Growth in seed dispersal studies has been fast-paced since the seed disperser effectiveness (SDE) framework was developed 17 yr ago. Thus, the time is ripe to revisit the framework in light of accumulated new insight. Here, we first present an overview of the framework, how it has been applied, and what we know and do not know. We then introduce the SDE landscape as the two-dimensional representation of the possible combinations of the quantity and the quality of dispersal and with elevational contours representing isoclines of SDE. We discuss the structure of disperser assemblages on such landscapes. Following this we discuss recent advances and ideas in seed dispersal in the context of their impacts on SDE. Finally, we highlight a number of emerging issues that provide insight into SDE. Overall, the SDE framework successfully captures the complexities of seed dispersal. We advocate an expanded use of the term dispersal encompassing the multiple recruitment stages from fruit to adult. While this entails difficulties in estimating SDE, it is a necessary expansion if we are to understand the central relevance of seed dispersal in plant ecology and evolution.

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Seed dispersal effectiveness revisited: a conceptual review

Ecosystems and Human Well-Being: Synthesis

Summary
1. It is often assumed that there is a trade-off between maternal provisioning and dispersal capacity, leading small-seeded species to disperse further than large-seeded species. However, this relationship between dispersal distance and seed mass has only been quantified for species from particular sites or with particular dispersal syndromes.

2. We provided the first large-scale, cross-species quantification of the correlations between dispersal distance and both seed mass and plant height. Seed mass was positively related to mean dispersal distance, with a 100-fold increase in seed mass being associated with a 4.5-fold increase in mean dispersal distance (R2 = 0.16; n = 210 species; P < 0.001). However, plant height had substantially stronger explanatory power than did seed mass, and we found a 5-fold increase in height was associated with a 4.6-fold increase in mean dispersal distance (R2 = 0.54; n = 211 species; P < 0.001).

3. Once plant height was accounted for, we found that small-seeded species dispersed further than did large-seeded species (R2 = 0.54; n = 181 species; slope = −0.130; P < 0.001); however, seed mass only added 2% to the R2 of the model. Within dispersal syndromes, tall species dispersed further than did short species, while seed mass had little influence on dispersal distance.

4. Synthesis. These findings enhance our understanding of plant life-history strategies and improve our ability to predict which species are best at colonizing new environments.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2745.2011.01867.x

Seed dispersal distance is more strongly correlated with plant height than with seed mass

1. Dispersal is fundamental to ecological processes at all scales and levels of organisation but progress is limited by a lack of information about the general shape and form of plant dispersal kernels. We addressed this gap by synthesising empirical data describing seed dispersal and fitting general dispersal kernels representing major plant types and dispersal modes. 
2. A comprehensive literature search resulted in 107 papers describing 168 dispersal kernels for 144 vascular plant species. The data covered 63 families, all the continents except Antarctica, and the broad vegetation types of forest, grassland, shrubland, and more open habitats (e.g. deserts). We classified kernels in terms of dispersal mode (ant, ballistic, rodent, vertebrates other than rodents, vehicle or wind), plant growth form (climber, graminoid, herb, shrub or tree), seed mass and plant height. 
3. We fitted 11 widely-used probability density functions to each of the 168 datasets to provide a statistical description of the dispersal kernel. The Exponential Power (ExP) and Log-sech (LogS) functions performed best. Other 2-parameter functions varied in performance. For example, the Lognormal and Weibull performed poorly, while the 2Dt and Power law performed moderately well. Of the single-parameter functions, the Gaussian performed very poorly, while the Exponential performed better. No function was among the best-fitting for all datasets. 
4. For 10 plant growth form/dispersal mode combinations for which we had >3 datasets, we fitted ExP and LogS functions across multiple datasets to provide generalised dispersal kernels. We also fitted these functions to sub-divisions of these growth form/dispersal mode combinations in terms of seed mass (for animal-dispersed seeds) or plant height (wind-dispersed) classes. These functions provided generally good fits to the grouped datasets, despite variation in empirical methods, local conditions, vegetation type and the exact dispersal process. 
5. Synthesis. We synthesise the rich empirical information on seed dispersal distances to provide standardised dispersal kernels for 168 case studies and generalised kernels for plant growth form/dispersal mode combinations. Potential uses include: a) choosing appropriate dispersal functions in mathematical models; b) selecting informative dispersal kernels for one’s empirical study system; and c) using representative dispersal kernels in cross-taxon comparative studies.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/1365-2745.12666

A synthesis of empirical plant dispersal kernels

Ecological disturbance is fundamental to the dynamics of biological communities, yet a conceptual framework for understanding the responses of faunal communities to disturbance remains elusive. Here, I propose five principles for understanding the disturbance dynamics of ants-a globally dominant faunal group that is widely used as bioindicators in land management, which appear to have wide applicability to other taxa. These principles are as follows: (1) The most important effects of habitat disturbance on ants are typically indirect, through its effects on habitat structure, microclimate, resource availability and competitive interactions; (2) habitat openness is a key driver of variation in ant communities; (3) ant species responses to disturbance are to a large degree determined by their responses to habitat openness; (4) the same disturbance will have different effects on ants in different habitats, because of different impacts on habitat openness; and (5) ant community responses to the same disturbance will vary according to ant functional composition and biogeographical history in relation to habitat openness. I illustrate these principles using results primarily from studies of ant responses to fire, a dominant agent of disturbance globally, to provide a common disturbance currency for comparative analysis. I argue that many of the principles also apply to other faunal groups and so can be considered as general ecological "laws." As is the case for ants, many impacts of habitat disturbance on other faunal groups are fundamentally related to habitat openness, the effects of disturbance on it and the functional composition of species in relation to it.

https://besjournals.onlinelibrary.wiley.com/doi/epdf/10.1111/1365-2656.12907

Responses of ant communities to disturbance: Five principles for understanding the disturbance dynamics of a globally dominant faunal group.

Seed dispersal by ants (myrmecochory) can be influenced by changes to ant assemblages resulting from habitat disturbance as well as by differences in disperser behaviour. We investigated the effect of habitat disturbance by fire on the dispersal of seeds of a myrmecochorous shrub, Pultenaea daphnoides. We also investigated the consequence of the seed relocation behaviours of two common dispersers (Pheidole sp. A and Rhytidoponera
 metallica) for the redispersal of seeds. Pheidole sp. A colonies did not relocate seeds outside their nests. In contrast, R. metallica colonies relocated 43.6 % of seeds fed to them, of which 96.9 % had residual elaiosome that remained attached. On average, R. metallica relocated seeds 78.9 and 60.7 cm from the nest entrances in burned and unburned habitat, respectively. Seeds were removed faster in burned than in unburned habitat, and seeds previously relocated by R.
 metallica were removed at similar rates to seeds with intact elaiosomes, but faster than seeds with detached elaiosomes. Dispersal distances were not significantly different between burned (51.3 cm) and unburned (70.9 cm) habitat or between seeds with different elaiosome conditions. Differences between habitat types in the frequency of seed removal, the shape of the seed dispersal curve, and the relative contribution of R. metallica and Pheidole sp. A to seed dispersal were largely due to the effect of recent fire on the abundance of Pheidole sp. A. Across habitat types, the number of seeds removed from depots and during dispersal trials most strongly related to the combined abundances of R. metallica and Pheidole. Our findings show that myrmecochory can involve more than one dispersal phase and that fire indirectly influences myrmecochory by altering the abundances of seed-dispersing ants.

Multiphase myrmecochory: the roles of different ant species and effects of fire.

The separate contributions of different vectors to net seed dispersal curves of diplochorous systems have rarely been characterised. In Australia, myrmecochory is a common seed dispersal syndrome and in the majority of such systems, seeds are initially dispersed ballistically. We measured ballistic and myrmecochorous seed dispersal distances in relation to canopies of Adriana quadripartita (Euphorbiaceae) and used a simulation model to estimate the net dispersal curve. We also compared seed removal rates and ant abundances under, and outside, plant canopies to examine how foraging patterns by ants may affect net dispersal. Overall ant abundance did not show a significant numerical response to seedfall; however, the abundance of the main seed dispersing ant, Rhytidoponera ‘metallica’ did. Despite this, seed removal rates did not differ significantly between canopy and open locations. Rhytidoponera ‘metallica’ account for 93% of observed seed dispersal events. On average, the ants dispersed seeds 1.54 m and in doing so, moved seed a mean radial distance of 0.76 m away from canopy edges. This contribution to net dispersal distance by ants is considerable since ballistic dispersal moved seeds a median distance of 7.5 cm. Our simulation model indicated that the combination of ballistic and ant seed dispersal is expected to result in seeds being transported a median net radial dispersal distance of 1.05 m from the canopy edge. Thus in this system, an important function of diplochory may simply be to move a higher proportion of seeds from under the canopy of parent plants than is possible by ballistic dispersal alone. This ‘dispersal-for-distance’ may result in reduced parent–offspring competition or may increase the probability that seeds reach rare safe sites for germination and recruitment.

Combining distances of ballistic and myrmecochorous seed dispersal in Adriana quadripartita (Euphorbiaceae)

Myrmecochory (seed dispersal by ants) is a common seed dispersal strategy of plants in fire-prone sclerophyll vegetation of Australia, yet there is little understanding of how fire history may influence this seed dispersal mutualism. We investigated the initial fate of seeds of two myrmecochorous plant species, the small-seeded Pultenaea daphnoides J.C. Wendl. and the large-seeded Acacia pycnantha Benth., in replicated burnt (3.25 years since fire) and unburnt (53 years since fire) forest plots in the Mount Lofty Ranges, South Australia. Specifically we measured (i) seed removal rates; (ii) the frequency of three ant–seed interactions (seed removal, elaiosome robbery and seed ignoring); (iii) the relative contribution of different ant species to ant–seed interactions; and (iv) the abundance of common interacting ant species. Rates of seed removal from depots and the proportion of seeds removed were higher in recently burnt vegetation and the magnitude of these effects was greater for the smaller-seeded P. daphnoides. The overall proportion of elaiosomes robbed was higher in unburnt vegetation; however, the decrease in elaiosome robbery in burnt vegetation was greater for P. daphnoides than for A. pycnantha. Ants ignored seeds more frequently in burnt vegetation and at similar rates for both seed species. In total, 20 ant species were observed interacting with seeds; however, three common ant species accounted for 66.3% of ant–seed interactions. Monomorium sydneyense almost exclusively robbed elaiosomes, Rhytidoponera metallica typically removed seeds and Anonychomyrma nr. nitidiceps showed a mix of the three behaviours towards seeds. Differences in the proportions of seeds removed, elaiosomes robbed and seeds ignored appeared to be largely driven by an increase in abundance of A. nr. nitidiceps and a decrease in abundance of M. sydneyense in burnt vegetation. Understanding how these fire-driven changes in the initial fate of myrmecochorous seeds affect plant fitness requires further investigation.

Kieren P. Beaumont

Papers

Multiphase myrmecochory: the roles of different ant species and effects of fire.

Combining distances of ballistic and myrmecochorous seed dispersal in Adriana quadripartita (Euphorbiaceae)

Interactions between ants and seeds of two myrmecochorous plant species in recently burnt and long‐unburnt forest sites

The effects of prescribed burning on epigaeic ant communities in eucalypt forest of South Australia

From salt to C: carbon sequestration through ecological restoration at the Dry Creek Salt Field