Nearly all papers on the evolution or ecological significance of seed size begin with two observations. First that, across the global flora, seed size varies over some 10 orders of magnitude, and secondly that, within species, seed size is remarkably constant (Harper et al. 1970; Haig 1989; Silvertown 1989). While the second observation implies that there is strong stabilizing selection on seed size, the first implies that the selected seed size varies widely between species.
An explanation for the apparent constancy within species was provided by the theoretical treatment of Smith & Fretwell (1974), who assumed that a plant has a fixed amount of resources to allocate to reproduction and that a decision must therefore be made concerning both the number and size of those offspring. Their model predicts that there will be a single optimum seed size that is evolutionarily stable (Lloyd 1987): individuals that produce seeds either smaller or greater than the optimum suffer reduced fitness, and consequently any observed variability in seed size is maladaptive (Lloyd 1987; Geritz 1995). However, it has been pointed out that the model fails to explain why the form of the seed size/offspring fitness function should vary so dramatically between species that share the same habitat (Geritz 1995; Rees & Westoby 1997).
However, if seedlings of other species form an essential part of the offspring's environment (for example through competition for establishment sites) and if seed size affects the competitive ability of seedlings (Pemadasa & Lovell 1974; Mack & Harper 1977; Law & Watkinson 1987), offspring fitness will depend not only on maternal provisioning of the species concerned, but also on the seed size of competing individuals (Geritz et al. 1988; Geritz 1995; Rees et al. 1996; Rees & Westoby 1997). Using these assumptions Rees & Westoby (1997) showed that, providing the advantage to larger seeds is capped in some way, this can lead to coalitions of species within which each species has a different seed mass. Because no single seed size is evolutionarily stable, new species can invade the community providing that their seed size is sufficiently different from that of resident species. If producing larger seeds results in reduced fecundity, the model is an example of a competition/colonization trade-off, in which seed mass determines both competitive and colonizing ability via the trade-off between seed size and seed number. Several theoretical treatments have demonstrated that competition/colonization trade-offs can allow the coexistence of a large number of species (Skellam 1951; Armstrong 1976; Hastings 1980; Tilman 1994).
It has been suggested that a competition/colonization trade-off based on seed mass might promote coexistence in sand-dune annual communities (Rees 1995; Rees et al. 1996); we explored this possibility with an overlapping group of species from a limestone grassland. The species have been observed to differ greatly in their seed masses (see Table 1), although it is not known whether large-seeded species do indeed suffer reduced fecundity. It is entirely possible that individuals of species that produce larger seeds will also capture more resources in total and hence produce more seeds as well as larger seeds, as is often found to be the case when such trade-offs are looked for within species (Venable 1992). In the natural community all species-pairs are spatially segregated (Turnbull 1998), meaning that all species experience an increase in the ratio of conspecific to heterospecific neighbours above that expected from their respective population sizes (Coomes et al. 1999). It has been suggested that such spatial segregation may result from local dispersal (Mahdi & Law 1987), although it may also indicate the presence of an underlying mosaic of patches with species specializing on different patch types (Mahdi & Law 1987; Law et al. 1993). In this case, seed size differences among species may represent such an adaptation to different types of microsite.
|Background seed production (10 cm–2)|
|Species||Seed mass (g)||1994||1995|
A seed-sowing experiment was devised to assess the advantage to larger seeds, and to test specifically for a competitive hierarchy based on seed mass. Seed sowing experiments have been used before to demonstrate colonization limitation (Putwain et al. 1968; Gross & Werner 1982; Fowler 1986; Kelly 1989; Eriksson & Ehrlen 1992; Tilman 1997) but not to answer more detailed questions about community structure. In this experiment the effect of any trade-off between seed size and seed number is annulled by sowing mixtures of seed composed of equal numbers of all species at a range of densities. As sowing density increases, each species should reach an increasing proportion of the available microsites and so there is less potential for winning-by-forfeit (sensuHurtt & Pacala 1996).
If the community is structured by spatial niche differentiation, each species will have a competitive advantage in some microsites, and will therefore be expected to win some sites even when all species reach all sites. In this situation there should therefore be no reduction in the number of species present as sowing density increases (Fig. 1a). However, if there is a clear competitive dominant, it is predicted that, once its colonization limitation has been overcome, this species will occupy all microsites and exclude all others (Tilman 1994; Pacala & Rees 1998). Hence, as sowing density increases, any competitive dominant is expected to make up an increasing fraction of the total number of recruits (Fig. 1b). If greater seed mass confers enhanced competitive ability we expect the competitive hierarchy to be based on seed mass.