Thomson et al. Seed dispersal distance is more strongly correlated with plant height than with seed mass.
Appendix S1: Predicted and actual relationships between seed mass and dispersal distance within dispersal syndromes with explanations.
Shows the predicted relationship (+ positive, - negative or ○ no relationship) for seed mass and dispersal distance within dispersal syndromes
and the preliminary reasoning for the predictions. The actual relationship between seed mass (SM) and mean (xˉ ) or maximum (Max) dispersal
distance, and seed mass controlling for plant height (SM + PH) and mean or maximum dispersal distance (N indicates prediction was wrong, Y
indicates prediction was right). Explanation for the actual result if different from the predicted relationship is given. Water (n = 8) and mean
distance for attachment (n = 4) were not analysed due to small sample sizes.
Predicted
relationship
Preliminary reasoning
Actual relationship
xˉ
SM
Max
Possible explanation for actual results (if
different to prediction)
SM+PH
xˉ
Max
Unassisted
+/○
Taller plants have larger seeds and wider
canopies (Thompson & Rabinowitz 1989; King
1990). We predicted a positive relationship
between seed mass and dispersal distance for
species with unassisted dispersal, because seeds
falling straight down from a wide canopy will
land farther from the base of the parent plant.
Once height is controlled for we expected no
relationship.
+
Y
+
Y
○
Y
○
Y
Wind
–
We predict a negative relationship because
heavier seeds may drop to the ground faster than
light-weight seeds, thus travelling shorter
distances (Greene & Johnson 1993; MullerLandau et al. 2008)
+
N
○
N
○
N
–
Y
Taller plants have larger seeds, and release
heights are important for species dispersal
distances. Large seeded species compensate for
heavier seeds with greater release heights
(Greene & Johnson 1986).
Ballistic
We predicted a negative relationship because
+
+
○
+
Small-seeded species may have low investment
1
Thomson et al. Seed dispersal distance is more strongly correlated with plant height than with seed mass.
–
Narbona, Arista & Ortiz (2005) suggested that
large-seeded species may have additional
structures on the seed to aid secondary dispersal,
and that these dispersal structures may reduce
dispersal distance. For two Viola species, the
species that invested less in its secondary
dispersal syndrome (elaiosome mass) had greater
ballistic dispersal distances (Ohkawara &
Higashi 1994).
N
N
N
N
in dispersal because they use secondary dispersal
syndromes such as wind or water. Large-seeded
species may compensate for weight e.g. by
producing high density seeds that increase
momentum and seeds being ejected individually
with greater accuracy (Stamp & Lucas 1983).
Ingestion
–
We predicted a negative relationship between
seed mass and dispersal distance for ingested
species, on the basis of observations from
previous studies. There was a negative
relationship between seed mass and dispersal
distance for 31 animal dispersed tropical tree
species (Muller-Landau et al. 2008) and for 21
species ingested by mallards (Soons et al. 2008).
○
N
○
N
○
N
○
N
The type of vector, vector behaviour and gut
passage times may be more important than
variation in plant traits for dispersal potential in
animals (Will & Tackenberg 2008). Crossspecies differences in reward size, seed number
per dispersal unit and protective structures will
influence dispersal distance and if seeds are
removed. These factors can be independent of
seed size. For ruminates there was no
relationship between seed mass and dispersal
potential (Bruun & Poschlod 2006).
Seed-caching
+
We predicted a positive relationship between
seed mass and dispersal distance for species
dispersed by seed-caching vertebrates, on the
basis of observations from previous studies
(Seiwa et al. 2002; Jansen, Bongers & Hemerik
2004; Xiao, Zhang, Wang 2005). The greater
potential energy gains from large seeds makes
them more attractive than small seeds to seedeaters according to optimal foraging theory
○
N
○
N
○
N
○
N
Our results are similar to those of Vander Wall
(2003) who also found no relationship between
seed mass and dispersal distance. Some vectors
may move lots of seeds as opposed to one seed at
a time which removes some of the constraints
predicted from optimal foraging theory.
Differences in seed nutrient content and
protective structures between plant species
means potential energy gain may not be
2
Thomson et al. Seed dispersal distance is more strongly correlated with plant height than with seed mass.
(Orians & Pearson 1979; Stephens & Krebs,
1986). Optimal cache spacing models predict that
high value seeds will be taken greater distances
than low value seeds (Stapanian & Smith 1978)
to stop density- or mass-dependent loss of caches
to other foragers.
Ant
+
We predicted a positive relationship because
larger ants transport seeds further (Gomez &
Espadaler 1998; Ness et al. 2004), and bigger
seeds are transported by larger ants (Pfeiffer,
Nais & Linsenmair 2006). Also large-seeded
species compensate for greater seed mass by
increasing elaiosome size (Edwards, Dunlop &
Rodgerson 2006) probably making their seeds
more or equally attractive to ants as seeds from
small-seeded species.
Attachment
○
We predicted no relationship because seed mass
has little effect on whether seeds attach to vectors
(Will, Maussner & Tackenberg 2007) and
retention times on fur for smaller seeds may be
longer, shorter, or equal compared to large seeds
(Kiviniemi & Telenius 1998; Couvreur et al.
dependent on seed mass alone (Vander Wall
2003).
○
N
○
N
○
Y
○
N
○
N
We suggest that seed mass had little influence on
dispersal distance because firstly when small ant
species find a large seed that is too heavy to
carry, they often remove the elaiosome in situ
(Edwards, Dunlop, & Rodgerson 2006), resulting
in large seeds being taken very short distances
(or not being moved at all). Secondly when
resources are scarce, the positive correlation
between worker size and load size disappears
(Pfeiffer, Nais & Linsenmair 2006), so ants will
target both large and small seeds. Lastly ants
may select seeds on the nutrient content of the
elaiosome not elaiosome/seed size. The nutrient
content of elaiosomes may be independent of
size, so although the elaiosome is larger the
nutrient reward may be smaller making them less
attractive to ants.
○
Y
3
Thomson et al. Seed dispersal distance is more strongly correlated with plant height than with seed mass.
2004; Couvreur, Verheyen & Hermy 2005;
Römermann, Tackenberg & Poschlod 2005).
Furthermore large-seeded species may cancel out
the negative impact of seed mass by increasing
the number of hooks, spikes or the stickiness of
seeds (Kiviniemi & Telenius 1998; Couvreur et
al. 2004).
Additional References
Bruun, H. H. & Poschlod, P. (2006) Why are small seeds dispersed through animal guts: large numbers or seed size per se? Oikos, 113, 402-411.
Couvreur, M., Vandenberghe, B., Verheyen, K. & Hermy, M. (2004) An experimental assessment of seed adhesivity on animal furs. Seed
Science Research, 14, 147-159.
Couvreur, M., Verheyen, K. & Hermy, M. (2005) Experimental assessment of plant seed retention times in fur of cattle and horse. Flora, 200,
136-147.
Edwards, W., Dunlop, M. & Rodgerson, L. (2006) The evolution of rewards: seed dispersal, seed size and elaiosome size. Journal of Ecology,
94, 687-694.
Jansen, P. A., Bongers, F. & Hemerik, L. (2004) Seed mass and mast seeding enhance dispersal by a neotropical scatter-hoarding rodent.
Ecological Monographs, 74, 569-589.
Kiviniemi, K. & Telenius, A. (1998) Experiments on adhesive dispersal by wood mouse: seed shadows and dispersal distances of 13 plant
species from cultivated areas in southern Sweden. Ecography, 21, 108-116.
Narbona, E., Arista, M. & Ortiz, P. L. (2005) Explosive seed dispersal in two perennial mediterranean Euphorbia species (Euphorbiaceae).
American Journal of Botany, 92, 510-516.
Ohkawara, K. & Higashi, S. (1994) Relative importance of ballistic and ant dispersal in 2 diplochorous viola species (Violaceae). Oecologia,
100, 135-140.
Orians, G. H. & Pearson, N. E. (1979) On the theory of central place foraging. Analysis of ecological systems (eds D. J. Horn, R. D. Mitchell &
G. R. Stairs), pp. 154-177. Ohio State University Press, Columbus.
4
Thomson et al. Seed dispersal distance is more strongly correlated with plant height than with seed mass.
Pfeiffer, M., Nais, J. & Linsenmair, K. E. (2006) Worker size and seed size selection in 'seed'-collecting ant ensembles (Hymenoptera :
Formicidae) in primary rain forests on Borneo. Journal of Tropical Ecology, 22, 685-693.
Römermann, C., Tackenberg, O. & Poschlod, P. (2005) How to predict attachment potential of seeds to sheep and cattle coat from simple
morphological seed traits. Oikos, 110, 219-230.
Seiwa, K., Watanabe, A., Irie, K., Kanno, H., Saitoh, T. & Akasaka, S. (2002) Impact of site-induced mouse caching and transport behaviour on
regeneration in Castanea crenata. Journal of Vegetation Science, 13, 517-526.
Soons, M.B., van der Vlugt, C., van Lith, B., Heil, G.W. & Klaassen, M. (2008) Small seed size increases the potential for dispersal of wetland
plants by ducks. Journal of Ecology, 96, 619-627.
Stamp, N. E. & Lucas, J. R. (1983) Ecological correlates of explosive seed dispersal. Oecologia, 59, 272-278.
Stapanian, M. A. & Smith, C. C. (1978) Model for Seed Scatterhoarding – Coevolution of Fox Squirrels and Black Walnuts. Ecology, 59, 884896.
Stephens, D. W. & Krebs, J. R. (1986) Foraging Theory. Princeton University Press, Princeton.
Vander Wall, S. B. (2003) Effects of seed size of wind-dispersed pines (Pinus) on secondary seed dispersal and the caching behavior of rodents.
Oikos, 100, 25-34.
Will, H., Maussner, S. & Tackenberg, O. (2007) Experimental studies of diaspore attachment to animal coats: predicting epizoochorous dispersal
potential. Oecologia, 153, 331-339.
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