Hitch-hiking in the wild: should seeds rely on ungulates?

Abstract

Seed dispersal is a key process in determining plant populations and community dynamics. Particularly, long-distance dispersal events (> 100 m for plants, Cain et al. 2000), though rare, are crucial to maintaining the connectivity between isolated populations necessary for plant species to survive at the regional scale (Will & Tackenberg 2008). Longdistance dispersal by large herbivores also accounted for the plant migration rates during the Holocene (Pakeman 2001). Understanding long-distance dispersal mechanisms and their consequences is critical in a context of rapidly changing environments under the effects of global change (Shupp et al. 2010). Because plants are fixed, their mobility relies on vectors that may be abiotic like wind (anemochory) and water (hydrochory), or biotic (zoochory) (Ridley 1930). Wind and water disperse few species over long distances (Mouissie et al. 2005, Schmidt et al. 2004) and wind is ineffective inside forest stands (Howe & Smallwood 1982). Birds and mammals are therefore the main vectors for long-distance seed dispersal within and among forest areas. While birds seem to disperse specific plants according to their diet (endozoochory), large herbivores, compared to small-bodied rodents, might be able to disperse many plant species in their fur due to a larger surface available for hitching seeds. For example, the role of roe deer (Capreolus capreolus) and red deer (Cervus elaphus) in the expansion of Green Hound’s-tongue (Cynoglossum germanicum Jacq.) through external dispersal has recently been highlighted (Boulanger et al. 2011). Wild ungulates provide useful models for the study of long-distance plant dispersal and distributional dynamics under changing landscapes. First, roe deer, red deer and wild boar (Sus scrofa) populations have been increasing in Europe, and particularly in France, since the 1970s. The number of animals killed by hunting has increased ninefold over the last forty years, reaching over 500,000 individuals for both roe deer and wild boar and 50,000 for red deer in 2010 (Source: ‘Reseau Ongules sauvages ONCFS-FNC-FDC’). Second, due to their large home ranges and high travel speed, wild ungulates move over long distances and through