Ecological trap

Article by Julie O'Connor, senior conservation partnerships officer, Sunshine Coast Council

The unfortunate giant jewel beetle that falls for a green beer bottle, the birdwing butterfly confusing its food for a toxic imposter, and dragonflies mistaking cemetery stones for pools of dark water, these examples are just some of the man-made ecological traps affecting some species.

Ecological traps, also referred to as evolutionary traps, can occur when a species is attracted to inferior habitat, not necessarily inferior quality per se, but inferior suitability for its reproduction and long-term survival.

Species generally evolve responses to environmental cues to guide their behavioural decisions. Sometimes, surprising features, such as lighting, in highly modified human habitats can replicate those cues so effectively that species are drawn to them just as surely as they are to the cues in their natural habitat.

An unusual example of such an ecological trap was discovered in a country cemetery in Hungary.

Researchers observed several Sympetrum species of dragonfly staking territories around black gravestones where they behaved in the same territorial manner that they did around water bodies. That is, individual dragonflies were observed adopting a favoured perch and defending intrusion by others; flying individuals repeatedly touching the ventral side of their bodies onto the top of the gravestone; and pairs in tandem position frequently circling the polished black surface of the gravestones.

The explanation behind this unusual behaviour lay in the way:

• light reflected from the polished black surfaces, and
• that the dragonflies were detecting this reflected light.

Previous research has found that some dragonflies prefer dark water and they find suitable water bodies on the basis of the horizontally polarised light reflected from the water surface. In that little Hungarian cemetery the polarisation features of the polished black gravestones were almost identical to the polarisation features of dark waterbodies. In extreme cases, such maladaptive responses can lead to localised extinctions.

A local example of an ecological trap can be found in the relationship between the Richmond birdwing butterfly and the introduced South American dutchman’s pipe vine Aristolochia elegans. The environmental cues perceived by the butterfly are similar enough in both the introduced plant and its native food plant Paristolochia praevenosa to induce the female to lay her eggs on either plant. Unfortunately, however, the leaves of the introduced dutchman’s pipe are toxic to the caterpillars when they feed on its leaves.

Most species require multiple cues to trigger certain behaviours, but this does not necessarily protect a species from falling into an ecological trap. For example, the male giant jewel beetle (Julodimorpha bakewelli) has been observed apparently using size, colour and texture cues to select green beer bottles as mates, even favouring the bottles over available females. Unfortunately, the beer bottle ticked all the cue boxes, with its deep colour, larger size and smooth texture of the bottle winning over the male.

While the ecological traps above were created by artefacts of human habitat, some species have perfected the art of creating their own ecological traps for other species. For example, carnivorous plants use a variety of methods to lure unwary insects to their death. Some pitcher plants use attractive interior patterns and nectar offerings to attract insects to the inside of the pitcher. Once on the inside, a flaky wax coating on the wall of the pitcher guarantees the insect’s slide into a cocktail of enzymes that convert the unfortunate victim into nutrients for the plant.

Several species of the Drosera genus found on the Sunshine Coast use an active flypaper tactic to catch insects. An insect’s contact with the sticky tentacles triggers the plant to close up, engulfing its prey for digestion and absorption of its nutrients. While the sundews don’t close at lightning speed (e.g. the tentacles of Drosera burmanii can bend 180° in around a minute), it is fast enough to seal the fate of the already stuck victim.

So, while the urban environment has certainly produced a variety of novel ecological traps, the concept has long been embedded in nature.

References

• Robertson, B. A. and Chalfoun, A. D. 2016. “Evolutionary traps as keys to understanding behavioural maladaptation” Current Opinion in Behavioural Sciences 12: 12-17
• Schlaepfer, M. A., Runge, M. C. and Sherman, P. W. 2002. “Ecological and evolutionary traps”, Trends in Ecology & Evolution 17: 10, 474-480.

Images by Alan Wynn & Ray Seddon