Contagious seed dispersal beneath heterospecific fruiting trees and its consequences

Abstract

2004. Contagious seed dispersal beneath heterospecific fruiting trees and its consequences. Á/ Oikos 107: 303 Á/308.An hypothesized advantage of seed dispersal is avoidance of high per capita mortality (i.e. density-dependent mortality) associated with dense populations of seeds and seedlings beneath parent trees. This hypothesis, inherent in nearly all seed dispersal studies, assumes that density effects are species-specific. Yet because many tree species exhibit overlapping fruiting phenologies and share dispersers, seeds may be deposited preferentially under synchronously fruiting heterospecific trees, another location where they may be particularly vulnerable to mortality, in this case by generalist seed predators. We demonstrate that frugivores disperse higher densities of Cornus florida seeds under fruiting (female) Ilex opaca trees than under non-fruiting (male) Ilex trees in temperate hardwood forest settings in South Carolina, USA. To determine if density of Cornus and/or Ilex seeds influences survivorship of dispersed Cornus seeds, we followed the fates of experimentally dispersed Cornus seeds in neighborhoods of differing, manipulated background densities of Cornus and Ilex seeds. We found that the probability of predation on dispersed Cornus seeds was a function of both Cornus and Ilex background seed densities. Higher densities of Ilex seeds negatively affected Cornus seed survivorship, and this was particularly evident as background densities of dispersed Cornus seeds increased. These results illustrate the importance of viewing seed dispersal and predation in a community context, as the pattern and intensity of density-dependent mortality may not be solely a function of conspecific densities.

Seed fate and seedling dynamics after masting in two African rain forest trees

“…Rather, in the understory, where the vast majority of seeds land, the stems (almost all of adult stature) of both species' function to deter both species seedling recruitment close to them, thereby generating a reciprocal apparent competition and a less host-specific Janzen-Connell effect (Janzen 1970, Carson et al 2008) that increases their establishment limitation because fewer ''safe'' sites are available in the forest. A similar effect of apparent competition, but non-reciprocal, was found for seed predation on a common understory tree (Cornus florida) in a temperate forest, which was a function of both its seed density and that of another co-dominant tree (Ilex opaca; Kwit et al 2004). Further, if Microberlinia seedling recruitment is also poorer in canopy gaps very close to Tetraberlinia stems because of overlapping dispersal (Fig.…”

“…In the latter, a negative indirect interaction occurs when one or both species' increased local abundance makes the other or both prey species more vulnerable to shared predators (Holt and Lawton 1994). These community level, predator-mediated interactions are often overlooked in both studies of masting and Janzen-Connell effects, despite their potential relevance to each (Schnurr et al 2002, Kwit et al 2004, Kitzberger et al 2007). …”

Hierarchical mechanisms of spatially contagious seed dispersal in complex seed‐disperser networks

“…For example, whereas seeds of Pyrus-Chamaerops and Juniperus-Rubus were positively associated at small spatial scales (i.e., 10 m), seeds of Pyrus-Pistacia and Chamaerops-Pistacia were positively associated at larger scales (i.e., ;400 m). These positive associations may be related to different ultimate causes, such as selection by individual frugivores of complementary fruit species (Whelan et al 1998), or the formation through the dispersal season of multispecific seed aggregates (Kwit et al 2004(Kwit et al , 2007J. M. Fedriani and T. Wiegand, unpublished data).…”

“…Thus, we clearly need robust analytical frameworks to characterize compound seed rains generated by multiple animal vectors dispersing several plant species at a range of spatial scales and to identify the underlying mechanisms. Seed dispersal in animal guts (i.e., endozoochory) is a pervasive process that often generates marked multispecific, spatially contagious seed dispersal (Schupp et al 2002, Kwit et al 2004. One overlooked approach allowing the unraveling of mechanisms of endozoochorous seed aggregation is the use of spatial point pattern analysis (Diggle 2003, Illian et al 2008.…”

Neighborhood effects on seed dispersal by frugivores: testing theory with a mistletoe–marsupial system in Patagonia

“…However, many studies of seed rain (i.e., seed deposition) indirectly document neighborhood effects on dispersal. Dispersal of zoochorous plants is contagious and patchy (Schupp et al 2002), with both seed rain and plant recruitment usually increasing in areas where fruiting resources are more dense or diverse (Howe and Smallwood 1982, Aukema and Martı´nez del Rı´o 2002, Fragoso et al 2003, Kwit et al 2004, Hampe et al 2008.…”