Influence of leaf chemistry on dietary choice and habitat quality of koala (Phascolarctos cinereus) populations in southwest Queensland

Abstract

Protecting high quality habitat is an important wildlife conservation action. Spatial and temporal variation in habitat quality in heterogeneous landscapes influences habitat use and population persistence. Populations living at the margins of species’ geographic ranges are particularly sensitive to fluctuations in habitat quality, especially if species occupy narrow ecological niches. For arboreal folivores, foliar chemical composition is a key factor influencing habitat quality. To understand the spatial and temporal dynamics of foliar chemistry and hence the habitat quality for an arboreal folivore species, I applied theories and methods from chemical ecology, nutritional ecology and landscape ecology to understand foliar chemical/folivore interactions in a seasonally changing environment.I used populations of koalas (Phascolarctos cinereus) in two semi-arid regions of Queensland, Australia, as a case study. Koalas are specialist folivores with complex feeding behaviour from Eucalyptus species. My aim was to identify the influence of foliar chemicals (moisture content, digestible nitrogen (DigN) and a toxin formylated phloroglucinol compounds (FPC) concentrations) and associated environmental factors on koala habitat use and diet across three rainfall seasons. I addressed three specific questions: 1) How do tree characteristics and environmental factors influence on spatial and temporal variation in leaf chemistry composition of koala food tree species? 2) What are relative influences of leaf chemistry, tree characteristics and environmental factors on koala habitat use and diet? and 3) Can we use the WorldView-3 satellite imagery to accurately map foliar nutrition at high resolution in koala habitats in semi-arid regions?The research was conducted in the Mulga Lands and Brigalow Belt South bioregions of southwest Queensland. Eucalyptus leaf chemicals, koala habitat use and diet were examined. A hierarchical sampling design was applied to select 34 sites (6-10 trees in each) from ten landscapes (each 10×10 km) across the 62,500 km2 area. Leaf samples were collected from 261 trees and repeated over three seasons with contrasting rainfall. Leaf moisture content, DigN and FPC concentrations were analysed in the laboratory. Koala habitat use was indicated by the presence of fresh faecal pellets at the tree scale. Koala diet composition was assessed by histological analysis of leaf cuticles from fresh faecal pellets collected from sites and the adjacent transects along creeks. Generalised mixed effects modelling was applied to analyse the influence of tree characteristics and environmental factors on foliar chemistry and the presence/absence of koala pellets. The relationship between foliar chemical composition and temporal variation in koala diet composition was also investigated. Two satellite images captured by WorldView-3 were used to extract tree spectral reflectance for eight sites from two landscapes. Spectral indices were calculated from the tree spectra. The correlations between spectral indices and foliar DigN concentrations were examined.Rainfall within the previous six-months and surface water availability were the primary determinants of leaf moisture and secondary determinants of foliar DigN and FPC. All foliar chemicals varied among the four eucalypt species sampled. The riparian species E. camaldulensis had higher leaf moisture content, DigN and FPC concentrations than the floodplain species E. populnea. Koala presence was positively influenced by foliar DigN concentration, tree size and long-term (three years) soil moisture. Koalas used taller E. camaldulensis in riparian areas and the long-term soil moisture in the Brigalow Belt South was positively associated with koala presence. More than 50% of the koala diet was from E. camaldulensis. E. coolabah was eaten more than E. populnea and E. melanophloia where it occurred. Koalas increased consumption of E. melanophloia in the dry season, probably to increase moisture intake but ate more E. populnea in wet seasons in response to higher DigN levels. Leaf moisture was lower in the dry season whereas DigN and FPC were more stable across seasons. The normalised difference index using bands ‘Coastal’ and ‘NIR1’ extracted from WorldView-3 satellite images was best correlated with DigN concentrations. The index was used to map foliar DigN at landscape scale.The significance of this research lies in demonstrating the importance of leaf moisture and DigN for semi-arid koala populations, especially in E. camaldulensis as the koala primary food tree species. Leaf moisture decreased in dry seasons and was a key factor limiting foliar nutrition for koalas. The high variation and concentration of foliar FPC in E. camaldulensis indicated koalas could cope with FPC through physiological tolerance and using trees with comparatively lower FPC. The temporal variation in diet of E. populnea and E. melanophloia revealed the supplementary function of secondary koala habitats. Therefore, preserving riparian habitats and surface water bodies is essential for the survival of western koala populations under a hotter and drier climate. Protecting secondary floodplain habitats is important for their long-term persistence. Because foliar DigN did not show strong and unique absorption features, the relationship between the best spectral index and DigN is indirect. The applicability of this index for mapping DigN in other areas needs to be verified.