Functional diversity and habitat preferences of native grassland plants and ground-dwelling invertebrates in private gardens along an urbanisation gradient

Urbanisation influences biodiversity and ecosystem functions. However, private domestic gardens provide habitats for many species. Challenging conditions in urban gardens may support species possessing certain traits, but exclude other species. Functional diversity is therefore often altered in urban gardens. We surveyed native grassland plants and ground-dwelling invertebrates (snails, slugs, spiders, millipedes, woodlice, ants, rove beetles), and compiled data on urbanisation (distance to city centre, percentage of sealed area) and garden characteristics. We furthermore derived data on traits and habitat preferences for the species recorded in the gardens from the literature and own measurements. The survey comprised 35 domestic gardens along a rural-urban gradient in the city of Basel, Switzerland and its surroundings.

Methods
Field survey methods:

We selected 35 gardens from a pool of 65 candidates offered in response to public calls. Selection criteria were the presence of grassland (at least 4 m2), accessibilty to researchers, and representation of gardens of different size and management types along the rural-urban gradient. Native grassland plants were recorded along a zig-zag line in grassland with effort proportional to the size of the grassland area in a garden. For total native plant species richness used as explanatory variable (see below), we complemented native plant species richness in grassland by recording the native plant species in the other habitat types along transect lines. These lines ran along the long axis of garden features. Furthermore, we considered plants at intervals of 2 m along the transect line to measure the height of the vegetation, which we used to calculate structural diversity of the vegetation. Invertebrates were collected using five pitfall traps and five hay bait traps (moist hay in a bag of coarse plastic mesh) in the grassland area of a garden. Traps were exposed three times for a week each. For gastropods, millipedes and ants, trap collections were supplemented by active search (ants: 3 x 15 min, millipedes: 3 x 30 min, gastropods: 1 x 30 min and in the case of gastropods sieving of soil samples (total 1 litre soil from 5–6 random surface soil samples per garden). For rove beetles we did not consider the subfamily Pselaphinae. For spiders and woodlice, we only considered adult individuals and for ants only workers. In the remaining invertebrate groups, we included all individuals that could be determined to the species level (99.4% of all individuals sampled). Abundance data was pooled over all methods and visits of the biodiversity survey for each garden.

Collection of data on traits and habitat preferences

Most data was derived from publications and online databases. In a few cases we complemented this with own measurements and observations. All sources are listed for the respective taxonomic groups in the file <Species_traits_sources.docx>.

Dates of data collection:

    Vegetation survey: 2018-07-24 to 2018-08-20

    Invertebrate survey: 2018-05-31 to 2018-10-18

    Survey of garden habitat types and landscape variables:

    2018-05-31 to 2018-10-18

    Trait data (from literature): 2018-11-01 to 2019-08-31

Geographic location of data collection:

35 private domestic gardens in the city of Basel, its suburbs and nearby villages in North-western Switzerland (47˚ 34' N, 7˚ 36' E). Information on the street addresses of the gardens is not provided to protect the privacy of the garden owners.

Software used:

All data was saved as workbooks in xlsx format using Microsoft Excel for Mac Version 16.11.1. A list of sources is given as a Microsoft Word docx file. Detailed definitions of variables and data files are given in a readme file save as a text file.