Multiple intrinsic and extrinsic drivers influence the quantity and quality components of seed dispersal effectiveness in the rare shrub Lindera subcoriacea

Abstract

Information about seed dispersal effectiveness (SDE) for plant species of conservation concern is rarely available to inform management strategies and actions. For Lindera subcoriacea (bog spicebush, Lauraceae), a rare endemic dioecious shrub of the southeastern United States, we examined the influence of two intrinsic and five extrinsic drivers on the number and proportion of seeds either dispersed, or predated pre- and post-dispersal. The number of seeds dispersed characterizes the quantitative component of SDE, while pre- and post-dispersal seed predation can affect the qualitative component of SDE. Using fruit counts, seed traps, and seed removal depots over multiple years, we estimated that approximately 28% of L. subcoriacea seeds are lost to pre-dispersal predation, 69% of seeds are dispersed, 3% of seeds fail to disperse, and 65% of dispersed seeds are predated post-dispersal. We observed substantial variation in these three processes among individuals. We also found that both intrinsic (plant height, crop size) and extrinsic (understory cover, time since last fire, conspecific fruiting neighborhood, substrate) drivers differentially influenced the three processes. We identified four generalist, seasonally frugivorous, avian visitors at fruiting individuals that likely act as variably effective dispersers, while the Northern Cardinal (Cardinalis cardinalis L.) is a seed predator. Rodent granivores were important pre- and post-dispersal seed predators. The magnitude of our pre-dispersal and post-dispersal seed predation estimates suggest that, given the low fecundity of L. subcoriacea, conservation strategies should emphasize facilitating dispersal and reducing the effects of seed predation.

Effects of Landscape Heterogeneity and Disperser Movement on Seed Dispersal

“…There are still additional considerations to be addressed when modeling L. subcoriacea seed dispersal, including various intrinsic and extrinsic drivers, and use of allometrically estimated versus observed avian parameters. For example, Hohmann et al [59] found that understory cover affected the magnitude of pre-dispersal seed predation and individual height affected L. subcoriacea SDE Q , but neither of these drivers were captured within our current model. The effects of vegetation management, such as prescribed fire, can also affect the availability of L. subcoriacea and other fleshy fruits in time and space [51,116] and influence L. subcoriacea SDE Q [59], but this variation was not included in our experiments.…”

“…Lindera subcoriacea is a rare, dioecious, avian-dispersed [59] shrub that inhabits several wooded wetland communities of the southeastern U.S. [51]. Small, isolated populations of L. subcoriacea have been identified in seven southeastern states [50].…”

“…Fort Liberty hosts at least 100 bird species, although the duration and seasonality of residency varies by species [63]. During the post-breeding period, which overlaps with peak dispersal/removal of mature L. subcoriacea fruit [59], a total of 22 seasonally-frugivorous bird species were observed on Fort Liberty (authors' unpublished data). A subset (n = 18) of the observed species had sufficient observations to develop species occupancy models [64].…”

“…LISU cells (n = 75) had a mean of 130.5 ± 33.5 (SE) and a median of 3 fruits. All fruits were considered mature (ripe) throughout modelling and no additional fruits were produced within the model; this summarized our field observations of fruit maturation trends [59]. Non-LISU locations varied within and among bird species and were a subset (n = 100) of the remaining cells in the landscape; their fruit availability was representative of their vegetation type and season (see Appendix S1).…”

“…Complete experiment descriptions are described in our Appendix S1. For each of the four experiments, we executed 30 replicates, each lasting for 7 simulation days, representing the mean duration of ripe fruit dispersal/removal [59]. Each bird species (n = 5) had their own set of experiments.…”