logs from a pine forest

What causes extinction?

Extinction is forever and there is a lot of it going on just now. The question is why?

All organisms evolve. 

Thanks to natural variation in anatomical, physiological and behavioural characteristics generated through genetic mutation and drift, individuals acquire new attributes that may be a benefit or 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.

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 allowing the species to persist as conditions change. It buys some time for future generations to track habitat change as warmer, drier conditions spread across the landscape.

If the climate is stable, dry soil capability for seedlings may be less of an advantage unless it is coupled with dispersal ability allowing the species to expand its range into already drier habitats. 

If the climate changes to be colder and wetter the mutation may have no effect at all or even be a disadvantage.

Human brains see this as the positive side of the evolutionary coin.

When does extinction occur?

Extinction happens when the last individuals of a species are no longer able to successfully reproduce unaided.

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 resources (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.

Extinction happens when the ability of the organism to gather sufficient resources is compromised to the point of failure to reproduce in all the remaining individuals.

This is much more likely when the species is rare or has a restricted distribution or both.  

It is also likely when environmental conditions change sufficiently to compromise the ability of extant individuals to reproduce. This is one reason why disturbance is so important to evolution.

Should environmental conditions change dramatically from what is usual, for example, a human being comes along with a chainsaw 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 may be unable to survive in new conditions. 

In fact, they have only three options.

  1. Hunker down in whatever safe place they can find and hope the disturbance goes away.
  2. Move to the nearest patch of intact forest but that might be hard for an orchid.
  3. Die

A glance at this image from Western Australia shows intact 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. 

What causes extinction?

The answer is disturbance. 

Specifically, 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 and in the midst of the Sixth Mass Extinction event is not really a surprise if you look at the way 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 as opposed to running at its own course 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 a surprise at all. If we do this to such a large proportion of the landscape, then what we know is that we are going to lose biological elements of that landscape. 

Any surprise we have at biodiversity loss is really misplaced. It shouldn’t be a surprise at all. It’s an inevitable consequence of human beings acquiring resources and making humans the priority. 

What sustainably FED suggests…

Elsewhere on this website and in our eCourses, you’ll see many references to extinction, disturbance and human appropriation of natural resources, often under the guise of population. 

At times we will even call them problems.

Only this is not because we lament the loss of species or habitat, sad as that is, we call them problems not because of extinction per se but because the appropriation of the landscape and converting energy into humans is actually the definitive challenge for humanity. 

Our challenge is not extinction but to cope with the consequences of rapid population growth. And when we say rapid we really do mean rapid to levels that historically no one would have even believed possible. 

But here we are at 8,000 extra people every hour. 

So we shouldn’t be surprised at extinction. Concerned certainly. That means that we might need to be more pragmatic about how some of 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 in order to keep those. 

Just as critical is how much extinction can we actually tolerate before it starts to impact our own likelihood of survival. 

It’s not a given that humans will live forever. And just about any type of dystopian future novel can tell you what it might look like if we don’t get sustainability right. And we are not very good at managing our resources in a way that looks to the future. 

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 can survive. Whether we will is up to us.

sustainably FED

Hero image modified from photo by Ales Krivec on Unsplash


Mark is an ecology nerd who was cursed with an entrepreneurial gene and a big picture view making him a rare beast, uncomfortable in the ivory towers and the disconnected silos of the public service. Despite this he has made it through a 40+ year career as a scientist and for some unknown reason still likes to read scientific papers.

Add comment

Subscribe to our explainer series

* indicates required

Most discussed