Native Plant Garden in Urban Backyard Supports Pollinator Biodiversity

Abstract

Urban ecosystems and human-altered environments pose different constraints on plant and animal communities than natural ecosystems, which can impact plant-pollinator interactions (Hennig & Ghazoul 2011). Habitat loss and fragmentation are increasingly occurring in and around urban areas, leading to changes in native plant and pollinator populations, the foraging behavior of pollinators, and the pollination success of native plants (Harrison & Winfree 2015). There have been efforts in urban areas around the world to plant pollinator gardens to support both native plants and threatened and endangered pollinator species. For this paper, a biodiversity survey was conducted to examine the level of species richness and species diversity found in a small urban backyard pollinator garden in Louisville, Kentucky. A survey of the species richness and diversity of the plants in the garden was conducted, as well as a survey of pollinators visiting the garden. It was hypothesized that the garden would contain a high level of plant species richness and a moderate level of plant species diversity. The plant survey was conducted using the method of direct counting individual plants. The pollinator identification survey was conducted through both direct observation and the identification of pollinators in photos taken in the garden. The results of the survey of the pollinator garden plant survey found a medium to a high level of species richness and a high level of species diversity. The pollinator survey identified over 20 different species of pollinating insects and animals. Based on the results of the survey, it is concluded that even a small backyard garden in an urban area can support a high level of plant and pollinator biodiversity, hence, pollinator gardens can be beneficial if planted in a way that reduces impact to wild remnants of native plant communities.

Introduction

  Urbanization is a major driver of declines in pollinator populations due to rapidly shrinking pollinator habitats. Public programs have been developed in many urban areas to educate and encourage the development of pollinator gardens (Fukase et al. 2016). While private gardens are seen as a potential means of pollinator conservation, the effectiveness of urban gardens in pollinator conservation is dependent upon the composition of plant species in the garden. Urban gardens can support pollinators by including plants that bloom throughout the spring, summer, and fall to provide sources of nectar and pollen across the seasons (United States Department of Agriculture 2016). Increased pollinator activity observed in natural areas and gardens with higher proportions of native plants suggest that the cultivation of native plant species should be a priority in urban gardens (Fukase et al. 2016). Natural areas tend to be diverse, so diverse gardens that closely resemble native plant communities will be resistant to diseases, pests, and weed infestations and therefore will be the most beneficial to pollinators (United States Department of Agriculture 2016) While pollinator gardens and monarch waystations have often proven to help support pollinators, there is a strong risk for negative effects associated with urban wildflower gardens due to the impacts of escaped non-native and invasive plant species (Johnson et al. 2017). Therefore, it is of vital importance for native plant and pollinator biodiversity that the type of planting be carefully considered. Because urban plantings can impact native wild remnant populations, the safest method for planting urban pollinator gardens is to utilize native plants which will support native specialist pollinators along with native and non-native generalists (Johnson et al. 2017). Further, there is a positive relationship between the amount of plant-pollinator interactions and the proportion of a garden planted with native plants (Fukase et al. 2016). While there is no relationship between the area of a garden and the abundance of pollinators, plant density does impact plant-pollinator activity, as does the proportion of native plants (Fukase et al. 2016). 

Materials and Methods

Location

The location of the survey is a garden measuring 3.5 x 1.5 meters in the backyard of a home in south-central Louisville, Kentucky. The homeowner converted an area formerly landscaped with non-native ornamental plants into a native plant garden with the intention of supporting pollinators, specifically monarch butterflies and native bees. Plants that support a variety of pollinators at various life stages and bloom across the seasons were selected. All plants selected are native to Kentucky or the Southeastern United States with the exception of Siberian Irises. These were included because they support pollinators when in bloom and the homeowner has been cultivating them for over 40 years. Some hybrid varietals of native plants were selected due to the limited local availability of wild varietals. 

Plant Survey Method

The plant survey was conducted using the method of direct counting individual plants. Species richness and species diversity were then calculated using the collected survey data. 

Species richness describes the number of different species found in an area. Species richness was determined using Menhinick’s Index, which uses the formula R = S/√N to demonstrate the relationship between the number of species and the number of individuals. A higher relative index indicates high species richness while a lower relative index indicates low species richness. 

Species diversity describes the number and kind of species found in an area. Species diversity was determined using Simpson’s Index, which takes into account not only the number of species present in a given area but also the relative abundance of each species. Simpson’s Index is calculated using the formula D = ∑n(n-1)/ N(N-1) and then Simpson’s Index of Diversity is calculated using 1-D. An index value of 1 represents the highest possible level of diversity while an index of 0 represents no diversity. 

Pollinator Identification Survey Method

The pollinator identification survey was conducted through both direct observation and the identification of pollinators in photos taken during the months of June, July, and August 2021. All pollinators identified were counted once. 

Results

The results of the survey of the native plant pollinator garden found a medium to a high level of species richness and a low level of species diversity. The Menhinick’s Index (R) for this survey is 1.42, indicating a medium to high level of species richness. The Simpson’s Index of Diversity (1-D) for this survey is 0.75, indicating a high level of species diversity. The pollinator survey identified 22 different species of pollinating insects and animals utilizing the garden. The survey results are shown in Tables 1 and 2. 

Images of Pollinators Identified

Discussion

Based on the results of the survey, it is concluded that even a small backyard garden in an urban area can support a high level of plant and pollinator biodiversity. This 5.25 meter2 urban backyard garden was planted with carefully researched native plants with the intention of supporting a variety of pollinators across various life stages. The ability of this garden to meet its functional design is evidenced by the species richness of plants along with the diversity of pollinators at different life stages identified within the garden. While this survey does demonstrate the ability of a small urban garden to support a biodiverse community of native plants and pollinators, further studies of pollinator species richness, diversity, and density in this and other urban pollinator gardens would contribute to a more thorough understanding of the relationships between plant-pollinator interactions, garden size, and native plant density. 

Literature Cited

Fukase J, Simons AM, Others. 2016. Increased pollinator activity in urban gardens with more native flora. Applied Ecology and Environmental Research 14:297–310. ALÖKI Applied Ecological Research and Forensic Institute Ltd. Available from http://www.aloki.hu/pdf/1401_297310.pdf.

Harrison T, Winfree R. 2015. Urban drivers of plant‐pollinator interactions. Functional ecology 29:879–888. Wiley. Available from https://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12486.

Hennig EI, Ghazoul J. 2011. Plant-pollinator interactions within the urban environment. Perspectives in plant ecology, evolution and systematics 13. Available from http://dx.doi.org/10.1016/j.ppees.2011.03.003.

Johnson AL, Fetters AM, Ashman TL. 2017. Considering the unintentional consequences of pollinator gardens for urban native plants: is the road to extinction paved with good intentions? The New phytologist 215. Available from http://dx.doi.org/10.1111/nph.14656.

United States Department of Agriculture. 2016. Kentucky Pollinator Handbook. Available from https://efotg.sc.egov.usda.gov/references/public/KY/KPH5a.pdf.

1. Brennan, Erin. “Native butterfly pollinators.” 2021. JPEG file.

2. Brennan, Erin. “Native and nonnative bee, fly, and beetle pollinators.” 2021. JPEG file.