Why are some plant species missing from restorations? A diagnostic tool for temperate grassland ecosystems

The U.N. Decade on Ecosystem Restoration aims to accelerate actions to prevent, halt, and reverse the degradation of ecosystems, and re-establish ecosystem functioning and species diversity. The practice of ecological restoration has made great progress in recent decades, as has recognition of the importance of species diversity to maintaining the long-term stability and functioning of restored ecosystems. Restorations may also focus on specific species to fulfill needed functions, such as supporting dependent wildlife or mitigating extinction risk. Yet even in the most carefully planned and managed restoration, target species may fail to germinate, establish, or persist. To support the successful reintroduction of ecologically and culturally important plant species with an emphasis on temperate grasslands, we developed a tool to diagnose common causes of missing species, focusing on four major categories of filters, or factors: genetic, biotic, abiotic, and planning & land management. Through a review of the scientific literature, we propose a series of diagnostic tests to identify potential causes of failure to restore target species, and treatments that could improve future outcomes. This practical diagnostic tool is meant to strengthen collaboration between restoration practitioners and researchers on diagnosing and treating causes of missing species in order to effectively restore them.

Terrestrial ecosystem restoration increases biodiversity and reduces its variability, but not to reference levels: A global meta-analysis

Ecological restoration projects often have variable and unpredictable outcomes, and these can limit the overall impact on biodiversity. Previous syntheses have investigated restoration effectiveness by comparing average restored conditions to average conditions in unrestored or reference systems. Here, we provide the first quantification of the extent to which restoration affects both the mean and variability of biodiversity outcomes, through a global meta-analysis of 83 terrestrial restoration studies. We found that, relative to unrestored (degraded) sites, restoration actions increased biodiversity by an average of 20%, while decreasing the variability of biodiversity (quantified by the coefficient of variation) by an average of 14%. As restorations aged, mean biodiversity increased and variability decreased relative to unrestored sites. However, restoration sites remained, on average, 13% below the biodiversity of reference (target) ecosystems, and were characterised by higher (20%) variability. The lower mean and higher variability in biodiversity at restored sites relative to reference sites remained consistent over time, suggesting that sources of variation (e.g. prior land use, restoration practices) have an enduring influence on restoration outcomes. Our results point to the need for new research confronting the causes of variability in restoration outcomes, and close variability and biodiversity gaps between restored and reference conditions.

Diversity and turnover of wild bee and ornamental plant assemblages in commercial plant nurseries

In human-modified landscapes, understanding how habitat characteristics influence the diversity and composition of beneficial organisms is critical to conservation efforts and modeling ecosystem services. Assessing turnover, or the magnitude of change in species composition across sites or through time, is crucial to said efforts, yet is often overlooked. For pollinators such as wild bees, variables influencing temporal turnover, particularly across seasons within a year, remain poorly understood. To investigate how local and landscape characteristics correlate with bee diversity and turnover across seasons, we recorded wild bee and flowering ornamental plant assemblages at 13 plant nurseries in California between spring and autumn over 2 years. Nurseries cultivate a broad diversity of flowering plant species that differ widely across sites and seasons, providing an opportunity to test for correlations between turnover and diversity of plants and bees. As expected, we documented strong seasonal trends in wild bee diversity and composition. We found that local habitat factors, such as increased cultivation of native plants, were positively associated with bee diversity in sweep netting collections, whereas we detected moderate influences of landscape level factors such as proportion of surrounding natural area in passive trap collections. We also detected a moderate positive correlation between the magnitude of turnover in plant species and that of bee species (as number of taxa gained) across consecutive seasons. Our results have implications for the conservation of wild bees in ornamental plant landscapes, and highlight the utility of plant nurseries for investigating hypotheses related to diversity and turnover in plant-pollinator systems.

Ten-year trends reveal declining quality of seeded pollinator habitat on reclaimed mines regardless of seed mix diversity

Plant-pollinator interactions represent a crucial ecosystem function threatened by anthropogenic landscape changes. Disturbances that reduce plant diversity are associated with floral resource and pollinator declines. Establishing wildflower plantings is a major conservation strategy targeting pollinators, the success of which depends on long-term persistence of seeded floral communities. However, most pollinator-oriented seeding projects are monitored for a few years, making it difficult to evaluate the longevity of such interventions. Selecting plant species to provide pollinators diverse arrays of floral resources throughout their activity season is often limited by budgetary constraints and other conservation priorities. To evaluate the long-term persistence of prairie vegetation seeded to support pollinators, we sowed wildflower seed mixes into plots on a degraded reclaimed strip-mine landscape in central Ohio, USA. We examined how pollinator habitat quality, measured as floral abundance and diversity, changed over 10 years (2009-2019) in the absence of management, over the course of the blooming season within each year, and across three seed mixes containing different numbers and combinations of flowering plant species. Seeded species floral abundance declined by more than 75% over the study, with the largest decline occurring between the fifth and seventh summers. Native and non-native adventive flowering plants quickly colonized the plots and represented >50% of floral community abundances on average. Floral richness remained relatively constant throughout the study, with a small peak one year after plot establishment. Plots seeded with High-Diversity Mixes averaged two or three more species per plot compared with a Low-Diversity Mix, despite having been seeded with twice as many plant species. Within years, the abundance and diversity of seeded species were lowest early in the blooming season and increased monotonically from June to August. Adventive species exhibited the opposite trend, such that complementary abundance patterns of seeded and adventive species blooms resulted in a relatively constant floral abundance across the growing season. Seeded plant communities followed classic successional patterns in which annual species quickly established and flowered but were replaced by perennial species after the first few summers. Long-term data on establishment and persistence of flower species can guide species selection for future-oriented pollinator habitat restorations.

Clair AL, Dolezal AG, Toth AL, O’Neal ME. North American Prairie Is a Source of Pollen for Managed Honey Bees (Hymenoptera: Apidae)

Prairie was a dominant habitat within large portions of North America before European settlement. Conversion of prairies to farmland resulted in the loss of a large proportion of native floral resources, contributing to the decline of native pollinator populations. Efforts to reconstruct prairie could provide honey bees (Apis mellifera) a source of much-needed forage, especially in regions dominated by crop production. To what extent honey bees, which were introduced to North America by European settlers, use plants native to prairies is unclear. We placed colonies with pollen traps within reconstructed prairies in central Iowa to determine which and how much pollen is collected from prairie plants. Honey bee colonies collected more pollen from nonnative than native plants during June and July. During August and September, honey bee colonies collected more pollen from plants native to prairies. Our results suggest that honey bees' use of native prairie plants may depend upon the seasonality of both native and nonnative plants present in the landscape. This finding may be useful for addressing the nutritional health of honey bees, as colonies in this region frequently suffer from a dearth of forage contributing to colony declines during August and September when crops and weedy plants cease blooming. These results suggest that prairie can be a significant source of forage for honey bees in the later part of the growing season in the Midwestern United States; we discuss this insight in the context of honey bee health and biodiversity conservation.

Evaluating Native Bee Communities and Nutrition in Managed Grasslands

Native pollinators are important for providing vital services in agroecosystems; however, their numbers are declining globally. Bees are the most efficient and diverse members of the pollinator community; therefore, it is imperative that management strategies be implemented that positively affect bee community composition and health. Here, we test responses of the bee and flowering plant communities to land management treatments in the context of grasslands in the upper Midwestern United States, a critical area with respect to bee declines. Twelve sites were selected to examine floral resources and wild bee communities based on three different types of grasslands: tallgrass prairie remnants, ungrazed restorations, and grazed restorations. Total bee abundance was significantly higher in ungrazed restorations than remnants, but there were no significant differences among grasslands in community composition or Shannon diversity. Across the three grassland types we also examined mass and lipid stores as nutritional health indicators in three sweat bees (Halictidae), Augochlora pura, Agapostemon virescens, and Halictus ligatus. Although there were no differences in lipid content, total average bee mass was significantly higher in Ag. virescens collected from ungrazed restorations as compared to remnants. Floral abundance of native and non-native species combined was significantly higher in grazed restorations compared to remnants and ungrazed restorations. However, ungrazed restorations had higher abundance and richness of native flowering ramets. These data suggest that bee abundance and nutrition are driven by high abundance of native flowering plant species, rather than total flowering plants.

Quantifying Temporal Variation in the Benefits of Aphid Honeydew for Biological Control of Alfalfa Weevil (Coleoptera: Curculionidae)

Sugar feeding by biological control agents, such as parasitoid wasps, may enhance their ability to control crop pests, although its importance is likely to vary greatly through space and time. Here we quantified temporal variation in the potential importance of sugar resources associated with honeydew secreted by the pea aphid (Acyrthosiphon pisum (Harris) (Hemiptera: Aphididae)) in determining levels of parasitism of the alfalfa weevil (Hypera postica (Gyllenhal) (Coleoptera: Curculionidae)) by its dominant parasitoid, Bathyplectes curculionis (Thomson) (Hymenoptera: Ichneumonidae) across irrigated alfalfa fields in Montana, United States over 5 yr. A positive association between parasitism of H. postica and A. pisum densities at the across-site scale was observed in 2 of 5 yr, with parasitism increasing twofold to fourfold over gradients in A. pisum density. The relationship was strongest in the 2 yr of lowest parasitoid relative to host densities, when increases in per capita effects of individual parasitoids would be expected to be particularly important. Acyrthosiphon pisum densities were at their lowest in these same years, suggesting that they may generally be sufficiently abundant that parasitoids are not limited by sugars in most years. This conclusion is supported by results of anthrone tests which revealed a high level of sugar-fed parasitoids (>50%) in a year of high aphid abundance. More studies, such as this one, that explore the frequency with which increasing sugar resource availability actually enhances parasitism levels in the field will be critical to gauge the broader potential of sugar resource addition (e.g., through flowering strips, banker plants or sugar sprays) to bolster biological control.

Pollination implications of the diverse diet of tropical nectar-feeding bats roosting in an urban cave

Background

Intense landscaping often alters the plant composition in urban areas. Knowing which plant species that pollinators are visiting in urban areas is necessary for understanding how landscaping impacts biodiversity and associated ecosystem services. The cave nectar bat, Eonycteris spelaea, is an important pollinator for many plants and is often recorded in human-dominated habitats. Previous studies of the diet of E. spelaea relied on morphological identification of pollen grains found in faeces and on the body of bats and by necessity disregarded other forms of digested plant material present in the faeces (i.e., plant juice and remnants). The main objective of this study was to examine the diet of the nectarivorous bat, E. spelaea, roosting in an urban cave at Batu Caves, Peninsular Malaysia by identifying the plant material present in the faeces of bats using DNA metabarcoding.

Methods

Faeces were collected under the roost of E. spelaea once a week from December 2015 to March 2016. Plant DNA was extracted from the faeces, Polymerase chain reaction (PCR) amplified at ITS2 and rbcL regions and mass sequenced. The resultant plant operational taxonomic units were searched against NCBI GenBank for identification.

Results

A total of 55 species of plants were detected from faeces of E. spelaea including Artocarpus heterophyllus, Duabanga grandiflora and Musa spp. which are likely to be important food resources for the cave nectar bat.

Discussion

Many native plant species that had not been reported in previous dietary studies of E. spelaea were detected in this study including Bauhinia strychnoidea and Urophyllum leucophlaeum, suggesting that E. spelaea remains a crucial pollinator for these plants even in highly disturbed habitats. The detection of many introduced plant species in the bat faeces indicates that E. spelaea are exploiting them, particularly Xanthostemon chrysanthus, as food resources in urban area. Commercial food crops were detected from all of the faecal samples, suggesting that E. spelaea feed predominantly on the crops particularly jackfruit and banana and play a significant role in pollination of economically important plants. Ferns and figs were also detected in the faeces of E. spelaea suggesting future research avenues to determine whether the 'specialised nectarivorous' E. spelaea feed opportunistically on other parts of plants.

Using Regional Climate Projections to Guide Grassland Community Restoration in the Face of Climate Change

Grassland loss has been extensive worldwide, endangering the associated biodiversity and human well-being that are both dependent on these ecosystems. Ecologists have developed approaches to restore grassland communities and many have been successful, particularly where soils are rich, precipitation is abundant, and seeds of native plant species can be obtained. However, climate change adds a new filter needed in planning grassland restoration efforts. Potential responses of species to future climate conditions must also be considered in planning for long-term resilience. We demonstrate this methodology using a site-specific model and a maximum entropy approach to predict changes in habitat suitability for 33 grassland plant species in the tallgrass prairie region of the U.S. using the Intergovernmental Panel on Climate Change scenarios A1B and A2. The A1B scenario predicts an increase in temperature from 1.4 to 6.4°C, whereas the A2 scenario predicts temperature increases from 2 to 5.4°C and much greater CO2 emissions than the A1B scenario. Both scenarios predict these changes to occur by the year 2100. Model projections for 2040 under the A1B scenario predict that all but three modeled species will lose ~90% of their suitable habitat. Then by 2080, all species except for one will lose ~90% of their suitable habitat. Models run using the A2 scenario predict declines in habitat for just four species by 2040, but models predict that by 2080, habitat suitability will decline for all species. The A2 scenario appears based on our results to be the less severe climate change scenario for our species. Our results demonstrate that many common species, including grasses, forbs, and shrubs, are sensitive to climate change. Thus, grassland restoration alternatives should be evaluated based upon the long-term viability in the context of climate change projections and risk of plant species loss.