Birds and guelder rose Viburnum opulus: selective consumption and dispersal via regurgitation of small-sized fruits and seeds

Plant Ecol (2009) 203:111122 DOI 10.1007/s11258-008-9514-y

Birds and guelder rose Viburnum opulus: selective consumption and dispersal via regurgitationof small-sized fruits and seeds

ngel Hernndez

Received: 24 July 2008 / Accepted: 19 September 2008 / Published online: 8 October 2008 Springer Science+Business Media B.V. 2008

Abstract The evolution of eshy fruit size, in particular in bird-dispersed plants, is believed to be inuenced by the size of seed-dispersing vertebrates through gape limitation. Also, it has been demonstrated that seed size correlates positively with fruit size, especially in single- or few-seeded fruits. However, there is little evidence of current selection pressure by disperser birds on fruit and seed size within populations of a particular plant species. In the present study, this aspect was investigated in guelder rose Viburnum opulus (Caprifoliaceae) fruit consumption by birds in an area in NW Spain. Guelder rose fruits are sub-globose drupes that can exceed 11 mm in width, with a single hard seed of up to8.5 mm in width. Most of the seeds were dispersed by the robin Erithacus rubecula (gape width \ 8 mm)

and a small thrush, the song thrush Turdus philomelos (gape width \ 11 mm), which swallowed the fruits whole, and some were destroyed by the bullnch

Pyrrhula pyrrhula. Most of the seeds were regurgitated rather than defecated by disperser birds, probably because seed size limited gut processing. The mean size of the ingested seeds was smaller than the mean diameter of Turdus droppings, which in turn was smaller than the mean size of the seeds of the

fruits available on the plants. As winter progressed, only larger fruits and seeds remained on the plants (seed and fruit size were positively correlated), and the size of ingested seeds increased. Thus, the largest fruits were consumed less by seed-dispersing birds and were exposed to seed-predators (bullnches and climbing rodents) for longer. Selection pressure on smaller guelder rose seeds must therefore be effective in the dispersal stage in the study area.

Keywords Fruit size Gape limitation

Gut processing Seed-dispersing birds

Seed size Selection pressure

Introduction

Most eshy fruit traits show signicant levels of phylogenetic inertia, or could be the result of selection by other environmental factors than seed-dispersing vertebrates (Fuentes 1992; Jordano 1995a; Cipollini and Levey 1997; Izhaki et al. 2002;...