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Keywords:

  • arthropods;
  • coarse woody debris;
  • ecological restoration;
  • herbivore;
  • insects;
  • intervention;
  • kangaroo;
  • microhabitat;
  • refugia

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

1. High densities of vertebrate herbivores can be a significant barrier to ecological restoration in many parts of the world because of their impact on vegetation biomass. A common method for managing vertebrate herbivores is the use of exclosure fences, but very few studies have examined how small-scale structural refugia (e.g. logs) can mitigate grazing impacts. We examined how beetles responded to experimentally manipulated kangaroo Macropus giganteus grazing levels using both exclosure fences and addition of logs over a 16-month period.

2. We analysed beetle responses across (a) one-hectare sites, by focusing on the interaction between grazing level and log volume, and (b) microhabitats, by focusing on the interaction between grazing level and microhabitat structure (in open ground or at experimental logs).

3. At the site scale, we detected significant negative effects of grazing and positive effects of logs on beetle abundance and species richness. Beetle trophic groups responded in the same direction across grazing levels with herbivores, detritivores and predators all having higher abundance and species richness at low grazing levels. Logs applied at 20 t ha−1 in clumped arrangements had the largest positive effect on beetles at low grazing levels. At the microhabitat scale, beetles sampled adjacent to experimental logs showed an increase in abundance and species richness compared with beetles sampled from open ground, indicating logs are acting as microhabitat buffers from grazing.

4. Synthesis and applications. A reduction in grazing level had benefits for the abundance and species richness of beetles at the site scale. Further benefits were achieved at both site and microhabitat scales when logs are used in combination with exclosure fencing. For ecological restoration, exclosure fences and logs can be used to manage the impacts of vertebrate herbivores at different spatial scales. The rapid response of beetles suggests there may be potential for cascading effects on other biota as a consequence of reduced grazing, including increased food availability for insectivorous vertebrates.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

High densities of vertebrate herbivores can be a significant barrier to ecological restoration in many parts of the world because of their direct consumption of plant biomass and alteration of plant habitat structure (Côtéet al. 2004; Tanentzap et al. 2009). High levels of grazing also can have indirect effects on many ecological processes such as plant seed dispersal and recruitment and the cycling of nutrients (Abensperg-Traun et al. 1996; Carline, Jones & Bardgett 2005; Beguin, Pothier & Côté 2011). This can lead to homogenization of habitat and loss of biodiversity (Rooney & Waller 2003; Reid & Hochuli 2007).

Insects comprise a significant component of ground-layer biodiversity in grasslands and woodlands grazed by vertebrates (Tscharntke & Greiler 1995; Gebeyehu & Samways 2003; Lindsay & Cunningham 2009; Woodcock et al. 2009). Many studies that test the effect of vertebrate grazing on insect communities have focused on the impact of livestock grazing (e.g. Dennis, Young & Gordon 1998; Kruess & Tscharntke 2002; Gebeyehu & Samways 2003; Batáry et al. 2007; Reid & Hochuli 2007). Fewer studies have examined the effects of native vertebrate herbivores on insects in natural systems (e.g. Rambo & Faeth 1999; Suominen et al. 2003; Den Herder, Virtanen & Roininen 2004).

In many grazed ecosystems, some level of grazing is considered necessary for promoting structurally and compositionally diverse plant communities (Olff & Ritchie 1998; Gordon, Hester & Festa-Bianchet 2004), which often support greater insect diversity (Dennis, Young & Gordon 1998; Stoner & Joern 2004; Schaffers et al. 2008). Managing for appropriate levels of vertebrate grazing is therefore a key objective for the management of plant and insect communities, particularly in cases where vertebrate herbivores occur in high densities (Dennis et al. 1997; Reid & Hochuli 2007).

Management of vertebrate herbivores has focused on the use of large-scale measures, such as fencing (Gordon, Hester & Festa-Bianchet 2004) or culling (Tanentzap et al. 2009), and more recently the reintroduction of top-order predators (Beschta & Ripple 2009). There has been little focus, however, on the manipulation of habitat structure at small scales to increase spatial heterogeneity in vertebrate herbivory patterns. Logs play an important ecological role in temperate woodlands and forests by providing focal points for water infiltration, seed aggregation, nutrient cycling, food and shelter for invertebrates and foraging sites for vertebrates (Harmon et al. 1986; Lindenmayer et al. 2002). They may also provide a significant small-scale structural refuge from grazing that allows the establishment and growth of grasses and plants (Milchunas & Noy-Meir 2002; Pellerin et al. 2010). However, the extent to which logs may moderate grazing impacts on insects is not well understood.

In south-eastern Australia, the dominant large native herbivore is the eastern grey kangaroo Macropus giganteus. Populations of this kangaroo species have increased in some areas over recent decades because of the absence of large predators and changes in land use (Viggers & Hearn 2005; ACT Parks Conservation and Lands 2010). As a consequence, populations have reached historically high densities of over two animals per hectare in areas of the Australian Capital Territory. We used a manipulative experiment to examine the response of ground-dwelling beetles to kangaroo grazing and addition of logs at two spatial scales. We predicted that overall abundance and species richness of beetles would be positively affected by reduced densities of kangaroos. This prediction was based on current understanding of two ecological phenomena: (i) resource availability and (ii) habitat structural complexity. First, community structure can be related to resource availability and interspecific competition, with an increase in resources at one trophic level affecting other trophic levels by redistributing matter and energy available to organisms and changing competition dynamics in the ecosystem (Hairston, Smith & Slobodki 1960; Chase et al. 2002; Schmitz 2008). High densities of vertebrate herbivores can reduce the availability of plant biomass for other organisms dependent on the same resource, including herbivores and detritivores (Chase et al. 2000; Ims et al. 2007). Secondly, species richness is often related to habitat structural complexity (MacArthur & MacArthur 1961; Tews et al. 2004). This is because more complex habitats can provide a greater variety of microenvironmental conditions, thereby reducing interspecific competition and facilitating the co-existence of species that utilize a diversity of niches (Hansen 2000; Finke & Snyder 2008). Simplification of habitat owing to high densities of vertebrate herbivores can therefore lead to a reduction in insect species richness (Dennis, Young & Gordon 1998). The addition of logs could increase heterogeneity in grazing pressure by altering spatial grazing patterns and providing small-scale refuges from herbivory (Schreiner et al. 1996; Pellerin et al. 2010).

In this study, we focus on the ground-dwelling beetle community in a remnant of temperate grassy eucalypt woodland in south-eastern Australia. Beetles are an abundant and diverse component of the fauna of grassy eucalypt woodlands (Barton et al. 2009; Gibb & Cunningham 2010), and they are a potentially useful insect group to test responses to vertebrate herbivory because of their potential for rapid response to habitat change. We asked the following questions: (i) Do differences in vertebrate grazing affect the trophic structure of beetle assemblages? (ii) Does the addition of logs interact with grazing level to affect beetle diversity at the hectare scale? (iii) Does microhabitat structure provided by logs interact with grazing level to affect beetle diversity at small scales because of a localized ‘refuge’ effect?

Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

Study Area and Experimental Design

We conducted our study in the Goorooyarroo Nature Reserve (149·18°E, 35·19°S) located near Canberra, south-eastern Australia (Fig. 1a). The nature reserve comprises grassy woodland with a heterogeneous cover of yellow box Eucalyptus melliodora and Blakely’s red gum Eucalyptus blakelyi. We used the experimental design of an established long-term experimental restoration project, the Mulligans Flat – Goorooyarroo Woodland Experiment (Barton et al. 2009; Manning et al. 2011). Briefly, we divided the nature reserve into 12 polygons with four 1-ha sites (50 × 200 m) located within each polygon (Fig. 1b). Each 1-hectare site included two observation plots of 25 m radius, separated by 100 m (Fig. 1c). We collected data in each observation plot, including pitfall trap sampling and ground-layer habitat characteristics.

image

Figure 1.  Study location in south-eastern Australia (a), experimental design showing spatial arrangement of grazing treatments and logs (b), the arrangement of pitfall traps in the one-hectare sites (c).

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We placed fences around groups of polygons (Figs 1b and 2a), with kangaroos removed from inside exclosures in December 2007. Total removal of kangaroos was not possible and some limited grazing by kangaroos still occurred inside exclosures. We assessed animal numbers four times inside exclosures (December 2007, June and November 2008, June 2009) and twice outside the exclosures (December 2007, June 2009). Over the duration of this study, the average density of kangaroos inside exclosures was c. 0·4 (±0·2 SE) animals per hectare, and 2·1 (±0·1 SE) animals per hectare outside exclosures. Where we refer to ‘low’ and ‘high’ grazing levels, this should be interpreted with regard to these animal densities.

image

Figure 2.  Two treatments were used in the experiment. (a) Exclosure fences were used to produce ‘low’ and ‘high’ kangaroo densities for the experiment. (b) Logs were applied across sites in different arrangements to increase ground-layer structure.

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In October 2007, we distributed over 1000 t of logs across 48 one-hectare sites in the reserve. The logs were from Eucalyptus tree species. We applied the experimental logs to the one-hectare sites in four different treatments: (a) control sites with no logs added, (b) 20 t ha−1 with individual logs evenly dispersed, (c) 20 t ha−1 with logs placed in clumps to mimic natural treefalls and (d) 40 t ha−1 with logs placed in both dispersed and clumped arrangements (Figs 1b and 2b). Dispersed logs allow for grazing immediately adjacent to the log, whereas logs in a clumped arrangement prevent grazing between the logs.

Beetle Sampling

We sampled beetles using a pair of pitfall traps, separated by 1 m, placed in open ground beyond the canopy of any tree at each end of every one-hectare site (Fig. 1c). In sites with 40 t ha−1 of logs added, we used additional pitfall traps to sample immediately adjacent to (a) clumped experimental logs, (b) dispersed experimental logs and (c) in situ logs, in addition to traps placed in open ground (Fig. 1c). In situ logs are logs that were already present in the study site prior to the experiment. We refer to clumped logs, dispersed logs, in situ logs and open ground as microhabitat elements. At every pair of pitfall traps, we measured maximum grass height and percentage grass cover within a 1-m2 quadrat. We opened pitfall traps for 2 weeks in April 2008 and 2009, respectively. Traps consisted of 250-mL plastic jars dug in flush with the soil surface, each with 100 mL of glycol as a preservative. This approach has previously resulted in c. 70% of total estimated species richness sampled from open-ground microhabitat (Barton et al. 2009).

We sorted adult beetles to family level using keys to the Australian beetle fauna (CSIRO 1991; Lawrence & Britton 1994; Lawrence et al. 1999) and then identified individuals to morphospecies (sensu Oliver & Beattie 1996), hereafter referred to as species. We placed each beetle species into one of three generalized trophic groups: herbivores, predators and detritivores (including fungivores). We assigned beetles to trophic groups based on the predominant feeding behaviour of the adults at the family or subfamily level as described in the literature (Table S1, Supporting information) (Lawrence & Britton 1994; Hunt et al. 2007).

Statistical Analyses

We used generalized linear mixed models (GLMMs) (Schall 1991) to examine the interactive effects of grazing level and year on maximum grass height and percentage grass cover measured at open-ground microhabitat across all sites. We also used GLMMs to examine beetles responses to the experimental treatments. We used a Poisson distribution and log-link function for the analysis of beetle abundance and species richness data and assumed overdispersion GenStat 12.0 (Lawes Agricultural Trust 2010). We used GLMMs to examine beetle responses to the experimental treatments at two spatial scales: (a) across all one-hectare sites and (b) across microhabitat types within sites with 40 t ha−1 of added logs. For all analyses of beetle data, we pooled each pair of pitfall traps to give one sample per microhabitat element per year. Some traps were interfered with by birds or kangaroos, but this represented only 1% of the total number of traps (9 of 768 traps) and was spread over both years of sampling. This was unlikely to bias our data, so we excluded these traps from our analyses. In each of our tests, we considered the hierarchical nature of the experimental design and used a random model of plots within sites within polygon.

Across one-hectare sites, we were first interested in the trophic response to grazing level as the differences in kangaroo density inside and outside the exclosures were likely to affect grass resource availability, regardless of log treatments. Therefore, we tested for differences in the abundance and species richness of trophic groups (herbivore, detritivore, predator) across grazing treatments (low, high) and year (2008, 2009). Here, our full fixed model considered interactions between year, grazing level and trophic group.

Secondly, we were interested in the interactive effect of grazing level and log addition on overall beetle diversity across one-hectare sites. The addition of logs to the one-hectare sites may enhance grazing intensity whenever kangaroos move through the site because of a reduced area available for grazing. This interactive effect should depend on the volume or arrangement of the added logs. We therefore tested for differences in the abundance and species richness of beetles in a full fixed model that considered interactions between year (2008, 2009), grazing level (low, high) and log treatments (none, 20 t dispersed, 20 t clumped, 40 t dispersed and clumped). At the site scale, we assessed those beetles collected from open ground only.

At the microhabitat scale, we were interested in how microhabitat structure affected beetle responses to grazing. At this scale, we assessed beetles sampled adjacent to experimental logs and in situ logs, as well as open ground only from sites with 40 t ha−1 of added logs. Here, our full fixed model considered interactions between year (2008, 2009), grazing level (low, high) and microhabitat element (clumped log, dispersed log, in situ log, open ground).

We expected impacts on the ground-dwelling beetle assemblages to be modest in April 2008 (4 months after exclusion) and stronger in April 2009 (16 months after exclusion). If there were significant effects of log addition or grazing level, they were therefore expected to be apparent in significant interactions between treatment and year. Hence, although we analysed means, we visually present in our figures the differences between means from 2008 to 2009 to emphasize the effect of the treatments over time. We present our figures with the standard error of difference of the predicted means from the full fixed model. Although we present the figures as interaction plots, the lines do not represent continuous grazing levels.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

We collected 1661 beetles comprising 166 species from 30 families. The dominant families were Carabidae with 22 species, Staphylinidae with 19 species, Scarabaeidae with 19 species and Curculionidae with 17 species. The three most abundant beetles were Atheta sp. (Staphylinidae), Formicomus sp. (Anthicidae) and Polylobus sp. (Staphylinidae), which together comprised 40% of the abundance of all individuals (Table S1, Supporting information).

We detected a significant interaction between grazing level and year for maximum grass height (Wald = 14·14, d.f. = 1, < 0·001), but not for percentage grass cover (Wald = 0·66, d.f. = 1, = 0·416). From 2008 to 2009, maximum grass height changed from an average of 0·41 to 0·60 m in low grazing sites but only changed from 0·38 to 0·42 m in high grazing sites.

Grazing Level and Beetle Trophic Structure

Abundance and species richness of trophic groups at the site scale showed a significant interaction with grazing treatment and a significant interaction with year (Table 1). Beetle herbivores increased in abundance in low grazing treatments (Fig. 3a), and both herbivores and detritivores increased in species richness in the low grazing treatment across years. In contrast, we found that predaceous beetles changed little in abundance and species richness across years in the low grazing treatments but decreased in abundance and species richness in the high grazing treatments (Fig. 3a,b). Regardless of whether the change was positive or negative, the overall change was always more positive in the low grazing compared with the high grazing for each trophic group for both richness and abundance.

Table 1.   Summary of GLMM testing for differences in (a) abundance and (b) species richness of trophic groups (herbivore, predator, detritivore) across grazing treatments (low, high) and year (2008, 2009). This analysis considers site-scale effects and used samples collected at open-ground locations from all plots
Fixed effectsWaldd.f.P
  1. GLMM, generalized linear mixed models.

(a) Abundance
 Year18·441<0·001
 Grazing1·5710·213
 Trophic group34·112<0·001
 Year × grazing17·901<0·001
 Year × trophic group18·142<0·001
 Grazing × trophic group7·8520·022
 Year × grazing × trophic group0·7820·676
(b) Species richness
 Year1·0010·318
 Grazing0·2210·641
 Trophic group25·462<0·001
 Year × grazing14·761<0·001
 Year × trophic group18·652<0·001
 Grazing × trophic group8·5620·015
 Year × grazing × trophic group0·2020·903
image

Figure 3.  Change in predicted mean (a) abundance and (b) species richness of beetle trophic groups sampled across experimental grazing levels and pooled across log treatments. Values above zero indicate an increase from 2008 to 2009. SED = standard error of difference.

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Interactions Between Grazing Level and Log Additions

At the site scale, we detected a significant three-way interaction between grazing level, log additions and year for beetle abundance and species richness (Table 2). Both beetle abundance and species richness showed the greatest increase across years in sites with 20 t of clumped logs in low grazing treatments (Fig. 4a,b). The dominant trend across all log treatments was a higher abundance and species richness of beetles in the low grazing treatment compared with the high grazing treatment (Fig. 4a,b). Surprisingly, the interaction between treatments meant that sites with 20 t clumped and 40 t of added logs had a larger negative change in beetle abundance and species richness relative to our control sites at the high grazing levels (Fig. 4a,b).

Table 2.   Summary of GLMM analysis testing for differences in beetle (a) abundance and (b) species richness across grazing treatments (high, low), log treatments (none, dispersed, clumped, 40 t) and year (2008, 2009). This analysis considers site-scale effects, using samples collected at open-ground locations from all sites
Fixed effectsWaldd.f.P
  1. GLMM, generalized linear mixed models.

(a) Abundance
 Year10·7110·001
 Grazing0·0010·946
 Log treatment4·0230·267
 Year × grazing52·931<0·001
 Year × log treatment7·3830·067
 Grazing × log treatment0·5730·904
 Year × grazing × log treatment11·9330·010
(b) Species richness
 Year0·0110·915
 Grazing0·910·344
 Log treatment3·5230·325
 Year × grazing37·291<0·001
 Year × log treatment1·6530·649
 Grazing × log treatment0·2830·964
 Year × grazing × log treatment11·0830·015
image

Figure 4.  Change in predicted mean (a) abundance and (b) species richness of beetles sampled from low and high grazing treatments with four log treatments. Values above zero indicate an increase from 2008 to 2009. SED = standard error of difference. Log treatments: none = no added logs, dispersed = 20 t of dispersed logs, clumped = 20 t of logs in clumps, 40 t = 40 tonnes of dispersed and clumped logs.

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Microhabitat and Grazing Level

At the microhabitat scale, we detected a significant interaction between grazing level and microhabitat type for beetle abundance and a near-significant interaction between grazing level, microhabitat type and year for beetle species richness (Table 3). Added logs (clumped or dispersed) always showed increased abundance and species richness of beetles (Fig. 5a). For species richness, however, the increase tended to be greater in the low grazing treatment for dispersed logs and greater in the high grazing treatment for clumped logs (Fig. 5b). In contrast, beetle assemblages at in situ logs responded in surprising ways with declines in abundance at low grazing levels that were greater than for high grazing levels (Fig. 5a). Notably, open ground always showed losses in species and abundance when under high grazing (Fig. 5a,b).

Table 3.   Summary of GLMM analysis testing for differences in (a) abundance and (b) species richness of beetles across grazing treatments (low, high), microhabitat type (open ground, in situ log, dispersed log, clumped logs) and year (2008, 2009). This analysis focused on microhabitat-scale effects and only considers samples from sites with 40 t of logs added
Fixed effectsWaldd.f.P
  1. GLMM, generalized linear mixed models.

(a) Abundance
 Year1·1810·279
 Grazing1·0010·326
 Microhabitat3·7330·296
 Year × grazing0·0710·789
 Year × microhabitat9·7730·023
 Grazing × microhabitat11·3930·012
 Year × grazing × microhabitat4·0730·258
(b) Species richness
 Year6·9110·009
 Grazing2·1510·157
 Microhabitat6·9930·077
 Year × grazing2·9710·087
 Year × microhabitat3·0530·388
 Grazing × microhabitat5·6530·135
 Year × grazing × microhabitat7·1230·073
image

Figure 5.  Change in predicted mean (a) abundance and (b) species richness of beetles at four microhabitat types in low and high experimental grazing levels. SED = standard error of difference. Microhabitat types: clump = clumped experimental logs, dispersed = single experimental logs, log = in situ logs already present in study site, open = open ground away from trees and logs.

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Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

We have shown that reduced vertebrate grazing can benefit beetle diversity at two spatial scales. Large-scale exclosure fencing had significant positive effects on the abundance and species richness of beetles. At the microhabitat scale, the addition of logs reduced the impact of high grazing levels relative to open ground. The significant interaction between these two treatments showed that both treatments have further benefits when used in combination. These two experimental treatments were chosen to counter the loss of ground-layer habitat structure because of grazing by overabundant kangaroos (ACT Parks Conservation and Lands 2010; McIntyre et al. 2010) and historic removal of logs through firewood harvesting and land clearing (Killey et al. 2010). Our results highlighted the strong potential for the combined use of exclosure fences and logs to reduce the effects of high densities of vertebrate herbivores and facilitate the restoration of beetle diversity at multiple scales.

Effects of Grazing Level and Log Addition

Inside exclosures with reduced kangaroo grazing, we found an increase in the abundance and species richness of beetles. Other studies of grazing impacts on insect communities generally show that reduced grazing intensity has a positive effect on insect diversity (Kruess & Tscharntke 2002; Gebeyehu & Samways 2003; Debano 2006; Dennis et al. 2008), although some studies show mixed responses (Batáry et al. 2007) or even a negative response (Fay 2003). This range of results shows that such studies are context-dependent, and differences in grazing intensity can limit direct comparisons across studies conducted in different environments.

We found that the effect of grazing differed among beetle trophic groups. Our initial prediction of a positive effect of reduced vertebrate grazing on beetle herbivores proved correct, although there was also a positive effect on detritivore species richness. In contrast, the absence of any notable response of predaceous beetles to lower grazing levels, but the strong negative response to the high grazing treatment (Fig. 3), was not anticipated. While it might be expected that herbivorous beetles will positively respond to decreased grazing because of changes in resource availability and grass structure (Hairston, Smith & Slobodki 1960; Gebeyehu & Samways 2003; Woodcock et al. 2009), the comparatively fewer individuals and species of predaceous beetles we detected in high grazing treatments could be related to a decrease in grass cover and structure that provides less shelter and food for a diversity of predators (Woodcock & Pywell 2010).

At the site scale, the effect of reduced grazing on beetle abundance and species richness depended on the volume and arrangement of logs (Fig. 4). In particular, logs arranged in clumps and applied at 20 t ha−1 had larger positive effects than our other log treatments for beetle abundance and species richness. All log treatments had positive effects on beetle abundance and species richness inside the exclosures with low grazing. Sites with added logs in the high grazing treatment, however, tended to have a negative effect on beetle abundance and species richness relative to our control sites. We suggest this is attributed to a ‘corralling effect’ that concentrated kangaroo grazing on the open areas between the logs.

At the microhabitat scale, beetles sampled immediately adjacent to the clumped and dispersed experimental logs showed similar changes in abundance across grazing levels (Fig. 5). This indicates that beetle abundance immediately adjacent to the introduced logs was not affected by grazing intensity, in strong contrast to open ground, which showed a decrease in abundance. This supports our hypothesis that the addition of logs provides localized grazing refugia. The presence of structural refugia has previously been implicated in plant recovery from grazing (Schreiner et al. 1996; Pellerin et al. 2010). After fires in 1988 in Yellowstone National Park, USA, fallen timber prevented grazing by elk Cervus canadensis, allowing the successful establishment of aspen seedlings (Turner, Romme & Tinker 2003). Another example from South America has been described where windstorms create gaps in Nothofagus forest. The resulting fallen timber prevents grazing by guanaco Lama guanicoe, thereby allowing seedling regeneration (Lindenmayer & Franklin 2002). A limited number of studies have also shown that grass tussocks can buffer the effects of grazing on arthropods in the presence of high grazing levels (Dennis, Young & Bentley 2001; Helden et al. 2010). Our study shows that logs can be added to influence the grazing patterns of vertebrate herbivores and provide microhabitat refuges for beetles.

Implications for Management

Our results have shown that reducing kangaroo densities to c. 0·4 animals per hectare (40 animals per km2) had significant positive effects on beetle assemblages and therefore suggests population densities near this figure would be beneficial for the restoration and maintenance of this component of the insect fauna. Further, logs appear to be acting as important centres for beetle activity, but the usability of the surrounding grassland away from the logs is significantly affected by the levels of grazing. Our results suggest the principle of landscape context, commonly described at landscape scales (e.g. Lindenmayer et al. 1999; Krauss, Steffan-Dewenter & Tscharntke 2003; Tubelis, Lindenmayer & Cowling 2004), also applies at fine spatial scales. This emphasizes the importance of appreciating scale from the organism’s perspective and approaching land management accordingly (Wiens & Milne 1989; Manning, Lindenmayer & Nix 2004).

Our results suggest three potential management scenarios available to mitigate the effects of high densities of vertebrate herbivores on insect diversity. First, exclosure fences could be used to exclude vertebrate herbivores from particular areas and to maintain desired levels of grazing. This would allow the widespread recovery of the ground-dwelling insect fauna through the promotion of grass biomass and structure and is limited in scale only by the size of the area fenced. Alternative options for general reductions in herbivore densities include culling (Tanentzap et al. 2009) or reintroduction of predators (Beschta & Ripple 2009). Secondly, logs can be added to increase ground-layer structure and provide localized, microhabitat refuges from grazing. In the presence of high vertebrate herbivore densities, however, this may put further pressure on the grassland at the site scale by concentrating grazing on areas interspersing the logs and potentially resulting in no net benefit. Thirdly, exclosure fences and logs could be used together to achieve benefits for insect diversity at both site and microhabitat scales. Both the second and third options would benefit further from the many additional ecological functions associated with logs such as enhanced water infiltration, nutrient collection and seedling establishment (Harmon et al. 1986; Lindenmayer et al. 2002). Consideration should therefore be given to the replacement of logs over the longer term. A sustainable input of logs into an ecosystem can be achieved by promoting a natural age structure of trees that encompasses a full spectrum of sizes within a tree population (Harmon et al. 1986; Killey et al. 2010).

The rapid response of beetles of a range of trophic groups suggests that there may be significant potential for positive cascading effects on other biota as a consequence of reduced grazing, such as increased food availability for insectivorous vertebrates (Dennis et al. 2008). Our study was restricted to beetles, and further research should focus on the responses of other invertebrate groups that are known to respond to vertebrate grazing, especially grasshoppers and spiders (Dennis, Young & Bentley 2001; Gebeyehu & Samways 2003). As part of achieving appropriate grazing levels by vertebrate herbivores, logs can be used to complement exclosure fences to lessen grazing pressure and increase grazing heterogeneity at small spatial scales. This will assist the restoration of grass-layer biomass and structure, with positive ramifications for insect diversity.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

We thank K Pullen, R Oberprieler and A Slipinski for the verification of some beetle morphospecies. We appreciate the helpful comments made on drafts of the manuscript by S Macfadyen and K Stagoll and two anonymous referees. Thanks to B Howland for providing kangaroo count data and M Westgate, S Hugh and H Weaver for assistance with pitfall trap collections. We thank D Iglesias, S Jeffries, S Lane, S Sharp, D Shorthouse, P Mills and G Woodbridge for their involvement with the project. P Barton was supported by a CSIRO OCE postgraduate scholarship. Funding for the project was provided by an Australian Research Council Linkage Grant (LP0561817).

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information
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Supporting Information

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

Table S1. Summary of beetles sampled from 2008 to 2009.

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