Abstract
The atmosphere is populated by a diverse array of dispersing insects and their predators. We studied aerial insect communities by tracking the foraging altitudes of an avian insectivore, the Purple Martin (Progne subis). By attaching altitude loggers to nesting Purple Martins and collecting prey delivered to their nestlings, we determined the flight altitudes of ants and other insects. We then tested hypotheses relating ant body size and reproductive ecology to flight altitude. Purple Martins flew up to 1,889 meters above ground, and nestling provisioning trips ranged up to 922 meters. Insect communities were structured by body size such that species of all sizes flew near the ground but only light insects flew to the highest altitudes. Ant maximum flight altitudes decreased by 60% from the lightest to the heaviest species. Winged sexuals of social insects (ants, honey bees, and termites) dominated the Purple Martin diet, making up 88% of prey individuals and 45% of prey biomass. By transferring energy from terrestrial to aerial food webs, mating swarms of social insects play a substantial role in aerial ecosystems. Although we focus on Purple Martins and ants, our combined logger and diet method could be applied to a range of aerial organisms.
Highlights
Most terrestrial animal species[1,2], many plants[3], and countless microorganisms[4] enter the Earth’s skies to forage, mate, evade predators, disperse or migrate
We focus on Purple Martins and ants, our method would be transferrable to other aerial predators and prey
Insect species are distributed in the air according to body size such that lighter species can occur at higher altitudes
Summary
Most terrestrial animal species[1,2], many plants[3], and countless microorganisms[4] enter the Earth’s skies to forage, mate, evade predators, disperse or migrate. Lighter insects tend to fly at higher altitudes than heavy ones[7,8,9,32], because they generally have lower wing loading, allowing them to stay aloft longer, fly in lower density air, and be more transported in rising air currents[2,11,33] In ants this relationship is captured by the Found or Fly hypothesis, which posits a tradeoff between flight ability and abdominal nutrient storage[34]. Female-calling species should fly near the ground regardless of body weight These predictions remain untested, as the difficulty of tracking small insects has precluded comparative studies of ant flight altitudes. We lack these data for any flying species, as individual altitude logging devices have only recently been developed[43,44]
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