Seed mass and the competition/colonization trade-off: Competitive interactions and spatial patterns in a guild of annual plants
Turnbull LA., Coomes D., Hector A., Rees M.
1. We used neighbourhood modelling to estimate individual-level competition coefficients for seven annuals growing in limestone grassland over 2 years. We calculated the relative strength of intra- and interspecific competition and related this to differences in seed size and plant size between targets and neighbours. 2. Significant differences in the impact of neighbours on each target species were observed in half the models fitted, allowing us to reject a null hypothesis of competitive equivalence. 3. In one year we found that as the seed size or plant size of neighbours increased relative to targets, so did their competitive effect. Although this is consistent with the competition/colonization trade-off model the competitive interactions were not sufficiently asymmetric to allow coexistence. In a second year we found only weak interspecific competition and no relationship with plant or seed size. 4. We found no overall relationship between competition coefficients and the degree of segregation, contradicting the spatial segregation hypothesis for coexistence. However, segregation was linked to differences in plant traits: when targets were smaller than neighbours the degree of segregation increased with relative neighbour size. 5. Most species were positively associated with each other due to a shared preference for otherwise unvegetated patches. The degree of association was negatively correlated with differences in plant and seed size, particularly when interspecific competition was weak. This might reflect (i) decreasing overlap in microhabitat use with increasing trait divergence or (ii) density-dependent mortality. 6. Seed size is a key trait within this group of species, determining both competitive and colonizing ability. The presence of such a competition/colonization trade-off undoubtedly stabilizes community dynamics although other mechanisms may also be at work.