Bird Diversity, Functions and Services across Indonesian Land-use Systems

Abstract

In tropical regions, rampant land-use conversion for agriculture threatens biodiversity hotspots. Forests are vanishing, and taxonomic diversity is severely affected by the transformation on a large scale. For mitigating the impacts of these land-use changes and reconciling human needs with biological conservation, we require effective and standardized biodiversity monitoring methods. Along with the biodiversity decline, functional diversity is also believed to suffer great losses, affecting ecosystem functions and their resilience. This also poses the question how ecosystem services will be affected. Focusing on the province of Jambi in Sumatra, Indonesia, I present results from studies on aboveground biodiversity, concentrating on birds. I investigated four different land-use systems: lowland rainforest, jungle rubber, rubber monocultures, and oil palm monocultures. The performance of autonomous acoustic sampling of birds was compared with classical bird point counts using a systematic review and meta-analysis, as well as field data. For the first time, I present methods for measuring the sound detection space: the sampling area of acoustic recorders, an important pre-requisite of any diversity comparison. I then document the changes in diversity of birds associated with the conversion of forests to plantations, and show how bird communities and feeding guilds are affected. The functional dimension of these changes is expanded next, when we analyse the relationship between species richness and functional diversity, and subsequently analyse the change in ecological function of birds, arboreal ants, and leaf-litter invertebrates using single-trait and multi-trait indices. Finally I demonstrate the role of birds and bats, as well as ants, on ecosystem functions and yield in a long-term and large-scale full-factorial exclusion experiment situated in young oil palm plantations. My results show that autonomous sound recordings systems are at least equal to traditional avian survey methods, generating datasets of indistinguishable completeness. However, acoustic sampling is superior to bird point counts in many other ways, not the least a lack of observer bias and guaranteed verifiability of results. Importantly, I show that the sound detection space of sound recorders is measurable and has a strong impact on any measure of diversity obtained from acoustic data. Using traditional bird point counts, we also confirm previous research, presenting strong declines in bird diversity and large differences in their communities, which also impact feeding guilds differently in our two study regions. The more general study of bird, ant, and invertebrate function demonstrates that multi-trait indices can mask trends that are better assessed with single-trait indices. Nevertheless, species diversity was strongly correlated with functional diversity and their relationship indicated low functional redundancy, as well as a shift towards lower trophic levels for all taxa and towards more mobile species in birds and ants. Contrary to our expectations, these functional changes did not seem to affect ecosystem functions or oil palm yield in our exclusion experiment: we found that the bird and ant exclusion only affected arthropod predators, but left ecosystem functions and yield untouched. I recommend promoting the use of autonomous acoustic sampling methods for their many advantages over traditional survey methods based on human observation. These methods still need to be developed and standardized, and I provide first elements of response by deriving sound detection spaces of acoustic recorders. However overall, acoustic sampling bears strong potential and could become a method of choice for assessing multiple animal taxa with one tool. In the discussion I further show that results from acoustic data are equivalent to those obtained from point counts, confirming that the taxonomic biodiversity losses we observed are robust and real. We can assert that land-use conversion detrimentally affects frugivores, possibly also insectivores, and positively affects omnivores. Also, the low functional redundancy we observed across all taxa predicts dire consequences of biodiversity loss for ecosystem functioning and resilience. Although we could not demonstrate a positive role of birds and ants on oil palm, we believe the observed trends to be specific to the region, which still experiences very low pest damage. However, we need to link animals to their function more directly with individual-based dietary data, and straightforward measurements of birds’ ecosystem functions. We also call for examining animal movement more thoroughly to reveal the true picture of the functional transformation associated with land-use change.