University of Tasmania
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Determinants of extinction risk in rails (Aves: Rallidae)

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posted on 2024-05-14, 04:54 authored by Lucile Leveque

Forty percent of bird species are declining worldwide and one in eight is threatened with extinction. With this situation continuously worsening, it has never been more important to understand and mitigate the causes of these biotic contractions, especially if we wish to avert further loss of species and ecosystem integrity this century. Rails (Aves: Rallidae) represent one of the most extreme cases of mass extinction within a modern vertebrate group. This once hyper-diverse bird family lost 54-92% of its species following waves of human contact during both the Holocene and the Anthropocene (starting 16th century) eras. Today, a third of the extant rail species are categorised as threatened or near-threatened.

My PhD thesis focuses on the vulnerability of rails to anthropogenic threats, with two major research themes: (1) global patterns of extinction and endangerment (Chapters 2-3), and (2) understanding factors that influence the susceptibility or success of a focal species of flightless rail (Chapters 4-5), Tribonyx mortierii, aka. the Tasmanian native hen. This species went locally extinct on continental Australia in the Holocene but thrives today on an island refugium at the southern extent of its former range, despite multiple threatening processes.

I begin by outlining the global context for rail conservation biology (Chapter 2), arguing that extant rail species follow two key pathways in the contemporary threatening pattern. One follows the same trajectory as extinct rails, whereby island endemic and flightless rails are most threatened, mainly due to invasive predators. The second, created by the recent diversification of anthropogenic activities, involves continental rails (generally in the Neotropics), with the most prevalent threat being agriculture, natural-system modifications (fire, dams, and water abstraction), and residential and commercial development.

In Chapter 3, I investigate the spatio-temporal variation in extinction and endangerment for rails. I found that all rail extinctions, Holocene and recent, took place on islands. While flightlessness or naivety to humans were the main intrinsic traits associated with Holocene rail extinctions, this pattern had shifted in recent extinctions towards body size being the main correlate: the smallest- and largest-bodied species being the most extinction prone. I also demonstrate that an elevated threat status for rail species globally was most strongly linked to island endemism and small clutch size, while no specific threatening process identified for island endemic species. This suggests that anthropogenic threats have potentially generated a "field of bullets" making it difficult to extract generalisations about vulnerability for contemporary island rails.

Few flightless rails have survived these successive waves of extinction, and even fewer are considered to have secure populations today. The Tasmanian native hen is one of the rare and emblematic cases of a surviving flightless rail. It was extirpated from the Australian mainland about 3,500 years ago and it is now endemic to Tasmania, where it remains widespread and populous, being one of only two flightless rail species globally to categorised as at Lower Risk of extinction (LC). To evaluate the long-term sustainability of this iconic species, I undertook a field programme to: (i) determine the factors that influence their occupancy across Tasmania and assess how climate change could impact the species' future distribution and relative abundance (Chapter 4), and (ii) identify the local-scale environmental and demographic stressors causing population declines (Chapter 5).

In terms of island-wide distribution, I found that native hens are predicted to shift parts of their distribution in Tasmania, following mainly the forecast changes in the summer rainfall patterns, with both increases and decreases in different parts of the island. Being positively associated with human land use and urban areas, I also demonstrated the capacity for native hens to thrive locally in the midst of large regions of otherwise unsuitable habitat (e.g., in urban enclaves with permanent water and grass), offering a potential for a higher suitability in Tasmania, depending on future trends in farming and urban development.

With respect to how local patterns and drivers might influence the viability of Tasmanian native hens, I investigated the survival and reproduction of a declining population isolated on Maria Island, in southern Tasmania (Chapter 5), using a comparison of historical data from the 1990s with field data collected in 2017-2018. I discovered that the native hen population on Maria Island has declined by 70% compared with two decades ago, with poorer-quality territories and a much lower reproductive rate. I hypothesised that the impact of over-grazing by introduced wombats and other macropods, coupled with climate variability, was likely to have affected water retention and vegetation cover across the population's territorial range. This would have led to reduced habitat quality and ultimately, to a reduction of the carrying capacity, as well as limiting recovery due to the lack of high-quality territories following a prolonged drought.

Overall, my thesis highlights the importance of understanding how the succession of extinction filters and threatening processes can shape the current and future vulnerability of a globally widespread bird family. Importantly, this research also provides in-depth insights from an understudied surviving flightless rail, delivering the first assessment of their conservation and emphasising unique ecological preferences under growing anthropogenic activities. As such, my thesis offers a timely contribution to the conservation biology of rails, and island biodiversity, globally.



School of Natural Sciences

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  • Unpublished

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