The biological diversity, recovery from disturbance and rehabilitation of mangroves in Darwin Harbour, Northern Territory

  • Kristin Nina Metcalfe

    Student thesis: Doctor of Philosophy (PhD) - CDU


    Surveys were done of the diversity, distribution and abundance of vertebrate and invertebrate fauna in mangroves in Darwin Harbour. The four main mangrove assemblages—the hinterland margin, tidal flat, tidal creek and seaward—were studied at three sites and, where possible, surveys reported annual and seasonal variation. Four disturbed sites were also surveyed to investigate the effects of anthropogenic disturbance. Vertebrate surveys involved trapping (mammals), ultrasonic detection (bats) and audio‐visual detection (birds). Invertebrates, and small resident fish, were sampled by quadrat and epifuanal searches, pitfall traps and anoxic mats in a composite methodology developed for this project.

    Field studies were also done to investigate factors which may delay forest recovery. A two‐year experiment in cyclone‐damaged and bulldozed forests tested for effects of shade, predation and damage from floating debris on seedling survival and growth. Multi‐species plantings at three degraded sites tested different rehabilitation techniques. Seedlings were either transplanted, nursery‐grown or directly implanted as propagules.

    Mammal diversity was low (13 species), and two herbivorous resident species comprised most captures, but arboreal mammal densities were remarkably high in the low‐intertidal. The rich, predominantly insectivorous, avifauna (70 species) included nine mangrove specialists but four species comprised half of all records. Most birds occurred amongst dense tree cover of the two seaward assemblages. Mangrove bats (11 species) were mostly small insectivorous microbats most prolific at the landward fringe but large flocks of flying foxes periodically foraged in the seaward assemblage.

    The invertebrate fauna was very diverse and dominated by crustaceans (31%), molluscs (31%) and worms (16%). Crustaceans were the most abundant, and molluscs the most diverse, faunal groups. Overall diversity and abundance of invertebrates increased to seaward and marked declines in the worm fauna occurred during the wet season.

    Of the grand total of 254 invertebrate taxa, 163 were recorded at undisturbed sites and 171 in disturbed. There were, in general, no significant differences in diversity or abundance between disturbed and undisturbed forests. Multivariate analyses, however, clearly demonstrated differences in species composition between the two types of forest.

    For instance, wading birds suited to foraging in waterbodies and clearings, were more common in some disturbed habitats. Polychaete worms appeared to be the most responsive indicators of environmental change, while gastropods and grapsid crabs in upper‐intertidal areas were potentially most vulnerable to urbanisation.

    The factors delaying the recovery of damaged forests were complex and varied with shoreline position. To seaward, recovery of cyclone‐damaged R. stylosa was hindered by herbivorous turtles whilst to landward, poor dispersal and lack of recruitment refuges prolonged recovery. Field methods developed to facilitate natural recruitment showed promise for fast‐tracking recovery of degraded ecosystems.
    Date of AwardFeb 2007
    Original languageEnglish
    SupervisorKeith Mcguinness (Supervisor)

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