2.1 Introduction history
Key sources of B. pertusa introduction were associated with pasture development, rehabilitation, and amenity.
2.1.1. Pasture development
The first Australian Herbarium samples of B. pertusa are from Queensland and date back to 1939, where it was introduced to the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Fitzroy Vale research facility, in Rockhampton (See Fig. 2 for place names), and to the agricultural college facility in Gatton. At Fitzroy Vale, B. pertusa was trialled within five 40 m2 plots; two sown with only B. pertusa and three with a mixture of B. pertusa, six other exotic grass species and the exotic leguminous shrub Stylosanthes gracilis (Miles 1949). At conclusion of the trial in 1942, B. pertusa was not included amongst the promising species enlisted for wider regional testing. Among the species considered promising were a number that have since spread widely and had substantial negative environmental impacts; namely Andropogon gayanus (Rossiter - Rachor et al. 2009; Setterfield et al. 2010), Chloris gayana, Cenchrus ciliaris (Fairfax, Fensham 2000; Jackson 2005), Melinis multiflora (van Klinken, Friedel 2017) and Panicum maximum (van Klinken, Friedel 2017).
Despite its poor performance in initial trials, B. pertusa continued to be introduced and tested across a number of different research facilities across the state. In the early 1950s, B. pertusa was being grown for seed at the CSIRO research facilities in Samford and Strathpine, in South-East Queensland. In 1955, the species was introduced at the Department of Agriculture and Fisheries (DAF) research stations in Emerald and Biloela, in central Queensland. In Emerald, B. pertusa was noted as “unimpressive for pastures because of its low productivity” (Bisset 1980). In 1978 B. pertusa was introduced to the DAF Brian Pastures research facility, near Gayndah, and in the early 1990s B. pertusa was included in pasture trails at CSIRO Landsdown station in Townsville (Jones 1997).
Although its value as fodder was contested, during the late 1980s and into the 1990s the species was recommended as a useful fodder crop, particularly in low fertility soils and to sustain high grazing pressure (Partridge, Miller 1991). A survey of 297 commercial beef producers in Queensland during 1996-97, found B. pertusa was used for pasture improvement on some properties in the central coastal region and Brigalow Belt North and in the Einasleigh Uplands (Bortolussi et al. 2005).
During the 1970s, B. pertusa was introduced to a number of regions across Queensland during trials to assess its value for soil conservation (Bisset 1980; Truong, McDowell 1985). B. pertusa was initially trialled for this purpose in research facilities in south-eastern Queensland (Amberley) (Truong and McDowell 1985). This trial led to a series of larger trials in 1979 and 1980 in south, central and northern Queensland, to evaluate the effectiveness of B. pertusa in stabilising farm waterways (Truong, McDowell 1985). B. pertusa was quick to germinate and establish high cover and deemed useful for stabilising the riverbanks and reducing water erosion. Its use for other stabilising works such as mining overburden and road and railway embankments was also promoted in this study.
During the 1980s the species was sown for mine rehabilitation at Blackwater, Theodore and Callide coal mines near Emerald (Truong, McDowell 1985) and was observed spreading naturally onto mine spoils at the Collinsville coal mine, near Mackay. The abundance of B. pertusa on gem-mine spoils in Rubyvale (inland from Emerald) is also likely associated with the rehabilitation of these mines in the late 1990s. It is difficult to determine how frequently the species was used for rehabilitation projects as detailed records were often not kept or were not made available (Silcock 1991). The Queensland Government’s Soil Conservation Guidelines (2015), however, suggest that B. pertusa, along with Cenchrus ciliaris (buffel grass) were used widely for rehabilitation projects on the lighter arid inland soils but “were no longer recommended due their weed potential”.
The final major source of B. pertusa introductions was for amenity purposes, such as lawns, airstrips and along road verges. It was particularly advocated as a lawn species in the drier parts of the state and as such most lawns in central and western Queensland are B. pertusa dominant (personal observation, March 4, 2020). Known plantings of B. pertusa for amenity in Queensland date to the 1950s with the species recorded as the lawn for the Bowen showgrounds and the airstrips at Cloncurry, Charleville and Bowen aerodromes (Bisset 1980). Sowing of the species for amenity purposes was also occurring in the Northern Territory throughout the late 1980’s (Cowie, Werner 1993). Its presence was recorded on the neighbouring Tiwi islands during this time (Fensham, Cowie 1998). The species is now common throughout the Northern Territory and its dominance is increasing in some grazed ecosystems (Robyn Cowley pers. comm. 2019).
It wasn’t until the mid-1960s that the number of B. pertusa herbarium records in Queensland started to increase, and its spread discussed within the literature. During the 1960s B. pertusa mostly occurred within the Bowen area, where the species was described to have ‘spread like wildfire’, during the 1960s and 1970s (Fig. 2a) (Bisset 1980). The landholders of Salisbury Plains, near Bowen first saw the species in 1964 and were ‘initially concerned by its aggression’ but noted an ‘improvement in management and production’ in comparison to pastures previously dominated by Heteropogon contortus (black spear grass) (Partridge, Miller 1991). Vegetation maps used for property evaluations in the 1960s and 1970s (Table 1, Appendix 1), also suggest the species was dominant across several properties in the Bowen region during this time.
By 1980 the species was observed forming ‘solid stands over whole paddocks between the coast and Bogie River’, which sits 50 km inland and runs parallel to the coast from Ayr to Bowen (Bisset 1980). The species was also noted as abundant and spreading along roadsides inland from Bowen and up towards Townsville. Transport of hay and cattle from the Bowen region is thought to have facilitated its spread into these regions (Bisset 1980). It was also during the 1980s when B. pertusa started to increase in occurrence inland of Mackay, and it is within this region today that B. pertusa is particularly abundant (Fig. 2b, e). Coal mining is an extensive enterprise in this region, and the use of B. pertusa to rehabilitate mine spoils during this time (Truong, McDowell 1985) is plausibly associated with this spike in occurrences.
Throughout the 1990s the number of records in the Charters Towers region increased considerably (Fig. 2c). B. pertusa was observed naturally spreading onto several properties during this time, often from the edge of roads or from neighbouring sown pastures (Bortolussi et al. 2005; O’Reagain, Bushell 2015). A grazing trial initiated in the region in 1992, found a steady increase in B. pertusa after the mid-1990s, particularly on heavily grazed sites (Ash et al. 2011). Its increase after this time was suggested to be associated with a series of below average rainfall years prior to 1996, followed by a run of higher rainfall years leading up to the 2000s (Ash et al. 2011). Also during the 1990s, B. pertusa was observed replacing substantial areas of H. contortus grasslands in central and northern Queensland, with Walker, Weston (1990) suggesting 100,0000 ha in the Burdekin Shire, 200,000 ha in the Dalrymple Shire and 500,000 ha in the Bowen Shire had been colonised by B. pertusa by the early 1990s.
During the 2000s the species appeared to expand between Mackay and Charters Towers (Fig. 2d). On a long-term (1998 to current) grazing trial near Charters Towers, the species was very infrequently recorded up until 2007, when it increased exponentially across all grazing treatments, but particularly on the heavily grazed treatments in the poplar box (Eucalyptus populnea) woodland (O'Reagain et al. 2022). Similar to the initial spike in the region in the 1990s, this increase on the trial was thought to be associated with the few years of above average rainfall following drought culminating in 2007. The significant increase in B. pertusa across the trial was coupled with significant declines in native perennial grass species, particularly of its native congener Bothriochloa ewartiana. Also, during the 2000s, B. pertusa appeared to increase in the Cape York Peninsula, which aligns with findings from Bortolussi, et al. (2005) who suggest the species was naturally spreading onto several pastoral properties in this region during this time.
In the 10 years between 2010 and 2020 B. pertusa continued to spread and increase in dominance in northern and central Queensland, and more recently has spread into southern parts of the state, near Gayndah (Fig. 2e). A producer survey conducted in this region in 2016 suggests that only in the last 5–10 years has B. pertusa become particularly noticeable and problematic (Spiegel 2016). Surveys conducted throughout Queensland’s grassy woodland ecosystem in 1995-6 across the southern and central Queensland found B. pertusa in only 1 of 207 survey sites. We resurveyed 92 of these sites in 2018 and found B. pertusa in 43 sites and at greater than 20% cover in nine sites (Lebbink et al. 2022). It’s spread within the Gayndah area has likely been even more recent with two producers here suggesting that although it has been present for ~ 25 years (mostly along roadsides) its dominance within the pasture has only become noticeable in the last five years (personal communication, 7 July 2019). Since its introduction to the Brian Pastures research station, near Gayndah, in 1978, B. pertusa has also spread considerably, replacing native Bothriochloa ewartiana pastures in some areas. In 2020 the key invasion area spanned 28,537,600 ha (calculated from a centroid around main occurrence points).
2.3 Habitat suitability and predicted spread
The occurrence and cover of B. pertusa across Queensland was largely predicted by climate (particularly mean temperature during the growing season (October to April)) and FPC of trees (Table 3, Figs. 3 and 4). B. pertusa mostly occurred in areas with a mean growing season temperature between 23 C˚ and 27 C˚ and in areas with low tree cover (< 40% FPC). Outside of these thresholds B. pertusa was very infrequently recorded. This temperature range is typical of most sub-humid and semi-arid regions of Queensland; as well as in India and south-east Asia, where B. pertusa is native.
Where B. pertusa was most likely to achieve high cover, was in areas with a mean growing season temperature of 25 C˚ and in areas with < 10% FPC. Its dominance at this temperature may be in part associated with high germination success, with Howden (1988) finding 80% germination at 25 C˚, compared to only 10% at 20 C˚ and 30 C˚. These results align with most other plant species distribution models, which find climate rather than soil or terrain variables, are the most important predictors of plant species occupancy (Syphard, Franklin 2009). This follows that landzone was not highly influential in predicting the occurrence of B. pertusa. B. pertusa can indeed occur across a wide range of land zones including clay to sandy soils. Although, it is not likely to occur across extremely weathered infertile substrates such as those which occur in the far west of the state. It is also well established that competition from trees (for light and nutrients) limits grass growth, particularly for shade intolerant species, such as B. pertusa (Jackson, Ash 1998; Setterfield et al. 2005).
For B. pertusa cover, rainfall variability was also a strong predictor, with high cover associated with moderate to high rainfall variability (index of variability ~ 1.0) (Table 3, Fig. 4). This response aligns with anecdotal reports suggesting B. pertusa increases after cycles of drought, followed by a period of above average rainfall. Although B. pertusa is not considered particularly drought tolerant, its stoloniferous growth strategy and large seed bank enables it to quickly regain space and resources in response to improved growing conditions (Ash et al. 2011; Howden 1988). Conversely, many co-occurring native perennial grass species are considered drought tolerant. Livestock grazing severely compromises this tolerance however, with many studies finding a decline in basal area and survival of native grass species during drought, particularly on intensely grazed sites (Ash et al. 2011; McIvor 2007; Orr, Reagain 2011). Even in response to improved growing conditions, the rate of recovery and recruitment of native grass species was low, and they were often replaced by B. pertusa (Ash et al. 2011). Thus, drought and grazing-induced competitive release, combined with the colonising ability and high grazing tolerance of B. pertusa, provides the ideal conditions for its proliferation.
Based on habitat suitability, B. pertusa still has considerable capacity to spread into new areas of Queensland and to increase in dominance within the coastal and sub-coastal regions where it already proliferates. In particular, it may become more prevalent in the western (towards Longreach and Charleville) and south-western (west from Gayndah), where it has not commonly naturalised (Fig. 5). It is not predicted to reach high cover in these areas however, because these regions are mostly outside the optimal temperature and rainfall range for high B. pertusa cover (Fig. 5). Where it already occurs in western Queensland, these limitations to growth have been observed; for instance, on a conservation property near Longreach, B. pertusa is common but restricted to seasonally inundated and water run-on areas, suggesting the moister and perhaps cooler microclimates of these habitats allow for its establishment. Results from the HSMs also suggest B. pertusa is more likely to occur and achieve high cover closer to waterways (Table 3, Fig. 5.). In southern eastern areas of Queensland, a combination of lower and less variable rainfall and high FPC in many areas has resulted in smaller and more dispersed patches of predicted suitable habitat, and lower predicted cover, than in northern Queensland. Predicted increases to temperature and rainfall variability under future climate change scenarios (IPCC, 2020) may change these range predictions for B. pertusa. Likewise, Cenchrus ciliaris (Martin et al. 2015) and other tropical exotic grass species (Gallagher et al. 2013), are expected to move southwards with the warmer winter temperatures predicted under climate change.
The predicted extent of B. pertusa presented here is based on environmental variables alone. As this paper has highlighted however land management, particularly grazing management is also a very important predictor of spread and dominance in this species. These habitat suitability models should be used in conjunction with information on land management to predict invasion risk more accurately.
2.4. Key learnings and recommendations
Using a combination of empirical and anecdotal data this paper describes the introduction and spread of a damaging invasive species Bothriochloa pertusa into Australia. By taking this approach we have gleamed important insights into both the environmental and societal factors involved in the spread of invasive plant species. We have distilled these learnings and propose five key factors associated with the spread for B. pertusa: propagule pressure, species traits, land management, climate, and cultural perception. Briefly we discuss each of these factors in turn before providing recommendations for their management.
As this paper highlights, B. pertusa was repeatedly introduced across Queensland, sometimes with significant propagule load per dispersal event (such as where it was deliberately sown for pasture or lawn). As a result, propagule load for this species has been far greater than what could have been achieved by natural processes of dispersal. B. pertusa is also far more fecund than common co-occurring native species (Howden 1988). High propagule load is a key factor attributed to the success of many invasive plants (Eschtruth, Battles 2009; Fensham et al. 2013; Warren et al. 2013) and it improves the likelihood of a species surviving demographic and environmental stochastic events which threaten small populations (Lockwood et al. 2005).
The physiological traits of B. pertusa have potentially provided it an advantage over native species in some contexts. Very few native grasses in Australia are stoloniferous, with most perennial species forming tussocks. Clonality is a trait often associated with invader success as it increases the species capacity to colonise, with options for sexual and asexual reproduction and can provide access to a wider resource pool (Hollingsworth, Bailey 2000; Keser et al. 2014; Wilfried et al. 2012). There is also evidence to suggest B. pertusa roots are more resource acquisitive than co-occurring native species (Lebbink et al. 2021a). Indeed, B. pertusa recovers rapidly after heavy grazing and drought and this is perhaps associated with these efficient resource acquisition strategies
By opening up space and resources, disturbance can make a ‘weed shaped hole’ for opportunistic invaders to establish (Buckley et al. 2007). The movement of B. pertusa across the landscape has been in close association with commercial grazing land uses and associated management practices (including land clearing and fragmentation) (Jones 1997; McIvor et al. 1996; Scanlan et al. 1996). There is also research to suggest the abundance of B .pertusa in protected areas, free from domestic grazing, is considerably lower than in adjoining lands grazed by cattle (Lebbink et al. 2021b). The pervasive and wide-spread use of land for intensive grazing systems in Queensland has shifted natural disturbance regimes in favour of B. pertusa invasion. Grazing intensity also appears to be important with less B. pertusa in areas conservatively grazed (O'Reagain et al. 2022).
The anecdotal and empirical data collated in this paper point to climate and particularly the cycles of drought and heavy rainfall often associated with El Nino and La Nina respectively, as important drivers of B. pertusa spread. Drought can reduce the cover of native understorey species and similar to other disturbances creates space and resources for opportunistic species such as B. pertusa to monopolise when conditions improve (Diez et al. 2012; Shea, Chesson 2002). Climate has also been associated with the spread of other exotic grasses in Australia including C. ciliaris (Buffel Grass) (Fensham et al. 2013) and Eragrostis curvula (African Lovegrass) (Roberts et al. 2021). Extreme climatic events can facilitate invasion by a) causing significant and widespread mortality of individuals increasing the available resources for invaders and/or b) reducing the resilience of resident species to respond to improved growing conditions (Diez et al. 2012). Invasion attributed to climate is therefore a factor of the species resilience (both native and invasive), as well as the duration, magnitude and timing of the climatic event (Diez et al. 2012). Understanding how global climate cycles such as La Nina and El Nino affect invasion success is an important area of future research, particularly under an uncertain climatic future.
The early introductions of B. pertusa were mostly occurring during a post-war era when there was a big push to develop pastoralism in Australia (Clements, Henzell 2010; Cook, Dias 2006b). Research into the productivity of native pastures was overrun by a campaign for ‘greener pastures’, exotic species bred for high fecundity, productivity and resilience (Cook, Dias 2006b; Driscoll et al. 2014). The aim of some influential agronomists of this time was indeed to replace all existing native pasture species with exotic pasture (Davies 1953). As such, effort to control escaped exotic pasture populations was likely negligible, particularly as this adventive behaviour was considered a good trait of productive pasture (Miles 1949). Thus, exotic pasture species such were B. pertusa were given a free pass to invade. Despite their environmental impacts being well known today, these species are in many cases still not considered or managed as ‘weeds’ under government legislations (Department of Agriculture and Water Resource 2017). Exceptions includes species such as Eragrostis curvula (African lovegrass) and Andropogon gayanus (gamba grass) which are mostly recognised as detrimental to the agricultural industry as well as the (Department of Agriculture and Water Resource 2017). Due to their potential importance economically, exotic pasture species such a B. pertusa are ‘taboo weeds’, and this has and continues to stifle attempts to reduce their spread and impact.
Propagule pressure, traits, climate, land management and cultural perceptions are all key factors implicated in the spread of B. pertusa. Changing approaches to land management and shifting cultural perceptions may help to improve the management of this species and its consequential spread and impact into the future. Disturbance, whether it be from grazing or drought, and the consequential opening of niche space seems to be an important driver of B. pertusa invasion. Ensuring niche gaps are minimal by managing for consistent ground cover may help to reduce the establishment and spread of this stoloniferous species (O'Reagain et al. 2018; O’Reagain, Bushell 2015). Improving grazing management practices to allow adequate rest and rotation of pastures and increasing grazing-protected areas may help to achieve this. This will encourage the persistence of native ground cover and improve the resistance and resilience of ecosystems to stochastic climatic events, and invasion by opportunistic invaders like B. pertusa. These approaches may also help to reduce the establishment of monocultures and reduce landscape scale propagule load. Acknowledging environmentally invasive pastures species, such as B. pertusa within key state and federal weed legislation is important to both improve public education around these species and to help channel resources into their management.