- You remember back in
the days right after the Permian-Triassic
Extinction event, with that giant flaming asteroid and
those methane explosions killed almost all of the
organisms on the planet? No, of course you don't,
because that happened two hundred and fifty
two million years ago and mammals weren't the thing yet. But that's kinda the
point of this episode, that asteroid was a disturbance to the ecology of the planet. The flora and the fauna and the 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 make up 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 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 the 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. (upbeat music) Disturbances happen in
ecosystems all the time, everyday a wildfire, 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 slash 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 procaryotes 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 gingkos 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 floor
barely survived extinction. Of all the dozens of species
of gingko 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. 'Cause plants rule the world, remember? Without plants, the animals in a community don't stand a chance and
primary successional species are often plants that
have wind borne seeds like lycophytes, or mosses
and lychens 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 outta 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 Permian-Triassic
extinction event took about four or five millions 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 fifty
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,
slight higher temperatures, less protection from weather, et cetera. 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 of tree, the soils will become different, the
mix of plants will become different and different
animals will wanna do business there because that little
niche suits there 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 five hundred 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 twentieth
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. Or 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 gonna look like a hundred years after a disturbance. Stochasticity is basically your element of unpredictable variability in anything. So, you could predict with
some accuracy what plants are gonna 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 a hundred years should always thought of as
probabilities, not certainties. Another difficulty 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 yeehaw 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
can 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 fallen 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 wipe out
level primary succession. But every nice, mid level
disturbance creates its own habitat at it's 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 even 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 U.S. great swaths of forests
were not allowed to burn. People considered the
purpose of forest to be wood production and you don't
wanna burn down some trees that are gonna 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 eight hundred thousand
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 twenty 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.