Invasive Species: Global Hitchhikers of Tourist Flows

Greetings to all the wonderful, magnificent curious readers out there!

Tourism often involves gatherings of multiple flows of individuals, vehicles and sea vessels from geographically wide-ranging parts of the world, providing a source of non-native (and potentially invasive) propagules including seeds and spores. Thus tourism creates key paths for transport of non-native species, a problem projected to increase in importance with further expansion of the sector.

This blog post was largely sparked off by a study hot off the press. It represents the first global meta-analysis of the link between tourism activities and the arrival of non-native species.

The results of the study show that globally non-native species are more abundant in areas featuring intensive tourism activity, with an overall significant large positive effect size (0.879, p>0.001) in sites where tourist-related recreation activities occurred. Although both land- and aquatic-based recreation activities pose a significant and large influence on non-native species abundance (Figure 1), their abundance is higher in aquatic environments, with boats having a greater effect than the other two vectors (horses and visitors/trails). 

Figure 1. Meta-analysis forest plot (+confidence intervals) displaying the mean effect size of overall and different recreational tourist activities on abundance of non-native species. Effect size values greater than 0 indicate non-native species abundance was greater at sites featuring recreational tourist activity occurred than in the controls. Confidence limits overlapping the dashed line are not significant. Source.

Across the meta-analysis the abundance of several types of aquatic non-native species, from seaweeds to bryozoans, was significantly greater in sites where tourism-related boating and yachting took place than control sites. Similar to national parks, marine reserves attract high tourist visitation rates and tourism-related recreational activities. This leads to the concentration of potential invasive species transport vectors such as diving equipment, boat ballasts and anchors and fouled hulls. Thus despite the terrestrial focus of the majority of studies on invasive species and tourism, recreational boating/angling is believed responsible for over 1/3 of non-natives pathways into Europe. For example the zebra mussel’s  spread into Ireland has been attributed particularly to vessels that are not regularly maintained and berthed for long periods, such as private cruisers used for tourism.

Nature-based Tourism and Invasive Species

Whilst tourism may not be the main way invasive species are spread (with shipping and industry being the two major pathways, it is one of the only pathways capable of introducing them to extremely remote parts of the world. Nature-based tourism (involving outdoor recreation and wildlife viewing) and new forms of “extreme/adventure tourism” are key growth areas in the sector. These are commonly based in comparatively pristine and high biodiversity sites such as national parks, and often in developing countries. Therefore as such forms of tourism continue to increase in popularity, an associated increase in the tourism-assisted passage of non-native species to these remote locations could have catastrophic consequences to local biodiversity. Flora and fauna residing in these habitats are often endemic and have evolved in isolation, and as such are likely to be less resilient to new pressures such as non-natives and any associated pathogens.

Aconcagua Provincial Park in Argentina is a protected area on a mountain region that experiences high levels of endemism, providing essential ecosystem services to the lowlands. It is a popular tourist destination amongst European hikers, and in the last 2 decades visitation has increased at approximately 10% annually, with > 6000 visiting hikers in 2011. A recent study recorded at least 21 non-native plant species in the Aconcagua region, with 16 of these only recorded within the park between 2001-2011, in tandem with increasing visitor numbers. All species were native to Europe and 8 are considered international invasive environmental weeds. Due to the very limited vehicle access and previous human use and isolated nature of the park, it is probable that tourists were the main vectors of non-native seed dispersal. Such propagules were likely carried in on their clothes, shoes and personal mountaineering equipment (which they may have previously used outside South America), and in the dung and fur of pack animals which can create microhabitats favourable for establishment of invasives in harsh environments. Furthermore, the dominance of European non-native species could reflect the increasing tourist flows from developed countries to remote natural habitats in developing countries through increased popularity in nature-based tourism.

Non-native plants were recorded more frequently in disturbed vegetation (52%) than in natural vegetation (38%), particularly those areas heavily trampled by hikers and pack animals. Popular recreational activities including hiking, off-road driving and mountain biking can cause habitat disturbance acting to destroy the flora and fauna of a site, changing species' niche dynamics and opportunities.  Non-native species often have particular success in disturbed areas as features such as their faster growth and reproduction rates allow them to rapidly colonise disturbed habitats. In this case, trampling on and off trails by tourists and pack animals favoured non-native establishment through reductions in native vegetation cover and increased soil erosion. Figure 2 shows how the occurrence of non-native species is concentrated along the main hiking pathways for mountaineers, where the maximum disturbance occurs.

Figure 2. Location of sites surveyed for the intensively used trails of the Horcones Valley in Aconcagua Provincial Park, Angentina. Sites with non-native species (black cirlces) and only native speces (white circles) are shown. Source.

Similarly, a study in 2014 found that survival of the invasive non-native species Poa annua was 33% higher on Antarctic sites with disturbed soils. P. annua competed with the native plant species for resources and space, having significantly negative effects on their photosynthetic performance and biomass. The authors suggest that disturbed soils contain higher levels of nutrients, so that like the mule dung in Aconcagua, human disturbance through tourism may generate microhabitats more favourable to the non-native species. This example is particularly worrying as Antarctica’s severe environmental conditions and isolation meant researchers thought it was pretty much immune to invasives. But – where there’s a will, there’s a way – and tourism has found that way! The non-native species recorded in Antarctica, such as Poa annua – are mainly found near scientific bases and tourism cruise landing areas. During the 21^st century, Antarctic visitor numbers have increased dramatically, mainly due to tourism, and now roughly 40,000 people - mostly tourists - visit each year. All the clothes, luggage and scientific equipment of Antarctic visitors may carry non-native seeds/spores, and an article in the National Geographic stated that, worryingly, on average tourists to Antarctica in 2007 each carried 2-3 seeds, and that 49-61% of these foreign plant materials were cold-adapted species. The limited species richness and simple community structures make Antarctica's ecosystems especially vulnerable to changes in plant communities occurring with non-native species establishment.

Of course the role of tourism in facilitating non-native species access to remote locations is not restricted to terrestrial environments. Whilst not necessarily linked to nature-based forms of tourism, long-distance yachting is potentially one of the very few transport vectors able to introduce non-native species to marine environments around highly remote ocean islands: ecosystems for which non-native introductions are believed to be the primary threat for biodiversity loss. For example, Palmyra Atoll, a remote island in the Pacific, holds one of the very last near-pristine reef ecosystems remaining globally. However in 2011, a study recorded 5 non-native invertebrate and algal introductions. Leisure-related yachts arriving from harbours like Hawaii, where all 5 of these species have been recorded as non-native, are the most probable source. In particular, the red alga Acanthophora spicifera was most likely transferred to Palmyra through small vessels’ hull fouling, as private yachts can enter Palmyra’s lagoons without standard checks for invasive hitchhikers. This and other algal invaders pose an ecological problem as they have the potential to alter the coralline algal dominated reef terrace environments.

Some Last Thoughts

The map below shows the location of the 32 studies included in Anderson et al.’s (2015) global meta-analysis. As you can see, a lot of them seem to be concentrated in North America, Europe and Australia. Furthermore, the number of studies included for calculating the effect size tourist activities in marine environments (n=8) was less than terrestrial environments (n=11) (Figure 1).

Map showing location of the 32 studies of invasive species included in Anderson et al.'s (2015) global analysis. Green dots = land; blue dots = marine and freshwater habitats. Source. 

In the above discussion, I hoped to rectify this bias somewhat towards developed countries and terrestrial environments in Anderson et al.’s (2015) study which highlights a wider problem of bias in the research field of tourism-related transport of non-native species. The examples I’ve drawn upon ultimately show that even isolated, high-elevation specifically protected areas, the coldest and driest places on Earth, and islands pretty much in the middle of nowhere, are susceptible to plant invasions, specifically via tourism.

Love this post but want more on those pesky little hitchhikers? Totally understandable. Check out my friend Ben’s blog, especially this post for a great insight into the impact invasives have on ecosystems!