Did Drosera evolve long scapes to stop their pollinators from being eaten?

Abstract

Background and aims: Insectivorous plants frequently display their flowers on the ends of long racemes. Conventional wisdom is that long racemes in insectivorous plants have evolved to provide spatial separation between flowers and traps, which consequently prevents pollinators from being captured. However, it is also possible that long racemes evolved for better seed dispersal or to make flowers more visible to pollinators.

Methods: Two sympatric insectivorous plants with identical pollinators were studied: Drosera cistiflora, with an upright growth form but a short raceme; and Drosera pauciflora, with a basal rosette of traps and a very long raceme. If long racemes evolved to protect their pollinators then D. cistiflora should capture more pollinators than D. pauciflora. However, if long racemes evolved to attract pollinators then taller flowers should receive more pollination visits than shorter flowers.

Key results: Examination of D. pauciflora and D. cistiflora traps revealed that no pollinators were captured by either species, suggesting that long racemes did not evolve to protect pollinators from being captured. Experimental manipulations of flower height in D. cistiflora showed that experimentally shortened plants received significantly fewer pollination visits than plants which were taller in stature.

Conclusions: Long scapes in Drosera and non-insectivorous plants probably evolved due to similar selective pressures such as pollinator attraction.

Fig. 1.

(A) Drosera cistiflora showing the small distance separating the traps and flower. (B) Drosera pauciflora showing with a long scape and large distance separating flowers from traps. (C) A large monkey beetle visiting a D. cistiflora flower for pollen. (D) A Tabanid fly on the petal of D. cistiflora. (E) The typical size of the prey items captured by D. cistiflora. (F) D. pauciflora with a large monkey beetle in the foreground and several smaller monkey beetles in the background.

Fig. 2.

The spectral reflectance of D. pauciflora and D. cistiflora, as indicated, where solid lines are averages and dotted lines are the standard deviations.

Fig. 3.

The number of insects observed from various functional groups on D. cistiflora and D. pauciflora, as indicated. NS, non-significant at P > 0·05.