The frill-necked lizard, hailing from Southeast Asia, is just one example of the numerous land animals that made their way to Australia. As we all know, Australia is renowned for its unique and peculiar creatures that cannot be found anywhere else on the planet, such as the platypus and the koala. However, what might surprise you is that many of these iconic critters actually originated from Asia and only recently arrived in Australia, at least in geological terms. This includes various lizards like the goannas and thorny devils, as well as other animals like hopping mice, flying foxes, and the kookaburra. Interestingly, the movement of animals between Asia and Australia has primarily been one-sided, with fewer representatives of Australian fauna found in Asia compared to Asian fauna in Australia. This begs the question: why is this asymmetrical situation the case?
To uncover the reasons behind this phenomenon, my colleagues and I conducted a study that focused on the distribution, habitat, climate, and plate tectonics of 20,433 species of land-dwelling vertebrates over the past 30 million years. The story begins over 200 million years ago when dinosaurs were still a new addition to the Earth, and Australia was part of the supercontinent Gondwana. Gondwana consisted of present-day Antarctica, South America, Africa, Australia, and India. Following the separation of Gondwana from another supercontinent called Laurasia, there was a loss of land connection between Australia and Asia. Subsequently, Gondwana itself began to disintegrate, with each fragment embarking on its independent journey. Many of these journeys eventually led back to Laurasia. For instance, India collided with Eurasia to form the mighty Himalayas, South America crashed into North America creating the Panama land bridge, and Africa collided with Eurasia forming the Mediterranean Sea. Meanwhile, Australia embarked on a collision course with Asia and eventually severed its final Gondwanan ties around 45 to 35 million years ago when it separated from Antarctica. At that time, Australia was positioned much farther south than its current location. As it drifted north, the increasing distance between Australia and Antarctica instigated the Antarctic circumpolar current, leading to a significant cooling of the planet. Isolated and exposed to these climatic changes, Australia became home to a distinct set of animals and plants that began to evolve.
Simultaneously, the collision of the Australian and Eurasian tectonic plates gave rise to numerous islands in the Indonesian archipelago, including Lombok, Sulawesi, Timor, and Lesser Sunda Isles. These islands do not belong to either the Australian continental shelf (known as Sahul), encompassing Australia and New Guinea, or the Asian continental shelf (known as Sunda), which includes Thailand, Malaysia, Singapore, Sumatra, Java, Borneo, and Bali. Instead, they occupy an in-between zone called Wallacea, named after Alfred Russell Wallace, a British naturalist from the 19th century who first noticed differences in the types of animals on either side of what is now called Wallace’s line. These islands essentially became stepping stones between two continents whose species groups had been separated for an incredibly long period. However, our research reveals that only certain types of animals were capable of successfully crossing these stepping stones and establishing themselves on the other side.
The first determining factor for species movement between Asia and Australia was their ability to traverse the ocean. Among all the animal groups that made this journey, birds were by far the most prevalent. However, this factor alone did not guarantee success. Animals also needed to adapt and thrive in their new surroundings, which often presented different environmental conditions. We discovered that animals capable of tolerating a wide range of wet and dry environments were more likely to successfully make the journey. This finding makes sense considering the wetness of Sunda and the dryness of Sahul, as those adaptable to a broader wet-dry spectrum had an advantage in moving between these regions. Nevertheless, one question remained: why did more animals move from Sunda to Sahul than the other way around?
To complete the puzzle, we had to examine how these critical factors—species dispersal and adaptation to new environments—evolved over time. The majority of animals that migrated from Asia to Australia were birds, including the ancestors of the kookaburra. Over time, we observe that Sunda remained dominated by lush tropical rainforests even before Australia separated from Antarctica. As the stepping-stone islands emerged, they, too, acquired the humid equatorial climate favored by the rainforest vegetation and later the animals from Sunda. However, Australia experienced a decline in similar rainforests, which were replaced by grasslands and woodlands in most areas. As a result, animals moving from Sunda, through the stepping-stone islands, to New Guinea and the northern tips of Australia in Sahul encountered a band of comparable humid tropical climate. On the other hand, most animals in Sahul evolved on the drier Australian mainland. Consequently, moving from mainland Australia through New Guinea and the stepping stones to Sunda required adapting to a substantially different environment. Moreover, Australian animals that did manage to reach the stepping-stone islands would likely have encountered competition from already established Sunda groups that thrived in their preferred tropical climate.
This long and intricate narrative emphasizes the crucial role that climate and geography play in shaping the evolution and distribution of different species. By taking a deep dive into the past, we can gain valuable insights into the world surrounding us. Simple questions like why kangaroos are absent in Asia while hopping mice reside in Australia have answers that span hundreds of millions of years. Ultimately, this research sheds light on the fascinating journey of species across continents and the intricate web of factors that contribute to the biodiverse planet we inhabit today.
More information: A. Skeels et al, Paleoenvironments shaped the exchange of terrestrial vertebrates across Wallace’s Line, Science (2023). DOI: 10.1126/science.adf7122 Provided by The Conversation This article is republished from The Conversation under a Creative Commons license. Read the original article. Citation: Why are there hopping mice in Australia but no kangaroos in Asia? It’s a long story (2023, July 9) retrieved 9 July 2023 from https://phys.org/news/2023-07-mice-australia-kangaroos-asia-story.html This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.
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Jessica Irvine is a tech enthusiast specializing in gadgets. From smart home devices to cutting-edge electronics, Jessica explores the world of consumer tech, offering readers comprehensive reviews, hands-on experiences, and expert insights into the coolest and most innovative gadgets on the market.
