Can Oceanic Island Endemic Partulid Tree Snails Survive the Anthropocene?

Abstract

Partulid tree snails are endemic to Pacific high oceanic islands and have experienced extraordinary rates of extinction in recent decades involving approximately half of the 120 described species. The main extinction agents are continental predators introduced as part of misguided biological control programs across Oceania. My research focused on surviving populations at either end of the familial range (eastern populations in the Society Islands and western populations in Near Oceania and the Marianas) that may hold clues as to how members of this clade may be able to survive in the Anthropocene. The first part of my dissertation concerned the island of Tahiti, where two endemic taxa, Partula clara and Partula hyalina, have differentially survived 40 years of predation by Euglandina rosea. Using historical databases, I found that higher clutch size was correlated with partulid survival on Tahiti. I further corroborated this association of survival with fecundity using birth rate data from captive populations and with parallel historical and captive demographic analyses of additional island populations: Moorea (Society Islands) and Guam and Saipan (Marianas Islands). I also tested the role of a putative ecological “solar refuge” on one of the two valley survivors on Tahiti, Partula hyalina, with a white high albedo shell. This species regularly occurs in forest edges being exposed to prolonged direct sunlight, and in interior forests. I hypothesized that these edge habitats create “solar refuges” for surviving P. hyalina in which ambient solar irradiation conditions are significantly higher than those tolerated by foraging E. rosea. My results corroborate the hypothesis, although any protective effect gained from being in a “solar refuge” is absent on overcast days. The second part concerns the enigmatic systematics, ecology and biogeography of Partulidae in the least-studied part of their familial range: the Near Oceania archipelagos of Papua New Guinea (PNG) and the Solomon Islands (SIs). A recent study on this regional fauna has uncovered highly atypical synanthropic distributions and cryptic multi-archipelago ranges, indicating a role for prehistoric humans in their distribution. From 2012-2016, I have collected partulid tree snails targeting known type locality sites in both Papua New Guinea (PNG) and the Solomon Islands (SIs) as well Remote Oceania (Santa Cruz archipelago). Many of these species are very poorly studied but their taxonomy has been recently revised. Specimens were genotyped using Next Generation Sequencing (NGS) ddRADseq and a mitochondrial COI gene approach. The results indicated that only 5 corroborated species are represented in the region. In particular, I recovered a discrete Near Oceania clade composed of two species in PNG and the main SIs archipelago instead of the 4 nominal species currently recognized. One, Partula grisea, is restricted to littoral forest edges in PNG and the other, Partula micans, is spread throughout both PNG and the SIs. Partula micans is the oldest available name and therefore has taxonomic priority. A notable feature of Partula micans is its strikingly aberrant synanthropic ecology, a characteristic that is unprecedented for Partula and strongly implicating prehistoric human introduction as the dispersal mechanism. Currently, I lack evidence of a source population for P. micans’ multi-archipelagic distribution raising compelling questions regarding its ecology and evolutionary life history. Finally, I summarized the main attributes enabling partulids to survive under anthropogenic impacts and highlighted prerequisites to developing a rational conservation strategy for the entire family.