Not that kind of native-ity: endemism and invasion of Australia

The endemics of Australia

Australia is world-renowned for the abundant and bizarre species that inhabit this wonderful island continent. We have one of the highest numbers of unique species in the entire world (in the top few!): this is measured by what we call ‘endemism’. A species is considered endemic to a particular place or region if that it is the only place it occurs: it’s completely unique to that environment. In Australia, a whopping 87% of our mammals, 45% of our birds, 93% of our reptiles, 94% of our amphibians 24% of our fishes and 86% of our plants are endemic, making us a real biodiversity paradise! Some lists even label us as a ‘megadiverse country’, which sounds pretty awesome on paper. And although we traditionally haven’t been very good at looking after it, our array of species is a matter of some pride to Aussies.

Endemism map
A map representing the relative proportion of endemic species in Australia, generated through the Atlas of Living Australia. The colours range from no (white; 0% endemics) or little (blue) to high levels of endemism (red; 100% of species are endemic). As you can see, some biogeographic hotspots are clearly indicated (southwest WA, the east coast, the Kimberley ranges).

But the real question is: why are there so many endemics in Australia? What is so special about our country that lends to our unique flora and fauna? Although we naturally associate tropical regions with lush, vibrant and diverse life, most of Australia is complete desert. That said, most of our species are concentrated in the tropical regions of the country, particularly in the upper east coast and far north (the ‘Top End’).

There are a number of different factors which contribute to the high species diversity of Australia. Most notably is how isolated we are as a continent: Australia has been separated from most of the rest of the world for millions of years. In this time, the climate has varied dramatically as the island shifted northward, creating a variety of changing environments and unique ecological niches for species to specialise into. We refer to these species groups as ‘Gondwana relicts’, since their last ancestor with the rest of the world would have been distributed across the supercontinent Gondwana over 100 million years ago. These include marsupials, many birds groups (including ratites and megapodes), many fish groups and a plethora of others. A Gondwanan origin explains why they are only found within Australia, southern Africa and South America (the closest landmass that was also historically connected to Gondwana).

Early arrivals and naturalisation to the Australian ecosystem 

But not all of Australia’s species are so ancient and ingrained in the landscape. As Australia drifted northward and eventually collided with the Sunda plate (forming the mountain ranges across southeast Asia), many new species and groups managed to disperse into Australia. This includes the first indigenous people to colonise Australia, widely regarded as one of the oldest human civilisations and estimated to have arrived down under over 65 thousand years ago.

Eventually, this connection also brought with them one of our most iconic species; the dingo. Estimates of their arrival dates the migration at around 6 thousand years ago. As Australia’s only ‘native’ dog, there has been much debate about its status as an Australian icon. To call the dingo ‘native’ implies it’s always been there: but 6 thousand years is more than enough time to become ingrained within the ecosystem in a stable fashion. So, to balance the debate (and prevent the dingo from being labelled as an ‘invasive pest’ unfairly), we often refer to them as ‘naturalised’. This term helps us to disentangle modern-day pests, many of which our immensely destructive to the natural environment, from other species that have naturally migrated and integrated many years ago.

Patriotic dingo
Although it may not be a “true native”, the dingo will forever be a badge of our native species pride.

Invaders of the Australian continent

Of course, we can never ignore the direct impacts of humans on the ecosystem. Particularly with European settlement, another plethora of animals were introduced for the first time into Australia; these were predominantly livestock animals or hunting-related species (both as predators and prey). This includes the cane toad, widely regarded as one of the biggest errors in pest control on the planet.

When European settlers in the 1930s attempted to grow sugar cane in the far eastern part of the country, they found their crops decimated by a local beetle. In an effort to eradicate them, they brought over a species of cane toad, with the idea that they would control the beetle population and all would be well. Only, cane toads are particularly lazy and instead of targeting the cane beetles, they just thrived on all the other native invertebrates around. They’re also very resilient and adaptable (and highly toxic), so their numbers exploded and they’ve since spread across a large swathe of the country. Their toxic skin makes them fatal food objects for many native predators and they strongly compete against other similar native animals (such as our own amphibians). The cane toad introduction of 1935 is the poster child of how bad failed pest control can be.

DSC_0867_small
This guy here, he’s a bastard. Spotted in my parent’s backyard in Ipswich, QLD. Source: me, with spite.

But is native always better?

History tells a very stark tale about the poor native animals and the ravenous, rampaging pest species. Because of this, it is a widely adopted philosophical viewpoint that ‘native is always best’. And while I don’t disagree with the sentiment (of course we need to preserve our native wildlife, and not the massively overabundant pests), there are rare examples where nature is a little more complicated. In Australia, this is exemplified in the noisy miner.

The noisy miner is a small bird which, much like its name implies, is incredibly noisy and aggressive. It’s highly abundant, found predominantly throughout urban and suburban areas, and seems to dominate the habitat. It does this by bullying out other bird species from nesting grounds, creating a monopoly on the resource to the exclusion of many other species (even larger ones such as crows and magpies). Despite being native, it seems to have thrived on human alteration of the landscape and is a serious threat to the survival and longevity of many other species. If we thought of it solely under the ‘nature is best’ paradigm, we would dismiss the noisy miner as ‘doing what it should be.’ The truth is really more of a philosophical debate: is it natural to let the noisy miner outcompete many other natives, possibly resulting in their extinction? Or is it only because of human interference (and thus is our responsibility to fix) that the noisy miner is doing so well in the first place? It’s not a simple question to answer, although the latter seems to be incredibly important.

Noisy miner harassing currawong
An example of the aggressive behaviour of the noisy miner (top), swooping down on a pied currawong (bottom). Despite the size differences, noisy miners will frequently attempt to harass and scare off other larger birds. Image source: Bird Ecology Study Group website.

The amazing biodiversity of Australia is a badge of honour we should wear with patriotic pride. Conservation efforts of our endemic fauna are severely limited by a lack of funding and resources, and despite a general acceptance of the importance of diverse ecosystems we remain relatively ineffective at preserving it. Understanding and connecting with our native wildlife, whilst finding methods to control invasive species, is key to conserving our wonderful ecosystems.

The history of histories: philosophy in biogeography

Biogeography of the globe

The distribution of organisms across the Earth, both over time and across space, is a fundamental aspect of the field of biogeography. But our understanding of the mechanisms by which organisms are distributed across the globe, and how this affects their evolution, can be at times highly enigmatic. Why are Australia and the Americas the only two places that have marsupials? How did lemurs get all the way to Madagascar, and why are they the only primate that has made the trip? How did Darwin’s famous finches get over to the Galápagos, and why are there so many species of them there now?

All of these questions can be addressed with a combination of genetic, environmental and ecological information across a variety of timescales. However, the overall field of biogeography (and phylogeography as a derivative of it) has traditionally been largely rooted on a strong yet changing theoretical basis. The earliest discussions and discoveries related to biogeography as a field of science date back to the 18th Century, and to Carl Linnaeus (to whom we owe our binomial classification system) and Alexander von Humboldt. These scientists (and undoubtedly many others of that era) were among the first to notice how organisms in similar climates (e.g. Australia, South Africa and South America) showed similar physical characteristics despite being so distantly separated (both in their groups and geographic distance). The communities of these regions also appeared to be highly similar. So how could this be possible over such huge distances?

Arctic and fennec final
A pretty unreasonable mechanism (and example) of dispersal in foxes. And yes, all tourists wear sunglasses and Hawaiian shirts, even arctic fox ones.

 

Dispersal or vicariance?

Two main explanations for these patterns are possible; dispersal and vicariance. As one might expect, dispersal denotes that an ancestral species was distributed in one of these places (referred to as the ‘centre of origin’) before it migrated and inhabited the other places. Contrastingly, vicariance suggests that the ancestral species was distributed everywhere originally, covering all contemporary ranges within it. However, changes in geography, climate or the formation of other barriers caused the range of the ancestor to fragment, with each fragmented group evolving into its own distinct species (or group of species).

Dispersal vs vicariance islands
An example of dispersal vs. vicariance patterns of biogeography in an island bird (pale blue). In the top example, the sequential separation of parts of the island also cause parts of the distribution of the original bird species to become fragmented. These fragments each evolve independently of their ancestor and form new species (red, and then blue). In the bottom example, the island geography doesn’t change but in rare events a bird disperses from the main island onto a new island. The new selective pressures of that island cause the dispersed birds to evolve into new species (red and blue). In both examples, islands that were recently connected or are easy to disperse across do not generate new species (in the sandy island in the bottom right). You’ll notice that both processes result in the same biogeographic distribution of species.

In initial biogeographic science, dispersal was the most heavily favoured explanation. At the time, there was no clear mechanism by which organisms could be present all over the globe without some form of dispersal: it was generally believed that the world was a static, unmoving system. Dispersal was well supported by some biological evidence such as the diversification of Darwin’s finches across the Galápagos archipelago. Thus, this concept was supported through the proposals of a number of prominent scientists such as Charles Darwin and A.R. Wallace. For others, however, the distance required for dispersal (such as across entire oceans) seemed implausible and biologically unrealistic.

 

A paradigm shift in biogeography

Two particular developments in theory are credited with a paradigm shift in the field; cladistics and plate tectonics. Cladistics simply involved using shared biological characteristics to reconstruct the evolutionary relationships of species (think like phylogenetics, but using physical traits instead of genetic sequence). Just as importantly, however, was plate tectonic theory, which provided a clear way for organisms to spread across the planet. By understanding that, deep in the past, all continents had been directly connected to one another provides a convenient explanation for how species groups spread. Instead of requiring for species to travel across entire oceans, continental drift meant that one widespread and ancient ancestor on the historic supercontinent (Pangaea; or subsequently Gondwana and Laurasia) could become fragmented. It only required that groups were very old, but not necessarily very dispersive.

Lemur dispersal
Surf’s up, dudes! Although continental drift was no doubt an important factor in the distribution and dispersal of many organisms on Earth, it actually probably wasn’t the reason lemurs got to Madagascar. Sorry for the mislead.

From these advances in theory, cladistic vicariance biogeography was born. The field rapidly overtook dispersal as the most likely explanation for biogeographic patterns across the globe by not only providing a clear mechanism to explain these but also an analytical framework to test questions relating to these patterns. Further developments into the analytical backbone of cladistic vicariance allowed for more nuanced questions of biogeography to be asked, although still fundamentally ignored the role of potential dispersals in explaining species’ distributions.

Modern philosophy of biogeography

So, what is the current state of the field? Well, the more we research biogeographic patterns with better data (such as with genomics) the more we realise just how complicated the history of life on Earth can be. Complex modelling (such as Bayesian methods) allow us to more explicitly test the impact of Earth history events on our study species, and can provide more detailed overview of the evolutionary history of the species (such as by directly estimating times of divergence, amount of dispersal, extent of range shifts).

From a theoretical perspective, the consistency of patterns of groups is always in question and exactly what determines what species occurs where is still somewhat debatable. However, the greater number of types of data we can now include (such as geological, paleontological, climatic, hydrological, genetic…the list goes on!) allows us to paint a better picture of life on Earth. By combining information about what we know happened on Earth, with what we know has happened to species, we can start to make links between Earth history and species history to better understand how (or if) these events have shaped evolution.