Local population density affects pollinator visitation in the endangered grassland daisy Rutidosis leptorhynchoides (Asteraceae)

Abstract

The spatial arrangement of plants has implications for their pollination. Dense patches of flowering plants can result in increased pollinator attraction and, consequently, higher pollination and seed set per flower. We investigated this effect in the endangered, self-incompatible Australian daisy Rutidosis leptorhynchoides F.Muell. (Asteraceae) by quantifying the effect of plant density on pollinator visitation and seed set in a wild population. Pollinator activity was investigated by direct observation of insect behaviour, by examining the pollen carried on candidate insect species, by video monitoring of visitation, and by tracking the movement of dye as a pollen analogue. Two native Lasioglossum species (Hymenoptera: Halictidae) were identified as the most frequent pollen-carrying visitors to inflorescences. Their visitation was significantly higher where plants were dense, but no statistical correlation between seed set and plant density was found. Florivory was identified as potentially having a substantial negative influence on seed set, but without any clear relationship to plant density. Given that R. leptorhynchoides has declined substantially across its range, and previous reintroduction attempts have often failed, our findings provide key information pertaining to knowledge of pollinator and florivore behaviour in relation to plant density. To maximise success of future management strategies, these animal-plant interactions should be captured in species recovery designs.