Rees et al. (2001) point out that it follows from the competition-colonization trade-off that recruitment of competitively dominant plants should be determined largely by the rate of arrival of their seeds. This means that herbivores that reduce seed production are more likely to reduce the density of dominant competitors than of subordinates. Recall that this is just what happened in the sand-dune study described in Section 16.4.4. In a similar vein, Carson and Root (1999) showed that by removing insect predators of seeds, the meadow goldenrod (Solidago altissima), which normally appears about 5 years into an old-field succession, became dominant after only 3 years. This happened because release from seed predation allowed it to outcompete earlier colonists more quickly.
Thus, apart from competition-colonization trade-off, succes-sional niche and facilitation, we have to add a fourth mechanism - interactions with enemies - if we are to fully understand plant successions. Experimental approaches, such as that employed to understand the role of seed predators, have also shown that the nature of soil food webs (Gange & Brown, 2002), the presence and disturbance of litter (Ganade & Brown, 2002), and the presence of mammals that consume vegetation (Cadenasso et al., 2002) sometimes play roles in determining successional sequences.
Figure 16.13 Relative growth rate (during the July-August 1994 growing season) of trembling aspen (+), northern red oak (•) and sugar maple (□) in relation to photosynthetic photon flux density (PPFD). (After Kaelke et al., 2001.)
an important role for seed prédation?
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