by Hilary Haskell
Post-closure, landfills require after-care and remediation. Additional after-care may involve restoring landfills into semi-natural habitats for flowering plants and insect pollinators. Tarrant et al. (2013) compared the floral characteristics, species richness and abundance, and pollinator assemblages at reference sites of ecological value and restored landfill sites. Pollinator populations were similar across the reference and restored sites. The authors found no differences in floral species richness or abundance between reference sites and restored sites. Some seasonal differences in species richness and abundance occurred at restored landfill sites. There are over 28,000 ha of landfills in England and Wales, and habitats are rapidly declining due to agricultural practices. Landfill restoration may provide a means to both remediate landfills and mitigate habitat loss.
Tarrant et al. (2013) researched the effectiveness of restoring closed landfill sites into semi-natural habitats for floral vegetation and insect pollinators. They compared species richness, abundance, and assemblage composition of insect-pollinated flowering plants at restored landfill sites and reference sites with established nature conservation value. The authors also considered differences between restored landfill sites that had been either naturally or artificially re-vegetated.
In Europe, an increase in agricultural land use has resulted in decreased population sizes and ranges for flowering plants and their pollinators. Pollination is a critical ecological service at both the ecosystem and societal level because both wild plants and crops depend on pollinators for survival. Although managed honey bees can also pollinate crops, most plants are pollinated by wild pollinators. Most research concerning landfill site restoration has primarily focused on plant diversity in the 1980s and 1990s in the United Kingdom. These studies have found low species richness at restored landfill sites, due to poor restoration and containment measures that allow for methane gas to escape from landfills, contaminate soil, and inhibit plant growth. In this study, the authors addressed whether or not improved remediation policies and standards for post-closure landfill sites have improved the potential for landfills to re-vegetate, support pollinators, and facilitate floral diversity.
Landfills are commonly restored for grassland stock grazing or semi-natural vegetation. Other sites have been successfully restored to promote habitat conservation, such as reclaimed opencast mines in North America. For habitat restoration purposes, there are approximately 2,200 working landfill sites in England and Wales that cover 28,000 ha. These landfills are closing at a rate of 100/year, and are then capped with clay and covered with 1m of soil before being hydro-seeded with liquid emulsion of grass seed, fertilizer, and binding agent. Alternatively, some landfills are left to regenerate naturally.
The authors considered nine restored landfill sites in East Midlands that were at least 0.5 ha in area. These sites are representative of restored landfill sites for the region. The landfills must have been restored to at least fifty percent of their natural state and have been in the process of restoration for at least four years. The reference sites included the closest grassland sites designated as Local Nature Reserves or Sites of Special Scientific Interest. These sites were at least 0.5 ha in area, and had similar local climates, regional plant and insect species pools, and landscapes. The reference sites were not under any type of pollinating insect conservation measures.
The authors conducted their survey with Standardized Plant Surveys from March through October in 2007 and 2008. These months include the most intense flowering period in the region, which results in higher pollination activity. All flowering plants were identified at the species level. Floral abundance and inflorescence size were used to calculate the floral cover, which represents the total available floral resources. Flower visitor surveys were conducted on warm, sunny days with little wind, per the Butterfly Monitoring Scheme. Insects that were able to pollinate plants through contact with the sexual part of flowers were captured for identification. The authors altered their methods in the second year of the study to increase capture rates, by surveying a spiral pattern transect. Tarrant et al. tested for normality using one sample Kolmogorov-Smirnov tests. For differences in means or medians, paired t=tests, one-way analysis of variances (ANOVAs), Mann Whitney U-tests, and Kruskal–Wallis tests. Pearson’s correlations were used for associations between variables. Non-metric multidimensional scaling (NMDS) was used to represent insect-pollinated flowering plant species composition and floral abundance between sites. The authors used this method due to its sensitivity to outliers and ability to map distances between flowering plant specimens.
The study’s survey area included 125 floral transects, covering 25,000m2 of area, with about 138,000 floral units from 98 plant species. The authors found restored landfill sites had 63 species of flowering plants with 19 species unique to the area, while the reference sites had 74 with 30 unique species. In 2007, the most common plant species at the landfill was Trifolium dubium, with a frequency of 6 out of 9 flowering plants. This flowering plant was also the most common at the reference site, with a slightly higher frequency of 7 out of 9. In 2008, the restored landfill site’s most common flowering plant was Trifolium repens, with a frequency of 3 out of 3, while the most common species at the reference site was Ranunculus acris, with a frequency of 3 out of 3. According to the NMDS analysis, the majority of species were clustered close together spatially, indicating an abundance of idiosyncratic species that occurred at only one site.
Both the landfill and reference sites exhibited similar mean annual richness for flowering plants, despite seasonal differences in richness between 2007 and 2008. In 2007, the restored landfill sites had a mean floral cover of 6.6 cm2/m2 and a combined total floral cover of 643 m2. The reference sites had a mean floral cover of 10.1cm2/m2 and a combined total floral cover of 342 m2.
The authors used 201 flower-visitor surveys, which resulted in sampling of 942 flower-visiting insects. In 2007, restored landfill sites had 156 insects from 30 species, while reference sites had 161 individuals from 37 species. In 2008, restored landfill sites had 405 insects with 41 species represented, while reference sites had 220 individuals from 40 species. The restored sites did not differ significantly in 2007 or 2008 from the reference sites in mean annual pollinator insect species richness per site, although there were some seasonal differences. The restored landfills and reference sites shared many of the same frequent native pollinators, including bumble bee species Bombus terrestris lucorum and Bombus lapidarius, as well as a variety of flies. Non-native honeybees were also frequent in the year 2007, but not in 2008. These findings indicate that restored landfills and the reference sites shared many of the same abundant pollinator insect species. According to the NMDS plots, almost all of the landfill sites were clustered closely together, indicating that they share many of the same flower-visiting species. Reference sites had a greater variance in their NMDS plots, demonstrating that species spread was greater at these sites. There was no significant difference in floral characteristics between artificially and naturally re-vegetated landfill sites.
Although the remediated landfill and reference sites demonstrated highly similar floral characteristics and pollinator insect species richness and abundance, there were significant differences between the sites in their floral cover’s seasonal plant species richness and abundance. The reference sites had greater richness and abundance in the spring, while the restored landfill sites exhibited this trend in the fall. The reference sites had greater mean cumulative seasonal floral cover in the first year of the study, while this was true for the restored landfill sites in the second year. The authors believe these differences may occur because reference sites are mown in the late summer, reducing floral cover. At restored sites, there may be less spring insect abundance due to lower floral cover or a lack of winter hibernation sites for pollinators.
This study supports similar research in restoration of British habitats and plant-pollinator relationships. Other studies that compared restored and natural meadows also found similar species richness and abundance between sites. But, these studies found few pollinators present at both sites. Most of these studies have focused more heavily upon recreating exact vegetation structures at remediated sites. This study focuses on the importance of restoring habitats to support pollinators and pollination of flowering plants, therefore addressing both pollinator and re-vegetation efforts. Pollination is essential for seed set, gene flow, and genetic diversity in plant populations.
Due to the number of shared species between restored and reference sites, a trend towards homogenization of insect pollinator populations may be occurring in England. If homogenization occurs, the common insects would adapt to intensive agricultural practices, while rarer species would be driven towards extinction due to their inability to adapt effectively. Although landfill restoration offers promise in remediating mass areas of land into functioning ecosystems, these findings may demonstrate decreases in biodiversity throughout the United Kingdom.
The ecosystems at restored landfill sites should be valued for the services they provided, especially pollination. Most consumers believe that managed bee populations are the most common pollinating insects, which is not the case. Relying solely on these managed bee populations does not provide the same ecosystem services as native insect pollinators, especially for certain crops.
Landfill remediation serves an important role in potentially restoring 28,000 ha of land across 2,200 landfill sites in the United Kingdom. These sites can provide habitats for species threatened by increased agricultural activity in the surrounding area. To maximize restoration potential, restored landfill sites should focus on increasing floral resources in the fall, thus providing a seasonal succession of plants for pollinators. Not only do remediated landfills provide habitats and ecosystem services threatened by increased agricultural intensity, but they also provide pollination for agricultural crops in the surrounding area.
Tarrant, S., Ollerton, J., Lutfor Rahman, M., Tarrant, J., McCollin, D., 2012. Grassland Restoration on Landfill Sites in the East Midlands, United Kingdom: An Evaluation of Floral Resources and Pollinating Insects. Restoration Ecology 21, 560–568. Full paper at: http://bit.ly/Zw9ZBr