The effects of plant density upon pollination success, reproductive effort and fruit parasitism in Cistus ladanifer L. (Cistaceae)

Abstract

Human activity often causes a decline in the local density of plant populations. Below some critical lower density, populations may suffer a progressive decline in reproductive success because of the difficulties associated with finding suitable mates. Therefore, to conserve endangered plant species it is necessary to understand in greater detail how changes in population density affect different determinants of plant reproductive success. We simultaneously recorded individual plant pollination success, reproductive effort and fruit parasitism in three populations of Cistus ladanifer L. in eastern Portugal. Pollination success declined significantly as distance to the nearest conspecific increased (p<0.001). However, reproductive effort and fruit parasitism showed the opposite pattern (both p<0.001). On average, plants farther than 1 m from their nearest conspecific suffered from three times more fruit parasitism compared to plants closer to a conspecific. Overall, net female reproductive output decreased as nearest neighbor distance rose (p<0.001). Thus, isolated plants were able to compensate only partially for reduced pollination success through increased reproductive effort. We conclude that management plans for plant populations should recognize that reproductive success is the accumulated result of several different processes, which may each respond to plant density in different ways.