Varley Gradwell 2010

Effects of habitat modification and fragmentation on dung beetle biodiversity and associated ecosystem functioning

Dr. Eleanor Slade, Dept of Zoology, Oxford.  

Increasingly, secondary forests, plantations and other human-modified habitats will dominate tropical landscapes and there is an urgent need to assess the capacity of these habitats to maintain biodiversity and ecosystem functioning in the long-term. Dung beetles provide an ideal focal guild for such studies as they are sensitive to habitat degradation and fragmentation and play a pivotal and easily-quantified role in tropical forests as nutrient recyclers and secondary seed dispersers.

This study aims to answer two questions: 1) How does the diversity and composition of dung beetle communities differ across habitats that have been modified to different degrees (primary forests, secondary forest of different fragment sizes, and oil palm plantations)? 2) How do rates of dung processing differ across these habitats and how do they relate to any observed biodiversity changes?

Study site and Methods

The study was conducted in Sabah, Malaysian Borneo where there is a gradient of forest types around Maliau Basin that are being intensively studied as part of the Stability of Altered Forest Ecosystems (SAFE) project (http://www.safeproject.net/). In July 2011 the secondary forest landscape will be experimentally fragmented by oil palm plantations, generating one of the world’s largest ecological experiments to quantify changes in ecological processes across a gradient of habitat modification and fragment The SAFE project has a quasi-experimental design with large-scale spatial replication of sites in each of three forest types: oil palm (OP), logged forest (LF) and old growth forest (OG), as well as the experimental fragmentation plots. The fragmentation of the forest by oil palm follows a split-plot design with six experimental blocks (A-F), each containing four plots: (1) 1 x 100 ha fragment, (2) 2 x 10 ha fragments, (3) 4 x 1 ha fragments; and (4) forest that will be converted to oil palm. Across all blocks, there will be a total of 42 fragments. Within the blocks plots have been marked out following a fractal design, which allows data to be studied across different spatial scales. The basic design consists of sets of three sampling points arranged in a triangle.

Fieldwork was carried out during February-March 2011 when dung beetle biomass, diversity and function peaks. I sampled the dung beetle community and the associated ecosystem function of dung removal at the second order points, a distance of 178 m between points, as dung beetles are relatively mobile and so this ensures independence between samples. Therefore, 27 points were sampled in each of the forest types (OG, LF, OP) and 16 points in each of the fragment blocks (A-F)

Results: Dung beetle diversity and composition

At each second order point a standard dung-baited pitfall trap was placed. Traps were baited with 25g human dung and beetles collected after 48 hours (Figure 4). The beetles were stored in 75% alcohol in a freezer and are now in the Oxford University Museum of Natural History (OUMNH) awaiting identification. All the necessary permits to carry out the research within Sabah and the SAFE/Maliau sites and export and import permits were obtained. I visited Dr Arthur Chung, my local collaborator, and a reference collection will be deposited in the FRC, Sandakan and OUMNH at the end of the project.

Large numbers of beetles were collected across all sites and in total nearly 10kg of beetles have been brought back for identification. Identification of the samples will begin this month and should be completed by December 2011. The dung beetle community diversity, biomass and composition will then be compared between the sites to assess changes across the land use gradient from old growth forest through secondary forest to oil palm. The data from the fragment plots also serves as the crucial baseline data before fragmentation begins, allowing a comparison of pre-fragmentation communities across a connected gradient of primary forest, logged forest and oil palm plantation and in the future will provide the baseline to map post-fragmentation community disassembly and associated ecosystem functioning changes over time across the different fragment sizes and isolations.

Results: Dung removal

Rates of dung removal were measured by monitoring standard-volume dung piles placed in the field. Fresh cattle dung was collected from a local oil palm plantation, homogenised and frozen for 24 hours to kill any invertebrates. Piles were placed at each of the second order points, with a minimum time between the pitfall trapping and dung removal experiment of two weeks. the dung removal experiment. 700g of dung was placed in the field with a plate as a rain-shield The dung was collected after 24 hours and re-weighed. Three moisture control piles of dung, from which all beetles were excluded using a fine mesh, were placed in each of the sites. This allowed the mass loss due to evaporation or mass gain due to excess rain to be accounted for.This prevented interference between the two components of the study, but was close enough in time that the dung beetle assemblages caught during trapping would be similar to those present during the dung removal experiment. 700g of dung was placed in the field with a plate as a rain-shield. The dung was collected after 24 hours and re-weighed. Three moisture control piles of dung, from which all beetles were excluded using a fine mesh, were placed in each of the sites. This allowed the mass loss due to evaporation or mass gain due to excess rain to be accounted for.

The preliminary results suggest that old growth forest has the highest rates of dung removal, and oil palm the lowest. However, there is a high degree of variability between both OG and OP sites. In the case of the OG forest this may be due to local differencesin soil types. Interestingly, OP3, which has the highest rate of dung removal is an older oil palm site, established in 2000 (OP1 and OP2 were established in 2006), and as such it has a much higher and denser canopy. In the logged forest blocks that are to be fragmented (A-F) there is a lot of variability in dung removal; block A has as much dung removal as old growth forest, while blocks D-F have similar dung removal to oil palm.

Once the dung beetle samples have been sorted and identified I will relate the dung beetle diversity, biomass and composition in each of the sites to the dung removal. Vegetation, soil and micro-habitat data exists for each of the second order points and this will also be examined to see if these may account for differences between sites in dung beetle diversity and composition and amount of dung removal.

Conclusions so far

The study has collected crucial baseline data on dung beetle community composition and associated ecosystem processes before fragmentation begins, allowing a comparison of pre-fragmentation communities across a connected gradient of primary forest, logged forest and oil palm plantation, and in the future will provide the baseline to map post-fragmentation community changes. The chance to collect ‘before and after data’ is rare and there are few studies which have been able to do this, and none from South-East Asia, thus this project provides a unique and exceptional opportunity to enhance our understanding of the consequences of habitat modification and fragmentation and the implications for forest management and conservation. While the results presented so far are very preliminary they suggest that even logged forest, in which most of the large dipterocarp trees have been removed, can maintain a high degree of ecosystem functioning, in some cases as high as in old growth forest. Moreover, although oil palm clearly has lower ecosystem functioning than old growth forest, these results suggest than some functioning is retained and that 10 year old oil palm, which has a higher canopy layer and superficially resembles a forest, can maintain levels of functioning similar to that of some logged forest plots.