Palm oil plantations require replanting every twenty to thirty years but its effects on biodiversity are widely understudied. In their latest research, Pashkevich and colleagues assess the impact of replanting on arthropod communities in an industrial plantation.

Biodiversity in oil palm plantations

Oil palm plantations are often in areas that were once tropical rainforests, and this conversion has resulted in widespread declines in biodiversity and the disruption of ecosystem processes. It is indisputably important to protect rainforests from further expansion of oil palm plantations, but we also need to understand how management of existing plantations affects biodiversity within these landscapes.

This is in part because oil palm plantations occupy vast areas of land across the tropics, and are therefore an important land use in their own right. Additionally, it has previously been shown that mature oil palm plantations (i.e., plantations that are at least 6 – 10 years old) can support a wide range of generalist species. Many of these species facilitate important ecosystem functions, such as pollination and pest control, and therefore it is vital that we understand how they are maintained.

Replanting of oil palm threatens biodiversity

Biodiversity within oil palm plantations is potentially threatened by replanting. Oil palms are replanted after about 20 – 30 years of growth but the process is highly destructive and drastically changes environmental conditions; it involves using large diggers which are used to obliterate the aboveground vegetation.

Many plantations across Southeast Asia (where the majority of palm oil is produced) have already been, or will soon be, replanted. Despite this, very little is known about whether replanting affects biodiversity and ecosystem functions within oil palm plantations..

To better understand how replanting of oil palm could affect biodiversity, we studied a chronosequence of oil palm in plantations in Sumatra, Indonesia.

The chronosequence spanned a replanting event, and included sites of mature first-generation oil palms (i.e., palms that replaced rainforest) and replanted second-generation oil palms aged 1, 3, and 8 years. Replanting of sites was done using recommended strategies, which represent how replanting will likely occur, or has already occurred, across Southeast Asia.

We focussed our biodiversity surveys on arthropods, which provide important ecosystem services such as pest control and pollination, and sampled the ground, understory and canopy microhabitats. We assessed changes in total arthropod abundance and order-level community composition, as well as specific changes in spider communities

Surprisingly, we found no differences in total arthropod abundance across the chronosequence, indicating no short- or long-term impacts of replanting in any microhabitat. But we did find that replanting caused long-term changes in the composition of arthropod communities in all microhabitats, owing to changes in abundance of particular arthropod orders..

In the first three years after replanting, the number of beetles (Coleoptera) on the ground decreased; the number of beetles, true bugs (Hemiptera), and booklice (Psocoptera) in the understory increased; and the number of earwigs (Dermaptera) and butterflies and moths (Lepidoptera) in the canopy decreased.

Additionally, the abundance and species richness of spiders decreased in some microhabitats, and replanting changed the composition of spider communities at the species-level.

Implications for long-term management of oil palm plantations

Our findings show that replanting of oil palm plantations using recommended management strategies has little impact on total arthropod abundance. This is a somewhat encouraging finding from a management perspective, since arthropods facilitate many important ecosystem functions..

However, the changes we observed in arthropod order-level composition and spider biodiversity could impact ecosystem processes in second-generation oil palm plantations; for instance, fewer spiders could lead to reduced pest control since they are predators of many insect pests.

Overall, our study demonstrates how replanting of oil palm, using current practices, might affect biodiversity. We now need to understand how individual replanting strategies support biodiversity and ecosystem processes during replanting events.

Encouragingly, such research is already taking place (e.g. the Riparian Ecosystem Restoration in Tropical Agriculture (RERTA) Project), and others like this will provide vital insight into the ecological impacts of replanting on existing oil palm plantations.

Read the full article, Assessing the effects of oil palm replanting on arthropod biodiversity, in Journal of Applied Ecology.