What causes extinction, let’s say, of the dodo, a flightless bird endemic to the island of Mauritius in the Indian Ocean?
Nobody knows what the huge pigeon looked like given the last widely accepted sighting of a dodo was in 1662 and previous descriptions and drawings were hard to verify. Many were made by illustrators who had never seen the animal.
Dodos had not seen humans either and had no fear of them, but there is no evidence that sailors plundering populations for provisions sent them to extinction. Nor was it likely that the resident humans on the island, that never exceeded 50 people in the 17th century ate them to death. People cleared much of the dodo’s forest habitat and introduced alien animals to the island, including dogs, pigs, cats, rats, and crab-eating macaques, which plundered dodo nests and competed for the limited food resources.
Nobody knows precisely what happened.
What we know is that after Dutch sailors’ first reports of the dodo in 1598, the species was probably extinct a century later.
Indeed, when the Dutch left Mauritius in 1710, the dodo and most of the large terrestrial vertebrates on the island were no more.
So what causes extinction? There is a one-word answer, but before we reveal that, let’s look at evolution and when extinction happens.
All organisms evolve.
The dodo was well suited to pre-human life in Mauritius. We know this because it existed as a distinct species to be seen by the intrepid explorers in their wooden ships from far away. It had evolved into its shape, behaviours and habitat over thousands of generations from a slightly different-looking ancestor.
The ancestors of the dodo each varied in anatomical, physiological and behavioural attributes generated through genetic mutation and drift. Some of these attributes were a benefit, and some were a hindrance. Benefits we call an adaptation to the environmental conditions in which that organism finds itself that gives the individual a better chance of survival, growth, and reproduction. Indeed, a benefit in evolutionary terms only occurs if the individual successfully reproduces.
How much help future generations get from a mutation depends on the environmental conditions and if those conditions are stable relative to the benefits the new attribute brings.
The dodo was flightless.
It had stubby wings, too small to lift the bulky animal off the ground. If the ability to fly offered no advantage to survival, growth or reproduction over not flying, then a mutation that made wings smaller had as good a chance of propagation as fully functional wings. Meanwhile, if a large body and big beak gave foraging advantages and no penalty for not flying, before long, dodos without expensive wings and flight muscles had the advantage. Biology sees no need to build and maintain unnecessary structures.
Individuals carrying the genes for big wings were at a disadvantage, and natural selection weeded them out. It is cruel like that.
Here is another example.
Suppose a tree species evolves a mutation that allows seedlings to survive longer when the soil is dry. This might be a considerable benefit if the climate is warming and the soil drying out faster between rain events. A mutation for more robust seedlings allows for regeneration and buys some time for future generations of the tree to track habitat change as warmer, drier conditions spread across the landscape.
If the climate changes to colder and wetter, the mutation may have no effect or even be a disadvantage.
When does extinction occur?
Extinction happens when the last individuals of a species can no longer reproduce unaided. The last lone rhino without a mate might persist for a few decades, and then the species is extinct.
Failure to reproduce comes about through
- death before reproduction, a failure to survive because the conditions were too severe or the food, water or nutrients were lacking for whatever reason from competition to a tsunami
- insufficient food or nutrients to either grow to reproductive size or channel resources into gametes or both
- absence of the necessary gametes, again for a range of reasons from failure to find a mate to a shortage of pollinators
Failure to reproduce for the last remaining individuals is the point of extinction.
And failure to reproduce is about the survival, growth and reproduction of individual organisms. Enough individuals must achieve successful reproduction for the species to persist.
This basic premise applies to everything from microbes to marsupials.
What causes extinction in the specific sense is the failure of all remaining individuals of a given species to gather enough resources to survive, grow and reproduce—the specific failure depends on the species and the circumstances.
Extinction is much more likely when the species is rare or has a restricted distribution, or both. The dodo on Mauritius had a restricted distribution and became rare when humans became an unfamiliar predator and altered their habitat.
Extinction is also likely when environmental conditions change sufficiently to compromise the ability of individuals to reproduce.
Should environmental conditions change dramatically from what is usual, for example, a human being comes along with an axe and chops a bunch of trees down, this is a disturbance outside the norm in terms of extent and intensity. Many organisms are pushed outside of their comfort zone and struggle in new conditions.
Any remaining individuals have only three options.
- Hunker down in whatever safe place they can find and hope the disturbance disappears and normality returns.
- Move to the nearest patch of intact forest, easy for an eagle, hard for an orchid or a dodo.
Adaptation through genetics happens quite slowly and extinction happens at the point when the environment no longer suits the evolved adaptations. So we are close to the one-word answer of what causes extinction.
Just stringing it out a little longer, look at this image from Western Australia that shows intact eucalyptus forest and forest cleared for agriculture. Now, it doesn’t take a genius to work out that the conditions on one side are very different to those on the other. Many of the organisms that exist in the three-dimensional conditions of the forest—places to hide, food to eat, shade from the sun—would struggle to find adequate conditions in the agricultural field.
Agricultural landscape in Western Australia. Photo by Alloporus
What causes extinction?
The one-word answer is disturbance.
Specifically, a disturbance that alters conditions beyond the tolerance of the last remaining individuals of a species sufficient to prevent them from reproducing.
Right now disturbance is everywhere.
To say that we’re in the Anthropocene amid the Sixth Mass Extinction event is not a surprise if you look at how the globe now looks compared to what it did just a few hundred years ago.
In the last two hundred years, humans have harnessed fossil fuels through machinery to radically transform the landscape. The vegetation is cleared and managed, and the organisms that were part of a slow, subtle change from predictable, natural disturbance can no longer cope. The changes that humans have brought about are radical, intense and acute.
People can sound incredulous and surprised by this reality, but it shouldn’t be surprising. If we disturb with axes, chains, fire and tractors such a large proportion of the landscape, then we know that we will lose biological elements of that landscape.
Any surprise we have at biodiversity loss is misplaced—it shouldn’t be a surprise at all.
Biodiversity loss through local and global extinction is an inevitable consequence of human beings acquiring resources and making humans the priority.
As we channel nature to our purposes humans disturb almost everything.
Disturbance causes extinction
Before we get all high and mighty or anthropocentric on disturbance, recall it has happened at a global scale many times before.
Human-induced mass extinction is at least the sixth big one and numerous small global extinction events are evident in the fossil record. Most of them are attributed to climate changes, especially ocean warming, and even the famous meteorite strike creating the boundary between the Cretaceous and Tertiary approximately 66 million years ago that took out the dinosaurs had a climate disturbance. Most of those creatures didn’t die in the blast but starved when the dust, circulating in the atmosphere, shaded the sun and messed with the plants.
Disturbance is what causes extinction and disturbances happen all the time.
The current human one is not unique. Nor is there an immorality to it because it is inevitable, simply a consequence of adaptation… human adaptation.
More than one extinction event recorded in the fossil record. By Marshall, Charles R. – “Forty Years Later: The Status of the ‘Big Five’ Mass Extinctions.” Cambridge Prisms: Extinction 1 (2023): e5. doi:10.1017/ext.2022.4., CC BY 4.0,
What sustainably FED suggests…
Disturbance is what causes extinction and right now the planet is disturbed by human activity, specifically activity in the last 200 years.
Extinction can be sad for us. It is tempting to focus on losses as specific events that are preventable and a bad thing that has a cause and someone to blame.
But if disturbance is what causes extinction, then it is going to happen because of what Homo sapiens, the species, has become.
Not only are we swallowers of exogenous energy and able to bend the environment to our own needs, but we are also abundant.
Our challenge is not extinction but to cope with the consequences of rapid population growth. And when we say rapid, we do mean rapid to levels that historically not even Thomas Malthus would have believed possible.
But here we are at 8 billion increasing at 8,000 extra people every hour. With this many people, disturbance is inevitable.
So we shouldn’t be surprised at extinction. If 50 people on a 2,000 km2 island along with a few visitors in wooden boats can push a flightless bird to extinction, what are 8 billion humans with container ships going to do?
It is time to become more pragmatic about our rhetoric around loss. Accept it for what it is. And then make some very tough but important choices around what types of biodiversity we want to keep and therefore what effort it will take—rewilding is a good way to begin the conversation.
So don’t be surprised when species go extinct. Be sad. Be disappointed. Be somewhat ashamed that we haven’t been able to fix this problem. That we haven’t been able to manage our use of the planet more effectively.
But recognize that humanity has a monumental challenge with both pessimistic and optimistic outcomes possible. Extinction is a risk that humanity faces.
We have to ask awkward questions, especially this one.
Just how much disturbance and extinction can we tolerate before it starts to impact our likelihood of survival?
We can survive. Whether we will is up to us.