By Wallace Meyer, Norine Yeung & Ken Hayes, 25th May 2021
Hawaiian land snails are recognized not only for their spectacular diversity (> 752 species) and beauty, but also because they are highly threatened (Fig. 1). Tragically, most Hawaiian land snail species are already extinct with current extinction rate estimates ranging from 30–100% depending on the family. Smaller, arboreal snails that primarily live on plants have fared better than ground-dwelling species and the larger, charismatic Oʻahu tree snails. However, we lack the baseline ecological data needed to make informed conservation recommendations for the extant species. Prior to our research, critical habitat requirements, such as which plants are necessary for snail survival, were unknown for these arboreal snail species. Identifying which plant species are preferred hosts for arboreal snails is essential for determining which plant species may facilitate the preservation of Hawaiian snail diversity, and for understanding how changes in native plant species abundances and distributions may influence remaining snail populations.
To fill these knowledge gaps, we have previously studied the plant preferences of native arboreal snails in wet forests on the islands of Hawaiʻi and Oʻahu, and now in Puʻu Kukui Watershed Preserve, a wet rainforest on west Maui (Fig. 2). Our research team consisted of: scientists from the Bishop Museum; researchers from several US institutions who arrived early for the 2018 American Malacological Society Annual Meeting; Hawaiian high school students interning at the Bishop Museum; and conservation practitioners from Maui Land & Pineapple Company, who manage the Preserve (Fig. 3).
Our team surveyed nine 5 x 5 m sites over 2 days at Puʻu Kukui Watershed Preserve. To determine plant preferences of each snail species we first surveyed plant assemblages at each site to determine the relative abundance of each plant species. Following these surveys, we examined every plant to see whether there were snails present (Fig. 4). Preference was determined when we found more snails on certain plants than expected by chance (i.e. the relative abundance of snails on a plant species was higher than the relative abundance of that plant). In addition to determining plant preferences of snails in the Preserve, we also compared these data to previous studies by our team on other Hawaiian Islands, to examine any shared preference patterns.
Our findings were consistent with results from other islands. Native arboreal snails in Puʻu Kukui Watershed Preserve preferred a small subset of understorey plants, rather than the most widespread and abundant tree and mid-canopy fern species found on all three islands. Furthermore, despite our studies including island endemic snails from multiple evolutionarily distinct families, we identified a consistent preference for one key understorey plant Hydrangea arguta (Fig. 5). Combined, these findings underscore the critical need for native understorey plant species, particularly H. arguta and others preferred by snails, and that ecosystems devoid of these native plants may not support many of the extant native snails. The extensive efforts required to preserve both endemic plants and snails are emphasized by the rapid reductions in the ranges of many native plants over the last century and the loss of understorey assemblages as a result of the activities of invasive pigs in forests that are not vigilantly protected from pig incursion.
Consistent plant preferences across evolutionarily distant island endemic land snail species in Hawaiian forests on three islands informs contemporary conservation strategies that aim to improve the management of extant native arboreal snails and stem biodiversity loss. These include: (1) identifying and protecting areas with diverse understorey plant assemblages; (2) where possible, restoring understorey assemblages in degraded areas, particularly near areas with extant snails; and (3) ensuring important plant species (e.g. H. arguta and others) are targeted in these protection and restoration efforts.
We hope this study helps spur future research examining plant conservation and restoration efforts for the benefit of native Hawaiian land snails and other terrestrial invertebrate species. We also hope it motivates others to help fill gaps in the baseline ecological knowledge required for making informed conservation decisions. Our study incorporated an inexpensive and tractable approach to collecting the data required for understanding the interactions between snails and plants. Understanding and preserving these critical interactions is a priority for biodiversity conservation.
The open access article Plants critical for Hawaiian land snail conservation: arboreal snail plant preferences in Puʻu Kukui Watershed, Maui is available in Oryx—The International Journal of Conservation.