67: Palaeoecology, and trees on ice

Most of us have probably heard of palaeontology, palaeo- derived from the Greek for ‘ancient’, -on- (from ontos) meaning ‘creature’, and -ology being the study of something, so the study of ancient creatures. We might know this best as the study of dinosaur fossils, though it also includes the study of plant fossils, bug fossils and really the fossils of any ancient creature. What is palaeoecology then? Well, it’s the study of ancient ecology, this being the relationships between organisms and each other, or organisms and their environment in ancient times. This week we’ll be having a look at what that means in practice- how does one do palaeoecology? What have we discovered with it? What can it tell us about the world many thousands or millions of years ago? If you’re curious to find answers to all these questions, and why a forest may be on ice, read on!

Palaeontology is the study of old or ancient creatures, whilst palaeoecology looks at the relationship between these creatures and each other, or their environment, although palaeoecologists do a lot of work with palaeontologists, climatologists, geologists and archaeologists (Picture: Instituto Nacional de Antropología e Historia, Mexico)

How it is done

Although palaeoecology is quite new as a field of research, only around since the 1950s, realistically palaeontologists have been doing it for much longer. Naturally once they discovered massive dinosaurs used to roam the earth, they got curious about what these dinosaurs ate or what kind of environment they lived in, along with other ancient ecological questions. There is a difference between these two fields nowadays however. A palaeontologist might study a fossilised tree to understand the tree itself. On the other hand a palaeoecologist might look at a fossilized tree trunk to understand the climate it experienced. This can be done by examining the tree rings. During a year of good weather, the tree can grow lots, so has a wide space between tree rings of that year. In a year of poor weather with little growth, the tree rings are much closer together. By looking at the tree rings of, say, a tree that lived to be one hundred, the palaeoecologist can examine what the weather did during that hundred year period because of how it affected the tree.

Sometimes the age of the fossils can teach important things about ecology at that time too. For example, in rocks aged around 450million years ago there are loads of fossils of different kinds of marine creatures. However, in rocks from around 430million years ago, there are barely any. Palaeoecologists have used this to infer that there was some kind of event or change (thought to be glaciation and falling sea levels) which caused a mass extinction of animals, known as the Ordovician-Silurian extinction. This isn’t something you can see from the bones, but you can deduce from their presence or absence, along with other evidence. The types of fossils present can also tell us a lot. If there are many desert plants we know that it was quite hot, or bog plants indicate it was quite wet. The health of fossilised bones can also show if populations of animals were malnourished, implying little food was available in their environment.

Models can be really important tools too, where you can predict what might happen to the movement of animal species if certain relationships exist, and then test that movement theory against the distribution of animal fossils in the fossil record, to see if it is true. There are many methods used within palaeoecology to deduce things about the past. So, if all this research has been done, what has it helped us learn about ancient times?

Fossils aren’t just t-rex skulls. You can also get fossilised plants like this fern on the left, or even fossilised pollen grains such as those shown in this microscopic image on the right (Picture: Debra A. Willard, USGS)

What we’ve learned

A lot of what we know about the distant past is thanks to palaeoecology, or supported by palaeoecology with crossover to climatology, archaeology and palaeontology too. Not all of it is in ancient history- some of what palaeoecology has discovered is related to humans. For example we know that humans domesticated plants and over time started to selectively breed them for traits that we wanted, because we can see how the pollen fossils of one brassica species slowly became more and more diverse until it eventually led to the creation of things like broccoli, cauliflower and mustard today, just like how we have bred wolves of the past into everything from St Bernard’s dogs to chihuahuas.

Another important example of things palaeoecology has taught is that our climate is changing in an unusual way. We know this because we can infer what the climates of the past were like from of fossils (e.g. if there’s a lot of marine fossils, sea levels may have been higher, so there may have been less ice) and how long or stable these periods of differences were. As mentioned earlier, it has allowed us to understand mass extinction events in the past, but it also tell us a lot about the resilience of species. We can see how species have adapted to changes in the climate before, such as during the last ice age, ending around 11,000 years ago (more on ice ages here). At that time huge ice sheets, many kilometres thick, extended down to central Europe. Species of birds and insects, and the ancestors of humans, moved south to avoid the ice, and then moved back north when these ice sheets melted, adapting to the changing climate through this migration.

Today we know that most species live along the equator (on the left the distribution of vertebrate animals, red being the highest down to blue, and on the right the distribution of plants with red being the highest down to white). Using climate data from the past, and fossil records of species which are still alive today, we can see how some plants and animals might have moves as the climate changed

Life on ice

Interestingly some very recent research published in 2018 by Dr Rolf Zale and Colleagues has shown that this running away from ice isn’t a technique used by all organisms. Whilst many birds and mammals fled south, some plants were seen to cling on to the edges of the ice sheets. This is not so surprising, as they were on the edge, near to soil and warmer climates, and even today we have many plants growing on permafrost (permanent underground ice, for more info click here). However, this recent study shows that plants can be hardier than expected. It took sediment cores dating back to this last ice age from a lake in Northern Sweden, which at the time would have been covered in many kilometres of ice. You would think that, sitting under an ice sheet, this sediment would have basically nothing in it, yet the researchers found not only pollen grains from grasses and sedges, but also heathery shrubs and even trees like larches. How could this happen? Surely you can’t have a forest under a glacier! Well, no, instead the study suggests that these plants grew on top of the ice at that time, and when the glaciers or ice sheets melted away many years later, this sediment got deposited onto the ground’s sediment layers (what’s the difference between a glacier and an ice sheet you ask? Find out here).

It’s one thing to have a few blades of grass clinging on to dear life at the edge of a frozen zone, even Antarctica only has 2 native plant species that we know of today. However, this research suggests that whole trees, which would have required rich soil layers to sustain them, were growing on top of thick ice far away from the edge of the ice sheet. It helps to answer questions such as why Sweden seemed to have a lot of plant life straight after the ice age, rather than being a barren rocky wasteland that slowly got colonised (more about plant colonisation here). Even today in some places like Alaska you can find whole forests growing on top of glaciers!

We have previously thought that there was little to no plant life at the centre of ice sheets like this one in Antarctica. However, a recent study suggests that during the last ice age there were plants as big as trees growing on top of many kilometres thick ice. (Picture: NASA/Public Domain)

It can be difficult to know what the past of our planet was like because we weren’t there to see, feel, touch and write about it. However, thanks to the deducing and inferring from fossil and chemical evidence in fields like paleoecology, we can get a strong idea of what it was likely to be like. This has just been a short introduction to the wonders of palaeoecology, which can teach us things like what the world looked like to trilobites millions of years ago, when and perhaps why agriculture originally rose amongst humans, and how animals and plants have responded to big changes in climate in the past, giving us important information about how we can mitigate, adjust and adapt to the changes going on in our climate today. As with all history, our past can teach us a lot about our present and what might be the best way to advance into our future.

For more info:

The recent research paper by Rolf Zale and colleagues about trees on glaciers:

https://www.sciencedirect.com/science/article/pii/S0277379117307771

A piece of palaeoecological research being done in Vietnam:

Music: kongano.com

Cover image: University of Exeter

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