Contrasting the effects of vegetation clearance on two insectivore communities and their prey at perennial streams in temperate Australia

Abstract

The movement of organisms and material between adjacent ecosystems is a ubiquitous process. Over the last three decades, many works have uncovered factors that influence the flux of spatial subsidies. The emergence of new ecosystems via riparian vegetation clearance, for instance, can impact the quantity and quality of the spatial subsidies that move between perennial streams and riparian zones, and this likely incurs complex responses from riparian consumers. This thesis asks two main questions: 1) how does environmental variables in both the donor (streams) and recipient (riparian zones) systems as a result of riparian vegetation clearance impact the relative quantity of active subsidies? 2) Do insectivores with different mobilities and foraging behaviours respond differently to the flux of spatial subsidies and does this interact with environmental variables? I answered these questions across four empirical chapters. This thesis focused on a biome identified as both impacted by agricultural intensification and comprising important perennially flowing freshwater (on the world’s driest inhabited continent): Australia’s temperate zone. In chapter two, I demonstrate longitudinal trends in riparian vegetation clearance at our study streams and tested models relating to spatial subsidies and riparian spider responses. I conducted vegetation surveys and monitored in-stream temperature at six perennial streams that run through a riparian vegetation clearance gradient, and related these to abundances, biomass and community composition of riparian spiders and their prey (including emergent aquatic invertebrates). In chapter three, I focused on the orb-weaving spider species, Tetragnatha valida and compared the relative contributions of low flux, high quality aquatic prey and terrestrial prey to its diet at perennial streams using stable isotope analysis. In chapter four, I investigated the role of riparian vegetation structure and the abundance and biomass of emergent aquatic prey in explaining variation in the foraging activity and community composition of insectivorous bats that occupy perennial stream habitats. Finally, in chapter five I continuously monitored the activity of insectivorous bats at a survey reach to investigate potential concordance between foraging activity, moon illumination and heat accumulation by the stream. This thesis represents ‘another string in the bow’ of spatial subsidy research that focuses on biomes and taxa that are seldom studied. The literature identifies that active subsidies, including emergent aquatic invertebrates, must be studied in the context of donor and recipient ecosystem dynamics. Despite this, few studies thoroughly measure these dynamics. The present study bucks this trend and extensively surveys relevant ecosystem characteristics including in-stream temperature and vegetation structure, and in-so-doing provides valuable context which underpins diverse riparian insectivore responses to the flux of spatial subsidies. By contrasting different modes of insectivory, this thesis provides new insight into the trophic dynamics of stream-riparian systems. Studies like these are important in a rapidly changing world.