Ground-layer vegetation of cleared woodlands (pastures) has lower biodiversity and different invertebrate assemblages to remnant woodlands in grazed landscapes of eastern Australia

doi:10.21203/rs.3.rs-2323385/v1

Introduction: Clearing of woodlands is used by graziers to promote pasture production, even though understanding of impacts of clearing on native fauna is lacking.

Aim/Methods: To evaluate impacts of clearing on biodiversity by comparing invertebrates associated with ground-layer vegetation in pastures to that of nearby uncleared woodlands. Two replicates of cleared woodlands (pastures) were compared with two woodlands at each of four locations. The adjacent riparian forest to each pasture and woodland site provided a geographic control, making four habitats. Invertebrates of ground-layer vegetation were sampled using three suction subsamples of 1m² at each site.

Results: Pastures had significantly lower order richness, herbivores, pollinators and macroinvertebrates (food for birds) than the woodlands, whereas the riparian forests closely resembled each other in all metrics. Invertebrate assemblages of pastures also differed from those of the woodlands, groundcover and leaf-litter correlating strongest with invertebrate composition.

Discussion: Findings of this study contrasted with another in recently cleared woodlands where few differences were observed. Our study differed in the much longer period since clearing (> 20 years cf. 5 years) and the dominance by introduced grasses (> 40% cover in pastures cf. <15% in woodlands) rather than native grasses.

Implications for insect conservation: Pastoralists have the capacity to improve outcomes for invertebrate biodiversity by maintaining groundcover above 80%, by encouraging native pastures over introduced species such as Buffel Grass and by retaining native woodlands. Biodiverse invertebrates benefit graziers by contributing to soil health, food webs that support pest control, pollination, herbivory of weeds and sustainable grass production.

clearing

Buffel Grass

Cenchrus ciliaris

rangeland management

remnant woodland

biodiversity

conservation

grassy woodlands

invertebrates

sustainable grazing practices

Grazing is the dominant land use in Australia (40% of the land mass) and almost 80% of Queensland’s rangelands are grazed (Department of Environment and Resource Management 2011). Graziers throughout the world traditionally manipulate the landscape to enhance pasture production, primarily by clearing trees or altering grazing pressure by varying stock numbers (Hall et al. 2016). Consequently, it is important that understanding of impacts of grazing management, including those associated with land clearing, on biodiversity of native fauna of woodlands be obtained.

Clearing of woodlands generally promotes grass growth (Walker et al. 1972, Walker et al. 1986), primarily by increasing insolation to ground-layer vegetation (Specht and Morgan 1981). However, while there are typically substantial increases in pasture biomass in the short-term, longer-term studies suggest that these gains may not be sustainable over longer time frames (> 20 years) due to nutrient rundown (Kaur et al. 2005, Sangha et al. 2005a, Radford et al. 2007). These studies also point to other issues such as loss of biodiversity and land degradation (Rolfe 2002, Sangha et al. 2005b). Besides altering pasture biomass, several other changes to the ground-layer vegetation are associated with removal of the tree layer. The reduction in shading of the ground-layer vegetation may raise soil temperatures, possibly influencing soil moisture (Lal and Cummings 1979, Hashimoto and Suzuki 2004). Less rainfall may be retained (Thornton et al. 2007), causing more rapid leaching of salts (Cowie et al. 2007) and elevation of soil pH with reduced nutrient availability (Sangha et al. 2005a). There is a loss of input of leaf and woody litter altering the quality of litter available (Sangha et al. 2006), potentially influencing food webs. Finally, there may be changes in dominance associated with colonisation by introduced pasture grasses or forbaceous weeds, particularly where heavy grazing occurs (Dorrough et al. 2006, Dorrough and Scroggie 2008, Kutt and Fisher 2011, Hall et al. 2017).

In Queensland, Buffel Grass (Cenchrus ciliaris) is one of the most important pastoral species for cattle grazing (Marshall et al. 2012). This species was introduced in the 1920s and is still widely sown. Although beneficial to graziers, Buffel Grass has the capacity to invade and expand its range into non-target habitats such as nearby woodlands (Eyre et al. 2009). Buffel Grass has been shown to reduce ground-layer plant diversity (Melzer et al. 2014, Fensham et al. 2015). Invasive grasses such as Buffel Grass may also affect the quality of the grass litter (Grigg 1999, Wolkovich et al. 2009).

Invertebrates are key components of any ecosystem but particularly for soils where they contribute to soil health by aeration and water infiltration, turnover of nutrients, pollination, herbivory and control of weed species (Stork and Eggleton 1992). They are also important as food for larger animals such as birds and other vertebrates (Hallmann et al. 2017). Diets of woodland and ground-foraging insectivorous birds in Australia are typically dominated by a few prey groups including beetles, ants, spiders, sucking bugs, flies, grasshoppers, caterpillars and lacewings (Major 1991, Gamez-Virues et al. 2007, Razeng and Watson 2012, Lindsay et al. 2014). While clearing will have obvious impacts on invertebrate fauna associated with woodlands such as those dependent on foliage or bark dependent invertebrates, impacts on those associated with the ground-layer vegetation are less obvious. While clearing generally leads to a decline in invertebrate biodiversity associated with ground-layer vegetation (Green and Catterall 1998, Bromham et al. 1999, Vasconcelos 1999, Mathieu et al. 2005, Houston et al. 2015, Majer et al. 2021), this is not always the case (Houston and Melzer 2018).

The objective of the present study was to evaluate the influence of clearing on invertebrate assemblages associated with ground-layer vegetation of grazing land by comparing long-cleared sites (i.e. >20 years) on alluvial plains to nearby uncleared remnant woodlands on the same soil type. Long-cleared sites were selected to ensure that the effects of clearing had sufficient time to alter ground-layer vegetation. As cleared sites in this region were dominated by Buffel Grass, changes to the ground-layer vegetation may reflect both clearing and Buffel Grass colonisation. Evaluation was based on biodiversity (order richness), assemblage composition and trophic structure (detritivore, herbivore and predator), food availability for insectivorous vertebrates and pollinator abundance. To provide context, measures of habitat structure were also examined.

Study Area

The study took place in the Fitzroy River basin of Central Queensland at four grazing properties between latitudes 22°48′ and 23°35’ south and longitudes 149°11′ and 150°02′ east: Isaac/Connors Rivers (IC); Mackenzie River (MK); Melaleuca Creek (FM), a tributary of the Fitzroy River and the Fitzroy River (FR). This area straddles the Tropic of Capricorn and lies ~ 100 km from the coast to the west of Rockhampton. The climate is typified by long, hot summers and mild winters (Hutchinson et al. 2005). Annual rainfall averages 653 mm at the nearest rainfall station (Riverslea TM, Australian Bureau of Meteorology), with the three summer months (December, January and February) accounting for almost half the annual rainfall. Annual pan evaporation rates in the region are high, approximately 2,100 mm per year (DES 2020).

Remnant vegetation consisted of riparian forests (> 30% canopy foliage projective cover (FPC)) along river edges dominated by Forest Red Gum (Eucalyptus tereticornis) and River She-oak (Casuarina cunninghamiana). Bordering the riparian forests, at slightly higher elevations, were woodlands (10–30% FPC) associated with gently sloping alluvial terraces. Dominant species of the terrace woodlands were Coolibah (Eucalyptus coolabah), Forest Red Gum and Brigalow (Acacia harpophylla). The former woodlands of the cleared adjacent alluvial terraces were typically well grassed and dominated in cover by an introduced pasture species, Buffel Grass.

While grazing regime varied between properties and paddocks, when stock numbers were annualised, all paddocks were grazed at levels typical of the commercial stocking rates in the region, which range from 0.1 to 0.3 cattle/ha (Kaur et al. 2005). Overall, riparian paddocks were stocked at slightly lower levels of grazing than the adjacent alluvial terraces.

Sites and study design

At each of the four locations, eight sites were established in two sets of four ‘habitats’, a pasture derived from cleared woodland on an alluvial terrace (TP) adjacent to a riparian forest (RFp); and a remnant woodland on an alluvial terrace (TW) adjacent to riparian forest (RFw). The two terrace sites (pasture or woodland) in each set had similar stocking rates, as did the two riparian sites.

The comparison between the terrace pasture and the terrace woodland provided the basis for establishing the influence of clearing on ground-layer invertebrates. The comparison between the two riparian forests that bordered each of the terrace sites acted as a geographic control in the sense that they should not have significantly different biodiversity/abundance metrics. If the two riparian habitats were to be significantly different, then this would cast doubt on the interpretation of significant differences in the two terrace habitats.

Sampling

Surveys were undertaken in Autumn (April-May) 2007. A 200 m transect aligned parallel to the river was established at each site. Habitat attributes were measured within a 10 m radius of each of three points (0, 100 and 200 m) along the transect. Percentage projective cover of ground-layer vegetation was visually estimated for grass, leaf litter, total litter (includes both leaves and woody debris), ground-layer vegetation (combined grass, forbs and foliage of low shrubs < 0.5 m height) and groundcover (combined ground-layer vegetation and litter). Percentage projective cover of important introduced species such as Buffel Grass, Green Panic (Megathyrsus maximus var. pubiglumis) and total invasive species cover (i.e. combined Buffel, Green Panic and invasive forbs such as Parthenium hysterophorus) were also recorded. Grass height was estimated from a measure of the height of tallest groundcover vegetation (cm) at 30 points spaced at 1m intervals along 3 radii at each point. The number of native species of ground-layer plants was also recorded.

To allow for the quantitative assessment of the abundance of invertebrates (numbers/m²) (King and Hutchinson 2007), soil surface, litter and grass-associated invertebrates were sampled using an MTD leaf blower acting as a suction sampler. The maximum air-flow velocity of this model (~ 70 ms_–1) exceeded the minimum (45.6 ms^–1) recommended for samplers of this type (Stewart and Wright 1995). The sampler was passed repeatedly over the vegetation and groundcover for 30 seconds within a randomly located 1 m² quadrat (Cagnolo et al. 2002) at each of 3 points along the 200 m transect (0, 100 and 200m). To avoid damage, invertebrates were collected in a fine-meshed bag placed in the mouth of the suction device.

Samples were washed through a fine meshed sieve (150 µ), sorted to Order (Naumann 1991) and enumerated. The three samples from each site were averaged to provide a sample estimate for each site. Exceptions to the order-level of classification were a few groups sorted to class, Chilopoda (centipedes), Diplopoda (millipedes) and Gastropoda (snails); and Hymenoptera was split into family Formicidae (ants) and Other Hymenoptera (wasps and bees). Collembola (springtails) were categorised as elongate-bodied (orders Poduromorpha and Entomobryomorpha) or globular springtails (order Symphypleona).

To evaluate trophic structure, invertebrate taxa with relatively uniform feeding habits and dominated by one trophic category were placed in one of three groups – detritivores (mites, springtails, book-lice, cockroaches and termites); herbivores (sucking bugs, grasshoppers, stick insects, thrips, butterflies and moths) and predators (spiders, pseudoscorpions, mantids, lacewings and wasps) (Naumann 1991, Houston and Melzer 2018). Due to a broad range of feeding habits, some taxa could not be assigned to a single trophic category (ants, beetles and flies).

Two measures of ecosystem service were evaluated based on abundance of various relevant invertebrate taxa: food availability for insectivorous vertebrates and pollination capacity. Taxa comprising the macrofauna were summed to provide a measure of the available food for birds and other vertebrates. Macrofauna comprise invertebrates > 2 mm and include most taxa sampled except those comprising the mesofauna (i.e. invertebrates < 2 mm; mites, springtails and pseudoscorpions). Macrofauna also include the most important pollinating taxa: wasps and bees, flies, butterflies and moths, beetles and thrips (Thien et al. 2000).

Analysis

A two-way analysis of variance (ANOVA) was used to test the influence of location and habitat on habitat attributes and arthropod metrics: order richness, total invertebrate abundance, abundant taxa (those comprising > 1% of the catch), abundance of each trophic category (detritivore, herbivore and predator), macrofauna and pollinators. A log10 (x + 1) transformation was used on abundance data to normalize distribution. If significant, a posteriori Tukey tests (for multiple pairwise comparisons) were used to identify significant differences between habitats (Quinn and Keough 2002). Habitat attributes with highly skewed distributions such as Buffel Grass and Green Panic (either mostly absent from or confined to riparian forests respectively) were tested using a non-parametric test, the Kruskal-Wallis ANOVA on ranked data.

Non-metric multidimensional scaling (nMDS) ordination was used to examine relationships between samples based on the order-level invertebrate assemblages. To reduce the influence of abundant taxa, data were square root transformed and sites were compared by applying the Bray-Curtis similarity index (Clarke and Warwick 2001). Ordinations were visualised in two-dimensional space, whereby sites closer together have a more similar assemblage than those further apart.

To evaluate influence of location and habitat on order assemblages, a permutational ANOVA was applied (PERMANOVA, PRIMER-e, v7) (Anderson et al. 2008). Pairwise tests were used to identify significant differences in invertebrate assemblages between habitats. Since this test is sensitive to data dispersion and may confound differences among groups with differences in scatter within groups, a multivariate homogeneity of dispersion test (PERMDISP, PRIMER-e, v7) was also applied (Anderson 2001). For all analyses, a p-value threshold of 0.05 was considered significant.

To ascertain which aspect of habitat structure (i.e., grass height, grass cover, ground-layer vegetation cover, leaf litter cover, total litter cover, groundcover, and invasive species cover) correlated with changes in invertebrate assemblage composition, a BEST analysis was applied using normalized Euclidean distance as the similarity measure (PRIMER-e v7) (Clarke and Warwick 2001). Because Buffel Grass and Green Panic were associated mainly with either terrace zones or riparian zones respectively, they were not included individually but as a component of the invasive species cover.

Ground-layer attributes of the four habitats

The two riparian forest habitats, irrespective of whether adjacent to woodlands or pastures on the adjoining terrace, resembled each other in all ground-layer habitat attributes (Table 1, Fig. 1), being relatively well vegetated with grass cover above 50% and more than 90% groundcover due to the combination of ground-layer vegetation (mainly grass but also forbs and low shrubs in the ground-layer), leaf and woody litter. Buffel Grass was present but comprised less than 1% of groundcover. Grass was relatively tall (averaging 30 cm), partly reflecting the dominance by tall, introduced grasses such as Green Panic which averaged 15–18% cover.

Table 1

Results of two-way ANOVA (location x habitat) on ground-layer attributes and invertebrate metrics and Tukey’s tests comparing the terrace pastures with the terrace woodlands and the two riparian habitats to evaluate the effect of clearing. *Due to non-normality, evaluations of Buffel Grass and Guinea Grass were based on the Kruskal-Wallis ranks test H (3, N = 32). Significant results at 0.05 shown in bold.
Metric	Location		Habitat		RiverxHabitat		Tukey's
	F_3,16	P	F_3,16	P	F_9,16	P	TP v TW	RFp v RFw
Habitat attributes
Grass height (cm)	1.581	0.233	2.482	0.098	0.299	0.964	na	na
Grass % cover	6.589	0.004	2.072	0.144	0.907	0.542	na	na
Total ground-layer vegetative % cover	1.520	0.248	2.827	0.072	0.955	0.508	na	na
Leaf litter % cover	0.086	0.967	6.951	0.003	2.380	0.062	0.004	0.993
Total litter % cover	0.887	0.469	8.326	0.001	2.228	0.078	0.001	0.937
Groundcover %	3.058	0.059	21.854	0.000	1.574	0.205	0.028	0.986
Total invasive species % cover	1.516	0.249	3.078	0.057	0.355	0.941	na	na
No. native plant species	3.755	0.032	2.736	0.078	1.507	0.227	na	na
Buffel Grass % cover*			17.962	0.000
Green Panic % cover*			10.935	0.012
Invertebrates
Order richness	0.191	0.901	4.082	0.025	3.033	0.026	0.023	0.802
Total abundance (no./m²)	1.270	0.318	1.096	0.380	1.743	0.159	na	na
Detritivore abundance (no./m²)	0.348	0.791	0.165	0.919	1.201	0.359	na	na
Herbivore abundance (no./m²)	1.048	0.398	10.941	0.000	2.442	0.057	0.001	0.998
Predator abundance (no./m²)	4.617	0.016	5.242	0.010	1.479	0.237	0.206	0.988
Macrofauna abundance (no./m²)	3.424	0.043	10.992	0.000	3.578	0.013	0.005	1.000
Mesofauna abundance (no./m²)	1.009	0.414	0.658	0.589	1.734	0.161	na	na
Pollinator abundance (no./m²)	5.824	0.007	8.824	0.001	4.639	0.004	0.016	0.866

Compared to the riparian forests, terrace woodlands had slightly less grass (35%) but similar amounts of litter (> 35%). At over 80%, groundcover was intermediate between the riparian forests and the terrace pastures, as was Buffel Grass cover (14%). Grass height was comparable to that in the pastures (15 cm compared with 18 cm).

While terrace pastures had similar amounts of grass cover to riparian forests (50%), the significantly lower amounts of leaf and woody litter cover (11%) meant that overall groundcover (76%) was significantly lower than in the other habitats (> 90% in riparian forests and 83% in terrace woodlands). Terrace pastures also differed in being dominated by Buffel Grass with an average of 40% cover compared with 14% in terrace woodlands and 1% in riparian forests. Lower amounts of leaf and woody litter reflected the absence of trees in these cleared woodlands.

Although species richness of native plants did not show significant variation across the four habitats, terrace pastures tended to have fewer species than the terrace woodlands (18 ± standard error 2 compared with 25 ± 2 in woodlands and 22–23 ± 3 in riparian forests).

Invertebrate biodiversity & trophic metrics

Over 20,000 invertebrates from 24 taxa were captured during the study. Eleven taxa comprised almost 98% of the catch, with representatives of all trophic categories – detritivorous: mites (23.5%), globular springtails (9.6%), elongate-bodied springtails (5.9%) and book-lice (5.2%); herbivorous: thrips (16.9%) and sucking bugs (11.0%); predatory: spiders (11.5%) and wasps (3.6%) and mixed: flies (4.2%), beetles (3.5%) and ants (2.8%). Remaining taxa were in low abundance (i.e. <1% of the catch) and included grasshoppers and crickets, cockroaches, moths and butterflies, mayflies, lacewings, praying mantids, pseudoscorpions, millipedes, isopods, phasmids, termites, silverfish and dragonflies.

Analysis of the biodiversity data (order richness) showed that pastures had significantly fewer invertebrate orders than the terrace woodlands (an average of 14 orders/site compared with 17), although there was a significant interaction with location and only two of the four locations, IC and MK, had lower order richness than the nearby terrace woodlands (Table 1, Fig. 2). The two riparian habitats resembled each other in this metric (averaging 16 to 17 orders/site).

Detritivorous invertebrates accounted for 45% of the overall catch, herbivores 29%, predators 15% and 11% unassigned (i.e. those orders with mixed feeding habits). Three of the four habitats had the expected abundance pattern with detritivores > herbivores > predators (Fig. 3). Terrace pastures were the exception having similar numbers of herbivores and predators. Abundance of detritivores was similar across the four habitats indicating that litter decomposition capacity of pastures resembled that of the woodlands and forests.

Analysis of abundance and trophic data showed that terrace pastures had significantly fewer herbivores than the terrace woodlands; whereas the two riparian habitats resembled each other in this metric. This pattern was consistent at all four locations. The same pattern was observed in all herbivorous taxa, Thysanoptera (thrips), Hemiptera (sucking bugs), Orthoptera (grasshoppers) and Lepidoptera (caterpillars) indicating that it was a universal trend in this group. Predator abundance was also relatively lower in terrace pastures than other habitats (e.g. the two riparian habitats) but not significantly lower than in terrace woodlands. These trends are illustrated for the most abundant taxa in each trophic group (Fig. 4).

Terrace pastures had significantly fewer macrofauna than the terrace woodlands (Fig. 5), showing that food availability for insectivorous vertebrates was reduced following clearing. Abundance of macrofauna also varied across location with significantly fewer macrofauna at IC than FR. Although the interaction was significant, this reflected a much higher abundance of macrofauna in riparian forest adjacent to woodlands at MK than the other three locations and a similar pattern for riparian forest adjacent to pastures at FR. In contrast, both terrace woodlands and pastures were consistent in macrofaunal abundance across the four locations.

Numbers of pollinators showed a similar pattern with significantly fewer pollinators in the pastures than the terrace woodlands (Fig. 5). There were also differences in location with more pollinators at FR than FM. Although there was a significant interaction term, this reflected differences in some of the riparian habitats at some locations, not the terrace pastures or woodlands which had consistent numbers at all four locations.

Invertebrate assemblage

Although some terrace pastures were relatively closely associated with other habitats (e.g. at FM and FR), seven of the eight pasture sites were to the right of the ordination while most sites comprising the other habitats were to the left or middle, indicating that terrace pastures had the most distinctive invertebrate assemblage of the four habitats (Fig. 6a). PERMANOVA on square root transformed abundance data confirmed this (location F_3,16 = 2.392, P = 0.002, habitat F_3,16 = 2.602, P = 0.002, location x habitat F_9,16 = 1.412, P = 0.074). Pairwise tests showed that pastures had a significantly different invertebrate assemblage to the three other habitats, including the terrace woodlands (pairwise tests, P < 0.01), and that riparian forests closely resembled each other in invertebrate composition; confirming that clearing has led to changes in ground-layer invertebrate assemblages. Terrace woodlands were found to resemble the riparian forests in invertebrate composition (pairwise tests: P > 0.05). Habitats had similar levels of dispersion (PERMDISP, homogeneity of dispersion, F_3,28 = 1.091, P = 0.401), indicating that differences in taxa composition between habitats obtained with PERMANOVA were not due to differences in dispersion.

The BEST routine using the seven selected habitat attributes indicated that groundcover (Spearmans r_s = 0.306) had the highest correlation with changes in invertebrate assemblage composition, the remaining attributes being < 0.3. The highest correlation was achieved with two attributes – groundcover and leaf litter cover (Spearmans r_s = 0.402). The four terrace pastures with the most distinct invertebrate assemblage to the right-hand side of the ordination were characterised by a combination of relatively low percentage groundcover (< 78%, Fig. 6b) and leaf litter cover (< 10%).

This study has demonstrated that grazing management relying on clearing of fertile grassy woodlands of the rangelands of Central Queensland leads to a reduction in biodiversity (i.e. order richness) of invertebrates associated with the ground-layer vegetation and a change in invertebrate assemblage composition. In addition, cleared pastures had fewer herbivorous invertebrates, food available for insectivorous vertebrates and pollinating insects compared to uncleared nearby woodlands and these differences were consistent at all four properties studied. Thus, clearing not only leads to biodiversity losses in the canopy layer of vegetation and associated fauna, but also in the biodiversity and resource base of the invertebrate fauna associated with the ground-layer vegetation. Implications for sustainability of such changes need to be considered within the wider context that has identified rundown of pasture productivity following clearing over longer time frames (Kaur et al. 2005, Radford et al. 2007).

Soil and ground-layer vegetation invertebrates are known to have important roles in supporting grazing production systems (Stork and Eggleton 1992). In particular, greater biodiversity of invertebrates has been shown to be associated with improved soil health and enhancing nutrient availability (Kemmers et al. 2013), indirectly supporting grass and cattle production. Lower diversity of invertebrate fauna was implicated in slower rates of litter decay in pastures compared with nearby woodlands (Grigg 1999).

Retention of woodland patches in the landscape matrix provides ecosystem functional benefits by promoting the abundance of macrofaunal invertebrates and pollinators. These provide a number of ecosystem services such as pollination, biocontrol of insect pests of pastures and weed outbreaks, benefitting both graziers and any crops grown in the vicinity (Potts et al. 2006, Holland et al. 2017, St. Clair et al. 2022). In addition, by enhancing the food available for insectivorous vertebrates, woodlands help support food webs and birds, which indirectly also benefit agricultural systems through predation on insect pests of crops and pastures, and controlling pest outbreaks (Gamez-Virues et al. 2007, Peng et al. 2020).

Reasons why clearing has led to a pasture that supports a reduced number of herbivorous invertebrates have not been determined. Another study, also using suction samplers, found no effect of clearing on biodiversity (order richness), total abundance, abundance of trophic groups or composition of invertebrate assemblages (Houston and Melzer 2018). However, that study took place in recently cleared paddocks (< 5 years) with native pastures of comparable biomass that resembled the composition of the ground-layer vegetation of the original woodlands (Hall et al. 2016). In contrast, clearing in the current study had occurred many years ago (> 20 years) and pastures were dominated by introduced pasture grasses such as Buffel Grass. Thus, it appears that clearing per se does not necessarily lead to changes in associated ground-layer invertebrates; but relate to further changes associated with the consequences of the clearing.

One possible explanation for the observed changes in biodiversity, trophic structure and invertebrate composition may relate to the relatively greater dominance by Buffel Grass in pastures than the uncleared terrace woodlands in the current study − 40% cover compared with 14% in terrace woodlands and 1% in riparian forests. Another study also reported lower invertebrate diversity in Buffel Grass pastures (Grigg 1999). A North American study of rangelands invaded by Buffel Grass found that the invaded paddocks had less invertebrates, particularly ants, beetles and spiders than native grasslands (Flanders et al. 2006). Most Australian studies were focussed on ant functional groups with mixed results, some ant groups declining, some increasing and others showing no change (Smyth et al. 2009, Williams et al. 2012, Bonney et al. 2017).

Consistent with other studies, Buffel Grass was found to be associated with lower native plant diversity in pastures (Melzer et al. 2014, Fensham et al. 2015). It is possible that this may have flow-on consequences for dependent fauna such as herbivores with specialist feeding preferences. A study of minesite rehabilitation pointed to such an impact. Corresponding with lower plant diversity than nearby native woodlands, there were fewer species of plant-feeding insects such as Hemiptera (sucking bugs) in the rehabilitated habitat (Moir et al. 2010, Orabi et al. 2010). Further, it is possible that Buffel Grass, as an invasive non-native species, has relatively fewer endemic herbivorous insect species feeding on it compared to native grasses (Cappuccino and Carpenter 2005), although further studies are needed to evaluate this.

Other explanations for the reduced numbers of herbivorous invertebrates in pastures compared to uncleared woodlands may relate to the quality of the grass. Graziers in northeastern Australia have long been aware of issues of “nutrient tie-up” in Buffel Grass pastures where productivity typically declines over time from the ‘tying-up’ of plant available nitrogen in the crowns, roots and organic matter of old grasses, resulting in reduced carrying capacity for cattle production (Peck et al. 2011, Clewett et al. 2021). Further, long-cleared pastures are prone to reduced nutrient availability and impacts on grass productivity (Kaur et al. 2005, Sangha et al. 2005a, Kaur et al. 2007). Irrespective of the cause, as well as impacts on cattle production, pasture rundown is likely to lead to less nutritious pasture grasses for herbivorous insects.

Differences in biodiversity and invertebrate assemblage of pastures and terrace woodlands were most pronounced in the two properties that used traditional grazing management approaches (i.e. continuous or long session grazing). In contrast, the two properties in which the pastures resembled the terrace woodlands in biodiversity attributes employed more modern grazing management approaches such as rotational grazing that improved grass productivity (Eaton et al. 2011). It is possible that the style of grazing management may have positive impacts on biodiversity outcomes, although more detailed studies are needed (McCosker 2000, Dorrough et al. 2002, Lindsay and Cunningham 2009, Eaton et al. 2011).

Management applications

Changes in assemblage composition and reduction in invertebrate order richness were most pronounced in pastures at two of the four locations. These sites had a relatively lower amount of groundcover (i.e. combined ground-layer vegetation and leaf litter) than the other pastures, indicating a link between retaining more groundcover and enhanced invertebrate biodiversity. Thus, pastoralists have the capacity to improve outcomes for invertebrate biodiversity by maintaining groundcover above 80%. In general, greater levels of ground-layer cover are recommended as a way to enhance sustainability of rangeland cattle production systems (Beutel et al. 2021). Rotational grazing practices that typically involve some spelling of paddocks may also promote conservation of biodiversity (Dorrough et al. 2002, Eaton et al. 2011, Houston et al. 2013, Houston and Black 2016), although not always (Dorrough et al. 2012).

Graziers interested in improving biodiversity on their property should consider encouraging native pastures over introduced species such as Buffel Grass. Most likely this would only be possible on a small scale due to the known capacity of Buffel Grass to colonise disturbed habitats.

Maintaining woodlands rather than clearing is another option. The terrace woodlands of this study had greater biodiversity, more pollinators and macroinvertebrates, and a more natural assemblage and trophic structure (i.e. comparable to the riparian forests) than the pastures. Retention of woodlands in the landscape enhances ecosystem services such as pollination, pest and weed control, including indirectly by supporting insectivorous birds and other vertebrates (Crisol-Martínez et al. 2016).

Acknowledgments

We thank Leif Black for his assistance with fieldwork and invertebrate sorting. This study was funded by the Fitzroy Basin Association.

Funding: This study was funded by the Fitzroy Basin Association. The authors have no relevant financial or non-financial interests to disclose.

Author contributions: WH and KW conceived and designed the research; WH, RB carried out the fieldwork and invertebrate identification; WH conducted all analyses, and wrote the manuscript; KW and RB provided assistance with editing and writing.

Data: The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

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No competing interests reported.

Ground-layer vegetation of cleared woodlands (pastures) has lower biodiversity and different invertebrate assemblages to remnant woodlands in grazed landscapes of eastern Australia

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

Methods

Study Area

Sites and study design

Sampling

Analysis

Results

Ground-layer attributes of the four habitats

Invertebrate biodiversity & trophic metrics

Invertebrate assemblage

Discussion

Management applications

Declarations

References

Additional Declarations

Status:

Journal Publication

Version 1