COMPETITION FOR POLLINATORS IN THE ECOLOGY OF CENTAUREA SCABIOSA L. AND CENTAUREA NIGRA L.

Abstract

SUMMARYThe insects which visit Centaurea scabiosa and Centaur̊ea nigra feed mainly on nectar and are likely to visit more frequently the species which produces most nectar. Nectar was extracted from plants in field populations and from plants transplanted into the Botanic Garden, to determine diurnal pattern and total quantity and concentration of nectar produced over 24 h periods.Florets of both species open in the morning and secrete nectar fairly constantly through the first day of floret opening. C. scabiosa florets continue to secrete into the second day when more florets start to open, but C. nigra usually ceases to secrete nectar around 18.00 h on the first day. C. scabiosa at all times produced more nectar than C. nigra, and on average produced about four times as much. On Hod Hill plants of C. scabiosa produced slightly more nectar than those on the Devil's Ditch. There was no difference between different C. nigra populations. Different plants within each population differed considerably, one plant producing up to six times as much as another under fairly uniform conditions. Less nectar was secreted in cool weather or on a cloudy day than on a warm sunny day, and capitula on shaded shoots produced less than those in the sun. Capitula produced at the beginning and end of the flowering season appeared to secrete less than those produced in the middle. There was a large difference between years in some plants and plants studied in August 1977 produced more than those studied at any other time.