Distribution and structure of arthropod communities in relation to resource patches and spatial scale in dryland woodland ecosystems

Abstract

In dryland ecosystems, resources such as water, nutrients and habitat are concentrated into discrete patches. This resource concentration occurs at fine (e.g. around trees, grasses or logs) and broad (e.g. habitat remnants within an agricultural matrix) scales. Arthropods, which include insects, spiders, and a range of other invertebrates, provide a range of critical ecosystem functions in drylands. Arthropods may be particularly sensitive to changes in resource concentration given their small size and habitat requirements. Limited research, however, has examined how arthropods respond to changes in resource concentration across different spatial scales. This thesis examines how the concentration of resources affects the distribution and structure of arthropod communities at multiple spatial scales in south-eastern Australia. Chapter 1 provides an overview of resource patchiness in arid and semi-arid ecosystems, and describes how it is known to affect the biota. Chapters 2 to 4 investigate how the fine-scale distribution of resources (plants, and plant-associated patches) affects the distribution and composition of arthropod communities at local (plant-plant) scales. Specifically, chapter 2 examines how plant species and density affect the plant-resident arthropod fauna in a semi-encroached shrubland, and demonstrates that plant species is the overwhelming driver of arthropod diversity in these communities. Chapters 3 and 4 examine how the multilayered resource patch created by a dominant tree (mallee, Eucalyptus spp.) structures arthropod communities in mallee woodlands. At fine-scales, the canopy patch beneath mallee trees supports a distinct suite of arthropods compared to inter-tree areas. This is influenced by fire, which has taxon-specific effects. Chapters 5 and 6 investigate how the concentration and health of remnant patches at landscape scales affects arthropods. Chapter 5 evaluates the use of common landscape health indices as indicators of arthropod (spider, ant, and beetle) biodiversity, illustrating that these indices show only weak or inconsistent relationships with arthropod biodiversity. Chapter 6 uses a multi-scale approach to investigate the drivers of ant community structure in a fragmented grassy box woodland. It indicates that finer-scale characteristics (particularly tree canopy cover and soil texture) drive ant communities within these landscapes. Chapter 7 summarises the findings and implications of the thesis, and proposes avenues for future research.