Plants to Encourage Biodiversity in the Garden. Natives vs Exotics

When back in the UK I always enjoy visiting the excellent gardens in and around London. One of my favorites is Wisley. Tucked away in the far North of Wisley Gardens is a ground-breaking research project called "Plants for Bugs". Here a research team  have set out to investigate whether the origin of the plants grown in gardens - be it British natives or introduced exotics - affects the sorts of insects that are attracted to a garden.
 

Map of Wisley Gardens

This four year experiment will examine the value of native and non-native plant assemblages for biodiversity leading to evidence-based advice for the ecological gardener.

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The Context of the Experiment

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.

Approach

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.

 

Further information

Download hand-out
More on Plants for Bugs
Plants for bugs blog
University of Roehampton

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 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.

For anyone who is interested in a first look at the results a joint event by the RHS and Wildlife Gardening Forum will take place on the 17/03/2014  Click here for more details.

Plant list for the Plants for Bugs project

Native Plants (UK)
Armeria maritima - Sea thrift
Buxus sempervirens - Common Box
Cytisus scoparius - Common Broom
Deschampsia cespitosa - Tufted Hair Grass
Dianthus deltoides - Maiden Pink
Dryopteris filix-mas - Male Fern
Eupatorium cannabinum - Hemp Agrimony
Geranium sanguineum - Bloody Cranesbill
Helianthemum  nummularium - Common Rockrose
Hyacinthoides non-scripta - English Bluebell
Knautia arvensis - Field Scabious
Leucanthemum vulgare - Ox-eye Daisy
Lonicera periclymenum 'Graham Thomas'  - Common Honeysuckle
Lythrum salicaria - Purple Loosestrife
Malva moschata - Musk Mallow
Molinia caerulea - Purple Moor Grass
Primula vulgaris - Primrose
Rosa rubiginosa - Sweet Briar
Scabiosa columbaria - Small Scabious
Stachys officinalis - Betony
Valeriana officinalis - Common Valerian
Veronica spicata - Spiked Speedwell
Viburnum opulus - Guelder Rose

Near-native plants:
Armeria juniperifolia - Juniper-leaved Thrift
Calamagrostis brachytricha - Korean feather reed grass
Dianthus plumarius - Cottage pink
Dryopteris wallichiana - Alpine Wood Fern
Eupatorium maculatum 'Orchard Dene' - Joe Pye weed
Genista lydia - Lydian broom
Geranium macrorrhizum - Bigroot Cranesbill
Halimium umbellatum - Umbel-flowered Sun Rose
Hyacinthoides hispanica - Spanish Bluebell
Knautia macedonica - Macedonican Scabious
Lonicera tragophylla - Chinese Honeysuckle
Lythrum virgatum 'Dropmore Purple'  - Wand Loosestrife
Malva alcea - Greater Musk Mallow
Primula japonica 'Miller's Crimson' - Japanese Primrose
Rhodanthemum hosmariense - Moroccan Daisy
Rosa rubrifolia - Red-leaved Rose
Sarcococca hookeriana var.humilis - Christmas box
Scabiosa caucasica - Caucasian Scabious
Stachys byzantina - Lamb's Ear
Stipa tenuissima - Mexican Feather Grass
Valeriana phu 'Aurea' - Golden Valerian
Veronica austriaca subsp.teucrium - Saw-leaved Speedwell
Viburnum sargentii - Sargent viburnum


Exotic plants:
Acaena microphylla - New Zealand Burr
Alstroemeria psittacina - Parrot Lily
Blechnum chilense - Chilean Hard Fern
Brachyglottis monroi - Monro's Ragwort
Callistemon rigidus - Stiff Bottlebrush
Carex testacea - Orange New Zealand Sedge
Diascia personata 'Hopleys' 
Eccremocarpus scaber - Chilean Glory Bower
Eryngium agavifolium - Agave-leaved Sea Holly
Euryops tysonii - Euryops
Fuchsia magellanica var. gracilis - Lady's Eardrops
Hebe rakaiensis - Rakai hebe
Leptinella squalida 'Platt's Black' - Leptinella 'Platt's Black'
Lobelia tupa - Devil's Tobacco
Mirabilis jalapa - Marvel of Peru
Nerine bowdenii - Bowden Cornish lily
Osteospermum jucundum - Boneseed
Oxalis adenophylla - Sauer Klee
Ozothamnus rosmarinifolius - Sea rosemary
Pittosporum tenuifolium - Tawhiwhi
Sisyrinchium striatum - Pale Yellow-eyed Grass
Uncinia rubra - Red Hook Sedge
Verbena bonariensis - Purple Top

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