It is generally accepted that some plants are better at supporting wildlife than others. However, wildlife planting guidance for gardeners is largely based on anecdotal evidence or, worse still, assumptions that have been shown to be untrue, for example that nettles in gardens will attract butterflies (Gaston et al. 2005).
One widely held assumption is that native plants are vital to attract wildlife to gardens. In fact, approximately 70% of plants in the ‘average’ garden are non-native yet these gardens are rich in biodiversity (Smith et al. 2006, Loram et al. 2008). Therefore it is possible that either native plants, which make up the minority of plants in the ‘average’ garden, are having a proportionally greater impact on wildlife than expected based on their abundance. or that non-native plants provide a more valuable resource for biodiversity than is usually assumed.
To begin to provide answers the Plants for Bugs project is testing the hypothesis that there is no difference in invertebrate diversity associated with assemblages of native, near-native and exotic garden border plants.
Plants for Bugs is a field experiment which compares invertebrate diversity on plots containing one of three plant assemblages (treatments) based on the geographical origin of the plants. These are:
- Native plants (naturally occurring in Britain and of British provenance where possible)
- Near-native plants (not native to Britain, but originating in the Northern hemisphere.
- Exotic plants (not native to Britain, but originating in the Southern hemisphere.
The experiment consists of 36 plots (each 3x3m) at two sites, one within RHS Garden Wisley and accessible to garden visitors (Howard's Field), and one at Deers Farm, Wisley Village. The layout follows a randomized split-plot design with six replicates of each treatment at each site (12 replicates in total). Each plot contains 14 plant species belonging to one of the treatments (i.e. native, non-native or exotic). Timber-edged woodchip guard rows of 1 m wide separate the plots.
The plant assemblages were designed to be as similar as possible across the treatments in terms of plant height, density and position within the plots. The plots were treated as ‘garden-like’ as possible, i.e. visually appealing and weed free.
Data collection and analysis
Protocols for collection and identification of invertebrates were established during the pilot year (2009). Where possible, collected invertebrates were identified to species and classified to guild (e.g. predators, herbivores, detritivores). The invertebrates were sampled on at least five occasions each year using pitfall traps and baited refuge traps for ground fauna, suction sampling for invertebrates found on plants, and direct observation of flying insect visitors.
In addition, a PhD project in collaboration with University of Roehampton is investigating and monitoring the soil fauna and function. This involved taking soil cores from the plots before extracting invertebrates using Tullgren funnels. Soil function was assessed using litter bags.
By the end of 2013 more than 80,000 invertebrates had been counted and identified, including 47 different species of ground beetle, more than 50 species of spider and 16 species of butterfly.
Measurements of additional factors that may affect invertebrate abundance and diversity have been made, including photographic records and assessments of soil moisture, flower number, canopy cover and plant volume.
Data analysis and interpretation
During the winter of 2013/14 analysis of the data will be carried out and the first results prepared for publication in the scientific literature. The results of the experiment will also be interpreted to provide advice for gardeners who wish to increase biodiversity in their own gardens.
Gaston K J, Warren P H, Thompson K & Smith R M (2005). Urban domestic gardens (IV): the extent of the resource and its associated features. Biodiversity and Conservation 14: 3327-3349
Loram A, Warren P H and Gaston K J (2008). Urban Domestic Gardens (XIV): The Characteristics of Gardens in Five Cities. Environmental Management42:361-376
Smith R M, Warren P H, Thompson K and Gaston K J (2006). Urban domestic gardens (VI): environmental correlates of invertebrate species richness. Biodiversity and Conservation15:2415-2438.
- Helen Bostock – Project manager
- Andrew Salisbury – Assistant project manager
- James Armitage - Botanical consultant
- Joe Perry - External consultant on statistics and plot design
- Mark Tatchell - External consultant
- Anna Platoni - Entomologist
The project is also supported by several RHS volunteers.