Invasive plants seriously impact our environmental, agricultural and forestry assets, and the ornamental plant trade is a major introduction pathway. The variety and extent of the ornamental plant trade is growing in reach and is increasingly facilitated by the internet (i.e., through e-commerce). A lack of surveillance and regulation of e-commerce has resulted in invasive species being widely traded on these platforms. Here, we investigated the extent of illegal trade in invasive plant species in Australia by collecting advertisements found on a popular public e-commerce website. Across a 12-month period we collected a total of 235,162 plant advertisements. From 10,000 of these advertisements (4.25% of total advertisements) we found 155 plant taxa advertised online that were prohibited to trade in at least one Australian State or Territory (12.5% of Australia’s total prohibited plant taxa). We detected 1,415 instances of invasive plants advertised, of which 411 breached local jurisdictional (i.e., State or Territory) laws. Opuntia cacti and invasive aquatic plants were traded in the greatest quantities. A variety of uses for plants prohibited to trade were reported by the sellers, with aquatic uses being the most popular (i.e., water filtering and habitat for aquatic animals). We used generalised linear mixed-effects models to test the effect of prohibiting the sale of invasive plants on the quantity and price of online advertisements. Despite Australia’s strict internal biosecurity regulations, we found that trade prohibitions had no influence on the quantity and price of trade in illegal invasive plants. Given this, and the extent of illegal invasive plants traded, we believe increased monitoring and regulation of online plant trade is warranted. We demonstrate that targeted searches using string matching is an effective tool for detecting e-commerce trade of invasive species. However, to obtain the most optimal outcomes, regulations should be coupled with increased cooperation from e-commerce platforms and public awareness campaigns. Future weed risk assessments should consider online trade as a key factor in the long-distance dispersal and propagule pressure of a plant. Jurisdictions would also benefit from greater alignment on plant trade prohibitions and revision of associated compliance policies.

Aquatic weeds, biosecurity, e-commerce, Opuntia, ornamental plants, prevention, surveillance, web scraping

Invasive plants can cause serious negative impacts to biodiversity, human health, and primary resource industries (Pyšek et al. 2020; Ward et al. 2021). The largest vector of new plant introductions and invasions is the global trade of ornamental plants, which is continually growing in both reach and quantity (Weber et al. 2008; Dodd et al. 2015; Faulkner et al. 2016; van Kleunen et al. 2018; Arianoutsou et al. 2021; Beaury et al. 2021; Rojas-Sandoval et al. 2022). Within this global trade, a pathway of serious concern is trade facilitated by the internet, hereafter termed e-commerce (Derraik and Phillips 2010; Lenda et al. 2014; Humair et al. 2015). E-commerce platforms facilitate long distance dispersal of invasive species and can often circumvent regulations (Giltrap et al. 2009; Derraik and Phillips 2010; Magalhães and Avelar 2012; Lenda et al. 2014; Humair et al. 2015; Beaury et al. 2021). As a result, e-commerce has proven challenging to monitor and enforce for biosecurity agencies (Derraik and Phillips 2010; Lavorgna and Sajeva 2021). Many invasive plant species are being traded online despite legislative regulations (Humair et al. 2015; Munakamwe and Constantine 2017; Beaury et al. 2021). Without intervention, it is predicted that online trade will lead to further invasive plant incursions (Humair et al. 2015; Peres et al. 2018; Beaury et al. 2021).

Australia has a highly endemic floral community that has been severely impacted by plant invasions (Broadhurst and Coates 2017; Bradshaw et al. 2021). Strict importation measures and risk assessment processes have been implemented by the Australian government to prevent the arrival of new alien-invasive plants (Pheloung et al. 1999; Walton 2001; Keller et al. 2007; Simberloff et al. 2013). Even so, Australia already has more than 29,000 introduced alien-plant species (Gallagher and Leishman 2014). There are also native Australian plants which have become invasive outside their indigenous range (Rose and Fairweather 1997; Morgan et al. 2002; O’Loughlin et al. 2015). Where plant species become invasive, or there is potential to be invasive, state and territory governments (‘jurisdictions’ hereafter) have the main responsibility for their management and control. A common control measure used by jurisdictions is to ‘declare’ invasive plant taxa in legislation as prohibited to trade within jurisdictional borders (simply ‘declared plant’ hereafter); with 1,236 taxa declared in one or more jurisdictions across Australia. These taxa are declared because they pose significant risks of environmental, economic and/or social impacts to natural ecosystems, agricultural and forestry production, and human communities. While legislation differs slightly between jurisdictions, generally it is prohibited to supply, sell, or transport declared plants, with fines issued for offences. However, e-commerce websites could circumvent traditional enforcement measures by trading without physical stores, sending plants by mail, or having buyers collect plants from private residences, resulting in a poorly regulated sector of the horticultural market (Munakamwe and Constantine 2017). Screening for invasive plants entering the country is also challenging due to the high volume of incoming international mail (Australian National Audit Office 2014). Therefore, surveillance of e-commerce is an essential tool for detecting and preventing plant invasions (Humair et al. 2015; Lavorgna et al. 2020; Duncan 2021; Stoett and Omrow 2021; Whitehead et al. 2021). E-commerce websites where members of the public post plant advertisements are particularly difficult to monitor. Some efforts have been made to monitor this trade within Australia, however the focus has been limited by time and resources to a handful of problematic species (Munakamwe and Constantine 2017).

To investigate the current invasion risk of e-commerce plant trade within Australia (i.e., internal trade, not international shipments into Australia), we applied web-scraping technology to monitor and record plant trade advertisements on a popular Australian e-commerce website over the course of one year. We investigated five research aims: (i) determine what proportion of plants advertised are prohibited to trade; (ii) determine the quantity and taxonomic composition of declared plants traded; (iii) determine whether current regulations reduce trade quantity or influence the price of declared plants in jurisdictions which prohibit trade versus those that permit trade; (iv) characterise the most frequently traded declared plants; and (v) document advertised plant uses to inform our understanding of the desire for declared plants. We seek to provide a clearer picture of the present risk of e-commerce trade and whether prescriptive laws reduce invasive plant trade. These results will help inform future policy decisions regarding the monitoring and prevention of invasive species occurring in the Australian plant trade.

Ornamental plant trade is the world’s leading pathway for invasive plant introductions and is greatly facilitated by internet e-commerce (Humair et al. 2015; Munakamwe and Constantine 2017; Peres et al. 2018; van Kleunen et al. 2018; Beaury et al. 2021). Our study represents the first investigation into the presence of the complete set of Australia’s declared invasive plants on e-commerce. On a single popular e-commerce website, we found hundreds of opportunities to purchase a wide variety of declared plants over the course of one year. This is despite the country’s strict biosecurity policies and a weed risk assessment that has been adopted by other countries (Gordon et al. 2008). Trade of invasive plants through e-commerce has been documented in other regions such as New Zealand (Derraik and Phillips 2010), the United States (US) (Maki and Galatowitsch 2004; Beaury et al. 2021) and European Union (EU) (Lenda et al. 2014; Humair et al. 2015). Australia shares similarities with the US and EU, having accessible e-commerce platforms and easily facilitated trade across jurisdictions with differing biosecurity regulations. Our findings contribute to this growing body of evidence calling attention to e-commerce as an invasion risk pathway that is establishing globally. In particular, we have quantified the risk of illegal online plant trade conducted by individuals rather than commercial nurseries, which is a challenging aspect of e-commerce to monitor and regulate. We highlight the need to review our approaches to managing invasive species in the face of an increasingly interconnected world.

The pace of the ornamental plant trade in Australia is increasing, where 2020 saw a record high number of plant sales in the nursery industry (Horticulture Innovation Australia 2021). Given this growth and the availability of invasive plants, online trade poses a serious invasion threat and demands greater scrutiny. Since declared plant taxa have already been determined as serious biosecurity concerns (i.e., declared in State/Territory laws), we argue that monitoring and interception of this trade is certainly warranted and should continue (Munakamwe and Constantine 2017). Low detection rates emphasise the challenge of capturing and regulating this trade. Given that our species accumulation did not approach a limit, it is likely that we have not captured the full diversity of declared plants traded online. It should also be noted that our study focussed on a narrow group of invasive plants (i.e., those that are currently declared as illegal to trade). Beyond the declared plants there are likely many other non-regulated, invasive plant species being traded on these e-commerce platforms that may still cause environmental harm (Beaury et al. 2021). Additionally, we only studied one e-commerce platform. A broader analysis of additional e-commerce platforms may reveal more declared invasive plant species that are available to the public.

In addition to the prohibited trade, declared plants were widely advertised in jurisdictions where they are currently permitted to trade. Just under half of the declared taxa and more than double the number of detections we found were located in the jurisdictions that did not prohibit sale. Some of the most frequently traded declared species are only prohibited to trade in one or two jurisdictions, despite many being known to be invasive in permitted jurisdictions. Some examples of invasive populations in permitted jurisdictions include: Lavandula stoechas in SA (Nicholson 2006), Orbea variegata in NSW (Hamilton et al. 2013), and Limnobium laevigatum in Queensland (Bickel et al. 2022). L. laevigatum was a particularly concerning example traded to a much larger extent in jurisdictions that did not prohibit its trade. We made 19 detections for L. laevigatum in prohibited jurisdictions and 69 in permitted jurisdictions. By using online trade data, we argue that jurisdictions should reconsider the risk of invasive species like L. laevigatum to determine if prohibition is warranted. A similar situation has been observed in the US (Beaury et al. 2021), another geographically large country with multiple states with their own governing legislations. Like in the US, we argue this type of trade can compromise the biosecurity of neighbouring jurisdictions (Beaury et al. 2021). For example, we found NSW and Vic. traded large quantities of species declared in neighbouring jurisdictions. This is especially concerning because the plant trade facilitates long-distance dispersal from plants mailed over long distances (Maki and Galatowitsch 2004). Despite the limitations of online trade data, it is still a valuable resource to help identify species or areas of concern (Kikillus et al. 2012). Thus, we suggest future weed risk assessments utilise data collected from monitoring e-commerce to factor in trade of invasive plants as a risk factor, even if occurring in other jurisdictions. Incorporating this may lead jurisdictions to consider a nationally consistent approach to plant declarations, similar to other control programs which have benefited from cross-border coordination (Pluess et al. 2012). As long as the trade of invasive plants persists somewhere with a country, the risk of natural or human mediated dispersal into vulnerable landscapes will remain.

While more consistent regulations among jurisdictions would provide the legal framework to address invasive plant trade, our results may suggest this is not a cure-all. We found that across declared plant taxa, there was no difference in the quantities of advertisements observed in prohibited and permitted jurisdictions. We also saw no significant effect on price, however our sample size was reduced to 20 declared taxa, making it difficult to draw a meaningful conclusion across all declared taxa traded. It is likely that jurisdictional regulations are reducing the total abundance of declared taxa in Australian plant trade, through compliance from traditional “brick-and-mortar” nurseries. It is important to note that the lack of effect on quantity we saw could be due to the limited size of our sample. Investigations across larger datasets, and across more e-commerce platforms, may reveal different results. However, if trade prohibition is not having an effect on the quantity of online trade, explanations from other plant trade studies may provide an answer. For one, sellers may perceive online trading of declared plants as low risk. This perception may be in part due to limited enforcement of e-commerce due to surveillance and legal challenges (Lavorgna and Sajeva 2021; Whitehead et al. 2021). Another reason may be a lack of awareness that these plants are invasive and that their trade is prohibited. Public awareness has been suggested by other studies into invasive plant trade, reporting that people are often unaware, lack the ability to correctly identify plants, or are misinformed about relevant legislation rather than knowingly breaking the law (Derraik and Phillips 2010; Martin and Coetzee 2011; Munakamwe and Constantine 2017). We suggest implementing web scraping surveillance tools to improve enforcement and to enhance public knowledge through awareness campaigns which improve invasive species management (Novoa et al. 2017; Cordeiro et al. 2020; Li et al. 2021). Further, e-commerce platforms can also play a role in prevention and should be engaged as a biosecurity stakeholder. Specifically, in agreement with other studies of the illegal plant trade, we recommend that relevant governments coordinate with e-commerce websites to prevent illegal trade (Derraik and Phillips 2010; Munakamwe and Constantine 2017). For example, e-commerce websites could provide information to people creating plant advertisements, warning them of plants that cannot be sold and to help identify those plants.

Given that plant trade is fundamentally human driven, we expected to observe a higher number of advertisements matching search terms and corresponding to declared plants in jurisdictions with larger populations. Consequently, in the targeted sample, we observed this trend with NSW and Vic. having the greatest number of total declared advertisements. Interestingly, NSW and Vic. also had the greatest proportion of total declared advertisements. However, in terms of prohibited advertisements, WA, SA, and NT had the highest proportions in the targeted sample. To explain this, we should consider the plants that jurisdictions have chosen to declare. Regulations are jurisdiction based, therefore differences in declarations arise between jurisdictions. WA declares the greatest number of plant taxa of any Australia jurisdiction (877 plant taxa), more than double that of the next highest jurisdiction. As a result, WA prohibits a larger proportion of Australia’s assemblage of declared plants. Complementary to this is that NT, SA, and WA declare highly traded declared species that other jurisdictions do not. Zantedeschia aethiopica is only declared in SA and WA, Opuntia ficus-indica is only declared in NT and WA, and Gazania spp. are only declared in SA. These species were frequently traded in SA and WA, thus the higher proportions are indicative of the regulations of these jurisdictions. However, NT prohibited advertisements were predominately for aquatic declared plants that are not exclusively declared in the jurisdiction. Evidently this is a popular group of plants traded in the jurisdiction, one that may benefit from targeted management campaigns.

We found that Opuntia cacti and aquatic invasive plants were among the most frequently traded declared plants. This is concerning given the historical extent of Opuntia impact on the Australian environment (Freeman 1992), and the invasiveness of the traded aquatic weeds Eichhornia crassipes and L. laevigatum (Riches 2001; Tidwell and O’Donnell 2010; Villamagna and Murphy 2010). It is possible that some traits that aid their invasion success could also lend to their popularity in trade. Opuntia cacti are easily propagated from cuttings and will do so readily when discarded from gardens (Smith 2006; Smith et al. 2011). E. crassipes and L. laevigatum can also reproduce vegetatively and in good conditions growers will quickly have an overabundance (Madsen and Morgan 2021; Prasetyo et al. 2021). This ease of excess could present sale as an attractive option to get rid of surplus plants, thus facilitating invasions. However, without further investigation into seller behaviour we cannot say how common this is. Similarly, it has been suggested that some Opuntia protective traits (e.g., spines and glochids) eventually lead owners to dispose of them. Smith et al. (2011) suggested that the irritating hairs (glochids) of Opuntia microdasys drive owners to dispose of the plants through dumping. We spoke with a compliance officer investigating Opuntia sales, who reported that sellers mention a desire to sell the plants in order to be rid of them (D. Swan 2021, pers. comm., 3 November). The high number of advertisements we observed of these taxa may indicate selling plants is an attractive alternative to disposal but this would require further investigation.

We demonstrated that targeted searches using string matching was a more effective means of detection than random sampling. We took a conservative approach by including common and generic names (e.g., pond plant) alongside scientific names in our effort to detect declared plants. Common and generic names are non-specific and can be shared by many plant species, contributing to a higher rate of false positives. However, we believe this approach is necessary to reduce the chance of missing advertisements for invasive species. Image recognition technology could be employed to further increase detection rate (Di Minin et al. 2019). However, the accuracy of image recognition is dependent on large, pre-identified image datasets and the quality of images provided (Xiong et al. 2021). The quality of images that we observed in advertisements varied greatly in resolution and often had complex backgrounds, a feature known to hinder the accuracy of image recognition (Xiong et al. 2021). We propose that string matching and other natural language processing methods are a cost-effective means for the semi-automated detection of invasive plants on e-commerce platforms.

The advertised uses for declared plants revealed some reasons why people desire them, which may complicate their management. We discovered a variety of uses advertised for declared plants, including food, medicine, cosmetics, and decoration (e.g., floral arrangements). However, the most commonly advertised uses fell into the ‘aquatic’ category, uses such as water-conditioning and providing habitat for aquatic pets. Perceived water-conditioning abilities could encourage people to introduce the plant into waterbodies (e.g., ponds and dams), risking dispersal into the surrounding environment. For example, we found E. crassipes traded which has been known to be intentionally introduced into waterbodies to help prevent algal blooms (Villamagna and Murphy 2010). It is important to consider people’s intended use of an invasive plant because prevention is often more of a cultural challenge than biological (Pfeiffer and Voeks 2008). Understanding the public’s desire for an invasive plant could help to tailor education campaigns or promote non-invasive alternatives. It is also important that public attitudes are understood to establish collaborative efforts between invested communities and policymakers, which will lead to optimal social and biosecurity outcomes (Virtue et al. 2004; Head 2017).

We observed the prohibited advertisement of invasive plants online in all Australian jurisdictions. This online trade creates many opportunities for the public to purchase and spread declared invasive plants around the country. As it stands, laws prohibiting the trade of declared plants have not halted prohibited advertisements of declared plants on public e-commerce. We suggest enhancing detection methods of illegal trade using web scraping techniques to improve enforcement. Jurisdictions should also focus on educating the public that certain plants are prohibited to trade while considering the desire that people have for these plants to help promote safe alternatives. Cooperation should be sought from e-commerce websites to prevent instances of illegal trade being facilitated on their platforms. For now, monitoring e-commerce is still needed and we have demonstrated that web-scraping is an effective tool. Data collected from monitoring e-commerce could also be utilised in future weed risk assessments with online availability incorporated as a risk factor. Beyond surveillance, jurisdictions should seek to better align the taxa they choose to regulate as the existing legal disparities could contribute to the persistence of invasive species being distributed within a country. Australia’s biosecurity, and that of other countries and regions, would benefit from more coordinated approaches to controlling the online trade of invasive species.

We, the authors, acknowledge we are all living and working on colonised land. Jacob Maher, Lisa Wood, Charlotte R. Lassaline, and Phillip Cassey live and work on Kaurna land. Oliver C. Stringham and Stephanie Moncayo live and work on Lenape land. John Virtue lives and works on Peramangk land. In all instances, we acknowledge and recognize the longstanding significance of these lands for these nations. The Kaurna, Lenape, and Peramangk people were violently displaced as a result of European settler colonialism yet remain closely connected with these lands and are their rightful stewards. We respect their custodianship of the land, value their past, present, and ongoing connection to the land and their cultural beliefs.

This work was supported by funding from the Australian Research Training Program and the Centre for Invasive Species Solutions (P01-W-003: Biosecurity surveillance of e-commerce and other online platforms for illegal trade in declared plants).