 2016, using a high-resolution Scheimpflug Lidar system. The monitored Lidar transect was about 520 m long and covered approximately 2.5 m3. The observed biomass spectrum was bimodal, and targets were separated into insects and vertebrates in a categorization supported by visual observations. Peak flight activity occurred at dusk and dawn, with a 37 min time difference between the bat and insect peaks. Hence, bats started to feed in declining insect activity after dusk and stopped before the rise in activity before dawn. A similar time difference between insects and birds may have occurred, but it was not obvious, perhaps because birds were relatively scarce. Our observations are consistent with the hypothesis that flight activity of bats is constrained by predation in bright light, and that crepuscular insects exploit this constraint by swarming near to sunset/sunrise to minimize predation from bats.

Highlights

Many insects show either diurnal or nocturnal patterns of activity associated with physiological traits such as, e.g. thermoregulatory and/or predator defence adaptations [1,2]
The feeding efficiency of birds is constrained by declining visual capacity at dusk [18,19], while the feeding activity of bats, which rely on echolocation, is believed to be constrained by predation pressure from raptorial birds in bright light [20,21,22,23]
We examined the following hypotheses: (i) foraging activity of birds and bats are constrained at dusk and dawn by decreasing light intensity and increasing predation risk, respectively, and (ii) crepuscular insects allocate the flight activity to dusk and dawn, when predation pressure may be relatively low owing to the constraints acting on the predators

Summary

Introduction

Many insects show either diurnal or nocturnal patterns of activity associated with physiological traits such as, e.g. thermoregulatory and/or predator defence adaptations [1,2]. There are insects that seem to lack such defensive traits and rather minimize the predation risk by concentrating their flight activity to short swarming periods around dusk and dawn [3,4] By adapting the latter strategy, they may employ predator satiation or selfish herd behaviour to reduce the predation risk at the individual level [5,6]. Such crepuscular insects are often the most prominent [7], and typically include many species of flies (Diptera) such as mosquitoes (Culicidae) [8] and midges (Chironomidae and Ceratopogonidae) [4,9], and many moths (Lepidoptera) [10,11] and bugs (Hemiptera). Counts of individuals or estimates of flight activity are subject to different biases inherent in the various methods and are, hard to compare across groups

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Results

Conclusion