The global risk of infectious disease emergence from giant land snail invasion and pet trade

BACKGROUND

Pathogen outbreaks mostly originate from animals, but some species are more likely to trigger epidemics. The giant land snail (Lissachatina fulica) is a widespread invader, a popular exotic pet, and a notorious vector of the rat lungworm, causing eosinophilic meningitis in humans. However, a comprehensive assessment of the risks of disease outbreak associated with this species is lacking.

METHODS

We assessed and mapped the risk of disease transmission associated with the invasion and pet trade of L. fulica. First, we conducted a review of the scientific literature to list all known L. fulica parasites and pathogens and query host-pathogen databases to identify their potential mammalian hosts. Then, to assess the potential for L. fulica to spread globally, we modelled its suitable climatic conditions and tested whether, within climatically suitable areas, the species tended to occur near humans or not. Finally, we used social media data to map L. fulica possession as an exotic pet and to identify human behaviours associated with increased risk of disease transmission.

RESULTS

Lissachatina fulica can carry at least 36 pathogen species, including two-thirds that can infect humans. The global invasion of L. fulica is climatically limited to tropical areas, but the species is strongly associated with densely populated areas where snails are more likely to enter in contact with humans. In temperate countries, however, climatic conditions should prevent L. fulica's spread. However, we show that in Europe, giant snails are popular exotic pets and are often handled with direct skin contact, likely increasing the risk of pathogen transmission to their owners.

CONCLUSIONS

It is urgent to raise public awareness of the health risks associated with L. fulica in both tropical countries and Europe and to regulate its trade and ownership internationally. Our results highlight the importance of accounting for multiple types of human-wildlife interactions when assessing risks of infectious disease emergence. Furthermore, by targeting the species most likely to spread pathogens, we show that it is possible to rapidly identify emerging disease risks on a global scale, thus guiding timely and appropriate responses.

Climatic niche shifts in introduced species

Predictions of future biological invasions often rely on the assumption that introduced species establish only under climatic conditions similar to those in their native range. To date, 135 studies have tested this assumption of 'niche conservatism', yielding contradictory results. Here we revisit this literature, consider the evidence for niche shifts, critically assess the methods used, and discuss the authors' interpretations of niche shifts. We find that the true frequency of niche shifts remains unknown because of diverging interpretations of similar metrics, conceptual issues biasing conclusions towards niche conservatism, and the use of climatic data that may not be biologically meaningful. We argue that these issues could be largely addressed by focussing on trends or relative degrees of niche change instead of dichotomous classifications (shift versus no shift), consistently and transparently including non-analogous climates, and conducting experimental studies on mismatches between macroclimates and microclimates experienced by the study organism. Furthermore, an observed niche shift may result either from species filling a greater part of their fundamental niche during the invasion (a 'realised niche shift') or from rapid evolution of traits adapting species to novel climates in the introduced range (a 'fundamental niche shift'). Currently, there is no conclusive evidence distinguishing between these potential mechanisms of niche shifts. We outline how these questions may be addressed by combining computational analyses and experimental evidence.

Increased acclimation ability accompanies a thermal niche shift of a recent invasion

Globalization is removing dispersal barriers for the establishment of invasive species and enabling their spread to novel climates. New thermal environments in the invaded range will be particularly challenging for ectotherms, as their metabolism directly depends on environmental temperature. However, we know little about the role climatic niche shifts play in the invasion process, and the underlining physiological mechanisms. We tested if a thermal niche shift accompanies an invasion, and if native and introduced populations differ in their ability to acclimate thermal limits. We used an alien ant species-Tapinoma magnum-which recently started to spread across Europe. Using occurrence data and accompanying climatic variables, we measured the amount of overlap between thermal niches in the native and invaded range. We then experimentally tested the acclimation ability in native and introduced populations by incubating T. magnum at 18, 25 and 30°C. We measured upper and lower critical thermal limits after 7 and 21 days. We found that T. magnum occupies a distinct thermal niche in its introduced range, which is on average 3.5°C colder than its native range. Critical thermal minimum did not differ between populations from the two ranges when colonies were maintained at 25 or 30°C, but did differ after colony acclimation at a lower temperature. We found twofold greater acclimation ability of introduced populations to lower temperatures, after prolonged incubation at 18°C. Increased acclimation ability of lower thermal limits could explain the expansion of the realized thermal niche in the invaded range, and likely contributed to the spread of this species to cooler climates. Such thermal plasticity could be an important, yet so far understudied, factor underlying the expansion of invasive insects into novel climates.

Smaller climatic niche shifts in invasive than non-invasive alien ant species

The globalization of trade and human movement has resulted in the accidental dispersal of thousands of alien species worldwide at an unprecedented scale. Some of these species are considered invasive because of their extensive spatial spread or negative impacts on native biodiversity. Explaining which alien species become invasive is a major challenge of invasion biology, and it is often assumed that invasiveness is linked to a greater ability to establish in novel climates. To test whether invasive species have expanded more into novel climates than non-invasive alien species, we quantified niche shifts of 82 ant species. Surprisingly, invasive species showed smaller niche shifts than non-invasive alien species. Independent of their invasiveness, the species with the smallest native niches and range sizes, experienced the greatest niche shifts. Overall, our results challenge the assumption that invasive species are particularly good pioneers of novel climates.