Tropical agroecosystem functions in a changing environment

In many tropical regions, forests have been extensively converted to various land uses, notably through the expansion of agriculture. Both biodiversity and associated ecosystem functions and services have been eroded in these human-modified tropical landscapes. My research investigates the effects of land-use change on invertebrate communities and ecosystem functioning from both fundamental and applied ecological perspectives, as well as the risks posed by global environmental change, mainly agriculture and climate change in tropical ecosystems. I focus on a human-modified tropical forest landscape associated with Panua Nature Reserve in Sulawesi, Indonesia. Despite its protected status, Panua Nature Reserve has seen extensive human disturbance, resulting in a mixture of forest, shrubland, and maize fields within the reserve. My study area is a tropical landscape shaped by small-scale farming. Small-holder farmers constitute the majority of tropical farmers and are severely marginalised.
I used a field experiment to evaluate the effects of habitat modification on decomposition, and explored how decomposition varied with substrate type and composition of the decomposer community (Chapter 2). By exposing leaf litters within litterbags and toilet rolls in the above-mentioned habitat types, I found that litter and cellulose decomposition differed across habitat types with highest decomposition was in the forest sites. The results suggest that even small-scale agriculture can reduce the decomposition of organic materials.
Using plasticine caterpillars (Chapter 3), I investigated predation by potential predator groups to determine the effect of habitat modifications caused by maize farms on predation, and to test whether predation of herbivorous insects is forest-dependent. Predation was significantly reduced in the most modified habitat, maize fields. In addition, predation of caterpillars is forest dependent, indicated by spillover from forest habitats to the maize farms. Similar to the results of the Chapter 2, the results of this chapter suggest that small-scale agriculture alters predation of herbivorous insects, and that predation of caterpillars in maize farms relies on the predator from nearby natural habitats.
In Chapter 4, I report a field mesocosm experiment to evaluate ecosystem functioning of the litter invertebrate community in response to simulated environmental change by manipulating (1) community composition (millipedes and other litter invertebrates), (2) soil moisture levels (dry, ambient, and wet) to represent extreme precipitation conditions, and (3) two distinct land uses (shaded and open areas representing the forest and agricultural land, respectively). The results highlight the importance of millipedes among invertebrate decomposers in maintaining decomposition in a changing soil environment.
Lastly (Chapter 5), I determined forest fragments' functional contribution to maize farmland by quantifying predation of caterpillars and cellulose degradation over six transects across forest and maize farmland. I found that ecosystem functions differed in the extent to which forested areas contributed to functions in adjacent farming. In this instance, predation of caterpillars was influenced by proximity to forest, but decomposition was not. This suggests that ecosystem functions have different dependencies on natural habitats.
In general, this thesis reveals that degradation of Sulawesi's forests causes a decline in ecosystem function. In the forest habitat, decomposition rates, insect predation rates, and pest control services were all considerably higher than in degraded habitats. Millipedes (Diplopoda) were found to be a noticeable and numerically dominant component of the macroinvertebrate community and have a major positive effect on nutrient recycling. To better understand the contribution of this under-studied taxon to ecosystem functionality in a changing environment, more research is needed to investigate how invertebrate communities in the agroecosystem landscape affect decomposition and soil nutrient input in different habitats and under altered climatic conditions.
This thesis findings suggest some practical implications in conservation. Despite often perceived negatively by public, invertebrates need to be conserved because of their important roles in the ecosystems. Discovering invertebrate biodiversity and understanding their benefits for the farmers are some of the keys in supporting invertebrate conservation. In the landscape level, sustainable agriculture practices, such as forest strip establishment and soil mulching in agricultural land, could potentially conserve invertebrates while maintaining agriculture production.

Advisor: Lewis, OT