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. Fruitremoval 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.