Life history traits as predictors of plant rarity, with particular reference to hemiparasitic Orobanchaceae

We made a comparison between life history and reproductive characteristics of a group of Dutch rare (30) and common (105) plant species, all dicotyledonous and insect-visited forbs. The traits life span, clonality, breeding system, seed production, seed dispersal, and soil seed bank longevity are considered. All trait values have been ranked according to their possible effect on the vulnerability of a species for extinction, where low values indicate a low risk and high values a high risk for a species. Rare and common plants differed significantly in four traits: seed production, breeding system, seed bank longevity and clonality. The discriminant analysis showed that 79% of the 135 species was correctly classified in the group they presently belong to. Especially species that are rare now but were much more common 50-100 years ago were classified as being common species, pointing at the detrimental effect of habitat loss these species encountered at which they have not yet been able to adapted to. The data set included eight hemiparasitic plant species (family Orobanchaceae), of which six are endangered in The Netherlands and two more common. Both rare and common hemiparasites scored high traits values, indicating that they are vulnerable for extinction. The hemiparasites had several characteristics in common: they are all annuals or biennials that have no means of clonal reproduction. They all have non-clustered, zygomorphic flowers that produce nectar and have a precise pollen presentation and are mainly visited by (bumble) bees. Mean life span was significantly shorter in the eight hemiparasitic Orobanchaceae than in the non-hemiparasitic Scrophulariaceae used for comparison (4 species). Overall, we concluded that hemiparasitic plants have a special combination of life history traits which makes them vulnerable for local and regional extinction.