Brown bear (Ursus arctos)
Brown bears consume the soft mast of at least 101 plant species spanning
24 families and 42 genera (García-Rodríguez et al. 2021). A
single fecal deposit from a brown bear can contain thousands of seeds
(Willson 1993), creating a massive seed dispersal event. While brown
bears do not form cooperative packs, social dominance and subordination
within populations has predictable effects on individual resource access
and space use. In salmon-supported populations, dominant individuals
(typically large males) displace subordinate individuals (particularly
females with cubs) at salmon streams (Ben-David et al. 2004;
Gende & Quinn 2004). Since time spent at a salmon stream is positively
correlated with salmon assimilation in brown bear diets and negatively
correlated with plant matter assimilation (Deacy et al. 2018), it
is likely that the monopolization of salmon streams by dominant
individuals leads to consistently greater quantities of seed-dispersal
services by subordinate individuals. Moreover, since the vast majority
of long-distance movement by bears is done by dispersing subadults
(Bartoń et al. 2019), these subordinate individuals are most
likely to disperse plant species beyond current range boundaries (Fig.
3A). Lamb and colleagues (2020) also show that urban habitats serve as
demographic sinks for brown bear populations, and urban populations are
supported by immigration by dispersing subadults. These subordinate,
subadult bears may therefore contribute substantially to the dispersal
and persistence of some plant populations in urban areas (Fig. 3B).
Since hunters typically target resident male bears (Gosselin et
al. 2017; Leclerc et al. 2019), this form of selective harvest
disrupts brown bear social structure, leading to increased infanticide
when immigrant males disperse into newly opened territories (Gosselinet al. 2017). Whether or not the persistent disruption of brown
bear social structure by selective harvesting generates cascading
effects on seed dispersal remains an open question. Moreover, it is
widely appreciated that brown bears use corridors to access otherwise
isolated habitat patches, and corridors are particularly important for
the dispersal of subadults into urban habitats (Clevenger & Waltho
2005; Ford et al. 2017; Lamb et al. 2020). Corridors
designed to facilitate brown bear movement through fragmented landscapes
may therefore have unappreciated benefits for the dispersal of many
bear-dispersed plant species.