Pollination, seed set and seed predation on a landscape scale

We analysed the combined effects of pollination and seed predation on seed set of Centaurea jacea in 15 landscapes differing in structural complexity. In the centre of each landscape, a patch of Centaurea plants was establish ed for standardized measurements of flower visitation, seed predation and s eed set. Both the number of flower-visiting bees and the proportion of flow er heads damaged by seed predators increased,,vith landscape complexity, wh ich was measured as the proportion of semi-natural habitats. The mean numbe r of seeds per flower head did not increase with the proportion of semi-nat ural habitats, presumably because of the counterbalancing effects of pollin ation and seed predation. For a subset of undamaged flower heads, the numbe r of seeds per flower head was positively correlated with the number of flo wer visits. Further reasons for the unexpected failure to detect a correlat ion between landscape complexity and seed set appeared to be changes in flo wer-visitor behaviour and the contrasting responses of honeybees and wild b ees to habitat context. Landscape analyses at eight spatial scales (radius of landscape sectors, 250-3000 m) showed that different groups perceived th e landscape at different spatial scales. Changes in pollinator numbers coul d be explained only at small scales (up to 1000 m), while the seed predator s also responded to large scales (up to 2500 m).