Plants for Bugs

RHS project team

Helen Bostock (RHS) and Dr Andrew Salisbury (RHS)

Partners

Wildlife Gardening Forum, University of Roehampton, Joe Perry and Mark Tatchell

Start date

05/01/2009 00:00:00

End date

31/12/2014 23:55:00

Keywords

Biodiversity, native plants, non-native plants, exotic plants, garden wildlife, field trial, Plants for Bugs.

The problem

It is generally accepted that some plants are better at supporting wildlife than others. However, planting guidance for gardeners is largely based on anecdotal evidence or, worse still, assumptions that have been shown to be untrue (e.g. that nettles are required to attract butterflies see Gaston et al. 2005).

There is a widely held belief that native plants should be paramount in wildlife gardening, however approximately 70% of plants in the ‘average’ garden are non-native (Loram et al. 2008) and are therefore often considered to be of less benefit to biodiversity. In contrast, studies of urban gardens suggest they are rich in biodiversity (Smith et al. 2006). Therefore it is possible that native plants, which make up the minority of plants in the ‘average’ garden are having a significantly greater impact than their numbers suggest or that non-native plants also provide a valuable resource for biodiversity.

We are testing the effect of selected assemblages of native and non-native plants on invertebrate abundance and diversity. This will test the hypothesis that there is no difference in invertebrate diversity associated with assemblages of native, near-native and exotic garden border plants. Findings from this study will only begin to answer these questions and this will stimulate further research.

Approach

The effect of the different plant assemblages on invertebrate abundance and diversity is being tested with a field experiment, designed to be representative of a garden border. Some attempt has been made to match flowering time and habit across native and non-native plant assemblages. The design and treatment is as follows:

There are three plant treatments (assemblages):

• 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 is made up of two replicate sites, Howard's Field, Wisley Gardens and 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 replicate consists of a 14 plant species 3x3m plot. Timber-edged wood-chip guard rows of 1m wide separate the plots.

A minimum of 14 (from a total of 24) plant species were selected for each treatment. The plant assemblages were designed to appear as similar as possible in terms of plant height, density and position in the plots.

Other considerations for the experimental design include: 

• The plots are treated as ‘garden-like’ as possible, e.g. visually appealing and weed free
• Weed control is carried out to prevent flowering and competition with the plant assemblages
• Individual plant needs, e.g. sun or shade requirements, were taken into account
• The plots are rabbit-proofed with wire fencing
• Irrigation has been carried out during plant establishment as necessary
• Wherever possible, plants were clonally propagated to ensure uniformity
• The Howard’s Field plots are accessible to garden visitors

Data collection and analysis

Invertebrate diversity:

Protocols for collection and identification of invertebrates were established during the pilot year (2009). Where possible, collected invertebrates are identified to species and classified to guild (e.g. predators, herbivores and detritivores). The invertebrates will be sampled on at least five occasions each year using the methods below: 

• Pitfall trapping and baited refuge traps for ground fauna
• Suction sampling for invertebrates found on plants
• Direct observation of flying insect visitors to the plots
• Soil fauna and function monitoring. This involves taking soil cores from the plots and extracting invertebrates using Tullgren funnels and soil function is assessed using litter bags. This is part of a PhD project in collaboration with Roehampton University.

At the end of 2013 approximately 80,000 invertebrates had been counted and identified, including more than 40 species of ground beetle from the pitfall traps, 30 species of spider from the suction sampler and 16 species of butterfly observed visiting the plots.

Other factors: 

• Measurements of additional factors that may affect invertebrate abundance and diversity are made on each plot. This includes photographic records of the plots and assessments of canopy and ground cover, soil moisture, flower number, seed counts, plant height and structure.

Writing up the experiment

The analysis of the flying insect visitors was completed at the end of 2014 and the first scientific paper submitted to a peer-reviewed journal. Once published, the information will be interpreted for the gardener. Analysis of the other data is on-going, and further publications and interpretation will be produced.

Benefits to gardeners

The study will give gardeners more confidence when deciding what to plant for wildlife. Surveys of urban gardens have indicated that non-native plant species are of benefit to garden biodiversity (BUGS project, see Gaston et al. 2005 and references therein).

The experiment has been rigorously designed to provide scientific evidence of the value of native and non-native plant assemblages for wildlife diversity which is of relevance to the gardener.

Further information

Project team

Helen Bostock - Project manager
Andrew Salisbury - Lead Entomologist
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

University of Roehampton

Plants for Bugs study
Plants for Bugs handout
Plants for Bugs blog
Plants for Bugs first paper on pollinators 
Interpretation Bulletin one

References

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 Management 42: 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 Conservation 15: 2415-2438.