Abundance and diaspore weight in rare and common prairie grasses

Abundance (g/m²) and diaspore weight are positively correlated in seven species of perennial grasses that occur in prairies. The rare grasses (<10.0 g/m²) have light dispersal units (0.06 to 1.76 mg); the common grasses (>10.0 g/m²) have heavy dispersal units (2.23 to 2.80 mg). This result is the first reported differentiating trait between related rare and common organisms occurring in same habitat.Three hypotheses that explain this phenomenon are compared; the third most likely holds. First, rare grasses may be rare because their small seeds are less successful in establishment than those of common grasses. Second, if the persistence of small populations is marginal, rare grasses may devote less energy (or other currency) to seed production. Third, rare grasses may be colonizers of spatially and temporally rare microsites appropriate for growth and thus have seeds adapted for longer distance dispersal than those of common grasses. This last hypothesis suggests a new pathway for the evolution of weeds.