Scale dependence shapes how plant traits differentially affect levels of pre- and post-dispersal seed predation in Scots pine

To understand the demographic consequences of seed predation, it is essential to scale-up losses over space and time. We tested how individual tree characteristics, forest attributes, and the local environment affected rates of both pre- and post-dispersal seed predation in Scots pine Pinus sylvestris, in Scotland’s ancient native pinewoods. The sampling strategy was hierarchical, allowing assessment of how the influence of these factors differed with spatial scale. This approach revealed that the fate of a tree’s seeds depended on many factors, at multiple scales. Levels of seed predation were highly variable over space and time. Seed predation by local post-dispersal seed predators (invertebrates and small mammals) varied at the finest spatial scales, whereas mobile avian pre-dispersal seed predators discriminated among seeds at every spatial scale. Tree crown shape affected removal of seed by all seed predators; both crossbills Loxia spp. and generalist avian granivores selected trees with broad crowns. However, trees with narrow crowns were selected by red squirrels Sciurus vulgaris, and the associated tall understory vegetation supported the highest levels of post-dispersal seed predation. Seed chemistry was an effective seed defence that reduced seed loss to all seed predators. In contrast, increases in physical cone defence were ineffective at reducing seed loss and showed a potential trade-off with investment in seed viability. Large seeds carried a survival cost, being preferentially consumed by squirrels and post-dispersal seed predators. All post-dispersal seed predators consumed a higher proportion of seeds at the times and locations where seed fall was greatest

Seed selection by crossbills Loxia spp. within cones of Scots Pine Pinus sylvestris

Why foraging animals sometimes leave patches before consuming all available food items is a key question in behavioural ecology. Abandoning some food would appear to be disadvantageous, yet optimal foraging theory demonstrates that this sometimes optimises energy intake rates. Crossbills Loxia are specialist avian granivores that forage on seeds within the cones of many species of conifer in the northern hemisphere. They often abandon a few seeds within cones they have fed upon. We assessed whether seeds left within Scots Pine Pinus sylvestris cones by foraging crossbills were of lower mass or in particular positions within the cone. Crossbills foraged on long cones which have more seeds per cone and heavier seeds, but avoided the longest cones, which typically have thicker scales that require more energy to open. Cones dropped by foraging crossbills contained over four seeds per cone, representing approximately one fifth of those originally present. Crossbills left mostly small or empty seeds ( 4 mg) were left behind, perhaps mistakenly overlooked during foraging. Such apparent preferential foraging on heavier seeds is probably advantageous, because of the higher energy reward per seed. To directly discriminate between seeds prior to extraction would reduce energy expenditure in foraging. This raises the question of how crossbills could attain this favourable outcome. While cones scales were closed an external cue would be required. After cone scale dehiscence, seeds would be visible to crossbills, allowing them to discriminate visually among seeds and selectively extract heavier seeds, leaving lighter seeds behind within the cone. Dropping cones when few seeds are encountered or as seed mass declines towards the distal scales could be additional components of crossbill foraging strategy