Gábor Horváth and odonatologist colleagues noticed that dragonflies were attracted to gravestones in a Hungarian cemetery. Although a number of species occurred in the area, only darters (Sympetrum species) visited the stones. The dragonflies were very particular in their choice of sites — they preferred large, black or dark grey, polished, horizontal stones clear of overhanging vegetation but with a nearby perching place. When the research team looked at the light reflected from the gravestones, they found it identical to that from other flat, dark surfaces, including still water.
- Fig. 1 (a) Male and female dragonflies (Sympetrum sp.) perching on the tips of sunlit iron railings in a cemetery in the Hungarian town Kiskunhalas. (b, c) Males of Sympetrum sp. perching near polished black tombstones. (d) A female Sympetrum sp. displaying touching behaviour at the shiny black plastic sheet used in the double-choice experiments. The photo shows the brief moment when she touches the test surface with her legs and ventral side of her body. (From Horváth et al., 2007)
The study shows that these Sympetrum species are sensitive to polarised light, using it to locate ponds and lakes for courtship and egg-laying. Mistaking gravestones for water every now and then is a small problem, but if it affects the dragonflies' reproductive success then it becomes an ecological trap (Schlaepfer et al., 2002).
The term 'ecological trap' applies to a situation where
in an environment that has been altered suddenly by human activities, an organism makes a maladaptive habitat choice based on formerly reliable environmental cues, despite the availability of higher quality habitat.
Under natural conditions — no puddles of oil, no black plastic sheets, no gleaming gravestones — horizontally polarised light is a dependable indicator of water, a reliable environmental cue. But when humans introduce materials with identical properties, the dragonflies are misled. Their habitat choice is now maladaptive, reducing their reproductive success and ultimately their numbers.
So far, the lure of the gravestones doesn't seem to have had an impact on the five Sympetrum species that frequent the cemetery but other dragonflies are known to lay their eggs on oil, shiny road surfaces and cars.
Horváth, G, Malik, P, Kriska, G & Wildermuth, H. (2007). Ecological traps for dragonflies in a cemetery: the attraction of Sympetrum species (Odonata: Libuellidae) by horizontally polarizing black gravestones. Freshwater biology 52: 1700–1709.
Schlaepfer, MA, Runge, MC & Sherman, PW. (2002). Ecological and evolutionary traps. Trends in Ecology & Evolution 17: 474–480.