The impacts of rats on the endangered native flora of French Polynesia (Pacific Islands): drivers of plant extinction or coup de grâce species?

Abstract

Although rats have clearly contributed to bird extinctions on islands, their role in plant extinctions is not as clear. Paleoenvironmental studies suggest rats were responsible for the demise of several island palm species. French Polynesia's islands provide an opportunity to evaluate ''modern'' impacts of rats on native flora. Our study shows that 15 threatened taxa (nine families) are damaged by rats. All 12 subjected to seed predation are woody plants with large-seeded drupes. Three experience severe predation and recruitment depression (Santalum insulare, Ochrosia tahitensis, Nesoluma nadeaudii).Three-year monitoring of Polynesian sandalwood (Santalum insulare) populations in Tahiti during rat control suggested that over 99% of fruits were eaten before ripening. Seed predation on sandalwood appeared to be lower on islands without black rats Rattus rattus. Studies from Indo-Pacific islands document rat impact on at least 56 taxa (28 families). Certain families (Arecaceae, Elaeocarpaceae, Rubiaceae, Santalaceae, and Sapotaceae) are particularly vulnerable to seed predation. Other soft-barked trees (Araliaceae, Euphorbiaceae, and Malvaceae) suffer from stem or bark damages, especially during dry seasons. Although rats depress seedling recruitment and alter vegetation dynamics, no evidence demonstrates that they are solely responsible for current plant extinctions. Most of French Polynesia's endangered species impacted by rats occur in severely degraded habitats. We therefore suggest that rats can be viewed more as coup de graˆce species (i.e., that give the final stroke of death), rather than as main drivers of plant extinctions. More research is needed to clarify the impacts of rat species and their importance in plant population decline or demise.

Disruption of recruitment in two endemic palms on Lord Howe Island by invasive rats

“…Past observations highlight the lack of ripe fruits on Lepidorrhachis plants unless mesh caging was applied to exclude rats from developing fruits (Moore 1966;Pickard 1980). The paucity of small juvenile plants in areas which have not been baited for rat control, both in our study plots and more generally throughout this unbaited area, supports the more general contention that the introduction of rats affects plant recruitment in a range of island ecosystems (Campbell and Atkinson 2002;Delgado García 2002;Towns et al 2006;Meyer and Butaud 2009;Traveset et al 2009). It is possible that rats also reduce seedling and juvenile growth as they have been observed eating palm leaf sheaths and causing leaf fall in both juvenile and adult palms consistent with similar damage observed on plants elsewhere (Traveset et al 2009).…”

Biology and Impacts of Pacific Island Invasive Species. 11.Rattus rattus,the Black Rat (Rodentia: Muridae)

“…Yamashita et al (2003) suggested that R. rattus may be facilitating the invasion of the nonnative tree Bischofia javanica in the Ogasawara Islands because R. rattus depredates the seeds of the dominant native tree Elaeocarpus photiniae-folius both before dispersal (27% -33% of the seed crop) and after dispersal (41% -100%). Several other studies of fruit and seed removal have occurred on Pacific islands that include R. rattus as part of the rodent community, but the particular rodent species responsible for such removals were not identified (e.g., Moles and Drake 1999, Uowolo and Denslow 2008, Meyer and Butaud 2009, Erwin and Young 2010, GrantHoffman et al 2010, Chimera and Drake 2011.…”

The slow demise of Easter Island: insights from a modeling investigation

“…All three simulated scenarios are broadly consistent with the major, undisputed paleoecological evidence: (i) the human population was decimated by the diseases introduced by Europeans after 1722 AD (Diamond, 2005;Hunt, 2007) and around 1850 AD the size of the population was between 1500 and 3000 individuals (Flenley and Bahn, 2003); (i) the rat population was large and peaked right after the arrival of the Polynesian settlers when palm seeds were very abundant and rats had a negative, although non-quantifiable, effect on tree growth (Steadman et al, 1994;Diamond, 2005;Hunt, 2007;Meyer and Butaud, 2008;Mieth and Bork, 2010), (iii) people substantially reduced the number of palm trees on the island and intensified agriculture over time (Flenley et al, 1991;Stevenson et al, 2006;Mann et al, 2008;Mieth and Bork, 2010;Ladefoged et al, 2013). Despite being all compatible with the above points, the three simulated scenarios reveal clear differences from one another in terms of human, rat, and palm tree population dynamics and, consequently, also in deforestation patterns.…”