Pollinator Diversity and Plant-Pollinator Interactions in Urban Green Spaces: Urban Green Spaces Study Notes

Global Importance and Context of Insect Pollination

Ecological Keystone Status: Flower visitors are defined as ecological keystones because they are responsible for the pollination of between $78\,\%$ and $94\,\%$ of all wild plant species.
Ecosystem Foundation: Pollinator diversity is the fundamental building block for maintaining ecosystem services and functions, enhancing the resilience of socio-ecological systems.
Agricultural and Economic Impact: * Insect pollination is a primary process supporting plant/animal population preservation and agricultural crop yields. * Approximately $75\,\%$ of all crop species require insect pollination. * The Global Economic Value of Animal Pollination: Estimated at $153 \times 10^{9}\,\text{Euros}$ .
Key Pollinator Species: * Honeybees ( $Apis\,mellifera$ ): Account for the largest proportion of pollinators worldwide in agricultural landscapes, pollinating one-third of human food and providing economic benefit via honey production. * Wild Insects: Perform a supplementary role to honeybees rather than acting as a substitute, due to specific pollination preferences.
Ethical Perspectives: Protection of species richness has intrinsic value based on both pathocentric animal ethics and natural-ethical motivations.

Drivers of Global Pollinator Decline

Anthropogenic Stressors: A dramatic global decline is noted in insect biomass, species richness, and individual numbers due to several factors: * Agricultural Chemicals: Exposure to a multitude of pesticides and combined exposure events over time and space. * Habitat Alteration: Loss of structural diversity, habitat degradation, and isolation resulting from monoculture cultivation. * Urbanization and Climate Change: Soil sealing due to increasing urbanization and shifting climatic conditions. * Non-native Species: The spread of invasive species and increasing parasite infections (e.g., $Varroa\,destructor$ or American foulbrood). * Apicultural Practices: Improper management leading to increased honeybee colony mortality.
Ecological Consequences: The loss of insects risks the massive occurrence of pests due to the absence of parasitic insect opponents and has caused population decreases in insectivorous birds due to lack of food resources.

Urban Environments as Potential Refuges

Positive Urban Effects: Cities can support resources for food, nesting, and hibernation. * Pesticide Absence: Significantly greater honey harvests are often recorded in urban bee hives compared to agricultural land hives due to lower pesticide exposure. * Flower Diversity: Cities generally possess higher flower diversity and longer vegetation periods in managed flower beds than rural areas.
The Concept of "Pollination Syndrome": Functional traits of plants that integrate morphological properties of blossoms and pollen vectors. Classification is based on flower classes according to Müller ( $1881$ ) and flower types according to Kugler ( $1970$ ). It identifies: * Abiotic pollination (water, wind). * Biotic pollination (specific animal vectors based on flower structure).
Urban Green Infrastructure (UGI): Core areas include parks, cemeteries, and community gardens, which serve as alternative refuges if managed and designed correctly.

Study Objectives and Research Questions

Objective: To assess different park types, management concepts, and structural properties regarding pollinator abundance, diversity, and the provision of the ecosystem service (ES) of pollination.
Research Questions: 1. How does park structure and management affect the pollinator community? 2. What are the differences in pollinator composition between standardized "designed beds" and existing public flower beds? 3. How relevant are insect-pollinated linden trees ( $Tilia\,spp.$ ) for urban pollination services? 4. What is the overall estimated pollination performance across different park types? 5. What recommendations can be made for urban planning and green space management?

Methodology: Investigation Sites and Categorization

Location: Aachen, Germany (medium-sized town, population $250,000$ ).
Investigation Sites: Eight urban green spaces and one rural reference site ( $2.75\,km$ from the city center).
Park Type Categorization: * Prestigious Park (e.g., Elisengarten): Representative space ( $8,000\,m^{2}$ ) in the city center; high maintenance, decorative plants, strict trimming. * Recreational Parks (e.g., Westpark, Nelson-Mandela Park, Frankenbergpark, Veltmanplatz): Central meeting places with leisure facilities (playgrounds, basketball courts). * Community Gardens (e.g., Vielfeld, Hirschgruen): Collectively managed, small areas ( $1,810\,m^{2}$ to $2,360\,m^{2}$ ) with high structural diversity, ecological maintenance principles (no pesticides, mowing only twice per year). * Cemetery (e.g., Ostfriedhof): Oldest used cemetery ( $220\,\text{years old}$ ), lush vegetation, mature trees, and regularly renewed flower beds. * Rural Reference: Extensively used meadows, orchards, and pastures at the rural-urban fringe.

Experimental Setup and Sampling Procedures

Timeframe: May to August $2016$ ; measurements between $10:00\,a.m.$ and $5:00\,p.m.$ in randomized repetition.
Designed Study Beds: Total area of $1\,m^{2}$ standardized across all sites. Components: * $Campanula\,carpatica$ (Campanulaceae): Bee syndrome. * $Antirrhinum\,majus$ (Plantaginaceae): Bumblebee syndrome. * $Rudbeckia\,hirta$ (Asteraceae): Generalist syndrome. * $Salvia\,patens$ (Lamiaceae): Hymenoptera syndrome.
Public Flower Beds: Representative $1\,m^{2}$ patches of existing park vegetation (e.g., $Nepeta\,x\,faassenii$ , $Salvia\,nemorosa$ , $Begonia\,semperflorens$ ).
Tilia Tree Selection: Genus chosen due to dominance in medium/large German cities. Species included $Tilia\,cordata$ , $Tilia\,platyphyllos$ , $Tilia\,tomentosa$ , and $Tilia\,dasystyla$ . Surveyed volume was $1\,m^{3}$ of the treetop.
Pollinator Morphogroups: Insects were non-invasively monitored and classified into: * Small wild bees (e.g., $Lasioglossum$ , $Halictus$ ). * Mason, leafcutter, carder, and resin bees (Megachilidae). * Large solitary wild bees. * Honeybees ( $Apis\,mellifera$ ). * Bumblebees (e.g., $Bombus\,terrestris$ ). * Hoverflies (e.g., $Episyrphus\,balteatus$ and other Syrphidae species).
Sampling Metric: Visitation rates ( $V_{\text{group}}$ ) were calculated per $30\,minutes$ per destination ( $1\,m^{2}$ bed or $1\,m^{3}$ treetop): $V_{\text{group}} = \frac{\text{no. of recorded individuals}}{\text{destination} \times 30\,min}$

Statistical and Network Metrics

Software: R ( $3.1.1$ ) with R Studio ( $1.0.136$ ).
Tests: Kruskal-Wallis (non-normal data), Dunn post-hoc with Bonferroni-Holm correction, Multivariate hierarchical cluster analysis (Bray-Curtis dissimilarity).
Network Metrics (Bipartite Package): * Weighted Nestedness: Measured using the Galeano et al. ( $2009$ ) estimator. Value found: $0.576$ for the species-level network. * Connectance: Realized vs. possible links. Value: $0.338$ for species level. * $H_{2}'$ Specialization: Measures interaction selectiveness. Value: $0.233$ . * Niche Overlap (Horn's Index): Value: $0.20$ for pollinators at species level.

Results: Comparison of Pollinator Communities

Overall Dataset: $187$ records for beds, $112$ for trees, and $7723$ total observed interactions.
Bed Visitation Rates by Park Type: * Rural Reference: $46.2 \pm 14.6$ individuals/ $30\,min$ . * Community Gardens: $43.7 \pm 23.0$ individuals/ $30\,min$ (highest urban abundance). * Prestigious Park: $21.0 \pm 12.6$ individuals/ $30\,min$ . * Recreational Parks: $20.8 \pm 15.5$ individuals/ $30\,min$ . * Cemetery: $17.8 \pm 10.1$ individuals/ $30\,min$ .
Morphogroup Trends: Wild bees and syrphids showed significantly lower visitations in urban parks compared to rural sites, except in community gardens where rates were comparable ( $19.0$ solitary bees and $13.0$ syrphids recorded).
Study vs. Public Beds: Public beds had higher absolute abundances (mean: $37.2$ ) than study beds (mean: $26.5$ ). However, public beds were dominated by eusocial colonizing insects (honeybees and bumblebees), while designed study beds successfully attracted higher proportional amounts of solitary bees and hoverflies.

Plant-Pollinator Interaction Network Analysis

Network Stability: The high weighted-interaction nestedness ( $0.576$ ) suggests a stable community composition and good connectivity providing re-colonization potential for urban green spaces.
Flower Constancy: Eusocial species ( $Apis\,mellifera$ , $Bombus\,terrestris$ ) exhibited high flower constancy, particularly on $Nepeta\,x\,faassenii$ and $Salvia\,nemorosa$ (Hymenoptera syndrome).
Syndrome Fidelity: Specialized pollination syndromes (e.g., bumblebee syndrome) were visited by the expected groups, but also regularly by solitary bees. Generalist and Hymenoptera syndromes attracted the widest variety of insect groups.
Aggregation Effects: As data transitioned from species-level to aggregated groups/syndromes, weighted nestedness decreased from $0.576$ to $-0.133$ , and niche overlap increased from $0.20$ to $0.82$ .

Role of Tilia Trees in Urban Pollination

Dominant Species: Tilia blossom visitors were overwhelmingly $Apis\,mellifera$ ( $59.8\,\%$ to $74.7\,\%$ of total).
Quantitative Impact: While visitors per $m^{3}$ of Tilia treetop were slightly lower than per $m^{2}$ of flower bed ( $12.9$ to $16.4$ visits/ $30\,min$ ), the total park visitation by trees is often $20\,\text{times}$ higher than beds due to sheer volume.
Resource Gap Implications: Tilia, especially $Tilia\,tomentosa$ , flowers late in the season. Mass mortality of bumblebees under Tilia is attributed to starvation once the trees finish flowering, highlighting the critical need for constant floral supplies in city beds to bridge nectar gaps.

Pollination Performance and Estimator Metrics

Total visitation Estimations (per 30 mins): * Total Public Bed Visitation: Approximately $144,000$ estimated visits across $3,504\,m^{2}$ of bed area. * Total Tilia Visitation: Approximately $3 \times 10^{6}$ estimated visits across $210,491\,m^{3}$ of total green volume.
Pollination Estimator (Performance Index): Calculated as total visitations divided by park area ( $m^{2}$ ). * Public Beds Index: * Community Gardens: $6\,\text{visits/m}^{2}$ . * Prestigious Park: $7.7\,\text{visits/m}^{2}$ . * Recreational/Cemetery: $0.1$ to $0.3\,\text{visits/m}^{2}$ . * Tilia Trees Index: * Community Gardens: $69.3\,\text{visits/m}^{2}$ . * Recreational Parks: $9.5\,\text{visits/m}^{2}$ . * Cemetery: $1.3\,\text{visits/m}^{2}$ .

Policy Recommendations and Management Conclusions

Near-Natural Concepts: Encourage management that reduces interventions (e.g., spontaneous vegetation, reduced mowing frequency) to promote habitats for solitary bees and syrphids.
High-Potential Areas: Recreational parks currently have low pollination performance but represent the highest potential for improvement through increased floral diversity.
Species Selection: * Use native, insect-pollinated plants and regional seed mixtures. * Plant time-delayed flowering trees to bridge food gaps: $T.\,platyphyllos$ , $T.\,cordata$ , $T.\,tomentosa$ , alongside $Acer\,pseudoplatanus$ , $Aesculus\,carnea$ , and $Robinia\,pseudoacacia$ .
Connectivity: Promote functional and structural connectivity between green infrastructure elements on a city-wide scale.
Integrative Management: Develop urban "Leitbilder" (guiding principles) that balance social services (recreation, well-being) with ecological functions like pollination habitat provision.