Rodent pest management and predator communities in oil palm plantations in Indonesia: a comparison of two contrasting systems

Rapid expansion of oil palm cultivation in Southeast Asia raises concerns about biodiversity conservation. Moreover, rats are invasive pests in oil palm plantations, often causing substantial damage. In Indonesia, rat control is generally based on field treatment using anticoagulant rodenticides and/or on reinforcement of predation by barn owls (Tyto alba), by providing nest boxes within the plantation. Rodenticide use is costly for the producer and can indirectly poison non-target species such as rat predators. Thus, biological control of rodent pests should be promoted, both from a conservation and a production points of view. Within the assemblage of rat predators, small carnivores may contribute to rodent population regulation. However persistence of small carnivores within oil palm plantations, their habitat use, their diet and their contribution to rodent control have been poorly investigated. We conducted a 3-year comparative study (2010-2012) in well-established oil palm plantations in Riau and Bangka provinces, in Indonesia: in both areas barn owls have been successfully introduced, but in Riau rat populations have been maintained at an acceptable level without the use of rodenticide for more than 10 years, whereas in Bangka intensive rodenticide applications did not prevent high levels of rat damage. We compared these two contrasted systems in term of predators community (barn owls and small carnivores) abundance and/or diet. Using a kilometric abundance index yielded from spotlight and faeces counts, we found that small carnivores were much more abundant in Riau plantations than in Bangka, and that the leopard cat (Prionailurus bengalensis) was the dominant species in Riau and absent from Bangka. We investigated the diet at community level and found no significant differences in frequency of occurrence or volume of small mammal’s food items in the faeces of small carnivores between Bangka and Riau; however, we found that the importance of vegetal food items in the diet of small carnivores was greater in Bangka than in Riau, thereby reflecting differences in predator community composition between both areas. Moreover, analyzing barn owls pellets content and number of eggs laid in nest boxes, we found that 1) the proportion of rats in barn owls diet was slightly less in Bangka than in Riau and prey taken as food were more diverse in Bangka, 2) breeding season was limited to one peak in Bangka comparatively to two peaks in Riau, thereby probably leading to a lowest food requirement in Bangka. Broadly, our results suggested that rat prey intake from the barn owl population and the small carnivore community would be less in Bangka plantations than in Riau. To further investigate barn owl selective predation on rat populations, we developed a model to assess the relative age of Rattus tiomanicus (the main barn owl prey in Riau plantations) from barn owl pellet macroremains. We also investigated spatial distribution of small carnivores within the oil palm habitat. We found no attractive effect of forest habitat or oil palm edge for the leopard cat and the common palm civet (Paradoxurus hermaphroditus), which may be encountered deep within the oil palm habitat, whereas the Malay civet (Vivera tangalunga) was always observed in the edge of the oil palm habitat. At the community level, our analysis of faeces spatial distribution showed an attractive effect of forest and oil palm edge habitats. These results support the hypothesis that, although the oil palm habitat may be habitable for some wild small carnivores species such as the leopard cat, where they supposedly forage at night, most species still need forest habitat for their survival in oil palm landscapes. Prey-predators relationship in agricultural landscape is a complex issue. Broadly, our results suggest that barn owls cannot regulate rodent population on their own, and that small carnivores probably play an important role, in the framework of a multi-factor hypothesis. To enhance small carnivores within oil palm plantations, the producer should adapt agricultural practices (e.g. rodenticide use and understorey vegetation management) and favor appropriate land-use such as retaining forest fragments within and surrounding the plantation.