Greenspace design for plant-insect interactions in the city
Creators: name: Berthon, Katherine
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 330404
subject: Land use and environmental planning
classificationCode: 410401
subject: Conservation and biodiversity

description: Urban green spaces form increasingly important habitats for animals in an increasingly urbanised world. Despite a growing body of evidence linking certain aspects of green space design to biodiversity outcomes, the best designs for attracting and maintaining biodiversity remain elusive. The prevailing mantra in consideration of animal biodiversity has traditionally been “build it and they will come”, despite an awareness in the urban ecological literature of taxon-specific responses to the quality and quantity of plant resources provided. The effectiveness of different greenspace designs for supporting biodiversity will be reflected in the ability of animals to successfully utilise greenspace resources.

However, previous studies have evaluated designs based on community aggregate outcomes, that is, species richness or diversity measures, which fail to recognise the underlying processes of community assembly that underpin both maintenance of biodiversity and the ecosystem functions which we rely on as ecosystem services. Given that recommendations on plant species combinations for use in urban green space is likely to be highly dependent on species-specific responses, they may be better informed by an ecological network approach, which focuses on how animals interact and utilise plant resources. This research develops a framework for understanding how greenspace design can influence biodiversity outcomes through an ecological network approach with plant-pollinator interactions as a model system. I posit that greenspace design influences plant insect interactions through influencing the availability and accessibility of interacting partners. This offers three main pathways for intervention to improve biodiversity: improving plant choices, manipulating the site and landscape factors to improve access to resources, and understanding the conditions that shape insect preferences.

Plant choice recommendations should be broad enough to allow for flexibility and context specificity across sites but should also target specific resource needs of pollinators for them to be both widely adopted and successful. Plant origin has been suggested as a good general rule for increasing biodiversity while maintaining that flexibility, based on the assumption that native plants will be better used by local animals. To test this assumption, I first undertook a systematic review on how plant origin influences biodiversity outcomes in urban greenspaces; and then followed this with a detailed investigation of pollinator preferences in highly maintained urban garden beds using both fine-grained pollen DNA metabarcoding techniques and course-grained observational approaches over a full year.

The systematic review found substantial variation in the definition of ‘nativeness’ across studies based on social-cultural values that delineate the temporal (e.g., pre-colonial) and geographic boundaries (e.g., cities, states, countries) of interest, many of which not represent ecologically robust divisions. However, the majority of studies also showed positive effects of native plants on improving biodiversity outcomes in urban greenspaces, proving that there is an ecological basis for inclusion of plant origin within plant selection frameworks, regardless of definitional ambiguity.

Similarly, my fieldwork showed that, overall, pollinators were more dependent on native plants, but that variation in flower abundance and diversity meant that the plants that were preferred most varied across seasons. For example, natives such as Alyogyne, Brachyscome and Wahlenbergia were highly preferred by pollinators during spring and summer, but native plant taxa such as Acacia, Correa and Plectranthus were more used during winter when other resources were scarce. Conversely, not all native plants were highly used, and some exotic plants also provided important resources, particularly Tanacetum during summer, and Choisya during winter. Metabarcoding work showed limited differences in the network structure of native and exotic garden beds, with only 43% of plants within garden beds being utilised by pollinators. Some pollinators also depended heavily on weeds, with native bees being particularly vulnerable to their removal from the landscape.

While pollinators showed strong and taxon-specific preferences for particular plant genera, there was an overarching positive effect of increasing diversity of flowering plants. This effect was consistent across both DNA metabarcoding and observational approaches and saw increased generalisation in plant-insect networks in sites with a high diversity of flowering plants. Floral abundance across seasons was also a large determinant of species-level specialisation, but pollinators showed contrasting responses, with some groups increasing in specialisation (e.g., beetles), and others becoming more generalised (e.g., native bees). Taken together, it is likely that increased floral diversity allows for increased recruitment of more specialised, or rarer pollinators that are more likely to find their preferred forage or interact with highly abundant flowering plants.

Even if appropriate species are planted, there is a possibility that insects will be unable to access those resources. To understand how a given plant-insect interaction might change across different landscape contexts, I deployed experimental ‘phytometres’ (sets of 5 functionally complimentary plant species) across 100+ sites in Munich, Germany, and used occupancy-detection modelling to measure the influence of site and landscape factors on probability of interaction. As with garden beds in Melbourne, pollinator groups responded uniquely to landscape and local factors, and increasing the quantity and diversity of neighbouring flowers increased the probability of interaction. Overall, small-scale site factors had a larger influence on probability of interaction than landscape scale factors, with only a few groups responding to site size or landscape connectivity. Placement of the phytometre on unsealed surfaces had a stronger effect (for increasing probability of interaction) than increasing the proportion of unsealed surfaces (i.e., habitat) on a site, indicating the importance of habitat heterogeneity and the potential for internal barriers for pollinators to access foraging resources. To investigate the question of accessibility of floral resources further, I then monitored the colonisation of invertebrates onto a newly retrofit green roof, using a before-after-control- impact design with reference sites. Here, too, I find that the composition, diversity and abundance of insects is driven by changes in floral richness and abundance, with the composition of the impact site becoming more similar to reference sites after the greening intervention. Several prominently used plants on ground sites (e.g., Brachyscome) never received a visitor on the newly retrofit roof, and some pollinator groups (e.g., native bees and native butterflies) never appeared on the roof. Therefore, while planting preferred plants is useful, strategic placement of and linkages between greenspaces is necessary for them to reach their full potential for supporting biodiversity.

Finally, a strength of this work has been the combination of coarse-grained and detailed information to provide useful, species-specific recommendations on plant choice. Using this information, I have provided several recommendations on the best plants to plant depending on the goal of your greenspace design: maximising diversity of pollinators, supporting specific groups of pollinators, or avoiding competition of native and exotic pollinators. Some general rules include: 1. Provide plant resources that match pollinator needs; 2. When in doubt plant native; 3. Plant a diversity of flowering plants; 4. Include a few ‘super attractive’ plants that support a lot of insects; 5. Plant according to pollinator preferences; 6. Consider seasonal complementarity and 7. Design for optimising access within and between sites.