Introduction
Methods
Study region
Data collection
Statistical analysis
α
-diversity) as well as the total number of species for each sampling period (γ-diversity). Changes in
α
-diversity per site were calculated using the formula: \(\Delta \alpha = \frac{\alpha [new] - \alpha [old]}{\alpha [new] + \alpha [old]}\). This means that Δα can range from −1 to 1 and that negative values represent a decrease in species richness, while positive values represent an increase. Changes in
γ
-diversity were calculated analogously. Generalized estimation equations were used to test for significant changes in
α
-diversity (dependent variable) over time (independent variable), using a Poisson distribution and sites as the grouping variable. A two-sided Pearson correlation test was used to establish whether changes in
α
- and
γ
-diversity were correlated across taxonomic groups. To visualise species-turnover rates, the fraction of occupied sites in 1988 was plotted against the fraction of occupied sites in 2005 for each species.Results
Local diversity
α
-diversity) over 17 years of calcareous grassland management (Fig. 2). Carabid beetles and weevils decreased in
α
-diversity, while true bugs and millipedes showed an increase in local species richness (Table 1). Local richness of plants, spiders, ants and woodlice did not change over time.
Trophic level | Taxonomic group | Δ α-diversity | p value | Δ γ-diversity |
---|---|---|---|---|
Primary producers | Plants | 0.012 | 0.588 | −0.045 |
Macrodetritivores | Woodlice | 0.015 | 0.785 | −0.125 |
Millipedes
|
0.081
|
0.025
| 0.157 | |
1st order consumers |
True bugs
|
0.371
|
0.008
| 0.293 |
Weevils
| −0.117
|
0.011
| 0.000 | |
Predators |
Carabid beetles
| −0.186
|
<0.001
| −0.206 |
Spiders | −0.004 | 0.922 | 0.009 | |
Ants | 0.020 | 0.508 | 0.050 |
Regional diversity and compositional variation
γ
-diversity) patterns generally followed local patterns (Pearson R2 = 0.75; df = 6; p = 0.006), with carabid beetles decreasing in
γ
-diversity, while
γ
-diversity increased for true bugs and millipedes (see Table 1). Few species had identical relative frequencies in both sampling periods (along diagonal in Fig. 3). This implies that considerable species turn-over occurred over time for all groups. This could cause increased biotic homogenization or differentiation, independent of local species richness changes. However, significant changes in compositional variation among sites was only found for two taxonomic groups. Carabid beetle communities became increasingly homogenized over time (Table 2), mainly because many initially rare species became rarer (Fig. 3). In contrast, millipede communities became increasingly differentiated (Table 2), due to some rare species becoming more prevalent as well as some common species becoming rarer (Fig. 3).
Trophic level | Taxonomic group | n sites | n species | ΔD
| p |
---|---|---|---|---|---|
Primary producers | Plants | 7 | 223 | −97.51 | 0.447 |
Macrodetritivores | Woodlice | 6 | 10 | −31.50 | 0.119 |
Millipedes
|
6
|
25
|
60.23
|
0.030
| |
1st order consumers | True bugs | 8 | 64 | 183.69 | 0.061 |
Weevils | 8 | 54 | −16.04 | 0.625 | |
Predators |
Carabid beetles
|
8
|
91
| −199.89
|
0.029
|
Spiders | 8 | 151 | 1.309 | 0.988 | |
Ants | 6 | 23 | 13.24 | 0.612 |