Independence of diversity measures
Correlation analysis, as expected, showed a strong correlation between site (total richness across four quadrats) and quadrat-level species richness (Table 2). It also showed strong correlations between Shannon diversity and both measures of species richness.
Table 2. Correlation matrix between species richness at a site level (Number of species) and quadrat level (Sp. m−2), species diversity (Shannon) and functional diversity indices: FRic = Functional Richness; FEve = Functional Evenness; FDiv = Functional Divergence; IV = index of variance. *0.05 ≥ P > 0.01, **0.01 ≥ P > 0.001, ***P < 0.001
| ||Number of species||Sp. m−2||Shannon||FRic||FEve||IV FRic||IV FEve|
|Sp. Density||0.89***|| || || || || || |
|Shannon||0.64***||0.76***|| || || || || |
|FRic||0.41*||0.11||−0.02|| || || || |
|FEve||−0.12||−0.01||−0.03||−0.05|| || || |
|FDiv||0.18||0.04||−0.10||0.06||0.20|| || |
|IV FRic||−0.31||−0.47**||−0.40*|| || || || |
|IV FEve||−0.12||−0.01||−0.02|| || ||0.32|| |
|IV FDiv||0.24||0.09||−0.04|| || ||−0.13||0.10|
Of the three possible correlations between the different measures of functional diversity, none were significant or even approached significance. This was the same for their indices of variance. Also, of the nine correlations between the measures of species diversity and functional diversity, only one was significant. This was a positive correlation between FRic and number of species recorded at a site level. However, there were also significant negative relationships between FRic’s IV and quadrat species richness and Shannon diversity. FRic was much lower than expected at high levels of species richness and diversity. Using the occurrence-weighted null model had no appreciable effect on the correlation between the measures of species diversity and the indices of variance (Table S2).
Difference of functional diversity indices from random
It is clear from the comparison of the measured FRic values with calculated values from random communities that 13 (of 30) sites show values ranked in the lowest 2.5% of those from simulated communities (Fig. 1a). The communities that showed this feature included those on arable land, fallows, silage fields, pastures and abandoned croft land. Communities with lower management intensities had ranks that were not significantly different from those expected. No community had a ranking of FRic in the top 2.5% of random draws for that abundance and richness pattern. The abundance-weighted null model produced a similar result (Fig. S1a).
Figure 1. Histograms showing the distribution of rankings of the measured functional diversity indices for each of the 30 sites as compared with the 999 calculated ranks; (a) FRic = Functional Richness; (b) FEve = Functional Evenness; (c) FDiv = Functional Divergence.
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The pattern was very different for FEve (Fig. 1b). Here, the rankings of the individual communities showed a spread across all possibilities. However, one heathland and one silage field showed higher than expected levels of evenness, and one abandoned silage field a lower than expected FEve. Again, a similar distribution of higher and lower than expected evenness values occurred as would be expected by chance from 30 tests. A similar pattern was also observed for FDiv (Fig. 1c), although here, only one site, an arable plot, showed an extreme value for this measure that put it in the highest 1% of the communities created by random draws. The abundance-weighted null models for FEve and FDiv produced similar results (Fig. S1b,c).
Relationship between functional diversity and the environment
The species-based measures of diversity showed some of the expected hump-backed relationship with productivity (Table 3, all reported quadratic relationships passed the Mitchell-Old & Shaws test). In particular, there was evidence of a peak in site species richness at intermediate levels of dead biomass (187 g m−2, R2 = 0.218, P = 0.036), as well as weak evidence for peaks at intermediate levels of live biomass (407 g m−2, R2 = 0.183, P = 0.065) and total standing biomass (582 g m−2, R2 = 0.182, P = 0.066). Similarly, species richness per quadrat peaked at 172 g m−2 dead biomass (R2 = 0.251, P = 0.018, Fig. 2a), Shannon diversity at 354 g m−2 live biomass (R2 = 0.208, P = 0.043) and 579 g m−2 total standing mass (R2 = 0.221, P = 0.035). There was weak evidence that quadrat richness peaked at 538 g m−2 total standing biomass (R2 = 0.181, P = 0.068). There was also a peak in site species richness (R2 = 0.283, P = 0.011) and weak evidence for a peak involving species richness per quadrat (R2 = 0.195, P = 0.053), for intermediate levels of soil nitrogen release.
Table 3. Significance of the fitted curves between the species and functional diversity indices and the selected environmental parameters. The results for the functional diversity indices and their index of variance are shown in the same column separated by ‘/’. The shape of the relationship is indicated by: + positive linear trend; − negative linear trend; U quadratic trend with a minimum within the data range; and ∩ quadratic trend with a maximum within the data range. The strength of the relationship is indicated by the number of symbols, for example: (+) 0.10 ≥ P > 0.05; + 0.05 ≥ P > 0.01; ++ 0.01 ≥ P > 0.001; +++ 0.001 ≥ P. FRic = Functional Richness; FEve = Functional Evenness; FDiv = Functional Divergence; IV = index of variance
| ||Number of species||Sp. m−2||Shannon||FRic/IV FRic||FEve/IV FEve||FDiv/IV FDiv|
|ANPP|| || || ||/||/||/|
|Live Biomass||(∩)|| ||∩||/||/||/|
|Dead Biomass||∩||∩|| ||/||−−−/−−||/|
|Vegetation C:N|| || || ||/+||/||/|
|log soil N release||∩||(∩)|| ||(−)/−−||UU/U||/|
|Disturbance frequency|| ||∩∩||(∩)||/−−||+/||/|
|Below-ground disturbance||∩||∩∩|| ||/||+/||/|
|Start of disturbance||(−)||−−||−||/++||(−)/||/|
|Vegetation height|| || || ||/+||/||/|
Figure 2. Example relationships between diversity indices and the environment: species richness per quadrat against (a) dead biomass and (b) biomass loss, index of variance (IV) of Functional Richness (FRic) against (c) loss soil nitrogen release and (d) the timing of the start of disturbance, and IV of Functional Evenness (FEve) against (e) dead biomass and (f) log soil nitrogen release. Fitted curves and their respective R2 and P-values are shown.
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The patterns in the functional diversity indices were very different. FDiv showed no significant relationships with any of the productivity measures used, nor did its IV. FRic was weakly negatively related to soil nitrogen release (R2 = 0.264, P = 0.087). However, its IV was significantly positively related to vegetation C : N (R2 = 0.145, P = 0.038) and to vegetation height (R2 = 0.264, P = 0.087) and negatively related to soil nitrogen release (R2 = 0.276, P = 0.003, Fig. 2c). As an illustration, two sites of contrasting FRic are shown in Fig. 3. The winter-grazed site, D2R, had much lower levels of soil nitrogen and higher vegetation C : N ratio than the arable site, D6CA, and considerably higher FRic for both the two traits illustrated and overall. FEve was strongly negatively related to both dead biomass (R2 = 0.378, P = 0.0003) and total standing biomass (R2 = 0.183, P = 0.018). It also showed a minimum at intermediate levels of soil nitrogen release (R2 = 0.331, P = 0.004). Its IV was also negatively related to dead biomass (R2 = 0.253, P = 0.005, Fig. 2e) and weakly to standing mass (R2 = 0.129, P = 0.051), and also showed a minimum at intermediate levels of soil nitrogen (R2 = 0.264, P = 0.016, Fig. 2f).
Figure 3. Functional Richness (FRic) (equal to the total area of the convex hull) of two plots showing extreme values of FRic for just two traits, specific leaf area (SLA) (mm2 mg−1) and seed mass (mg). •: D2R, a winter-grazed site and ○: D6CA, an arable site.
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In general, there were also strong, quadratic relationships between the species diversity measures and the measures of the degree of disturbance (Table 3). Of the nine possible relationships between the degree of disturbance and species diversity, five were significant at P < 0.05 (range of R2: 0.200–0.414), and a further two relationships had P-values < 0.1. As an example, the relationship between quadrat species richness and biomass loss is shown in Fig. 2b. There were also significant, negative relationships between the start of disturbance and both species richness per quadrat (R2 = 0.277, P = 0.003) and Shannon diversity (R2 = 0.147, P = 0.036), as well as a weak relationship with site species richness (R2 = 0.101, P = 0.086). This indicated that species diversity was higher where disturbance occurred earlier in the year.
Neither FRic nor FDiv showed any significant relationships with the chosen measures of disturbance. However, the IV of FRic was strongly negatively correlated with disturbance frequency (R2 = 0.222, P = 0.009) and biomass loss (R2 = 0.228, P = 0.008), and positively with the start of disturbance (R2 = 0.271, P = 0.003, Fig. 2d); the observed declined in relation to the expected value as disturbance increased in intensity or arrived earlier. FEve showed a positive relationship with the frequency of disturbance (R2 = 0.264, P = 0.003), the severity of disturbance (R2 = 0.184, P = 0.010) and the time since below-ground disturbance (R2 = 0.213, P = 0.020). It also showed a weak positive relationship with time of disturbance (R2 = 0.095, P = 0.097). Its IV was unrelated to disturbance. Indices of variance calculated with the occurrence-weighted null model produced similar patterns (data not shown).