ric phase is highly unlikely. Here we provide clear support for sympatric speciation in a case study of two species of palm (Arecaceae) on an oceanic island. A large dated phylogenetic tree shows that the two species of Howea, endemic to the remote Lord Howe Island, are sister taxa and diverged from each other well after the island was formed 6.9 million years ago. During fieldwork, we found a substantial disjunction in flowering time that is correlated with soil preference. In addition, a genome scan indicates that few genetic loci are more divergent between the two species than expected under neutrality, a finding consistent with models of sympatric speciation involving disruptive/divergent selection. This case study of sympatric speciation in plants provides an opportunity for refining theoretical models on the origin of species, and new impetus for exploring putative plant and animal examples on oceanic islands.
Disruption of recruitment in two endemic palms on Lord Howe Island by invasive rats
Invasive species may have negative impacts on many narrow range endemics and species restricted to oceanic islands. Predicting recent impacts of invasive species on long-lived trees is, difficult because the presence of adult plants may,mask population changes. We examined the impact of introduced black rats (Rattus rattus) on two palm species restricted to cloud forests and endemic to Lord Howe Island, a small oceanic island in the southern Pacific. We combined estimates of the standing size distribution of these palms with the proximal impacts of rats on fruit survival in areas baited to control rats and in unbaited areas. The size distribution of palms with trunks was comparable across baited and unbaited sites. Small juvenile palms lacking a trunk (<50 cm tall) were abundant in baited areas, but rare in unbaited sites for Lepidorrhachis mooreana, and rare or absent in 3 out of 4 unbaited Hedyscepe canterburyana sites. All ripe fruits were lost to rats in the small fruited L. mooreana. Fruit removal was widespread but less (20–54%) in H. canterburyana. Both palms showed evidence of a reduced capacity to maintain a juvenile bank of palms through regular recruitment as a consequence of over 90 years of rat impact. This will limit the ability of these species to take advantage of episodic canopy gaps. Baiting for rat control reduced fruit losses and resulted in the re-establishment of a juvenile palm bank. Conservation of both endemic palms necessitates control (or eradication) of rat populations on the unique cloud forest summits of the island.
Floristics and structure of the mossy cloud forest of Mt Gower summit, Lord Howe Island
The summit of Mt Gower, Lord Howe Island (31°33’8, 159°05’E), is a small area of 27 ha supporting mossy cloud forest. This study describes patterns in the floristic composition and structure of the vegetation of the summit, in relation to a range of environmental variables. A total of 42 vascular plant species was recorded, 86% of which are endemic to Lord Howe Island, and 17% of which are found only on the summit and upper slopes of Mt Gower and adjacent Mt Lidgbird. A complete species list for Mt Gower is presented, including species from the present survey and all previously recorded species. The composition of the vegetation differed In gullies and on ridges, reflecting differences in substrate rockiness and soil moisture, and was also influenced by the number of bird burrows and aspect. The summit of Mt Gower is of high conservation significance, due to the restricted distribution of the habitat type it represents, its relatively undisturbed state and high levels of endemism. Potential threats to the summit vegetation include the impacts of climate change, seed predation by introduced rats, and the introduction of weed species and exotic pathogens such as Phytophthora cinnamomi.