Marooned on the Houtman Abrolhos

Within a global biodiversity hotspot, one of the highest latitude true coral reefs in the world, the oldest European structure in Australia, a rich and colourful marine environment, perhaps the most infamous murderous mutiny in marine history, intensive human destruction of habitat and still little known to the most of the world, the Houtman Abrolhos archipelago provides a wonderful setting and a rich resource for biological study. The reptile fauna were once part of the mainland but have been isolated for the last 6 or 7 millennia, allowing us to predict likely changes of the same species in fragmented mainland habitats created over the last century.

This remote archipelago, 50 km off the coast of Western Australia, contains the oldest European structure in Australia: Wiebbe Hayes Fort. This stone structure was built in 1629 by marooned soldiers who survived the mutiny and massacre that followed the shipwreck of the Dutch vessel, Batavia. The soldiers were indeed fortunate to have fresh water, pooled in limestone solution holes, abundant wildlife and avoid the long, hot and dry summer.

Wiebbe Hayes Fort, constructed in 1629 on West Wallabi Island to protect soldiers from intermittent raids by mutineers from the shipwrecked Dutch merchant vessel, Batavia.

But human destruction on the Abrolhos has not been confined to our own species. During the late 19th century, a guano industry harvested phosphate rich soil, derived from internationally important seabird breeding populations, devastating the soil and vegetation of 16 islands. This was also the catalyst for introducing exotic biota. During the last 70 years, a thriving and lucrative western rock lobster industry developed with fishers’ huts built on 22 islands, and schools and airfields on three. This collage of environmental changes has modified the abundance and challenged the persistence of many species. In dramatic contrast is the sustainable environmental use by Aboriginal Australians that occupied the region for over 50000 years; although they were not resident on the Abrolhos at the time of European arrival.

FROM THE COVER: How, RA, Cowan, MA, Teale, RJ, Schmitt, LH. (2020) Environmental correlates of reptile variation on the Houtman Abrolhos archipelago, eastern Indian Ocean. J Biogeogr. 47: 1– 12.

Insular populations have long fascinated biologists. The faunas of the Galapagos and eastern Indonesian archipelagos led Charles Darwin and Alfred Wallace to their profound insights of fundamental evolutionary principles.

The Abrolhos is our most recent evolutionary palette. It comprises 170 islands that have been isolated from mainland Australia on several occasions during the periodic fluctuations in Pleistocene sea levels; the most recent isolation beginning around 6500 years ago. This disconnect provides the opportunity to examine short-term, fundamental evolutionary changes in biota on islands of different sizes and geomorphology, in three defined groups separated from one another and the mainland.

This study evolved from our multi-decadal research of insular populations in temperate and tropical Australia, and Wallacea. With our colleague Darrell Kitchener, we illustrated the significance of sea barriers to the evolution of mammalian and reptilian taxa across eastern Indonesian. Many fauna show differences between adjacent islands that remained separated throughout the past 2 million years. Others illustrated morphological and genetic divergence of adjacent islands separated for only the past few thousand years.

Our interest in the Abrolhos’ isolated populations was piqued by the recent studies of David Pearson and Zoe Hamilton on the iconic python and spiny-tailed skink that showed inter-island differences in morphology and genetics. The archipelago is also the ‘type’ locality for seven Australian reptile species, making the collection of tissue from these defining populations imperative to understanding the evolutionary relationships of taxa. Our program focused on determining the diversity of reptile assemblages and populations on islands that represented a range of sizes, geomorphologies, habitats and covered all three island groups. Another principal objective has been to compare morphological and genetic variation of island taxa to their mainland counterparts.

(1) Pigeon, Oystercatcher, Tattler and West Wallabi islands in the northern Houtman Abrolhos. (2) Spiny-tailed Skink on West Wallabi 2011. (3) Dwarf Bearded Dragon basking on East Wallabi, 2012. (4) The endemic Abrolhos Dwarf Bearded Dragon (Pogona minor minima) is found on only a few islands of the Houtman Abrolhos archipelago in the eastern Indian Ocean. The Abrolhos is best known for centuries-old Dutch shipwrecks and the location of the massacre of the Batavia survivors, however, it also has a diverse reptile assemblage whose species show marked morphological differences between islands. Photos by Ric How.

Decades of collaborative research with fellow West Australians allowed us to ‘keel-haul’ colleagues into providing their assistance on six field-trips – all for the princely sum of a week or so on a chartered vessel among one of Australia’s most fascinating and beautiful archipelagos! The Indian Ocean surrounding the Abrolhos greatly enhances the rich biodiversity. Small colonies of rare Australian Sea Lions ‘haul-out’ on shorelines of several islands, while turtles and spectacular coral reefs, the world’s southernmost, provide constant distractions during field programs.

The excitement of discovering a new species cannot be overstated but, despite over 100 years of opportunistic reptile collecting, we also documented many new populations on islands and three previously unrecorded species. Remarkably, most populations showed size differences between islands and we await with interest their imminent genetic appraisal. However, none of seven environmental correlates provided a consistent explanation for the observed inter-island differences in morphology. An exceptional behavioural observation was of a mating aggregation of pythons in a limestone solution cavern adjacent to Wiebbe Hayes Fort where a large receptive female was entwined by five attendant males, all with a singular intent. Modified by European occupancy the Abrolhos may be, but the reptile fauna remain steadfastly focused on fundamental evolutionary behavior!

Mating aggregation of Carpet Pythons in a limestone solution cavern on West Wallabi Island, adjacent to Wiebbe Hayes Fort.

Abrolhos reptile assemblages and their richness are significantly correlated with island area, geomorphology and native plant species richness. The proposed development of eco-tourism infrastructure on the Abrolhos requires consideration of conservation measures designed to protect this diversity of islands, assemblages and species. Reptiles isolated on Abrolhos islands also inform the extinction debt inherent in more recently isolated populations on the mainland, a consequence of agricultural and urban development over the past 100 years, and requiring active management for their persistence.

Written by:
Linc Schmitt1 & Ric How2
1 Emeritus Professor, School of Human Science, The University of Western Australia and Research Associate, Department of Terrestrial Zoology, Western Australian Museum
2 Adjunct Professor, School of Human Science, The University of Western Australia and Research Associate, Department of Terrestrial Zoology, Western Australian Museum

We would like to thank Roy Teale, Mark Cowan and Jason How who commented on this blog.

Published by jbiogeography

Contributing to the growth and societal relevance of the discipline of biogeography through dissemination of biogeographical research.

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