The effect of the non-native superb lyrebird (Menura novaehollandiae) on Tasmanian forest ecosystems

Some 900 species of bird have been introduced throughout the world but the research effort regarding their ecological impact as non-native species has been minimal and largely based on ad hoc observations. In particular, the impact of non-native birds on non-avian components of native biota and ecosystem function are poorly understood. I addressed this knowledge gap by investigating the effect of the non-native superb lyrebird (Menura novaehollandiae) on native soil- and leaf litter-dwelling forest invertebrates, seedling survival and ecosystem processes within the wet eucalypt forests of Tasmania, Australia. The superb lyrebird is a predator of invertebrates and is an ecosystem engineer capable of turning over hundreds of tonnes of soil and leaf litter per hectare every year. The absence of any native equivalent-sized predator of invertebrates or native species capable of such large-scale habitat modification within Tasmanian wet forest means that the superb lyrebird may have a significant negative effect on Tasmanian forest ecosystems. I used a multifactorial approach consisting of field surveys and manipulative experiments to examine the impact of the superb lyrebirds at a number of spatial and temporal scales. Firstly, I surveyed six sites, three invaded by lyrebirds and three without lyrebirds to investigate the patterns of association between macroinvertebrate assemblage structure and abundance and the presence of superb lyrebirds. I found that the presence of superb lyrebirds was associated with lower abundance and taxonomic richness of invertebrates, higher evenness and altered assemblage composition but the magnitude of this relationship was strongly dependent on small-scale variation in microhabitat. To establish any causal link between the presence of lyrebirds and patterns in invertebrate assemblages and seedling numbers, I conducted two manipulative field experiments that examined the short and long term influence of superb lyrebird disturbance. The physical disturbance of soil and leaf litter immediately reduced the abundance and taxonomic richness of macroinvertebrates, those that inhabit leaf litter being more affected than generalists and soil dwelling taxa. However, the influence was short-lived: the abundance of generalist and soil dwelling taxa was similar to that of individuals in undisturbed areas within 21 days. Similarly, a longer-term experiment found no evidence of impact on invertebrates after approximately two months. Next, I used a multi-scale survey to determine how the magnitude of the effect of superb lyrebirds on invertebrate assemblages varied across different spatial scales. While their effects on invertebrates were profound at small spatial scales and short timeframes, they were weaker over longer timeframes and at intermediate and landscape scales. In general, mesoinvertebrates showed a weaker response to the presence of superb lyrebirds than did macroinvertebrates. Thus, although superb lyrebird scratching causes obvious changes to the structure of the forest floor of Tasmanian wet eucalypt forests; it appears that their disturbance is neither frequent nor intense enough to result in lasting changes in biotic communities. Finally, I tested the influence of superb lyrebirds on ecosystem function through experiments on the effect of their activity on several ecosystem processes (decomposition, nitrogen cycling and soil respiration). Superb lyrebirds increased decomposer potential but did not appear to influence soil respiration or pH. The concentration of inorganic nitrogen was lower at lyrebird sites; this may have been linked to their disturbance but the lack of any experimental treatment effects weakens the strength of this inference. Overall, it is unlikely that the presence of superb lyrebirds will significantly affect functioning of mature forest ecosystems, as they are resilient to all but extreme perturbation such as wildfire. This thesis represents an integrated and holistic examination of the ecological impact of a non-native bird. In doing so it makes a substantial contribution to global understanding by demonstrating that non-native birds can have an influence, albeit a limited one in this case, on native biota and ecosystem function.