RELATIONSHIPS BETWEEN SEED MASS, PLANT HEIGHT AND DISPERSAL DISTANCE WITHIN DISPERSAL SYNDROMES
Surprisingly, seed mass had little influence on mean and maximum dispersal distances within dispersal syndromes, especially after accounting for plant height. This lack of relationships could be because species adjust their investment in energetic rewards for dispersers or dispersal structures (primary or secondary) to increase dispersal distances (Appendix S1). We examine relationships between seed mass and dispersal distances within each dispersal syndrome in Appendix S1. There was a positive relationship between mean dispersal distance and plant height within all dispersal syndromes, even for ant-dispersed and seed-caching species where experimental depots are commonly used (Fig. 4). Seeds dispersed from experimental depots negate the influence of canopy width, because dispersal distance is measured from a depot and not the plant base. This suggests that the positive relationship for tall species is not dependent on the drop distance from the canopy edge, but in fact tall species have adapted to disperse long distances, possibly to escape the reduced survival associated with parental proximity (Janzen 1970; Howe & Smallwood 1982; Hyatt et al. 2003). Wind is often considered a long-distance dispersal syndrome, but we found that species dispersed by animals through ingestion, attachment or seed-caching actually disperse much further than do species with wind-dispersed seeds (Fig. 2). Our findings could be attributed to certain dispersal syndromes being easier to track for long distances (ant or seed-caching) compared with other types of syndromes (wind and water). However, overall, species that disperse seeds using biotic vectors appear to be better dispersers, gaining long dispersal distances, than species using abiotic dispersal vectors. We found few studies that gave the total dispersal distance of combined dispersal syndromes for species that use multiple dispersal syndromes (e.g. Beaumont, Mackay & Whalen 2009). Most studies only examined dispersal distances of par-ticular dispersal syndromes within species or conversely used seed traps without defining all the vectors. Therefore, our data were typically species’ dispersal distance for one syndrome and did not represent entire dispersal kernels of individual species. Broadening research from singular syndromes to more complete dispersal kernels for species is an important future research direction (Vander Wall &Longland 2004). There are multiple definitions for dispersal capacity, which can include or exclude survival to reproductive maturity (Howe & Smallwood 1982; Levin et al. 2003). Our study used two measures of dispersal capacity that excluded recruitment: mean and maximum seed dispersal distance. However, some species use directed dispersal, where seeds are dispersed to local sites with relatively higher probabilities of survival over sites that are further away from the parent plant (Howe & Smallwood 1982). Species using directed dispersal may have higher seedling and juvenile survival rates over passively dispersed seeds, making them more efficient dispersers with greater dispersal capacities (Wenny 2001). Our use of simple dispersal distance meant we excluded other sources of variation, such as seed predation and germination rates, but we could not account for directed dispersal. Despite this and differences between methodologies, our results indicated underlying trends between dispersal capacity and plant traits. Future work using measures of dispersal that include recruitment would complement our findings.