Revue internationale d'écologie méditerranéenne International ...

INTRODUCTION
ecologia mediterranea, tome 29, fascicule 2, 2003, p. 153-164
INTERPRETING GERMINATION RESULTS BASED ON SEED SIZE, MASS AND ECOLOGICAL… ◆
Most studies on the evolution or ecological significance
of seed size begin with two observations. Firstly,
across the global flora, seed size varies over some 10
orders of magnitude, and secondly, within species, seed
size is remarkably constant (Harper et al., 1970; Silvertown,
1989). An explanation for the apparent constancy
within species was provided by the theoretical treatment
of Smith and Fretwell (1974), who assumed that a plant
has fixed amount of resources to allocate for reproduction
(reproductive efforts) and that a decision must therefore
be made concerning both the number and size of the
offspring.
Differences in seed size among species may represent
an adaptation to different types of micro-sites
(Silvertown, 1989). If species compete for establishment
sites, large seeded species must have some advantage
to counterbalance the reduction in their seed production
(Lindsay et al., 1999). This advantage could take a
number of forms: (i) small-seeded species could suffer
higher levels of predation, (ii) large-seeded species being
the best competitor for all sites, (iii) safe-micro-sites
for the large-seeded species being much more frequent
than those for small-seeded species. The latter form is
in agreement with numerous experimental studies that
have shown that larger-seeded species are better able to
establish and survive under a wide range of conditions
(Westoby et al., 1997).
There are a number of arguments as to why large
seeds should be advantageous in low moisture conditions.
Baker (1972) and Salisbury (1975) argued that the large
seed weight may enable the seedling to allocate proportionately
more to root development and so produce
extensive root system quickly. For example, in Central
Australia seedlings issued from larger seeds had larger
roots up to 10 days after germination than seedlings from
smaller seeds of 32 species (Jurado & Westoby, 1992).
Several studies have shown that intra-specific mortality
rates in the field are higher for smaller seedlings under
water stress (Parker, 1982; Cook, 1980). Thus, the larger
root systems of seedlings from large seeds may provide
an advantage by allowing access to soil moisture at
deeper levels. The final argument concerning the likely
advantage of large seed size concerns the energetic cost
of tolerating low moisture conditions. Large seeds provide
a larger metabolic reserve for seedlings than small seeds.
Morphological and physiological characteristics, which
confirm drought tolerance, are energetically expensive, so
that the probability of establishment in low soil moisture
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