Grazing alters insect visitation networks and plant mating systems

Abstract

Summary1. Many flowering plant species have a facultative or obligate dependence on insect pollination for reproductive success. Anthropogenic disturbance may alter these species interactions, but the extent to which structural changes to plant-pollinator networks affect plant species mating systems is not well understood. 2. We used long-term livestock grazing of a birch wood ecosystem to test whether disturbance of this semi-natural habitat altered floral resources, the structure of plant-insect visitation networks and the mating system of a focal plant species, Cirsium palustre. 3. Grazed habitat had a higher species richness of floral resources for pollinators. Visitation networks in grazed habitats were larger, more diverse, with an increase in the number of pollinators per plant species. Controlling for sampling effects, however, showed networks in grazed habitats were less nested and revealed a positive correlation between network connectance and floral species richness. 4. Network connectance was negatively related to C. palustre outcrossing rate within grazed and ungrazed sites. However, on average, the effects of grazing, including greater mean connectance, produced higher overall outcrossing rates and more pollen donors compared with ungrazed habitat. The number of different pollen donors, spatial genetic structure and mating among close relatives were all correlated with greater extent of suitable C. palustre habitat in the landscape, consistent with the effects of increasing plant population size but limited seed dispersal. 5. Pre-adaptation of C. palustre to disturbance coupled with a preponderance of highly dispersive flies attracted to the greater food resources in grazed habitat is a likely mechanism underpinning this increased pollen transport. 6. Habitat modification by long-term mammalian grazing fundamentally shifted visitation network structure and the state of a plant mating system, indicating how ecosystem disturbance can cascade across levels of biological organization through altered interspecific interactions. Cirsium palustre retains flexibility to bias reproduction towards selfing where pollen donor diversity is limited; such reproductive flexibility may be an important mechanism structuring plant populations in human-modified landscapes.

Variation in Insect Assemblage and Functional Groups along a Grazing Gradient in an Arid Environment

“…Previous works showed that the disturbances generated by human activity caused pollinator and plant diversity declines worldwide [5][6][7][8][9][10][11]. One of the main human-disturbance that may affect plant-pollinator interactions is the introduction of livestock for grazing in natural areas [12][13][14][15].…”

Landscape alteration and habitat modification: impacts on plant–pollinator systems

“…Sociality is another trait affecting vulnerability to landscape alteration. Social bees are central location foragers tied to the colony location, consequently they are more sensitive to the distance to forage resource patches in the surrounding landscape [20 ,38] than nonsocial insects with free-living progeny, such as Diptera [38,39 ]. Even within social bee taxa, species-specific differences in mobility and dispersal range will govern responses to habitat loss and/or fragmentation.…”

The resilience of forest fragmentation genetics—no longer a paradox—we were just looking in the wrong place

“…With many tree species being animal pollinated (at least in tropical systems), the genetic diversity and fitness of progeny of woody plants sired in fragmented landscapes are expected to decline due to shifts in plant-mating patterns (for example, reduced outcrossing rate and/or pollen diversity due to habitat fragmentation and/or changes in pollinator community composition and behavior, Vanbergen et al, 2013). This hypothesis is tested by Breed et al (2015a) in a case study of three ecologically divergent Eucalyptus tree species, and from a meta-analysis of the literature.…”

“…A continued focus on these three areas is likely to be scientifically fruitful and deliver real progress on management strategies for trees in impacted landscapes. In particular, a more quantitative approach to linking changes in pollination to mating systems to fitness is likely to yield important insights on the mechanisms driving the changes we observe (for example, Vanbergen et al, 2013). We look forward to reviewing new hypotheses, developments and latest research in this area in 10 years time!…”