Early Land Plants Made The World Muddy

Cooksonia is one of the earliest land plants to have evolved. Photo by Matteo De Stefano/MUSE licensed under CC BY-SA 3.0

Try to picture the world before life moved onto land. It would have been a vastly different landscape than anything we know today. For one, there would have been no soil. Before life moved onto land, there was nothing organic around to facilitate soil formation. This would have changed as terrestrial habitats were slowly colonized by microbes and eventually plants. A recent paper published in Science is one of the first to demonstrate that the rise in certain sediments on land, specifically mud-forming clays, coincided with the rise in deep-rooted land plants.

This was no small task. The research duo had to look at thousands of reports spanning from the Archean eon, some 3.5 billion years ago, to the Carboniferous period, some 358 million years ago. By looking for the relative amounts of a sedimentary rock called mudrock in terrestrial habitats, they were able to see how the geology of terrestrial habitats was changing through time. What they found was that the presence of mudrock increased by orders of magnitude around the same time as early land plants were beginning to colonize land. Before plants made it onto land, mudrocks comprised a mere 1% of terrestrial sediments. By the end of the Carboniferous period, mudrocks had risen to 26%.

This begs the question, why are mudrocks so significant? What do they tell us about what was going on in terrestrial environments? A key to these questions lies in the composition of mudrocks themselves. Mudrock is made up of fine grained sediments like clay. There are many mechanisms by which clay can be produced and certainly this was going on well before plants made it onto the scene. The difference here is in the quantity of clay-like minerals in these sediments. Whereas bacteria and fungi do facilitate the formation of clay minerals, they do so in small quantities.

A little bit of moss goes a long way for erosion control!

The real change came when plants began rooting themselves into the earth. In pushing their roots down into sediments, plants act as conduits for increased weathering of said minerals. Roots not only increase the connectivity between subsurface geology and the atmosphere, they also secrete substances like organic acids and form symbiotic relationships with cyanobacteria and fungi that accelerate the weather process. No purely tectonic or chemical processes can explain the rate of weathering that must have taken place to see such an increase in these fine grained minerals.

What's more, the presence of rooted plants on land would have ensured that these newly formed muds would have stuck around on the landscape much longer. Whereas in the absence of plants, these sediments would have been washed away into the oceans, plants were suddenly holding onto them. Plant roots act as binders, holding onto soil particles and preventing erosion. Aside from their roots, the rest of these early land plants would have also held onto sediments via a process known as the baffling effect. As water and wind pick up and move sediments, they inevitably become trapped in and around the stems and leaves of plants. Even tiny colonies of liverworts and moss are capable of doing this and entire mats of these would have contributed greatly to not only the formation of these sediments, but their retention as well.

The movement of plants onto land changed the course of history. It was the beginning of massive changes to come and much of that started with the gradual formation of soils. We owe everything to these early botanical pioneers.

Photo Credit: [1]

Further Reading: [1]

Invasion of the Earthworms

Photo by Rob Hille licensed under CC BY-SA 3.0

As an avid gardener, amateur fisherman, and a descendant of a long line of farmers, I have always held earthworms in high regard. These little ecosystem engineers are great for all of the above, right?

Not so fast! Things in life are never that simple! Let's start at the beginning. If you live in an area of North America where the glaciers once rested, there are no native terrestrial worms in your region. All of North America's native worm populations reside in the southeast and the Pacific northwest. All other worms species were wiped out by the glaciers. This means that, for millennia, northern North America's native ecosystems have evolved without the influence of any type of worms in the soil.

When Europeans settled the continent, they brought with them earthworms, specifically those known as night crawlers and red wigglers, in the ballasts of their ships. Since then, these worms have been spread all over the continent by a wide range of human activities like farming, composting, and fishing. Since their introduction, many forests have been invaded by these annelids and are now suffering heavily from earthworm activities.

As I said above, any areas that experienced glaciation have evolved without the influence of worms. Because of this, forests in these regions have built up a large, nutrient-rich, layer of decomposing organic material commonly referred to as "duff" or "humus." Native trees, shrubs, and forbs rely on this slowly decomposing organic material to grow. It is high in nutrients and holds onto moisture extremely well. When earthworms invade an area of a forest, they disrupt this rich, organic layer in very serious ways.

Worms break through the duff and and distribute it deeper into the soil where tree and forb species can no longer access it. Worms also pull down and speed up the decomposition of leaves and other plant materials that normally build up and slowly create this rich organic soil. Finally, earthworm castings or poop actually speed up runoff and soil erosion.

All of this leads to seriously negative impacts on native ecosystems. As leaves and other organic materials disappear into the soil at an alarming rate via earthworms, important habitat and food is lost for myriad forest floor organisms. In areas with high earthworm infestations, there is a significant lack of small invertebrates like copepods. The loss of these organisms has rippling effects throughout the ecosystem as well. It has been shown that, through these activities, earthworms are causing declines in salamander populations.

It gets worse too. As earthworms speed up the breakdown of the duff or humus, our native plant species are suffering. They have evolved to germinate and grow in these rich, organic soils. They rely on these soils for survival. As the nutrient rich layers get redistributed by earthworms, native plant and tree populations take a hit. Spring ephemerals have been hit the hardest by earthworm invasions for these reasons and more. There is very little recruitment and, in time, many species are lost. For small seeded species like orchids, earthworms can even consume seeds, which either destroys them outright or drags them down deeper into the soil where they cannot germinate. Earthworms have also been shown to upset the mycorrhizal fungi networks which most plant species cannot live without.

Top Left: Forest soil horizons without earthworms; Top Right: Forest soil mixed due to earthworms; Bottom Left: Forest understory diversity without earthworms; Bottom Right: Forest understory diversity with earthworms. Credits: [1]

So, what can we do about this? Well, for starters, avoid introducing new populations of earthworms to your neighborhood. If you are using earthworms as bait, do not dump them out onto land when you're done. If you must get rid of them, dump them into the water. Also, if you are using worm castings in your garden, it has been recommended that you freeze them for about a week to assure that no eggs or small worms survive the ride. If you are bringing new plants onto your property, make sure to check their root masses for any worm travelers. Remember, no worms are native if you live in a once glaciated region.

Sadly, there is not much we have come up with at this point for dealing with the current earthworm invasion. What few control methods have been developed are not practical on a large scale and can also be as upsetting to the native ecology as the earthworms. The best bet we have is to minimize the cases of new introductions. Earthworms are slow critters. They do not colonize new areas swiftly. In fact, studies have shown that it takes upwards of 100 years for earthworm populations to migrate 1/2 mile! Armed with new knowledge and a little attention to detail, we can at least slow their rampage.

Further Reading: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

On Soil and Speciation

Lord Howe Island. Photo by John Game licensed under CC BY 2.0

Many of you will undoubtedly be familiar with some variation of this evolutionary story: A population of one species becomes geographically isolated from another population of the same species. Over time, these two separate populations gradually evolve in response to environmental pressures in their respective habitats. After enough time has elapsed, gradual genetic changes result in reproductive isolation and eventually the formation of two new species. This is called allopatric speciation and countless examples of this exist in the real world.

At the opposite end of this speciation spectrum is sympatric speciation. Under this scenario, physical isolation does not occur. Instead, through some other form of isolation, perhaps reproductive or phenological, a species gives rise to two new species despite still having contact. Examples of this in nature are far less common but various investigations have shown it is indeed possible. Despite its rarity, examples of sympatric speciation have nonetheless been found and one incredible example has occurred on a small oceanic island off the coast of Australia called Lord Howe Island.

Howea belmoreana and Howea forsteriana [SOURCE]

Lord Howe Island is relatively small, volcanic island that formed approximately 6.4–6.9 million years ago. It is home to four distinct species of palm trees from three different genera, all of which are endemic. Of these four different palms, two species, Howea belmoreana and Howea forsteriana, are quite common. Interestingly enough, H. forsteriana, commonly known as the kentia palm, is one of the most commonly grown houseplants in the entire world. However, their horticultural value is not the most interesting thing about these palms. What is most remarkable is how these two species arose. 

Multiple genetic analyses have reveled that both species originated on Lord Howe Island. This is kind of odd considering how small the island actually is. Both palms can regularly be found growing in the vicinity of one another so the big question here is what exactly drove the evolution of their common ancestor? How does a single species growing on a small, isolated island become two? The answer is quite surprising.

Howea belmoreana Photo by John Game licensed under CC BY 2.0

When researchers took a closer look at the natural histories of these two species, they found that they were in a sense isolated from one another. The isolation is due to major phenological or timing differences in their reproductive efforts. H. forsteriana flowers roughly six weeks before H. belmoreana. Flowering time is certainly enough to drive a wedge between populations but the question that still needed answering was how do such phenological asynchronies occur, especially on an island with a land area less than 12 square kilometers? 

As it turns out, the answer all comes down to soil. Individuals of H. belmoreana are restricted to growing in neutral to acidic soils whereas H. forsteriana seems to prefer to grow in soils rich in calcarenite. These soils have a more basic pH and dominate the low lying areas of the island. Growing in calcarenite soils is stressful as they are poor in nutrients. This physiological stress has caused a shift in the way in which the flowers of H. forsteriana mature. When found growing on richer volcanic soils, the researchers noted that the flowers mature in a way that is more synchronous, not unlike the flowers of H. belmoreana.

Thanks to their attention to detailed life history events and conditions, researchers were able to show that soil preferences caused a phenological shift in the flowering of these two related species. Because they flower at completely different times when growing on their respective soil types, enough reproductive isolation was introduced to disrupt the random mating process of these wind pollinated palms. As soon as such reproductive biases are introduced, speciation can and will occur.

Photo Credits: [1] [2] [3]

Further Reading: [1]