1 Sampling of gardens
Gardens were selected following a stratified non-random sampling design by visually assessing aerial images and during field visits. The independent strata comprised i) garden type (domestic vs. allotment), ii) a habitat heterogeneity/management intensity gradient ranging from extensively managed gardens with a high vertical vegetation structure and a high proportion of native plant species to intensively managed gardens with a low vertical vegetation structure and a high proportion of alien plant species (Grün Stadt Zürich, 2010), and iii) an urbanization gradient which ranged from densely built-up to more peripheral areas of the city. The urbanization gradient was subsequently quantified as the proportion of sealed surface (i.e. built and paved) in 500-m radius circles around each garden (XXX, submitted). To ensure statistical independence among observations (i.e. gardens) no adjacent garden lots were sampled, and gardens were maximally spaced across the entire city to include all urban districts. Additionally, with two exceptions, only one garden lot was sampled per allotment garden area. Non-independence due to positive spatial autocorrelation can arise among nearby or adjacent gardens due to “social mimicry” effects (e.g. Hunter and Brown, 2012). Note, however, that we did not sample front or easement gardens, where spatial autocorrelation is usually detected. The average distance among gardens was 4.5 km (sd ± 2.2 km, 0.1–11 km).
Private garden owners were approached initially by letter and thereafter by phone to arrange a meeting. If no phone number was available, they were contacted in person. We received approx.
10 refusals, but most garden owners contacted agreed to participate in the study. Recruitment of allotment garden tenants was approached differently. Some allotment garden tenants contacted us in response to a call we placed in a magazine distributed to allotment gardeners in Zurich. We enlisted some of these volonteers but also had to refuse many who did not fulfill the stratification criteria. Further allotment garden tenants were recruited by contact in person.
Checking for spatial effects
Potential spatial autocorrelation in the response variables (i.e. species richness) was investigated by computing Moran’s I autocorrelation indices (Popescu et al., 2012). In addition, we examined the putative presence of spatial structure in the residuals of all models by using semivariograms (Pepesma, 2004). See figures S9 to S11 and Table 1 below.
References
Frey, D. & Moretti, M. A comprehensive survey of cultivated and spontaneously growing vascular plants in urban gardens. Data in Brief, submitted.
Grün Stadt Zürich, 2010. Kartierschlüssel Biotoptypenkartierung Stadt Zürich [Maping code for urban habitats]. City of Zurich, unpublished document.
Pepesma, E.J. (2004). Multivariate geostatistics in S: The gstat package. Computers &
Geosciences, 30, 683-691.
2 Popescu, A.-A., Huber, K. T., & Paradis, E. (2012). ape 3.0: new tools for distance based
phylogenetics and evolutionary analysis in R. Bioinformatics, 28, 1536-1537.
3 Supplementary Figure S1. Residual plots for the species richness model of total plant species richness (Model 1). For this model, the garden-owner reported habitat heterogeneity index includes expert-collected data on the presence/absence of compost heaps, piles of branches, rock piles, trees and shrubs (as well as the 11 garden-owner reported habitat types).
4 Supplementary Figure S2. Residual plots for the alternative species richness model of total plant species richness (Model 1), including the habitat heterogeneity index based only on the 11 exclusively garden-owner reported land-use types and features. For this model, the garden-owner reported habitat heterogeneity index does not include the expert-collected data on the presence/absence of compost heaps, piles of branches, rock piles, trees and shrubs.
5 Supplementary Figure S3. Residual plots for the species richness model of native plant species richness (Model 2). For this model, the garden-owner reported habitat heterogeneity index includes expert-collected data on the presence/absence of compost heaps, piles of branches, rock piles, trees and shrubs (as well as the 11 garden-owner reported habitat types).
6 Supplementary Figure S4. Residual plots for the alternative species richness model of native plant species richness (Model 2), including the habitat heterogeneity index based only on the 11 exclusively garden-owner reported land-use types and features. For this model, the garden-owner reported habitat heterogeneity index does not include the expert-collected data on the presence/absence of compost heaps, piles of branches, rock piles, trees and shrubs.
7 Supplementary Figure S5. Residual plots for the species richness model of cultivated plant species richness (Model 3). For this model, the garden-owner reported habitat heterogeneity index includes the expert-collected data on the presence/absence of compost heaps, piles of branches, rock piles, trees and shrubs (as well as the 11 garden-owner reported habitat types).
8 Supplementary Figure S6. Residual plots for the alternative species richness model of cultivated plant species richness (Model 3), including the habitat heterogeneity index based only on the 11 exclusively garden-owner reported land-use types and features. For this model, the garden-owner reported habitat heterogeneity index does not include the expert-collected data on the presence/absence of compost heaps, piles of branches, rock piles, trees and shrubs.
9 Supplementary Figure S7. Residual plots for the species richness model of spontaneous plant species richness (Model 4). For this model, the garden-owner reported habitat heterogeneity index includes the expert-collected data on the presence/absence of compost heaps, piles of branches, rock piles, trees and shrubs (as well as the 11 garden-owner reported habitat types).
10 Supplementary Figure S8. Residual plots for the alternative species richness model of spontaneous plant species richness (Model 4), including the habitat heterogeneity index based only on the 11 exclusively garden-owner reported land-use types and features. For this model, the garden-owner reported habitat heterogeneity index does not include the expert-collected data on the presence/absence of compost heaps, piles of branches, rock piles, trees and shrubs.
11 Supplementary Figure S9. Semivariograms of residuals of the species richness models of total (A), native (B) cultivated (C) and spontaneous (D) plants species averaged over all observations and independent of the direction. Semivariances were computed with the R package ‘gstat’
(Pebesma, 2004). In all plots values are close to 1, indicating that residuals are not more similar or dissimilar to each other than expected by chance and hence are uncorrelated. For these
models, the garden-owner reported habitat heterogeneity index includes the expert-collected data on the presence/absence of compost heaps, piles of branches, rock piles, trees and shrubs (as well as the 11 garden-owner reported habitat types).
12 Supplementary Figure S10. Scatterplots displaying the total (A), native (B), cultivated (C) and spontaneous (D) number of plant species (plan species richness; S) versus garden area for each of the investigated 83 gardens.
13 Supplementary Figure S11: Scatterplots displaying the total (A), native (B), cultivated (C) and spontaneous (D) number of plant species (plant species richness; S) versus the urbanization gradient, which was quantified as the proportion of sealed surface (i.e. built and paved) in 500-m radius circles around each garden.
14 Supplementary Figure S12. Extended habitat heterogeneity instrument, including the garden features compost heaps, piles of branches, rock piles, trees and shrubs.
15 Supplementary Table S1. The autocorrelation index Moran’s I of the response variables. SD;
standard deviation of I under the null hypothesis (i.e. no autocorrelation). P; P-value of the test of the null hypothesis. Values were computed with the R package ‘ape’ (Popescu et al., 2012).
Response variable of each model
I observed I expected SD P
Model 1:
All plants
-0.043 -0.012 0.022 0.156
Model 2:
Native plants
-0.029 -0.012 0.021 0.439
Model 3:
Cultivated plants
-0.043 -0.012 0.022 0.161
Model 4:
Spontaneous plants
-0.034 -0.012 0.022 0.322
16 Supplementary Table S2. Alternative models including garden area as a predictor variable.
Native and spontaneous plant species richness (Models 2 and 3) are significantly associated with garden area, but the model fit did not substantially change with respect to original models without area as a predictor. For these models, the garden-owner reported habitat heterogeneity index includes the expert-collected data on the presence/absence of compost heaps, piles of branches, rock piles, trees and shrubs (as well as the 11 garden-owner reported habitat types).
Dependent variable of each model
Intercept Garden-owner reported species richness
Garden-owner reported habitat heterogeneity
Garden area Explained variance (Adjusted R2)
β1 SE β2 SE β3 SE β 4 SE R2
Model 1:
All plants
4.74*** 0.02 0.10** 0.03 0.17*** 0.03 0.00NS 0.02 0.49
Model 2:
Native plants
4.14*** 0.03 0.07* 0.03 0.17*** 0.03 0.07* 0.03 0.50
Model 3:
Cultivated plants
4.28*** 0.04 0.13** 0.04 0.24*** 0.04 -0.06 NS 0.04 0.43
Model 4:
Spontaneous plants
3.67*** 0.03 -0.00NS 0.03 0.10** 0.03 0.09** 0.03 0.22
NS = not significant; * P < 0.05; **P < 0.01; ***P < 0.001.
17 Supplementary Table S3. Alternative models including the habitat heterogeneity index based only on the 11 exclusively garden-owner reported land-use types and features. For this model, the garden-owner reported habitat heterogeneity index does not include the expert-collected data on the presence/absence of compost heaps, piles of branches, rock piles, trees and shrubs. Note that differences between this table and Table 1 in the article are minimal.
Dependent variable of each model
Intercept Self-reported plant species richness
Self-reported habitat heterogeneity
Explained variance (Adjusted R2)
β1 SE β2 SE β3 SE R2
Model 1:
All plants
4.74*** 0.02 0.10*** 0.03 0.15*** 0.03 0.46
Model 2:
Native plants
4.14*** 0.03 0.10* 0.03 0.15*** 0.03 0.39
Model 3:
Cultivated plants
4.28*** 0.04 0.14*** 0.04 0.23*** 0.04 0.41
Model 4:
Spontaneous plants
3.67*** 0.03 0.02NS 0.03 0.09** 0.03 0.09
NS = not significant; * P < 0.05; **P < 0.01; ***P < 0.001.
