Rees et al. (2001) drew together a diversity of experimental, comparative and theoretical approaches to produce some generalizations about vegetation dynamics. Early successional plants have a series of correlated traits, including high fecundity, effective dispersal, rapid growth when resources are abundant, and poor growth and survival when resources are scarce. Late successional species usually have the opposite traits, including an ability to grow, survive and compete when resources are scarce. In the absence of disturbance,
... that predicts a stable species composition and the time taken to reach it an ideal theory of succession should predict and explain a trade-off between colonization and competitive ability?
late successional species eventually outcompete early species, because they reduce resources beneath the levels required by the early successional species. Early species persist for two reasons: (i) because their dispersal ability and high fecundity permits them to colonize and establish in recently disturbed sites before late successional species can arrive; or (ii) because rapid growth under resource-rich conditions allows them to temporarily outcompete late successional species even if they arrive at the same time. Rees and his colleagues refer to the first mechanism as a competition-colonization trade-off and the second as the successional niche (early conditions suit early species because of their niche requirements). The competition-colonization trade-off is strengthened by a further physiological inevitability. Huge differences in per capita seed production among plant species are inversely correlated to equally large variations in seed size; plants producing tiny seeds tend to produce many more of them than plants producing large seeds (see Section 4.8.5). Thus, Rees et al. (2001) point out that small-seeded species are good colonists (many propagules) but poor competitors (small seed food reserves), and vice versa for large-seeded species.
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