Being one of the world’s biodiversity hotspots, Southeast Asia’s rainforest is home to an extremely high density of species. However, the region has lost a high proportion of its rainforest as a result of logging and conversion to agriculture since the early 1970s, causing declines in species diversity across wide-ranging taxa. Studies have found that this loss has been driven by changes in microclimatic conditions, resources (for feeding, breeding, protection against predators, and refuge during extreme weather events), and connectivity. Invertebrates, in particular, have been found to decline in richness, abundance, and biomass with land-use change (although logging seems to have much less of an impact than converting forest to agriculture), causing concerns over the various functions they support in the ecosystem, including nutrient recycling, pollination, and biological control.

Despite an increasing number of studies, the effects of habitat change and alternative management options on many invertebrate taxa remain unknown, making studies assessing them a priority for informing targeted and successful conservation efforts. In this thesis, I investigated the impacts of rainforest logging and conversion to oil palm on semi-aquatic bugs (Gerromorpha, Hemiptera), representing aquatic communities, as well as management decisions by oil palm smallholders within existing plantations on butterflies (Rhopalocera, Lepidoptera), representing terrestrial communities. Studies took place in Sabah, Malaysian Borneo for semi-aquatic bugs (Chapters 2 – 4) and Selangor, Peninsular Malaysia for butterflies (Chapters 5 & 6). In Selangor, I also studied the effects of environmental conditions at a smaller scale, particularly how habitat structure and complexity within smallholder plantations affected the resource-use behaviour of butterflies.

In Chapter 2, I developed length-biomass equations which can be used to predict the biomass of semi-aquatic bugs from their body lengths. Biomass can be a good indicator of ecosystem function but obtaining these data can be costly and difficult. I found that power regression equations gave the most accurate estimations of biomass across life stages, particularly when taking into account the body forms of semi-aquatic bugs. In Chapter 3, I investigated the impacts of forest conversion for logging and oil palm on semi-aquatic bug communities. Despite being sensitive to anthropogenic disturbance and ecologically important (they are predators of invertebrates and prey for some invertebrates and vertebrates), semi-aquatic bugs are little studied in the region. The abundance and species richness were lower following forest conversion, whilst total biomass was not affected, potentially indicating the robustness of prey availability for predators of semi-aquatic bugs. In Chapter 4, I assessed the effects of within-stream physical structure and maintaining forested margins around oil palm streams on semi-aquatic bugs. I found that, at the small-scale (along 10-meter transect), there was a significantly higher abundance of semi-aquatic bugs in oil palm streams with forested margins than those without, as well as significantly different community composition. However, species richness and total biomass remained unaffected.

In Chapter 5, I investigated the effects of smallholder management decisions, in terms of replanting and crop choices after replanting (monoculture vs polyculture), on butterfly assemblages. I found that smallholders managed their plantations in widely different ways, resulting in differing habitat structure and complexity across plantations, but that broad management decisions (immature monoculture, immature polyculture, and mature monoculture plantations) did not significantly impact the density or species richness of butterflies. Despite this, finer scale differences, such as more understory vegetation, including nectar sources for adult butterflies, as well as polyculture farming, increased the density of butterflies. In Chapter 6, I studied the impacts of habitat structure and complexity, associated with management decisions, on the resource-use behaviour of butterflies. Although data were limited, I found that the novel methods developed for this chapter are promising and can provide detailed information at a small scale, which could be applied in other habitat types.

In conclusion, this thesis found that semi-aquatic bugs are sensitive to rainforest logging and conversion to oil palm. I also demonstrated that conservation management around streams (by maintaining forested margins) and within plantations (by maintaining understory vegetation including hostplants and nectar sources, as well as polyculture farming) can increase the abundance of semi-aquatic bugs and butterflies, respectively. I also demonstrated that agricultural habitats do not support forest-dependent species, at least within the two taxa studied here. Therefore, in addition to the evident negative impacts on many other taxa that existing studies have found, this confirms that preventing further forest conversion remains a priority for biodiversity conservation. Nevertheless, conservation management options I have identified open up opportunities to better support persisting species within altered habitats, particularly oil palm systems, potentially increasing biodiversity and associated ecosystem processes across the wider landscape.