A preliminary assessment of the response of a native reptile assemblage to spot-spraying invasive Bitou Bush with glyphosate herbicide

Summary

During recent work examining the effects of Bitou Bush (Chrysanthemoides monilifera ssp. rotundata) invasion on native reptile assemblages in coastal heathland vegetation in Eastern Australia, unplanned spot-spraying of glyphosate occurred at some of our experimental sites invaded by Bitou Bush. We used this unexpected herbicide application as an opportunity to provide a preliminary assessment of the short-term impacts on reptiles of glyphosate spot-spraying of Bitou Bush. Using an M-BARCI design, we compared reptile assemblages among uninvaded (reference) sites, invaded (control) sites and invaded and sprayed (impact) sites before and after spraying. We found no significant short-term (7 – 10 months) differences in reptile abundance, species richness or assemblage composition among invaded, uninvaded and sprayed sites before and after glyphosate application. We cautiously interpret our results to generate a preliminary finding that spot-spraying of Bitou Bush with glyphosate appears not to have a deleterious effect on reptile assemblages at seven and ten months following herbicide application. While we would not recommend basing management decisions on the outcomes of our study alone, we suggest that our findings can be used to assist in the development of strategic analyses of glyphosate impacts on native flora and fauna.

Introduction

The exotic plant species Bitou Bush, Chrysanthemoides monilifera ssp. rotundata (L.) Norlindh, is a widespread invader of coastal areas of Eastern Australia (Australian Weeds Committee 2010). Invasive Bitou Bush is listed as a Weed of National Significance in Australia as it poses a threat to a number of native plant, bird and mammal species (Coutts-Smith & Downey 2006).

Application of the herbicide glyphosate by spot or aerial spraying is considered an effective broad-scale method of controlling Bitou Bush Department of Environment & Conservation 2006). The highly diluted concentrations of glyphosate used in herbicide applications are unlikely to bioaccumulate in large quantities, with more recent formulations generally considered safe for terrestrial animals (Baylis 2000). Nevertheless, there are concerns about the potentially deleterious effects on nontarget plant and animal species of spraying weeds such as Bitou Bush with glyphosate Department of Environment & Conservation 2006). Considering that Bitou Bush is thought to be utilised as habitat by the Diamond Python (Morelia spilota spilota) and several native skink species (Winkler et al. 2008), it needs to be determined whether glyphosate spraying of Bitou Bush has deleterious effects, or indeed, if it has little impact on native reptile assemblages.

During our recent work examining the effects of Bitou Bush invasion on native reptile assemblages at ten sites (five invaded compared with five uninvaded) in coastal heathland vegetation in Eastern Australia, unexpected glyphosate spot-spraying of Bitou Bush occurred at two of the five invaded sites. The glyphosate spraying resulted in near to 100% mortality of Bitou Bush at these two study sites. We used this unexpected application of glyphosate herbicide as an opportunity to provide a preliminary assessment of the short-term impacts on reptiles of glyphosate spot-spraying of Bitou Bush. We employed the M-BARCI design (e.g. Lake 2001) to compare reptile assemblages among uninvaded (reference) sites, invaded (control) sites and invaded and sprayed (impact) sites before and after spot-spraying. As our study was not initially set up to provide a comprehensive assessment of the response of native reptile assemblages to glyphosate spraying of Bitou Bush, we interpret our findings cautiously and point to ways in which future large-scale manipulative experiments might address the broader issue of potential collateral damage to native biodiversity as a result of the use of glyphosate to manage a range of weed species.

Methods

Study sites

Prior to the unexpected glyphosate spot-spraying, our initial study design consisted of a total of ten sites, with five uninvaded sites and five sites invaded by Bitou Bush. Sites were selected in National Parks and public reserves in the Botany Bay and Kurnell Peninsula area of Sydney, NSW. All sites were located near the coast at elevations 7–42 m and the minimum distance between any two sites was 1 km. Three large sites (50 m × 50 m) and two small sites (50 m × 20 m) were selected for invaded vegetation, with a matching number and type of sites in uninvaded vegetation. The two small sites were selected in place of large sites based on the limited availability of large sites, which was determined after an exhaustive survey of the study area. The two small sites could not be extended in width to 50 m as marked changes in the slope of the sites precluded slope remaining constant across the sites. To account for differences in site dimensions, reptile surveys were standardised using transects of the same length (50 m), and data for each transect within a site were combined to give abundance and species richness totals for each site (standardised on a per m² basis).

Reptile surveys were planned for each site in March/April 2010 (autumn), November 2010 (spring) and February 2011 (summer). Soon after we completed the autumn 2010 survey, two invaded sites (one large site and one small site) were subjected to glyphosate herbicide treatment of Bitou Bush by the NSW National Parks and Wildlife Service in late May to early June. We continued to perform reptile surveys at these two sprayed sites (and at the other eight sites) during spring 2010 and summer 2011. The revised experimental design used in the present study consisted of five uninvaded (reference) sites, three invaded (control) sites and two invaded and sprayed (impact) sites that were all surveyed before and after glyphosate spot-spraying.

All study sites were located in vegetation adjacent to well-formed tracks or roads as Bitou Bush primarily invades areas of disturbance. Sites were selected with no evidence of recent or ongoing weed control or recent fire, as these could act as confounding factors influencing reptile assemblages. Thick patches of Bitou Bush comprised 40% cover on average at invaded sites and formed a mosaic with native vegetation. The native vegetation at all sites was characterised by low dense shrubs, such as Coast Wattle (Acacia longifolia var. sophorae) (Labill.) Court and Banksias (Banksia spp.) Permanent 4-m-wide × 50-m-long search transects were established within each study site, running parallel to the vegetation edge. The first transect was located 2.5 m from the edge, and further transects were positioned at 15-m intervals from each other and away from the vegetation edge, parallel with the first transect. Four transects (a total search area of 800 m²) and two transects (total search area 400 m²) were used in large and small plots, respectively. This arrangement was used to ensure consistency of sampling effort in the disturbed edges and to ensure transect independence.

Reptile surveys

Reptiles were sampled once per season using time-constrained, diurnal-active searches. Order of site visits during each survey period was randomised. All searches were conducted in warm, sunny conditions with ambient temperatures in excess of 20°C and between the hours of 0900 and 1100 Eastern standard time (EST) or 1500 and 1700 EST. Searches were constrained to 15 min per transect (total 60 min per large site and 30 min per small site) and consisted of haphazardly turning rocks and logs, lifting loose bark and raking leaf-litter along each transect. Surface-active reptiles sighted within sampling transects were also recorded. Duration of time-constrained searches was based on a pilot assessment of the time required to efficiently search a transect.

Statistical analysis

Reptile abundance and species richness were analysed using separate generalised linear models (GLM) in SPSS v.20 (IBM, Ireland). This type of model is robust to the unbalanced design necessitated in this study. ‘Condition’ was a fixed factor with three levels (uninvaded, invaded, sprayed). ‘Time’ was a fixed factor with three levels (autumn, spring, summer) and an interaction term for Condition x Time was included in the models. In the analyses, the emergence of a significant Condition x Time interaction, linked to lower reptile abundance and/or richness in the sprayed sites after spraying, would indicate that glyphosate spot-spraying was associated with declines in reptile biodiversity. In contrast, no significant Condition x Time interaction in both models would indicate that glyphosate spot-spraying of Bitou Bush did not have a significant effect on either reptile abundance or species richness. We used Wald chi-squared tests for tests of statistical significance. Data were checked for normality using Kolmogorov–Smirnov tests and a normal probability distribution and identity link function were specified in the models.

Results

There was no significant effect of Condition (Wald χ²₂ = 0.45, P = 0.80) or Time (Wald χ²₂ = 0.02, P = 0.99) on reptile abundance (Fig. 1a). There was no significant Condition × Time interaction (Wald χ²₄ = 1.37, P = 0.85), indicating a nonintrusive effect of glyphosate spot-spraying of Bitou Bush on reptile abundance.

Figure 1. Mean (± SE) (a) reptile abundance and (b) reptile species richness at uninvaded, invaded and sprayed sites, before (autumn 2010) and after (spring 2010 and summer 2011) application of glyphosate herbicide to Bitou Bush at sprayed sites.

There was no significant effect of Condition (Wald χ²₂ = 4.16, P = 0.13) or Time (Wald χ²₂ = 0.68, P = 0.71) on reptile species richness (Fig. 1b). We also found that there was no significant Condition × Time interaction (Wald  = 3.21, P = 0.52), indicating that glyphosate spot-spraying of Bitou Bush did not have a significant effect on reptile species richness.

Reptile species composition was remarkably similar among invaded, uninvaded and sprayed sites before and after spraying. Jacky Lizards (Amphiblurus muricatus), Garden Skinks (Lampropholis guichenoti) and Delicate Garden Skinks (Lampropholis delicata) were commonly found in all three site conditions before and after spraying. Copper-tailed Skinks (Ctenotus taeniolatus) were absent only from invaded sites in prespraying surveys but were common in all other site conditions at all sampling times. The Eastern Water Skink (Eulamprus quoyii) was recorded at uninvaded sites in prespraying and postspraying surveys but was only recorded in pre-spraying surveys at sprayed sites. The Marsh Snake (Hemiaspis signata) was absent from sprayed sites prior to spraying but present after spraying.

Discussion

Given the opportunistic nature of our study and the small number of impact sites (two in total), we cautiously interpret our results to generate a preliminary finding that spot-spraying of Bitou Bush with glyphosate appears not to have a deleterious effect on reptile abundance, species richness or composition at seven and ten months following herbicide application. Clearly, it will be of great value to utilise a larger number of impact sites in future to examine the impacts of glyphosate on reptile as well as other faunal and floral assemblages. Nevertheless, we believe our finding is probably a reliable indication of the effects of glyphosate on reptile assemblages for three reasons.

First, our survey efforts within each site were comprehensive and more than adequate to obtain robust measures of the structure of reptile assemblages. The survey areas of 800 m² for large sites and 400 m² for small sites are comparable with other published studies that have described exotic plant influences on reptile assemblages (e.g. Barrows & Allen 2010). Second, our finding is consistent with previous work (Lindsay & French 2004a) that found no effect of spraying Bitou Bush with glyphosate on the abundance or composition of leaf-litter invertebrates. Third, when we initially designed the larger study to examine invasive impacts of Bitou Bush on native reptiles, we predicted that there would be no significant difference in reptile assemblages between invaded and uninvaded sites based on a conceptual framework that we had developed previously (Martin & Murray 2011). We predicted that in systems where an invasive plant species is similar in growth form to the invaded habitat, there should not be substantial impacts on resident reptile assemblages. Habitat surveys of the coastal heathland sites used in the present study, indicated that invading Bitou Bush does not differ substantially in growth form from native shrub species in the vegetation (e.g. Coast Wattle and Banksias).This contrasts with a previous study that found Bitou Bush invasion was associated with cooler and darker leaf-litter conditions and higher soil moisture when compared with native vegetation (Lindsay & French 2004b). An important distinction between the two studies is the extent of Bitou Bush cover. Bitou Bush averaged approximately 40% cover at invaded sites in our investigation, whereas the minimum cover of Bitou Bush reported by Lindsay and French (2004b) was 70%. Increased cover of exotic plants is associated with greater levels of habitat modification (Martin & Murray 2011). Thus, our finding in the present study that there are no substantial differences in reptile assemblages between uninvaded and invaded sites is not surprising. Furthermore, the fact that we then found no differences in reptile assemblages among uninvaded, invaded and sprayed sites before and after spot-spraying is to be expected if glyphosate is not having a deleterious effect on the abundance, richness and composition of reptiles.

Considering together the three issues outlined above, we believe that our study provides reliable preliminary evidence of the nonintrusive effects of spot-spraying Bitou Bush with glyphosate. Nevertheless, more work needs to be carried out to provide a larger body of evidence for nonintrusive impacts of glyphosate in general. We would not recommend basing management decisions on the outcomes of our study alone, but rather, our findings can be used to develop more strategic analyses of glyphosate impacts.

It is possible, for instance, that control of Bitou Bush may reduce reptile abundance during particularly short time frames (i.e. less than six months) when the impacts may be felt more intensely. Bitou Bush provides cover to protect some small skink species from predators, which include introduced cats and dogs (Winkler et al. 2008). Larger, planned studies with increased replication are needed to determine whether reptile abundance is reduced within six months of spraying.

The relatively small number of reptiles encountered in surveys reflects the size of sprayed Bitou Bush patches. While use of larger sites may have increased reptile numbers that we observed, the use of longer transects at the unsprayed sites would have resulted in confounding the observation of effects of glyphosate spot-spraying with the effects of increasing native vegetation and unsprayed bitou bush. Consequently, while our results are relevant for spot-spraying of smaller Bitou Bush patches, we cannot extrapolate our results to larger spray events such as might occur in aerial spraying of large stands.

Further studies are also needed to determine the response of reptiles to Bitou Bush control that does not use glyphosate such as physical removal, burning and biological control. This would allow the efficacy and impacts of available control methods to be compared. The longer-term response of reptiles to Bitou Bush control also requires further study. The period covered by our investigation represents only a single reptile breeding season. Longer-term investigations are needed to determine the ongoing response of reptiles to Bitou Bush control over several years. This work should be a priority, given the increasing need to control Bitou Bush due to its invasive spread and the detrimental impacts of this spread on other native species of plants, birds and mammals (Coutts-Smith & Downey 2006).

Acknowledgements

We thank Kaiya Donovan and Michael Hand of the NSW National Parks and Wildlife Service for assisting with access to sites in the Kamay Botany Bay National Park and for providing details of glyphosate treatment of Bitou Bush. We also thank Paul Price of Sutherland Shire Council and Nethan Kana of Kurnell Land Fill Company for assistance with access to sites. We are grateful to Lilith Mordecai for commenting on a draft of the paper. This study was conducted under UTS Animal Care and Ethics Committee Animal Research Authority No 2009-352A and NSW Office of Environment and Heritage Scientific Licence No S13059.