Effects of simulated squirrel branch landings on ants and other arboreal arthropods

Wind, rain, and the activities of vertebrates frequently dislodge arthropods from tree surfaces, resulting in arthropod "rain" beneath trees. The probability of falling likely differs among taxa based on their adhesive capacities and ability to anticipate disturbance. We quantified the motion of red oak (Quercus rubra Linnaeus [Fagales: Fagaceae]) leaves, and the composition of arthropods dislodged from woody vegetation, following disturbance mimicking branch landings by eastern gray squirrels (Sciurus carolinensis Gmelin [Rodentia: Sciuridae]). Simulated squirrel landings caused terminal leaves on red oak branches to move rapidly (up to 100 cm s^-1) upward and inward toward the tree trunk during the first 50 ms, and dislodged 30% of the arthropods present on a branch. The composition of fallen arthropods was similar to those that remained on the branches, but the proportion dislodged differed among taxa. We used a small catapult to test the effect of leaf movement on the adhesive capabilities of workers of the carpenter ant Camponotus pennsylvanicus DeGeer (Hymenoptera: Formicidae). Arboreal ants presumably anticipate disturbance; thus, they were either "warned" or "unwarned" before each launch. Most ants (98%) were dislodged by the catapult. Warning did not affect the average (± SE) initial velocity (126.0 ± 72.0 cm s^-1) or maximum height (22.8 ± 9.3 cm) of launched ants. However, both variables were positively associated with ant mass. The results illustrate the limited ability of many arboreal arthropods to avoid the consequences of a common natural hazard and improve our understanding of the challenges imposed by an arboreal lifestyle.