Seed Size and Plant Growth Form as Factors in Dispersal Spectra

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Abstract
All vascular plant species were listed on eight sites near Sydney, Australia. Four sites were on infertile and four on fertile soil. Each of the 335 species was classified according to seed mass (measured as fresh mass of embryo plus endosperm), morphological adaptations for dispersal by different vectors, and growth form. The infertile-soil sites had more species and more cover adapted for dispersal by ants, and the fertile-soil sites had more species and more cover with fleshly fruits adapted by vertebrates. We tested the hypothesis that this difference could be interpreted as a secondary correlate of seed size or of growth form. Species with smaller seed mass or growing to < 2 m tall were significantly more likely to be adapted for dispersal by ants relative to vertebrates. An indirect association via growth form was capable of accounting for up to 84% of the relationship between soil type and dispersal mode, an indirect association via seed mass for up to 23% of the relationship; the two together could account for a maximum of 85%. The indirect association via seed mass was relatively weaker because seed masses proved not to be very different between these soil types. There remained a significant residual tendency for species in any given seed size class and growth form to be more likely to be dispersed by vertebrates relative to ants on fertile soils than on infertile soils.