Multi-scale effects of nutrition on an arboreal folivore

Abstract

Habitat loss is a leading cause of decline in animal populations and identifying suitable habitats are essential for the conservation and management of wildlife. Our ability to identify suitable habitats is reliant on our understanding of the factors that influence the expansion, persistence and loss of animal populations. Nutrition underpins animal growth and reproductive success and is therefore a key factor in animal population dynamics. Both nutrients and anti-feedant secondary metabolites affect the feeding behaviour of folivores such as the koala at local scales. How this translates to whole landscapes remains unknown in any system. I suspect that the availability of plant nutrients and secondary metabolites varies across large Australian forest landscapes, which would explain why there are islands of suitable habitat in a sea of uninhabitable forest. Our understanding of how nutrition regulates herbivore population density is limited by our ability to collect plant chemistry data across landscapes. This typically involves the collection and analysis of hundreds and thousands of plant samples. Two global near infrared calibration models to predict forage quality across large landscapes in an extensive dataset are introduced in Chapter one. Two biologically-relevant nutritional traits were predicted, available nitrogen (NA) and formylated phloroglucinol compounds (FPCs). I discuss techniques for developing robust predictive models, facilitating the integration of forage quality into landscape ecology. In Chapter two, I examined the role of nutrition in explaining the striking differences in koala population densities across its extensive range of habitats. I travelled across the wide distribution of the koala to collect the largest collection of eucalypt leaves for forage quality analysis. I found that forage quality explained variation in transcontinental patterns in herbivore population densities. There was a positive association of nitrogen (as proxies for protein) traits and a negative association of FPCs with koala density. Further, the effect of nutrition remained significant even when other environmental/landscape/climatic variables were accounted for in the model. To my knowledge, this was the first study to show how the effects of nutrition on animal populations can scale up to large landscapes. In Chapter three, I examined changes in forage quality to describe the cascading negative effects of disturbance on a vulnerable animal population. Following intensive logging and/or wildfires, Eucalyptus sieberi dominate the landscape and replace what was once a heterogeneous forest. I combined two large datasets, an large field survey of koala activity, and an extensive forage quality data set. I found that koalas were highly unlikely to be found in these E. sieberi dominated (i.e. intensely disturbed) forests. Further, the nutritional quality of forage quality of E. sieberi was the poorest of the eucalypt species sampled in the area, with very low concentrations of NA, and extremely high concentrations of unsubstituted B-ring flavanones (an herbivore-deterrent PSM specific to Monocalyptus species). I then demonstrated the process by which disturbance alters the suitability of habitat through changes in forage quality. I simulated the change in tree species composition, from heterogenous forest to E. sieberi and showed that forests dominated by E. sieberi are nutritionally-poor and unlikely to be suitable habitat for the koala. In Chapter four, I described how variation in plant chemistry can inspire baffling bark-eating behaviour in a “fussy leaf eater”. Koalas in the Monaro region of New South Wales, Australia eat bark from select individuals of a single species, E. mannifera. I revealed that the bark from these chewed trees were significantly higher in sodium and proposed that this remarkable feeding strategy has aided the persistence of a folivore in an otherwise mineral-poor environment. Further, I highlighted that the availability of sodium decreases increasing elevation above sea level and discussed the implications of these findings for the future conservation and management of this iconic animal. In this thesis, I explore the multi-scale effects of forage quality on an iconic, yet vulnerable specialist folivore, the koala. Each study contributes to our understanding of animal population requirements and we can use this information for effective conservation planning of wildlife populations