Mangrove-avifauna relationships in Aotearoa New Zealand : conservation insights from banded rail (Gallirallus philippensis) ecology : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Conservation Biology at Massey University, Auckland, New Zealand
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Date
2023
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Massey University
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Abstract
Among terrestrial vertebrates, birds are the most ubiquitous taxa in mangroves globally, using these habitats to breed, roost, and forage. However, within the past half century, the large-scale loss and fragmentation of mangrove forests throughout much of their distribution has corresponded with declines in populations of mangrove-using avifauna. Despite these declines, remarkably little is known about the avifauna that inhabit mangrove forests, nor the ecological relationships that exist between birds and mangrove habitats. The absence of this understanding presents a significant barrier to effective avifauna conservation in mangrove environments.
The ecological relationships between the banded rail (Gallirallus philippensis assimilis) and mangroves (Avicennia marina var. australasica) in Aotearoa New Zealand are poorly understood, reflecting a lack of scientific research addressing mangrove avifauna globally. The study of banded rails has been hindered by their cryptic behaviours and exacerbated by mangroves being logistically challenging habitats to work in because of their intertidal nature, dense vegetation, and muddy substrates. The paucity of research in this field is a concern given that New Zealand’s population of banded rails – largely restricted to mangroves in the coastal and estuarine regions lining the northern shores of the North Island – is declining and categorised as ‘at-risk’ of extinction. Mangroves in New Zealand are globally anomalous, having expanded rapidly in recent decades and having been subject to intensive management predicated on vegetation removal. In this context, understanding the importance of mangrove habitats for banded rails is of ecological interest and conservation concern. This thesis elucidates the ecological relationships between banded rails and mangroves in the context of recent mangrove expansion and contemporary management practices (especially in terms of removals) in New Zealand by (1) reviewing mangrove management practices and their effects on avifauna, (2) determining the relative habitat quality of mangroves for banded rails, (3) establishing and implementing a reliable survey method for banded rails, and (4) quantifying banded rail habitat selection and use in saltmarsh-mangrove complexes of northern New Zealand.
First, to understand the extent, configuration, and repercussions of mangrove removal in New Zealand, I reviewed all legal mangrove removals until 2020, using resource consent documents from relevant regional authorities. I determined that the area of mangrove removed is small relative to mangroves’ contemporary area and expansion. Decisions regarding mangrove removal largely prioritised human-centric desires for recreational spaces rather than principles of ecological restoration. In addition, I showed that an ecological understanding of the repercussions of removal on avifauna is limited by insufficient monitoring. Drawing on limited data, I suggested that mangrove removal creates a conservation trade-off, benefitting species that use open habitats, such as waders and shorebirds, at the expense of mangrove-using avifauna. I then emphasised the importance of addressing the drivers of mangrove expansion rather than its symptoms, situating New Zealand’s management response in the theory of invasion biology.
Second, I assessed the habitat quality of New Zealand’s mangroves for banded rails, using a resource-based approach. I quantified the abundance and diversity of macrofauna – key food resources to banded rails – collected from a stratified sampling regime across four habitat zones in four saltmarsh-mangrove complexes, determining that old-growth mangroves held the highest abundance and biomass of banded rail food resources. Additionally, I assessed the availability of these resources to banded rails using existing literature and field-based observations, theorising that mangroves provided the highest availability of food resources within saltmarsh-mangrove complexes.
Third, I established and implemented a survey method novel to the study of banded rails (and cryptic marsh birds more broadly). I determined that a combination of camera traps and drift fences (CDF) provides an effective method for surveying banded rails in intertidal habitats, capable of providing both presence-absence data and inferences into banded rail movement patterns. I observed banded rail movements between saltmarsh and mangrove habitats to be correlated with temporal and tidal cycles, the first time banded rail habitat use has been assessed in relation to environmental cues. I explored the applicability and value of the CDF method as a monitoring tool, suggesting the method could support new research avenues for cryptic species and complement monitoring methods used for banded rails in New Zealand.
Fourth, I quantified the habitat selection and habitat use patterns of banded rails at a home-range scale in saltmarsh-mangrove complexes, assessing data from GPS biotelemetry via resource selection ratios and a generalised linear mixed effects model. I determined that banded rail home ranges are largely restricted to saltmarsh and mangrove habitats finding that individuals select for mangrove habitats to support foraging efforts, select for saltmarsh habitats as roosting grounds, and generally avoid open habitats such as mudflats and residential gardens. I showed that habitat use may vary among individuals, noting two individuals that chose to roost in mangrove habitats overnight – a novel observation for this species. Additionally, biotelemetry findings confirmed movement patterns observed by camera traps in that banded rail habitat use was mediated by temporal and tidal cycles. Banded rails were significantly more likely to use mangroves during the day, whereas saltmarshes were primarily used at night and during high tides.
Combining insights from research findings and existing literature, this thesis demonstrates that mangrove habitats play an important role in supporting banded rails. While mangroves are not a prerequisite for the survival of banded rail individuals, mangroves represent preferred habitats and support banded rail behaviours such as foraging and roosting. Viewed from a population perspective, mangroves help maintain banded rail populations by providing habitats to the majority of the country’s banded rail population. Three observations from this thesis are particularly relevant to conservationists and coastal managers in New Zealand: (1) mangroves are not uniform habitats; mangrove forests may appear structurally similar, but can be functionally different in their ability to support avifauna populations, (2) mangroves are more important to banded rails than previously understood or quantified; mangroves are preferred as foraging habitat to banded rails, can support roosting behaviours, and may make small patches of adjacent saltmarsh or terrestrial scrub viable breeding habitats, and (3) mangrove removal is likely to adversely affect local populations of banded rail, but more research is required to understand the nuances of these effects.
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Gallirallus, Habitat, Monitoring, Mangrove forests, Management, New Zealand