Maternal fecundity does not affect offspring germination – An empirical test of the sibling competition hypothesis

Seed dormancy is often described as a bet-hedging strategy in temporally variable and unpredictable environments such as deserts, or as a possibility to escape local crowding caused by sibling competition. Because the risk of experiencing sibling competition increases with increasing seed family size, a negative relationship between maternal fecundity and offspring germination fraction is predicted. Since competition is the main reason for this relationship, this effect should be stronger in more competitive environments. Unfortunately, there is very limited and contradictory empirical support for the sibling competition hypothesis, and habitats differing in competition intensity have never been compared. Using field collected seeds, we examined the effect of seed family size on germination fractions in three common dryland winter annuals along a steep climatic gradient where competition intensity increases with increasing productivity. Our findings clearly contradict the sibling competition hypothesis. There was no relationship between maternal fecundity and germination fractions in any of the ten combinations of species and environments but high variation between coexisting families. In combination with related studies, our findings suggest that other factors, such as variation in the abiotic environment or generalised competition may be more important constraints for the evolution of optimal germination than competition among siblings.

► Empirical test of the ‘sibling competition hypothesis’.
► We use three annual plant species in four different environments.
► Results contradict the ‘sibling competition hypothesis’.
► Sibling competition does not explain seed dormancy.
► Generalised competition and abiotic factors appear more important.