Ecological Succession: Change is Good - Crash Course Ecology #6

Summary

This episode of Crash Course: Ecology explores the concept of ecological succession, illustrating how disturbances like the Permian-Triassic Extinction Event can pave the way for new life. It explains primary and secondary succession, emphasizing the role of plants in rebuilding ecosystems and the importance of moderate disturbances for maintaining biodiversity. The video challenges the notion of stable climax communities, advocating for the natural process of change and disturbance as essential for a healthy ecosystem.

Takeaways

• 🌏 The Permian-Triassic Extinction Event, occurring 250 million years ago, wiped out most life on Earth, setting the stage for ecological succession.
• 🌱 Ecological succession is the process by which the makeup of a community changes over time, starting from the aftermath of a disturbance.
• 🔥 Disturbances like fires, floods, and windstorms are common in ecosystems and lead to predictable changes despite their unpredictability.
• 🌳 Primary succession occurs in lifeless areas, where pioneer species like prokaryotes, protists, and non-vascular plants begin to colonize.
• 🌲 Secondary succession follows primary succession, often after smaller disturbances, and can lead to the development of complex ecosystems like forests.
• 🕰 Primary succession is a lengthy process, potentially taking hundreds of thousands to millions of years for full recovery.
• 🛠 Soil development is a critical part of succession, as it provides the foundation for plant growth and supports biodiversity.
• 🔄 Change is the only constant in ecology; ecosystems are always in flux, with no true 'stability'.
• 🎯 The concept of a climax community, once thought to be the end goal of succession, has been replaced by the understanding of ecosystems as dynamic and ever-changing.
• 🌡 Factors like temperature and precipitation play a significant role in shaping the microclimates within ecosystems, influencing plant and animal life.
• 🔄 The Intermediate Disturbance Hypothesis suggests that moderate disturbances are ideal for promoting biodiversity and ecosystem health.

Q & A

What major event is referred to as the 'Permian-Triassic Extinction Event'?

The Permian-Triassic Extinction Event, also known as the 'Great Dying,' is an extinction event that occurred about 250 million years ago and resulted in the loss of nearly 90% of marine species and about 70% of terrestrial vertebrate species.

What is ecological succession?

Ecological succession is the process by which the structure of a biological community evolves over time following a disturbance, such as a fire, flood, or asteroid impact. It involves the predictable changes in species composition and ecosystem structure that occur over time.

What is the difference between primary and secondary succession?

Primary succession occurs in an area where no life previously existed, such as after an asteroid impact or where a glacier has retreated. Secondary succession occurs in areas where a community has been disturbed but where soil and some life forms remain, such as after a forest fire.

Why are pioneer species important in ecological succession?

Pioneer species are important in ecological succession because they are often the first to colonize a disturbed area. They can survive in harsh conditions and help to build or rebuild the soil, paving the way for other species to move in and establish themselves.

What role do plants play in primary succession?

In primary succession, plants play a crucial role as they are often the first organisms to colonize an area. They help to build or rebuild the soil through the process of growth, death, and decomposition, creating a foundation for other life forms to inhabit.

What is the significance of soil in ecological succession?

Soil is significant in ecological succession because it provides the necessary nutrients and environment for plant growth. The development of soil over time is a key indicator of the progression of succession, as it allows for the establishment of a more diverse range of plant and animal species.

How does the concept of 'stochasticity' relate to ecological succession?

Stochasticity, or randomness, in ecology refers to the unpredictable variability in ecological processes. It affects ecological succession by introducing elements of uncertainty, making it impossible to predict with absolute certainty the future state of a community after a disturbance.

What is the Intermediate Disturbance Hypothesis?

The Intermediate Disturbance Hypothesis suggests that moderate disturbances, which are neither too small to have an effect nor too large to destroy the entire ecosystem, are ideal for promoting biodiversity. These disturbances create new niches and habitats, leading to a more diverse and stable ecosystem.

Why is biodiversity important in ecosystems?

Biodiversity is important in ecosystems because it contributes to the stability and health of the environment. A high level of biodiversity means that there are many different species and niches, which can better withstand disturbances and provide a range of ecosystem services.

How have human activities affected ecological succession and biodiversity?

Human activities, such as preventing natural forest fires or altering landscapes for development, can disrupt the natural process of ecological succession. This can lead to a decrease in biodiversity and an increase in catastrophic events, such as large-scale forest fires.

What is the historical perspective on ecological succession and how has it changed?

Historically, ecologists viewed succession as leading to a stable 'climax community.' However, modern understanding recognizes that ecosystems are dynamic and never truly stable, with change and disturbance being constant factors that contribute to biodiversity and ecosystem health.

Outlines

00:00

🌿 Ecological Succession and Disturbances

The paragraph discusses the concept of ecological succession, using the Permian-Triassic Extinction Event as a dramatic example of how ecosystems can change over time. It explains that after such a massive disturbance, the planet's ecology was reset, allowing for new species to emerge and fill the void left by the extinction. The paragraph also touches on the idea that smaller, more frequent disturbances can lead to a more robust and diverse ecosystem, as they create opportunities for change and adaptation. The process of primary and secondary succession is introduced, with primary succession occurring in lifeless areas and secondary succession following smaller disturbances in already established ecosystems.

05:02

🔍 The Complexity of Ecological Succession

This paragraph delves into the historical perspective of ecological succession, where ecologists once believed that communities would reach a stable 'climax community' after a series of changes. However, it is now understood that ecosystems are inherently unstable and subject to constant change, with no true end point to succession. The concept of stochasticity, or randomness, is introduced to explain the unpredictability of ecological outcomes. The paragraph also discusses the idea that ecosystems are not单一 communities but rather complex mosaics of habitats, formed by a series of disturbances over time. The Intermediate Disturbance Hypothesis is presented, suggesting that moderate disturbances are crucial for maintaining biodiversity and ecosystem health. The paragraph concludes with a call to embrace natural disturbances as a normal and beneficial part of ecological processes.

Keywords

💡Permian-Triassic Extinction Event

The Permian-Triassic Extinction Event, also known as the Great Dying, was the most severe extinction event in Earth's history, occurring around 250 million years ago. It wiped out about 96% of marine species and 70% of terrestrial vertebrate species. In the video, this event is used as a dramatic example of ecological disturbance, setting the stage for primary succession and illustrating the concept of how life can recover and evolve after a mass extinction.

💡Ecological Succession

Ecological succession refers to the process by which the structure of a biological community evolves over time. It is a fundamental concept in ecology, demonstrating how life responds to changes in the environment. The video discusses both primary and secondary succession, showing how communities change and develop after a disturbance, such as the extinction event mentioned or smaller-scale events like forest fires.

💡Disturbance

In ecology, a disturbance is an event that disrupts an ecosystem and creates opportunities for change. The video script uses various examples, such as an asteroid impact, fires, floods, and windstorms, to illustrate how disturbances can initiate the process of ecological succession and lead to the development of new habitats and communities.

💡Primary Succession

Primary succession is the process by which life colonizes a previously uninhabited area or an area where the original vegetation has been completely removed. The video explains that this type of succession occurs in 'lifeless, post-apocalyptic wastelands' and is characterized by the gradual establishment of pioneer species that can thrive in harsh conditions without competition.

💡Pioneer Species

Pioneer species are the first organisms to colonize an area after a disturbance, often in harsh conditions. They are typically hardy, able to grow in places with little soil or where conditions are otherwise inhospitable. The video mentions prokaryotes, protists, non-vascular plants, and lycophytes as examples of pioneer species that initiate primary succession.

💡Soil Development

Soil development is a critical part of ecological succession, as it provides the foundation for plant growth and, subsequently, supports animal life. The video emphasizes the importance of soil in succession, describing how it is built up over time through the growth, death, and decomposition of plants, particularly during primary succession.

💡Secondary Succession

Secondary succession occurs in areas where the existing vegetation has been disturbed but not completely destroyed, allowing for a quicker recovery of the ecosystem. The video contrasts this with primary succession, noting that secondary succession can happen over shorter time frames and involves the reestablishment of plant and animal communities after smaller disturbances like fires or floods.

💡Biodiversity

Biodiversity refers to the variety of life in an ecosystem, including the number of different species and their genetic diversity. The video script highlights the importance of biodiversity in creating a stable and healthy ecosystem, explaining that high biodiversity is a sign of a mature ecosystem and that disturbances can contribute to increased biodiversity by creating new niches for species.

💡Stochasticity

Stochasticity, in the context of ecology, refers to the random or unpredictable elements that affect ecological processes, such as succession. The video explains that stochasticity introduces variability and uncertainty into ecological outcomes, making it impossible to predict with absolute certainty how a community will look in the future.

💡Intermediate Disturbance Hypothesis

The Intermediate Disturbance Hypothesis suggests that moderate disturbances are most beneficial for maintaining biodiversity in an ecosystem. The video script explains this concept by stating that such disturbances create a variety of habitats at different stages of succession, which in turn support a wide range of species and contribute to overall ecosystem health.

💡Climax Community

A climax community is an old concept in ecology that refers to a stable state of an ecosystem that has reached the end of succession, with a predictable set of species. The video script points out that this idea has been largely replaced by the understanding that ecosystems are dynamic and constantly changing, with no true 'climax' state of stability.

Highlights

The Permian-Triassic Extinction Event, also known as 'The Great Dying,' wiped out most life on Earth, setting the stage for ecological succession.

Ecological succession is the process by which the structure of an ecological community evolves over time following a disturbance.

Disturbances such as fires, floods, or windstorms are common in ecosystems and trigger predictable changes despite their unpredictability.

Primary succession occurs in lifeless areas and is characterized by pioneer species colonizing the disturbed site.

Pioneer species, such as prokaryotes, protists, and non-vascular plants, are the first to colonize disturbed areas due to lack of competition.

The Permian-Triassic event led to the rise of simpler vascular plants like lycophytes, replacing the complex gymnosperm forests.

Ginkgo biloba is a living fossil, the last of its kind from the time of the Permian-Triassic event, highlighting the rarity of species survival post-extinction.

Plants play a crucial role in primary succession by building or rebuilding soils, which is essential for the establishment of animal communities.

Secondary succession follows primary succession and occurs in areas where the soil already exists, leading to rapid changes in the community structure.

Intermediate Disturbance Hypothesis suggests that moderate disturbances are ideal for promoting biodiversity and ecosystem health.

Ecosystems are never truly stable; they are dynamic and constantly changing, with disturbances being a natural part of this process.

Stochasticity, or randomness, plays a significant role in ecological succession, making it impossible to predict community outcomes with certainty.

The idea of a climax community, a stable endpoint of succession, has been challenged by the understanding that ecosystems are always subject to change.

Biodiversity is a sign of a mature ecosystem, achieved through a complex mosaic of habitats created by disturbances over time.

The historical suppression of natural disturbances, such as forest fires, has led to catastrophic consequences, highlighting the importance of intermediate disturbances.

The recovery of Yellowstone National Park after the 1988 fires demonstrates the resilience of ecosystems and the beauty of ecological succession.

Heraclitus's philosophy that change is the only constant is a fundamental principle in understanding ecological succession.

Transcripts

You remember back in the days right after the Permian-Triassic Extinction Event,

when that giant flaming asteroid and those methane explosions killed almost all the organisms on the planet?

No, of course you don't, because that happened 250 million years ago when mammals weren't a thing yet.

But that's kind of the point of this episode: that asteroid was a disturbance to the ecology of the planet.

The flora and fauna and soils were largely wiped out, leaving a blank canvas for the organisms that survived

-- and there weren't really all that many of them -- to fill in as they could.

What happened after the Permian-Triassic "disturbance" is a dramatic example of ecological succession,

how the makeup of a community changes over time, starting from, like, the day after a disturbance.

Just, usually the disturbance is a little less...disturbing.

The study of how ecological communities change doesn't just look at huge, long periods of time

or the effects of some apocalypse, succession can easily happen over a season in a park

or in just a few days in a patch of land as small as your garden.

And this might come as a surprise, but disturbances that shake up that status quo within a community

actually serve to make that community better in the long run.

Because much like life and the entire universe, succession is all about change

and change is how a universe full of nothing but hydrogen came to include a planet full of life.

[Theme Music]

Disturbances happen in ecosystems all the time, every day, a fire, a flood, a windstorm.

After these unpredictable events, ecologists kept seeing predictable, even orderly, changes in the ecosystem.

How life deals with these disturbances is an important key to understanding ecosystems.

First, let's note that a tree falling in the forest and a comet falling in the forest,

while both disturbances, are different levels of disturbance.

Likewise, there are a couple of different types of succession.

The first type, the one that happens after the asteroid hits, or the glacier plows over the landscape,

or the forest fire/volcano burns the verdant ecosystem into pure desolation --

that's called a primary succession, when organisms populate an area for the first time.

The jumping off point for primary succession is your basic, lifeless, post-apocalyptic wasteland.

You're probably thinking, "That place sounds terrible, who would ever want to live there?"

Well actually, there is one tremendous advantage to desolate wastelands: no competition.

A lot of organisms don't mind settling down in the more inhospitable nooks and crannies of the planet;

these pioneer species are often prokaryotes or protists,

followed by non-vascular plants, then maybe some extra-super-hardy vascular plants.

There are tons of organisms that make their living colonizing dead places; it's their thing.

Like before the Permian-Triassic extinction, there were these dense forests of gymnosperms,

probably full of species a lot like the conifers and ginkgos and cycads we still have today.

But after the asteroid hit, the big forests died and were replaced by lycophytes,

simpler vascular plants like the now-extinct scale trees and today's club mosses.

While they might have had a hard time competing with the more complicated plants during the good times,

the rest of the Paleozoic flora barely survived extinction.

Of all the dozens of species of ginkgo that were around back then,

only one still exists, completely genetically isolated -- a living fossil.

It's important to remember that when we talk about primary ecological succession,

we're talking about plants pretty much exclusively.

Because plants rule the world, remember?

Without plants, the animals in a community don't stand a chance,

and primary successional species are almost often plants that have wind-borne seeds, like lycophytes,

or mosses and lichens that have spores that blow in and colonize the area.

And the outcome of a primary successional landscape is to build or rebuild soils,

which develop over time as the mosses, grasses, and tiny little plants grow, die, and decompose.

Once the soils are ready, slightly bigger plants can move in,

at which point we move on to secondary succession, and then it's game on;

a whole redwood forest could develop out of that!

But primary succession takes a long, long time, like hundreds, maybe thousands of years in some places.

In fact, the recovery of these big gymnosperm forests after the Permian-Triassic Extinction

Event took about 4 or 5 million years.

Dirt may seem unglamorous to you, but it is alive and beautiful and complicated, and making good soil takes time.

Now secondary succession isn't just the next act after primary succession has made a place livable after some disaster,

it's usually the first response after a smaller disturbance

like a flood or a little fire has knocked back the plants that have been ruling the roost for a while.

Even a disturbance as small as a tree crashing down in the woods can make a tiny patch of

forest more like it was 50 years ago,

before that one tree got so huge and shady.

In that tiny area, there will suddenly be a different microclimate than in the rest of the forest,

which might have more sunlight, slightly higher temperatures, less protection from weather, etc.

And just like every other ecosystem on Earth, this tiny patch of forest will be affected

by temperature and precipitation the most, which will be different in different parts of the forest.

So as a result of the fallen tree, the soils will become different, the mix of plants will become different,

and different animals will want to do business there because that little niche suits their

needs better than other little niches.

So the question becomes, when does succession stop and things get back to normal?

NEVER. Because change doesn't end. Change is the only constant, people! You know who said that?

Heraclitus, in 500 BC, so it's been true since at least then. Consider it a lesson in life.

And as ideas in ecology go, it's actually a pretty new way of looking at things.

See, back in the early 20th century, ecologists noticed a tendency of communities to morph over time,

but they also saw succession in terms of a community changing until it ultimately ended

in what they called a climax community,

which would have a predictable assemblage of species that would remain stable until the next big disturbance.

Well, maybe that's what seemed to be happening, but ecological succession is actually a lot more complicated than that.

For starters, there's a little thing called stochasticity, or randomness,

which prevents us from ever knowing exactly what a community is going to look like 100 years after a disturbance.

Stochasticity is basically your element of unpredictable variability in anything.

So you can predict with some accuracy what plants are going to take over a glacial moraine after the ice has receded

because the seeds of some colonizer species typically make it there first.

But unpredictable things, like weather conditions during early stages of succession, can end up favoring another species.

The point is, scientists' attempt to predict what a community ends up looking like in 100

years should always be thought as probabilities, not certainties.

Another difficult with the whole model of a climax community has to do with the idea

of an ecosystem eventually stabilizing.

That word "stable," whenever it's used in a sentence that also includes the word "ecology,"

you can pretty much be sure it's being used wrong, because stability never happens.

There are always disturbances happening all the time in every ecosystem.

A small portion of the forest might burn, a windstorm might take out a bunch of trees,

some yee-haw might rent himself a backhoe one weekend and clear himself a little patch of heaven

on the mountain beside his house because he's got nothing better to do.

Who knows? Stuff happens!

So instead of ending in some fixed, stable climax community,

we now know that an ecosystem is in later successional stages if it has high biodiversity

-- lots and lots of biodiversity.

And the only way biodiversity could be high is if there are tons of little niches for all those species to fit into.

And the only way there could be that many niches is if, instead of a single community,

an ecosystem is actually made up of thousands of tiny communities --

a mosaic of habitats where specific communities of different organisms lived.

Such mosaics of niches are created by disturbances over time, with everything always changing here and there,

but it's important that these disturbances be of the right kind and the right scale.

Because it turns out that the kind of disturbances that have the greatest effect on biodiversity

are the most moderate disturbances.

When ecologists figured this out, they decided to call it the Intermediate Disturbance Hypothesis,

because it hypothesizes that intermediate disturbances -- not too big and not too little -- are ideal.

See, just a little disturbance, like a falling tree or something, isn't really enough to change the game.

On the other hand, a really severe disturbance like getting covered with lava would take

the community all the way back to asteroid-wipeout level primary succession.

But every nice mid-level disturbance creates its own habitat at its own stage of succession with its own unique niches.

More niches means more biodiversity, and more biodiversity means more stability and healthier ecosystems.

Even if two disturbances happen in two different areas with roughly the same climate at the same time,

the stochastic nature of ecosystems mean that the two areas might recover in completely different ways,

leading to even more niches and more biodiversity.

Now, this does not mean that you should go rent a backhoe tomorrow and cut a swath into the wilderness.

It's just suggesting that a medium level of disturbance is natural and normal and good for an ecosystem;

it keeps everybody on their toes.

And like I said, disturbance happens, and by and large we should let it happen.

This too is a relatively new idea in ecology.

In fact, for most of the history of public land management in the US, great swaths of

forest were not allowed to burn.

People considered the purpose of forests to be wood production,

and you don't want to burn down some trees that are going to make you a bunch of money.

But because of the lack of intermediate disturbances over a long period of time,

we ended up with catastrophic fires like the one that torched Yellowstone National Park back in 1988.

A single lightning strike totally annihilated almost 800,000 acres of public forest

because the ecosystem hadn't been allowed to indulge in a nice, leisurely burn every now and then.

But now, those forests have undergone more than 20 years of succession,

and some parts have even re-burned at a more intermediate level,

creating a nice, high-biodiversity mosaic of habitats.

And it's gorgeous, you should come visit it sometime.

And that is ecological succession for you. How destruction and disturbance lead to beauty and diversity.

Just remember what my main man Heraclitus said and you'll be good: the only constant is change.

Thank you for watching this episode of Crash Course: Ecology,

and thank you to everyone who helped us put this episode together.

If you want to review any of the concepts that we studied today, there's the table of contents over there.

And if you have any questions or ideas or comments, we're on Facebook and Twitter and

of course, down in the comments below. We'll see you next time.