Oecologia (2010) 163:141–152 DOI 10.1007/s00442-010-1567-7 123 PLANT-ANIMAL INTERACTIONS - ORIGINAL PAPER Delayed response in a plant–pollinator system to experimental grassland fragmentation Hans-Peter Rusterholz · Bruno Baur Received: 16 June 2009 / Accepted: 13 January 2010 / Published online: 13 February 2010  Springer-Verlag 2010 Abstract The fragmentation of natural habitat is consid- ered to be a major threat to biodiversity. Decreasing habitat quality and quantity caused by fragmentation may lead to a disruption of plant–pollinator interactions and to a reduc- tion in sexual reproduction in plant species. We conducted a 6-year Weld experiment to investigate the eVects of small- scale fragmentation on plant–pollinator interactions and genetic diversity in the self-compatible Betonica oYcinalis. We examined the abundance and composition of pollina- tors, the foraging behaviour of bumblebees and the perfor- mance, outcrossing rate and genetic diversity of B. oYcinalis after 2 and 6 years in experimentally frag- mented nutrient-poor, calcareous grassland in the northern Swiss Jura mountains. Fragments of diVerent size (2.25 and 20.25 m 2 ) were isolated by a 5-m-wide strip of frequently mown vegetation. Control plots of corresponding size were situated in adjacent undisturbed grassland. Experimental grassland fragmentation altered the composition of B. oYcinalis pollinators and reduced their Xower visitation rate. Furthermore, the foraging behaviour of bumblebees was changed in the fragments. After 6 years of fragmenta- tion seed weight was higher in fragments than in control plots. However, the densities of B. oYcinalis rosettes and inXorescences, plant height and inXorescence length were not aVected by fragmentation. The outcrossing frequency of B. oYcinalis growing in fragments was reduced by 15% after 2 years and by 33% after 6 years of experimental frag- mentation. This resulted in a signiWcant reduction of the genetic diversity in seedlings emerging in fragments after 6 years. Our study shows that small-scale habitat fragmen- tation can disturb the interaction between B. oYcinalis and pollinators resulting in a reduced outcrossing frequency and genetic diversity in plants growing in fragments. However, the response to fragmentation was considerably delayed. This Wnding strengthens the claim for long-term Weld experiments with proper replications and controls to assess delayed eVects of habitat fragmentation. Keywords Betonica oYcinalis · Genetic diversity · Pollinator foraging behaviour · Outcrossing frequency · Self-compatibility Introduction The fragmentation of natural habitats is generally consid- ered to be a major threat to biodiversity (Saunders et al. 1991). Fragmentation reduces the area suitable for organ- isms and leads to isolation and decreased size of remnant populations in plants and animals (Gilpin and Soulé 1986). The disadvantages suVered by small populations involve greater sensitivity to environmental and demographic sto- chasticity (Holsinger 2000), which together with a loss of genetic diversity and inbreeding depression result in a high risk of local extinction (Krauss et al. 2004). Furthermore, eVects of habitat fragmentation may lead to the disruption of biotic interactions such as parasitism, seed dispersal or pollination and hence, can aVect species with previously stable populations (Kearns et al. 1998; Groppe et al. 2001). Communicated by Rebecca Irwin. Electronic supplementary material The online version of this article (doi:10.1007/s00442-010-1567-7) contains supplementary material, which is available to authorized users. H.-P. Rusterholz (&) · B. Baur Section of Conservation Biology, Department of Environmental Sciences, University of Basel, St. Johanns-Vorstadt 10, 4056 Basel, Switzerland e-mail: hans-peter.rusterholz@unibas.ch