The Impact of Habitat Alteration on the Population Dynamics of a Declining Woodland Bird in the Mount Lofty Ranges

Abstract

Biotic invasions are among the main drivers of ecosystem change and contribute to species declines. In the southern hemisphere, perennial native understorey plants have been largely outcompeted by fast-growing annual crop grasses and herbs. This significant compositional change has altered patterns of seed production. Granivores such as the diamond firetail (Stagonopleura guttata) are likely to be affected by such alterations. This species has been declining nationally since large-scale land-clearance prior to 1980. Focussed, species-specific research is required to identify the threats and their interactions that contribute to ongoing declines. This thesis examined an isolated meta-population in the Mount Lofty Ranges (MLR), South Australia, subsisting in heavily degraded grassy woodland dominated by exotic annual grasses. The hypothesis that changes in grass seed phenology associated with invasion by exotic annual species has resulted in seasonal food shortages was tested. An assessment of the seed resources available to ground-foragers in the southern MLR confirmed that introduced, annual species dominate the understorey. Total seed biomass was over seven times greater in spring (4.08 g m-2) than in autumn, when biomass was just 0.53 g m-2. High spring seed biomass was predominantly produced by annual grasses. The subsequent drop in biomass coincided with breaking autumn rains, implicating mass-germination as the cause. Variation in seed biomass was attributed to seasonal changes in the seed abundance of annual weedy grasses and forbs. Diet analyses found that S. guttata diets mirrored the abundance and diversity of seasonal resources. However, during winter, when grass seed biomass was scarce, S. guttata relied heavily on the seeds of the drooping sheoak (Allocasuarina verticillata). A mark-resight study determined that S. guttata populations were affected by seasonal resource fluctuations. Stagonopleura guttata encounters were strongly correlated with seed abundance and S. guttata densities reflected seed biomasses. To further explore the reason for low numbers of S. guttata in autumn and winter, an in-field food supplementation experiment was conducted. The food-supplemented population had significantly higher survival than the non-supplemented population, indicating that food is limiting for S. guttata. Juveniles were less efficient foragers than adults, highlighting their vulnerability during times of seed scarcity. However, food supplementation appeared to increase the proportion of juveniles that survived their first winter. The transition of independent fledglings from spring/summer breeding into young breeding adults is critical for recruitment and is likely to be hampered by winter food shortages. Overall densities of S. guttata in the southern MLR were 0.023–0.062 birds/ha, lower than the minimum viable population estimate of 0.069 birds/ha. As such, this meta-population is not sustainable at current densities. To improve the persistence of S. guttata, it is essential that the grassy woodlands of the region are managed to create consistent, year-round food resources, particularly during winter. This requires the restoration of perennial native grasses in the understorey that seed more consistently and over longer time periods than annuals. In addition, rehabilitation of A. verticillata, a key resource, will improve food availability during times of scarcity.