Scholtz, E.J. and DeSantis, L.R.G. (2020). Invasive species, not environmental changes, restrict the population and geographical range of the quokka (Setonix brachyurus). Journal of Zoology, https://doi.org/10.1111/jzo.12765
In 2013, the quokka was named the “Happiest Animal in the World” by the Huffington Post. After one look at these Australian marsupials and their natural ‘smiles,’ it’s hard to imagine that these highly endemic and elusive medium-sized mammals occur on only a few islands and a fraction of their prior geographic range. Their affable demeanor also makes them a prime candidate for photos—hence the rise of the “quokka selfie” (really, Google it and you can see your favorite celebrity posing with a quokka).
While their cuteness is hard to match, their ecological history tells a more complex story about competition, predation, and extirpation from much of their native land.
Today, quokkas are heavily concentrated on Rottnest Island (off the west coast of Western Australia), as well as on Bald Island and a few mainland locations in Western Australia. Although quokkas were once documented to be widespread throughout southwestern Australia, they have since been listed as “vulnerable” by the IUCN due to a significant decline in their population beginning in the 20th century.
The motivation behind this study was to help understand the stark decline of mainland quokka populations and their subsequent isolation on coastal islands. There are several existing hypotheses in the literature that could describe why quokka populations began to dramatically decrease. Past studies have attributed the decline to climatic and vegetative change, others have pointed to overhunting, and some focus on the correlation between the introduction of non-native species and the decline of the quokka population.
To answer this question further, we used dental microwear texture analysis (DMTA) and stable isotope analysis to understand the dietary ecology of the quokka over time, including at different geographic locations. Specifically, we wanted to know if the dietary ecology of the quokka changed from several thousand years ago (during the Pleistocene) to the present and across different geographical locations, to essentially help answer if their restricted range is due to limited habitat availability.
Our results showed that quokkas maintained a browse diet throughout time and across geographical locations. Fossil and modern quokkas on the mainland of Western Australia ate very similar vegetation, but modern quokkas living on coastal islands ate drier and tougher foods than expected. Additionally, there was an apparent shift in the feeding environment of the quokka: Pleistocene populations (~36-27 thousand years ago) inhabited more open forests and shrublands, while modern populations inhabited denser and wetter forests.
These results do not indicate that a significant shift in vegetation restricted the quokka’s population or geographic range, but instead that the presence of non-native species likely impact the geographic range of the native quokka.
In 1788, European settlers introduced the goat to Australia (a competitor with the quokka for vegetation). In the 1850s, settlers introduced the European red fox (predator of the quokka) and the European rabbit (another competitor with the quokka). Soon, these invasive species spread rapidly across the continent and reached the western coast of Australia by the early to mid-1900s. At the same time, quokka populations began to decline in the mid-1900s.
Our results indicate a similar change across time and locations. Prior to European arrival, quokkas lived in more open environments on mainland Western Australia and ate a browse diet. After European arrival and the introduction of invasive species, quokkas began to inhabit denser forests (likely to escape predation and competition). Thus, populations on the mainland became limited to specific locations. Meanwhile on coastal islands, where there are no invasive species that serve as direct competitors or predators, quokkas continue to persist in their greatest numbers. On these small islands though, there is limited vegetation and habitat, so the quokkas must eat tougher and drier food than what is preferred and inhabit more open landscapes than on the mainland. Unfortunately, the lack of fresh water sources (aside from plant water and precipitation) on Rottnest Island leads to high mortality rates in quokka populations during the summer months when temperatures are high and precipitation is limited.
Further, Rottnest Island has changed dramatically since the arrival of Europeans and the most recent increase in tourism during the past few decades, with human food supplementing the diets of quokkas around the settlement area today.
In reality, these perceived ‘happy animals’ are restricted to highly fragmented and limited geographic ranges in large part due to invasive species. With the drought durations and temperatures on the rise across much of Australia, and fires eliminating large swaths of forested habitat and forest inhabitants (most notably, koala populations being reduced by 30% or more in New South Wales, Australia during the 2019-2020 fires), it is even more likely that quokkas may face similar fates. To put this all in perspective, the entire geographic range of quokkas is only a fraction of the size of the forests that were completely decimated by fires in one state during one year in Australia.
This study not only aims to inform modern conservation issues regarding the quokka, but it also lays out a framework for using the fossil record to help inform the ecology of species over time.
Elinor Scholtz and Larisa DeSantis (Authors of blog post and article)