Scharf, I., Gilad, T., Taichman, Y. and Subach, A., 2021. Urban Pit-Building Insects Are Attracted to Walls for Multiple Reasons. Biology, 10(7), p.635 https://doi.org/10.3390/biology10070635
Many animals ambush their prey in order to eat. One example of a great ambush tactic is the use of pit-fall traps. Pit-fall predators, like Wormlions, dig out a small cavity with their tails and wait for their prey to fall into the pit (figure 1). While the prey struggles to regain mobility, the Wormlion strikes, securing its meal.
A pit-fall is only as good as the location it is deployed
Cities are complex environments that can have many areas that may not be suitable for some species. For example, Wormlions cannot build pit-falls on roads and sidewalks. Instead, they need areas with plenty of loose fine-particle soil. However, cities can also increase the abundance of Wormlion prey such as ants, spiders and other small insects. Additionally, both the abundance of prey and pit-falls appear to be higher near walls. This observation led Inon Scharf, (University of Tel Aviv) and colleagues to study why Wormlions pit-falls building behavior in the city. Specifically, the team studied where pit-falls are made, what are soil conditions near walls, how walls affect the movement of prey, and whether Wormlions move towards walls in controlled experiments.
Sharf studied 16 groups of pit-falls deployed across the campus of Tel-Aviv University. All of the pitfalls were built within 80 cm of the nearest wall. Six of the sixteen groups occurred as close as 10 cm away from the wall and 8 of the groups were significantly closer to the wall than expected by chance. Then, the team explored whether soil conditions were better for pit-falls closer to walls. Wormlions prefer to build pit-falls where the soil is loose, deeper and with finer particles. Additionally, the pit-fall needs to be in an area that gets plenty of shade so that the Wormlions do not overheat. Walls could provide shade and also function as a fence that accumulates loose soil particles blown by the wind. They found that in 5 out of 16 cases the soil was deeper when it was closest to walls, but there was no link between soil depth and pit-fall distance to walls. The proportion of small soil particles was similar across the sites studied. Positions closest to the walls were indeed more shaded than the soil 40 cm or more away from the wall.
Do walls affect prey abundance and capture?
Walls and other artificial structures could impact the abundance of prey. To test this idea the researchers built their own pitfalls traps close and far way from to walls. By contrasting the captured prey at pitfalls close to and far away from walls, the researchers could see if pitfalls near walls capture more prey. This experiment found that in one site predators were more abundant near walls but in the second site, predators were more abundant away from walls. Interestingly, Wormlions built their traps where prey was more abundant. In the site where predator abundance was highest away from walls, Worlmlions built their trap further away from the wall. At that particular site, there was an abundance of vegetation away from the wall, which likely influence prey abundances. In a separate experiment, they placed Wormlions at random distances away from the wall, and over time the Wormlions moved closer to the wall. This suggests that Wormlions can relocate according to preference. To test why walls might increase predator capture, the researchers also built simulations where they modeled random prey movement along a grid in which one side was a wall. The simulation showed that pit-fall traps near walls can be highly successful when prey moves along the wall and thus encounter the pitfall more frequently. However, when too many pit-falls are placed adjacent to walls the predators can outnumber the prey, decreasing the advantage of deploying a pit-fall in a saturated area.
A new hope for pit-falls in the city
Pit-falls are highly effective for capturing prey. Many researchers rely on pit-fall trapping to collect taxa such as insects, small reptiles, and amphibians. Pit-falls have also inspired great science fictions tales such as the fall of Boba Fett in Returns of the Jedi! However, as many who have used pit-fall traps will tell you, they are only as good as where you deploy them. The research by Scharf and colleagues details where Wormlions deploy pitfalls in the city. It appears that walls can increase predator capture, but this depends on whether prey moves along the wall or not.
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