Impacts of invasive species on island land snail fauna

概要

To understand the effect of non-native species on native ecosystems is one of the most crucial issues in biodiversity conservation. Non-native species sometimes cause extinction, especially on islands and the majority of the cause of community decline is predation. Land snails are one of the most threatened groups of organisms. Like other taxa, the reduction of land snail fauna has caused especially on islands. The major reasons for their decline and the extinction are habitat destruction and invasive predators. In the present study, I focus on relationships between invasive species and native land snail fauna.

In chapter 2, I investigated how the African big-headed ant (BHA, Pheidole megacephala) has affected native achatinellid snails because these snails are excellent indicators of the impact of ants and they have high conservation value in Ogasawara. In 2015 I surveyed the Minamizaki area of Hahajima Island of Ogasawara, designated a core zone of the World Heritage Site (WHS), for BHA. In Minamizaki, I determined the distribution and density of achatinellid snails in 2015 and compared these data with their distribution and density in 2005. The survey area was entirely covered with forest. I also tested whether BHA preyed on achatinellid snails in the laboratory. The African big-headed ant was present in the forested areas of Minamizaki. Achatinellid snails were absent in 19 out of 39 sites where BHA was present, whereas in other areas densities of the snails were ranged from 2 to 228 individuals/site. In the laboratory, BHA carried 6 out of 7 achatinellid snails and a broken shell was found. The snail distribution and density comparisons and results of the feeding experiments suggest that the presence of BHA has contributed to the decline of achatinellid snails in forests in the survey area. Yet, BHA is not on the official list of invasive alien species. Stakeholders using the list of invasive species to develop conservation programs should recognize that invasiveness of non-native species differs depending on the ecosystem and that official lists may not be complete.

In chapter 3, I investigated the origin of Bipalium muninense on the Ogasawara Islands. Bipalium muninense has been considered as an endemic species on the Ogasawara Islands. However, previous studies documented that B. muninense have caused the decline of the native snail fauna in the islands. I conducted molecular analyses of flatworms from the Ogasawara Islands and the Ryukyu Islands. A molecular phylogeny and species-delimitation analyses based on the mitochondrial COI gene revealed that B. muninense and some cryptic species inhabit the Ryukyu Islands. A haplotype network analysis documented no genetic divergence among the individuals on the Ogasawara Islands. These results suggest that B. muninense in Ogasawara is the introduced species from the Ryukyu Islands.

In chapter 4, I investigated the effects of multiple predators on land snail fauna in the Daito Islands. On the Daito Islands are declining due to the New Guinea flatworm (NGF, Platydemus manokwari). The African big-headed ant and 2 small flatworms have also invaded the islands but no study has examined their effects on snails. I investigated how the predators have affected native snails on the Daito Islands. In 2017, I surveyed these 4 predators and snail species at 100 sites throughout the islands’ forests and recorded dead shells on the forest floor to estimate species composition of past fauna. I found that NGF and BHA affected species richness and density of native snails. The richness and density of native snail species were lowest at the sites where both predators co-existed. Endemic snail snails were negatively affected by NGF whereas small snails were negatively affected by BHA. The effects of the other 2 flatworms were not detected. These findings suggest that resistance against the impacts of certain predators differ among snail lineages. Traits that confer resistance to snail species toward one type of predator do not necessarily play the same role with different types of predators. As such, it is crucial to consider the unique impacts of each invasive species on each native species.

In chapter 5, I evaluated the program to exterminate the African big-headed ant (BHA, Pheidole megacephala) started on Hahajima of the Ogasawara Islands in March 2016 for conservation of in an attempt to conserve the land snail fauna. In this program, boundary treatment with toxic baits has been conducted by the Ministry of the Environment, Japan. I surveyed the densities of BHA and land snails during 2016–2019. I made a state-space model for BHA, achatinellid snails, and Mandarina snails by using the Bayesian method to divide the effects of BHA and extermination on land snails. I also assessed the effect of the toxin on the surrogate species, Bradybaena similaris, in a laboratory setting by offering toxic and non-toxic bait to the snails. The density of BHA decreased at the sites where toxic baits were placed, while no clear trend was detected in BHA density at other sites. The density of achatinellid snails increased as BHA decreased, while the density of Mandarina showed no clear tendency. The state-space model showed a negative effect of BHA on land snails, and a positive effect of the extermination of BHA on achatinellid snails. The effects of BHA and the effects of extermination BHA on Mandarina were both slightly negative. In the feeding experiment, no effect of the toxin was detected on the mortality or mature size of the snails. Boundary treatment for BHA is useful to eradicate BHA and mitigate its impacts. Although controlling an invasive predator by toxin baits can promote recovery of native snails, the present study suggests that careful consideration of its toxin side effect on snails is required.