Disentangling direct and indirect effects of habitat fragmentation on wild plants' pollinator visits and seed production

Abstract

Habitat fragmentation threatens plant and pollinator communities, as well as their interactions. However, the effects of landscape fragmentation on the pollination of wild plant species are not well understood yet, partly because there are many correlated features in fragmented landscapes (e.g., decreased patch size, increased isolation, and patch complexity) whose influences are difficult to disentangle. Using a structural equation modeling approach, we assessed the direct and indirect effects of landscape fragmentation (patch size, isolation and complexity, percentage of surrounding land in forest) on the abundance, functional‐group richness, and evenness of pollinators of 24 habitat fragments within an agricultural landscape in Southern Norway. In addition, we studied how these variables affected visitation rates (visits per flower) and seed production (seed set, seed mass) in the four most abundant plant species in the area. Flower abundance was higher in larger and complex patches and decreased with the percentage of forest in the surroundings, while flower richness increased with patch complexity. We found a direct negative relationship between patch complexity and the overall number of pollinator visits that the habitat fragments received. Apart from this direct landscape effect, pollinator visits were mostly affected by the floral communities, with overall flower abundance and richness increasing both total number of pollinator visits and pollinator‐group richness, and flower richness having an additional negative influence on pollinator‐group evenness. Interestingly, we did not find any direct link between visitation rates and reproductive success for any of the study plant species. Instead, several landscape variables directly affected species seed production, although the effects of landscape on seed production were highly species specific. Patch complexity had a negative effect on seed production in two of the four focal species, while other components of the landscape had species‐specific effects. Increasing fragmentation of agricultural landscapes affects pollination interactions at the community level and the reproduction of wild plants. However, understanding the effects of fragmentation on seed production requires going beyond estimating visitation rates, since landscape effects on plant reproduction are not always related to overall interaction frequencies.

Forest fragmentation modifies the composition of bumblebee communities and modulates their trophic and competitive interactions for pollination

“…Consistent with previous studies in Scandinavia 42 , the relationship of bumblebee abundance and richness with flower density varied along the flowering season, probably because bumblebee populations are much larger in the middle of the season 41,57 and then the resources become more limiting 42 . It is also in concordance with the positive relationship between bumblebee visitation and blooming density found in other studies 15,58,59 , and with a previous study carried out in our study system that showed a strong influence of the flowering resources on the whole pollinator community 5 . It is not surprising that, in systems as the Scandinavian temperate forests, which are generally poor in flowers 60 , flower availability plays such an important role in bumblebee distribution.…”

“…However, these important pollinators are suffering worldwide declines 1,2 , which have been mainly attributed to the loss and fragmentation of natural and semi-natural areas due to landuse changes 1,3 . Such negative effects of habitat fragmentation are related to the reduction in the extent of natural habitats 1 , to changes in the quantity and quality of flowering resources 4,5 and nesting sites 6 and their spatial configuration 7 , as well as to changes in habitat edge/area relationships that could affect habitat suitability 8 (see Fig. 1 for a conceptual diagram).…”

Impacts of beekeeping on wild bee diversity and pollination networks in the Aegean Archipelago

“…For any given area, an increase in the number of different cover types leads to a decrease in the amount of each (Duelli 1997), which might not allow a large growth of populations of species that are benefitted from certain habitats. Finally, as was true for wild bee richness, we also found that flower abundance tended to be positively related to wild bee abundance, supporting the idea that a larger number of flowers in a community attract a larger number of pollinators (Hegland and Boeke 2006, Lázaro et al 2020).…”

Solar park management and design to boost bumble bee populations

“…Habitat shape can also have implications as edge length increases as habitats become more complex in shape [25,26]. High edge to area ratios can increase exposure of pollinator populations to pesticides [27] and negative relationships between habitat patch complexity and pollinator visitation have been reported [28]. Conversely, edges themselves can be important pollinator habitats [29].…”