Pollen limitation and inbreeding depression in an ‘old rare’ bumblebee‐pollinated grassland herb

Abstract

Habitat fragmentation and reduction of population size have been found to negatively affect plant reproduction in 'new rare' species that were formerly common. This has been attributed primarily to effects of increased inbreeding but also to pollen limitation. In contrast, little is known about the reproduction of 'old rare' species that are naturally restricted to small and isolated habitats and thus may have developed strategies to cope with long-term isolation and small population size. Here we study the effects of pollen source and quantity on reproduction of the 'old rare' bumblebee-pollinated herb, Astragalus exscapus. In two populations of this species, we tested for pollen autodeposition, inbreeding depression and outbreeding depression. Caged plants were left unpollinated or were pollinated with pollen from the same plant, from the same population or from a distant population (50 km). Additionally, we tested for pollen limitation by pollen supplementation in four populations of different size and density. In the absence of pollinators, plants did not produce seed whereas self-pollinated plants did. This indicates a self-compatible breeding system dependent on insect pollination. Both self-pollination and, in one of the two populations, cross-pollination with pollen from plants from the distant population resulted in a lower number of seeds per flower than cross-pollination with pollen from plants from the resident population, indicating inbreeding and outbreeding depression. Pollen addition enhanced fruit set and number of seeds per flower in three of the four populations, indicating pollen limitation. The degree of pollen limitation was lowest in the smallest but densest population. Our results suggest that, similar to 'new rare' plant species, also 'old rare' species may be at risk of inbreeding depression and pollen limitation.

Genetic and reproductive characterisation of seasonal flowering morphs ofGentianella bohemicarevealed strong reproductive isolation and possible single origin

“…; Becker et al . ). Given only four early‐flowering populations today, such a scenario seems very likely.…”

“…Low visitation rates are often a result of population decline, small population sizes and/or fragmentation and isolation of populations due to disturbed mutualism between pollinators and plants (e.g. Luijten et al 1999;Mustarj€ arvi et al 2001;Becker et al 2011). Given only four early-flowering populations today, such a scenario seems very likely.…”

Pollinator limitation causes sexual reproductive failure in ex situ populations of self-compatible Iris ensata

“…If an empty fruit exhibited evidence of natural damage or fruit predators, it was discarded. Seed set of each treatment was expressed as the proportion of ovules per flower that developed into seeds (seed set = seed number/ovule number) (Becker et al 2011). Flowers that did not develop into a fruit were treated as fruits with 0 seeds per flower.…”

Explaining Rarity of the Dry Grassland Perennial Astragalus exscapus

“…The species is self-compatible but is obligately pollinated by insects (bumblebees). Without pollinators the flowers do not produce seeds (Becker et al 2010). Though insect damage (herbivory) is rare in A. exscapus, the plants are often infected by the Astragalus exscapus-specific rust Uromyces jordianus Bubák, which becomes visible when it produces rust-coloured urediospores on the leaves in May.…”

“…However, reproductive potential of A. exscapus could have been reduced when its habitats were very small and surrounded by extensive forest in the mid-Holocene. The former can be assumed because A. exscapus depends on bumblebees for pollination (without pollinators A. exscapus does not produce seeds; Becker et al 2010), and bumblebees are known to avoid woodland habitats (Westrich 1989). Therefore, many populations of A. exscapus could have gone extinct due to probably low reproduction in that time, contributing to the present day rarity of the species.…”