3. Results
3.1 Species composition
In 2009 and 2010, 3048 individuals belonging to 210 species of 32 beetle families were
collected, using sweep nets. Four predator species; Anthophagus Omalinus, Malthodes Guttifer, Malthodes Fuscus and Malthodes Brevicollis and two herbivore species; Micrelus Ericae and Absidia Schoenherri comprised 65% of all the individuals collected. The three largest families;
Cantharidae, Curculionidae and Staphylinidae contituted 81% of the total individulas. Species composition varied significantly between different habitats: A) power-line corridor, B) edge of power–line corridor, C) forest edge, D) forest, distance into forest equal to one half of the corridor width, and E) 100 m into the forest from the corridor/forest edge. The results of the CCA showed that Site explained 34 % of variation (Monte-Carlo permutation test: Pseudo-F50,197: 2.02, p < 0.001, 999 permutations). In order to find significant additional variation by habitat after the variation related to site had been explained, the partial constrained ordination was performed. The results of the CCA showed that the variable habitat explained 2.4 % (Monte-Carlo permutation test: Pseudo-F4,193: 1.84, p < 0.001, 999 permutations). Plots in the center of the power-line corridor A) had a species composition very different from plots in the forest (D, C and E), and plots on each side of the edge (B and C), also differed substantially in species
composition (Figure 3). Detritivores as Cryptophagus abietis, Acrotrichis rugulos and Corticarina obfuscata were most associated with forest habitat (C and D). Predator Dasytes niger and herbivores as Polydrusus undatus,Rhampus pulicarius and Lochmaea suturalis where most associated with early succession habitats (A and B).
12 Figure 3 CCA plot showing differences in species composition of beetles captured in the field layer between five different habitats. Beetles captured with sweep nets in 51 different locations, in five different habitats (power-line corridor (A), edge of power–line corridor (B), forest edge (C), forest, distance into forest equal to one half of the corridor width (D), and 100 m into the forest from the corridor/forest edge (E)). The diagram shows abbreviated species names (red) and centroids of habitats types (blue).
13 3.2 Species richness
The species accumulation curves of the different habitats A, B, C, D and E did not seem to level off (Figure 4). This indicates that not all available species were collected in any of the habitats.
The early succession stage (A) had higher species richness than the edge to the early succession stage (B), wheareas the forest edge (C) was higher in species richness than the forest interior (D).
The early succession forest (A and B) has higher species richness than the later forest succession (C and D).
Figure 4 Species accumulation curves plotted for the five habitats. The x-axis shows the number of plots for collecting of beetles by sweep nets (see Figure 2). The y-axis show the cumulative numbers of species recorded. The habitat under the power-line (A) had the largest species richness and the lowest was in the forest habitat 100 m from the center of the power-line (E) and the forest habitat (D). The edge of the early succession habitat (B) had higher richness than the forest edge (C). The largest differences in species richness were found between the habitat in the center of the power-line had larger species richness, than the forested habitats (D and E). The vertical lines show the corresponding standard deviation.
plots
14 In the family accumulation plots, most of the curves seem to approach an asymptote
(Appendix 2). This is natural since there are fewer beetle families than beetle species, and thus easier to collect all or most of the families. The edge plots of the early succession stage (B), i.e.
power-line corridor, had the highest beetle family richness. The early succession forest, i.e.
center of power-line corridor and interior forest has approximately the same family richness.
Habitat and the field layer explanatory variables grass, dwarf shrubs and deciduous shrubs individually influenced the species richness (p < 0.05; Table 2), whereas there was no significant effect of herb (Table 1). Further analyses were performed using the variables with p<0.10 in Table 1. Pairwise correlation tests showed that none of the field layer variable were correlated (r>0.5). The full (most complex model) included these variables and second order interactions between these field layer variables and habitat. Model selection was performed using backward elimination by sequentially removing terms with the highest p-value, and always removing the interaction term before main effects. Habitat, grass and dwarf shrub were the only variables that significantly enhanced species richness (Table 2).
Table 1 Relationships between species richness and individually tested field layer vegetation
explanatory variables, using log link function and Poisson distribution. Beetle species captured in 2009, in 20 different locations and 2010 in 31 different locations. Random effect: site p < 0.0001 in all
analyses.
Explanatory variables Df Log (likel) χ2 F p
Habitat 4,953 3.13 0.0143
Grass 1,956 12.6 0.0004
Dwarf shrubs 1,956 4.92 0.0269
Deciduous shrubs 1,956 18.3 <.0001
Herbs 1,956 2.52 0.1126
15 Table 2 Results of relationships between species richness and explanatory variables. Response variable was number of beetle species. Results of generalized linear models, with log link function and Poisson distribution. a) Only field layer vegetation as explanatory variables. b) Field layer vegetation and habitat as explanatory variables. Field layer variables were measured as percentage cover within subplots in Figure 2. Beetles were sampled at 51 different sites, and site was modeled as random effect. Habitat was power-line corridor (A), edge of power–line corridor (B), forest edge (C), forest, distance into forest equal to one half of the corridor width (D), and 100 m into the forest from the corridor/forest edge (E).
Explanatory variables Df Log (likel) χ2 F P
a)
Fixed effects
Grass 1,954 11.1 0.0009
Dwarf shrubs 1,954 11.1 0.0009
Deciduous shrubs 1,954 12.4 0.0004
Random effect
Site 1 -1700 120 <.0001
b)
Fixed effects
Habitat 4,950 3.06 0.0160
Grass 1,950 12.2 0.0005
Dwarf shrubs 1,950 10.9 0.0010
Deciduous shrubs 1,950 13.8 0.0002
Random effect
Site 1 -1694 121 <.0001
Type Ⅲ Wald-F tests of fixed effects. Log likelihood tests of random effect.
16 Table 3 Parameter estimates and associated standard errors for model b) in Table 2.
Estimate (β) SE
Habitat A 0.26 0.080
Habitat B 0.15 0.080
Habitat C 0.41 0.075
Habitat D 0.29 0.078
Habitat E 0.26 0.083
Grass 0.0075 0.0021
Dwarf shrub 0.0063 0.0019
Deciduous shrub 0.0266 0.0071
Frequency plots of grass, dwarf shrub and deciduous shrub cover is shown in Appendix 3. In the early succession forest (A and B) deciduous shrub and grass were most abundant, while in the later succession forest (C and D) dwarf shrub were more abundant (Figure 5).
Estimated mean species richness was highest in forest edge plots (C), followed by plots in center of the power-line corridors (A), with the lowest richness in edge plots in power-line corridors (B) and forest interior (D) (Table 3, Figure 6, 7 and 8). Species richness increased with increasing cover of grass (Table 3, Figure 6), deciduous (Table 3, Figure 7) and dwarf shrubs, (Table 3, Figure 8).
17 Figure 5 Percentage cover of environmental variables (deciduous shrub, grass and dwarf shrub) within each habitat (power-line corridor (A), edge of power–line corridor (B), forest edge (C), forest, distance into forest equal to one half of the corridor width (D), and 100 m into the forest from the corridor/forest edge (E).
Figure 6 Species richness increased with increasing amount of grass cover. Predicted average species richness increased with higher density of grass cover up to 40 %. The center of the power-line (A) and the forest edge (C) had the largest average increase in species richness, respectively. The forest (D), the power-line edge (B), and the habitat 100 m in the forest from the power line (E), had approximately the same increase in species richness.
0 5 10 15 20 25 30 35 40
A B C D E
deciduouse shrub grass
dwarf shrub
18 Figure 7 Species richness increased with increasing amount of deciduous shrub cover. Predicted average species richness increased with higher density of deciduous shrub cover up to 12 %. The center of the power-line (A) and the forest edge (C) had the largest average increase in species richness, respectively.
The forest (D), the power-line edge (B), and the habitat 100 m in the forest from the power line (E), had approximately the same increase in species richness.
Figure 8 Species richness increased with increasing amount of dwarf shrubs cover. Predicted average species richness increased with higher density of dwarf shrub cover up to 60 %. The center of the power-line (A) and the forest edge (C) had the largest average increase in species richness, respectively.
The forest (D), the power-line edge (B), and the habitat 100 m in the forest from the power line (E), had approximately the same increase in species richness.
19 3.3 Edge effects on both sides of the edge (early and later successional stages)
The mean species richness in the center of the power-line (A), i.e. in the center of the early successional stage forest, had on an average 0.25 species more per plot than the edge to the early succession forest (B). The forest edge (C) had on an average 0.29 species more per plot, than the forest habitat situated at the same distance from the forest edge as of the width of the corridor (i.e., habitat D) (Figure 9).
Figure 9 Estimated mean species richness (number of species) and associated standard errors. Beetles were captured by sweep netting in plots of 4 m x 5 m in five different habitats: species richness in the centre of the power-line, early succession habitat (A), the early succession edge habitat (B) the forest edge habitat (C), and in plots located one half corridor width into the forest (D), and in plots 100 m into the forest (E). Plots in the forest edge zone had higher richness than plots located farther within the forest, whereas the opposite was found for early successional stage forests in power-lines, in power-line corridors, plots along the forest edge had lower species richness than plots in the center of the corridor.
When comparing only the two types of edge plots (B and C in Figure 10), was the mean number of beetle species on an average 0.32 higher in the forested edge (C) plots than in the early power-line edge (B) plots. Grass and dwarf shrub significantly enhanced spices richness in these two plots (Table 4).
Habitats
20 Table 4 The average species abundance was greater in the forested edge habitat (C) (e(0.4441-0)= 1.56 species), than in the early succession forest edge (B) (e0.4441+(-0.2311)
= 1.24 species ), with an average 0.32 species per plot (a total of 51 x 4 plots). Grass and dwarf shrubs were the only significant environmental effects that enhanced species richness in the edge effect habitats (B and C). Results of generalized mixed models with response variable species richness, log link function and poison distribution. Type 3 Wald F-test of fixed effects and log likelihood F-test for random effects are reported.
Explanatory variables df F value Pr > F Log
(likel) χ2
Standard
Fixed effects Estimate error
Habitat B 0.2130 0.0757 1,349 9.32 0.0024
The early succession forest habitat (A) had higher beetle species diversity than the other habitats except from the forest edge (C). Habitat C had higher diversity than the forest habitat (D) and the forest habitat 100 m from the center of the power-line (E). Steep curves indicate a large variation in abundance among different beetle species, i.e. lower evenness (Figure 10). For the family richness, the diversity for the early succession stage edge (B) was higher than other habitats except the early succession forest (A). Habitat (A) was more diverse then the forest edge (C) and the forest habitat a 100 m from the center of the power-line (E). The diversity for beetle families was lowest for the forested habitat 100 m from the center of the power-line (E) (Appendix 4).
21 Figure 10 The Renyi diversity profile shows the difference in evenness, species richness and biodiversity among species between the different plots, based on aggregated data from 51 sites. The figure shows Renyi diversity profiles for each habitat. Steep curves indicate lower evenness among beetle species.
The starting position on the left (alpha = 0) indicate that A, B and C has the largest beetle species richness, ranked in respective order. It is not possible to separate E and D. The antilog (eH-value) for alpha
= 0, shows the number of species richness. It is not possible to decide which site was the most diverse since the lines were crossing. Alpha = infinitive shows that the later forest edge habitat (C) had the least number of the most dominating species and the forest habitat a 100 m from the center of the power line habitat (E) had the largest number of dominating species. The forest habitat (D) and the early succession forest edge (B) have approximating equal numbers.
3.5 Species abundance distribution
The empirical cumulative distribution function plot (ECDF, Figure 11) shows that approximately 85% of the species had abundance less than 10-2 = 0.01, i.e. 1% of all captured beetle
individuals. This shows that most species had low abundance for all habitats. There was no significant difference in species abundance between the two distributions that appear to be most dissimilar in Figure 11, i.e. between forest edge (C) and forest interior (E)
(Kolmogorov-Smirnov test: D = 0.147, p = 0.337).
22
Forest, one-half corridor width into forest (D) Forest, 100 m into forest from edge (E)
Figure 11 The empirical cumulative distribution function (ECDF) with the beetle abundance showed in proportions of all individuals observed for the given habitats: A) power-line corridor, B) edge of power–
line corridor, C) forest edge, D) forest, distance into forest equal to one half of the corridor width, and E) 100 m into the forest from the corridor/forest edge, in all sites. The species abundance divided by total number of individuals on a log10 scale is shown on the x-axis. Species is ranked from highest to lowest abundance, and the ranks (divided by total species richness within each habitat) is shown on the y-axis.
The slope is indicative of evenness, and as the slopes is sharply vertical, this implies that the evenness is high.
3.6 Functional groups
Number of individuals and number of species differed between habitats and between functional groups. There was also a significant interaction between habitat and functional group (Table 5 and 6, Figure 12 and 13). This means that the relative proportion of individuals and species within different functional groups differed among habitats. Interestingly, the herbivore/predator ratio shifted from strong herbivore-bias in the center of the power-line corridor (early
successional stage forest) to a strong predator-bias 100 m into the later successional stage forest, with gradual change in the ratio in the plots in between these different habitats (Figure 12 and 13).
23 Table 5 Factors influencing the number of beetle individuals captured per 4 m x 5 m plot with sweep netting in the field layer vegetation. Wald F test of fixed effects and likelihood ratio tests of random effects. Individuals captured in 2009, in 20 different locations and 2010 in 31 different locations.
Generalized mixed models with log link function, negative binomial distribution, and Gaussian hermite quadrature approximation to the likelihood.
Explanatory variables df Log (likel) χ2 F P
Fixed effects
Habitat 4,397 5.16 0.0004
Functional group 3,397 292 <.0001
Habitat×functional group 12,97 6.92 <.0001
Random effect
Site 1 -3929 176 <.0001
Figure 12 Estimated mean (±SE) number of individuals within different functional groups of beetles.
Beetles were captured by sweep netting within 4m x 5m plots (see Figure 2) at 51 sites, in five different habitats: The center of the power-line habitat (A), the early succession edge habitat (B), forest edge habitat (C), forest habitat (D) and the forest habitat, a 100 m from the center of the power-line (E). PR = predators, HB = herbivores, DE = general detritivore and dead wood feeders and fungivores are grouped into the category ‘OTHER’.
0 0.5 1 1.5 2 2.5
DE HB OTHER PR DE HB OTHER PR DE HB OTHER PR DE HB OTHER PR DE HB OTHER PR
A A A A B B B B C C C C D D D D E E E E
24 Table 6 Factors influencing the number of beetle species captured per 4 m x 5 m plot, with sweep netting in the field layer vegetation. Wald F test of fixed effects and likelihood ratio tests of random effects. Species captured in 2009, in 20 different locations and 2010 in 31 different locations.
Generalized mixed models with log link function, negative binomial distribution, and Gaussian hermite quadrature approximation to the likelihood.
Explanatory variables df Log (likel) χ2 F p
Fixed effects
Habitat 4,397 4.79 0.0007
Functional group 3,397 230 <.0001
Habitat×functional group 12,97 5.59 <.0001
Random effect
Site 1 -2970 117 <.0001
Figure 13 Estimated mean (±SE) number of species within different functional groups of beetles. Beetles were captured by sweep netting within 4m x 5m plots (see Figure 2) at 51 sites, in five different habitats:
The center of the power-line habitat (A), the early succession edge habitat (B), forest edge habitat (C), forest habitat (D) and the forest habitat, a 100 m from the center of the power-line (E). PR = predators, HB = herbivores, DE = general detritivore and dead wood feeders and fungivores are grouped into the category ‘OTHER’.
25
4. Discussion
4.1 Species composition and functional groups
As predicted, there was a significant effect of habitat in analysis of beetle species composition.
The positions of the centroids of the different habitats in the CCA plot, and the arrangement of the different species (Figure 3), show that plots in the centre and along the edge of the power-line corridor had a species composition that differed substantially from plots in the forest. The greatest difference was between plots in the center of the power-line corridors, and plots in the forest interior. This is not surprising, as they reflect the greatest contrast in habitat conditions.
Interestingly, plots along the edge of the power-line corridor differed substantially from plots in the edge zone within the forest. Kaila et al. (1997) found a similar trend that the early and later successional forest stages varied significantly, due to various environmental factors and access to resources. Hansson (1994) found that the vertebrate species composition varied between clearcut and forest interior, because some species have different adaption to disturbance.
Detritivores as Cryptophagus abietis, Acrotrichis rugulos and Corticarina obfuscata were most associated with edge to the forest interior and forest interior. Predator Dasytes niger and
herbivore as Polydrusus undatus,Rhampus pulicarius and Lochmaea suturalis where most associated with the early succession power-line corridor and the edge to the early succession power-line corridor. This correspond well with Ewers & Didham (2008) that found highest abundance of detritus feeders in the forest interior and that they declines closer to the edge, and herbivores are more abundant in open habitat.
There was a significant interaction between habitat and functional group. That the proportion of different beetles functional groups differed among habitats. This is probably because of the different ways of utilizing food resources. Various functional groups are strongly related to their feeding habits (Lassau et al. 2005). Interestingly, the herbivore/predator ratio shifted from strong herbivore-bias in the center of the power-line corridor (early successional stage forest) to a strong predator-bias 100 m into the later successional stage forest, with gradual change in the ratio in the plots in between these different habitats. This is the same as Ewers and Didham (2008) found for herbivore beetles, i.e. they were abundant in the clear cut and declined towards
26 the forest interior. There is less chance to be depredated in the open habitat (Halme & Niemelä 1993), i.e. predators are more associated with forest interior (Hunter 2002; Elek & Lövei 2007) and a more complex habitat in the early succession stage gives more shelter from predators (Lassau et al. 2005), and there is more plant food available (Halme & Niemelä 1993).
4.2 Species richness
I found substantial differences in species richness among habitats. When looking at total species richness in each habitat aggregated over all sites and plots, species richness was higher in power-line corridors, i.e. in early successional stage forest, than in forest. Within power-power-line corridors, species richness was higher in the center than along the edges, whereas the opposite pattern was found for forest, in forest, species richness was higher in the forest edge than in plots located farther into the forest.
The beetle species richness in the early succession stages appeared to be positively affected by the frequent clearing of vegetation, which increases the cover of grass, deciduous shrubs and dwarf shrubs. It has been found that clearcuts, urbanized areas and small forest fragments have larger species richness, than forest interior, because of many open habitat species (Heliölä et al.
2001; Elek & Lövei 2007; Gagné & Fahrig 2011). Lassau et al. (2005) found that in higher complex habitats will create more different niches that can be utilized for the different beetle species, i.e. the clearance of vegetation under the power-line disturbed the habitat such as, more food is available.
Species in indigenous forest are lost after disturbance, but overall spices richness and diversity increases (Lewis & Whitfield 1999). Specialist beetle species in the interior forest do not cope well in disturbed patches, since they are more specialized, with large body sizes, and more affected by synergistic effects, such as fragmentation and edge effects (Collinge & Forman 1998;
Davies et al. 2004). In contrast, beetle species in the clearcuts are in general small sized and generalists (Collinge & Forman 1998; Gibbs & Stanton 2001; Elek & Lövei 2007).
27 4.3 Edge effects
When analyzing mean species richness per plot, controlling for among site effects, a different pattern emerged for species richness, than when looking at total species richness in each habitat aggregated over all sites and plots aggregated data, as above: The forest edge had the highest estimated mean species richness. Thus, the forest edge had higher mean species richness than the forest interior habitats. In contrast in the power-line corridors, plots along the edge had lower mean species richness than plots in the centre of the corridor. When comparing only the edges, the forested edge had significant higher species richness than the early succession forest edge.
Grass and dwarf shrub cover enhance the species richness in the two edges. The early succession forest had larger beetle species richness than the later forest succession. Some open habitat species move through the edge and into the forest interior (Niemelä et al. 1993; Spence et al.
1996). Open habitat species and forest interior species moves to the forest edge and increase the species richness in the forest edge (Magura et al. 2001; Baker et al. 2007; Roume et al. 2011).
The sensitivity to edge is different for early succession species and forest interior species (Ries &
Sisk 2010), because resource distribution in relation to edge differs between the two habitats.
Sisk 2010), because resource distribution in relation to edge differs between the two habitats.