Results

Species richness at TF

Figure 1: TF taxon diversity. B = butterfly; P = pollinator; G = ground dwellers *** p < 0.001; ** p < 0.01; * p < 0.05

A total of 22 species of butterflies were recorded in 82 butterfly counts (surveys with no species name recorded were excluded) at 42 TFs with a mean of 1.76 species per survey. The average number of species recorded per TF was 3.88, with a maximum of 9. The most widespread species were small white (24 TFs), large white (18 TFs), and common blue (8). The most abundant were small white, large white and small heath. Simpson diversity index (SDI) varies with species richness (SR) - TFs with higher SR and SDI have greater butterfly diversity.

Within each taxonomic group, SR and SDI were significantly correlated, but there was little association between groups apart from SDIs for ground dwellers (ants excluded) and pollinators ?@fig-ft-but-sr-1.

SPAC

(a) Butterflies

(b) Pollinators

(c) Ground-dwellers
Figure 2: Species accumulation curves derived from EW surveys

For butterfly species, 22 butterfly species (gamma diversity) were observed in 51 surveys across 41 TFs. As SACs show (Figure 2), saturation is nearly achieved implying that the vast majority of butterfly species occupying these areas have were spotted in the surveys. For ground-dwellers and pollinators, saturation was reached at 13 and 10 species groups respectively, and for both is achieved within a relatively small number of surveys. [discussion - relatively difficulty of observing butterflies, relatively less ‘effort’ per survey - may need fewer pollinator and ground-dweller surveys to estimate gamma diversity].

##3 Survey level analysis

Model outcomes

# A tibble: 7 × 7
  effect   group          term            estimate std.error statistic  p.value
  <chr>    <chr>          <chr>              <dbl>     <dbl>     <dbl>    <dbl>
1 fixed    <NA>           (Intercept)      1.79       0.0387    46.2    0      
2 fixed    <NA>           month            0.00948    0.0314     0.302  0.763  
3 fixed    <NA>           mean_age_days    0.0316     0.0599     0.528  0.597  
4 fixed    <NA>           y                0.0759     0.0353     2.15   0.0318 
5 fixed    <NA>           mean_ht          0.133      0.0683     1.95   0.0509 
6 fixed    <NA>           shannon         -0.112      0.0424    -2.65   0.00807
7 ran_pars tiny_forest_id sd__(Intercept)  0.179     NA         NA     NA      

X.Intercept.

mean_age_days

mean_ht

month

shannon

y

AICc

1.8

0.16

-0.118

0.077

863

1.8

0.031

0.14

-0.113

0.076

865

1.8

0.16

0.0085

-0.118

0.077

865

1.8

0.15

-0.113

866

1.8

0.121

-0.069

0.066

867

1.8

0.032

0.13

0.0095

-0.112

0.076

867

1.8

0.040

0.11

-0.106

867

1.8

0.102

0.067

868

1.8

0.15

0.0080

-0.113

868

1.8

0.116

-0.069

868

# A tibble: 7 × 7
  effect   group          term           estimate std.error statistic    p.value
  <chr>    <chr>          <chr>             <dbl>     <dbl>     <dbl>      <dbl>
1 fixed    <NA>           (Intercept)      1.60      0.0462    34.7    1.64e-263
2 fixed    <NA>           month            0.0265    0.0445     0.595  5.52e-  1
3 fixed    <NA>           mean_age_days   -0.0648    0.0674    -0.961  3.37e-  1
4 fixed    <NA>           y               -0.0682    0.0454    -1.50   1.34e-  1
5 fixed    <NA>           mean_ht          0.169     0.0762     2.21   2.68e-  2
6 fixed    <NA>           shannon         -0.0638    0.0509    -1.25   2.10e-  1
7 ran_pars tiny_forest_id sd__(Intercep…   0.0642   NA         NA     NA        

X.Intercept.

mean_age_days

mean_ht

month

shannon

y

AICc

1.6

0.096

-0.067

522

1.6

0.106

522

1.6

0.136

-0.061

523

1.6

0.124

-0.059

-0.066

523

1.6

0.086

0.036

-0.071

524

1.6

-0.048

0.142

524

1.6

0.099

0.028

524

1.6

-0.042

0.128

-0.066

524

1.6

-0.072

0.196

-0.074

524

1.6

-0.065

0.179

-0.071

-0.065

524

# A tibble: 7 × 7
  effect   group          term            estimate std.error statistic   p.value
  <chr>    <chr>          <chr>              <dbl>     <dbl>     <dbl>     <dbl>
1 fixed    <NA>           (Intercept)      8.11e-1    0.0979    8.29    1.13e-16
2 fixed    <NA>           month            2.43e-2    0.102     0.238   8.12e- 1
3 fixed    <NA>           mean_age_days    1.65e-2    0.137     0.120   9.04e- 1
4 fixed    <NA>           y               -6.94e-2    0.105    -0.664   5.07e- 1
5 fixed    <NA>           mean_ht         -2.16e-1    0.150    -1.44    1.50e- 1
6 fixed    <NA>           shannon         -4.11e-3    0.108    -0.0380  9.70e- 1
7 ran_pars tiny_forest_id sd__(Intercept)  5.01e-6   NA        NA      NA       

X.Intercept.

mean_age_days

mean_ht

month

shannon

y

AICc

0.81

-0.19

187

0.82

188

0.81

-0.21

-0.071

189

0.82

-0.109

189

0.81

0.028

-0.21

189

0.81

-0.19

0.0175

189

0.81

-0.19

-0.0099

189

0.82

-0.0545

190

0.82

-0.025

190

0.82

0.0043

190

Model outcomes

Descriptive analysis of SR by taxa at TFb

(a) TFb SR distribution

(b) SR by taxon class

Figure 3: Species richness from NBN data

(a) Class by land cover

(b) Land cover by class

Figure 4: Species richness by taxon class and land cover

As Figure 12 shows insects, vasculat plants and birds are the most diverse taxa across all habitat type. Insect and plant diversity are greatest in residentail gardens; bird and mamal diversity are siialr betwen gardens, grasslant and arable land.

Summary of environmental predictors

Table 1:

Environmental predictor summary table

Category

Metric

Mean

Sd

Q25

Q75

Blue_infrastructure

still_water

21590

39716

2451

21590

Blue_infrastructure

watercourse

39287

135997

3753

39287

Blue_infrastructure

nearest_water

276

251

82

410

Built

built_area

2086421

653916

1694140

2591003

Climate

tf_temp_2020

10.88

0.76

10.65

11.17

Climate

tf_temp_2022

11.27

0.70

11.02

11.56

Climate

tf_rain_2020

70

20

59

71

Climate

tf_rain_2021

61.5

13.1

54.6

63.1

Climate

tf_rain_2022

55.8

14.7

47.8

57.5

Climate

mean_spring_rain

145

29

130

153

Climate

mean_spring_temp

9.28

0.66

9.02

9.55

Connectivity

nc_y

10.0

3.8

7.0

12.0

Connectivity

lnk_y

136

95

58

205

Connectivity

slc_y

113078

131052

26100

144700

Connectivity

msc_y

21423

53115

5630

21423

Connectivity

ccp

0.48

0.22

0.32

0.66

Connectivity

lcp

0.0046

0.0110

0.0000

0.0046

Connectivity

cpl

2.7

1.1

1.9

3.2

Connectivity

ecs

94889

124745

17650

117182

Connectivity

awf

6042740107

17522014830

173363627

3765155663

Connectivity

iic_y

0.0026

0.0071

0.0000

0.0000

Green_infrastructure

perc_tree

0.0187

0.0245

0.0038

0.0220

Green_infrastructure

perc_garden

0.201

0.089

0.136

0.260

Green_infrastructure

perc_veg

0.41

0.16

0.32

0.50

Green_infrastructure

wood_area_sum

227252

202858

82726

327279

Green_infrastructure

gi_prop

0.55

0.18

0.42

0.68

Green_infrastructure

tree_density_m_2

0.73

0.36

0.49

0.98

Greenness

green_prop

0.30

0.21

0.12

0.44

Greenness

ndvi

0.51

0.10

0.46

0.57

Greenness

savi

0.263

0.071

0.218

0.306

Greenness

gbndvi

0.28

0.12

0.23

0.35

Greenness

evi_2

0.310

0.086

0.256

0.361

Landscape

aggregation_index

92.4

2.1

91.0

94.0

Landscape

modified_simpsons_diversity_index

0.89

0.31

0.69

1.11

Landscape

perimeter_area_fractal_dimension

1.317

0.027

1.303

1.335

Landscape

patch_richness

8.3

1.6

7.0

9.0

Landscape

shape_index

47.1

8.2

41.1

52.4

Linear_features

hedge_len

3870

2898

1575

5692

Linear_features

road_length

40002

11801

33248

47869

Tf_char

tf_age_y

579

204

572

655

Tf_char

total_trees

142

60

100

200

Tf_char

elevation

80

51

32

121

Tf_char

lon

-1.79

1.11

-2.13

-1.16

Tf_char

lat

53.00

1.39

52.36

53.32

Many indicators are highly correlated or cluster as shown in Figure 5 which orders pair-wise correlations of scaled (normalised) metrics by hierarchical cluster. Rainfall metrics cluster and are inversely correlated with temperature indicators. Gardens, built areas and roads form a separate cluster as do connectivity metrics, and vegetation/‘greenness’ indicators.

(a) Non-significant correlation are masked by an X
Figure 5: Correlogram of environmental predictors

(a) Non-significant correlation are masked by an X
Figure 6: Correlogram of bio predictors

Gamma diversity

Results

Mean TF butterfly SR was 2.29 (survey sites = 42) and varied between 1 and 8. Taxon richness also varied between 1 and 13 (mean = 6.2, sites = 102) for pollinators and 1 to 10 (mean = 5.03, sites = 81) for ground dwelling invertebrates respectively.

Figure 7: Correlation between TF butterfly SR and insect and butterfly TFb SR

As Figure 7 shows, TF butterfly richness is uncorrelated with TFb butterfly or insect SR.

Intrinsic TF variables were then fitted to butterfly SR using GLMM with a negative binomial error structure and TF as a random effect. The 10 models with the lowest AICc fitted with the dredge function of the MuMIm R package are shown in ?@tbl-butterfly-model (Bartoń (2023)). Each row of the table shows the coefficients of the explanatory variables - the number of surveys (n) is included in all 10 models, with mean age at time of survey (scaled) included in four. Survey month and weighted tree height are included in 2 candidate models.

Model selection can be difficult. It has been common practice in ecology to average models with with an AICc within 7 of the model of the lowest AICc but this has been criticised for leading to faulty inferences about relevant predictors (Galipaud et al. (2014); Walker (2017)).

An alternative is use penalised regression where predictors are removed

Ground dwelling invertebrates

GDM

Generalised dissimilarity models (GDM) were fitted to community matrices generated for each TF site for butterflies, pollinators and ground dwelling invertebrates to test the hypothesis that community composition changed as forest aged. Modelling followed the method set out by Mokany et al. (2022).

Figure 8: Partial dependence plots for butterfly GDM

Figure 9: GDM results for ground-dwellers

Figure 10: GDM results for ground-dwellers
Table 2:

Variable importance

taxon

Important variables

All predictors

Butterflies

Geographic...1

0.00

Butterflies

shannon...2

0.28

Butterflies

mean_ht...3

0.54

Pollinators

Geographic...4

0.20

Pollinators

mean_ht...5

0.58

Pollinators

tf_age...6

0.28

Ground dwellers)

Geographic...7

0.00

Ground dwellers)

shannon...8

0.68

Ground dwellers)

mean_ht...9

0.66

Ground dwellers)

tf_age...10

0.00

Relationship between intrinsic tiny forest variables and tiny forest species, richness

Impacts of characteristics of surrounding area on tiny forest, biodiversity line Abiotic. Biotic

Env

{#fig-glmnet}

taxon

metric

estimate

Butterfly

gi_prop

0.18352018

Butterfly

n

0.03240214

Butterfly

lnk_y

0.00093757

Butterfly

hedge_len

0.00000651

Butterfly

watercourse

0.00000115

Butterfly

wood_area_sum

0.00000012

Butterfly

nc_y

-0.00009932

Butterfly

tf_rain_2021

-0.00303056

Butterfly

perc_tree

-6.97509738

Ground-dwellers

lat

0.02982828

Ground-dwellers

n

0.01232032

Ground-dwellers

tf_age_y

0.00014862

Pollinators

n

0.04499600

Figure 11: Butterfly predictors

Environmnental predictors of taxon richness

The results of glment modelling are shown in Figure 11. For butterflies, green infrastructure, linear features (water courses and hedges), wooded areas, rainfall and connectivity metrics variables were found to be weakly positively associated with butterfly SR, whereas tree cover was strongly negatively predictive. For pollinators and ground-dwellers, SR was associated with the number of surveys, and increased with forest age and latitude for ground-dwellers.

(a) Class by land cover

(b) Land cover by class

Figure 12: Species richness by taxon class and land cover

Estimated SR for small areas based on random sampling

Estimated SR for small areas based on SAR

Figure 13: Relationship between log habitat area and log SR

Figure 14: Relationship between log habitat area and log SR

Sampling


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(a) All species
Figure 15: Species accumulation curve for randomly sampled 4Ha sites

Butterfly richness estimated by sampling

Based on 300 (of 10,000) randomly sample 4Ha pseudo-controls, median butterfly richness was , with a percentile range of Butterfly diversity for TFs was 22 which is at the 95th centile of the distribution. Butterfly richness was highest in suburban areas approaching near saturation at 34 species. Richness in arable areas, improved grassland and deciduous woodland were lower but based on smaller samples.For a single 4Ha site the highest SR was in fen habitat (15 species), with arable (12), suburban (11), grassland (10) and woodland (8).

References

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